A second possible explanation is that even though gamma band activity is important in tinnitus perception, it has also been proposed that this gamma band activity only leads to conscious perception if this activity is connected to a larger network involved in conscious perception (van der Loo et al., 2009; De Ridder et al., 2011a). Gamma band activity, which normally waxes and wanes, and is spatially restricted to small areas, actually is nested on low

frequency activity, predominantly theta activity, in order to connect to widespread larger networks, both for normal cognition (Canolty et al., 2006; Lisman and Buzsaki, 2008) and in tinnitus (De Ridder et al., 2011a). In a recent study it has been demonstrated that auditory attention control is mediated via gamma band activity in different brain areas, which were connected via theta activity, the phase of which determined gamma synchronization (Doesburg et al., 2012). Thus it could have been better to select theta connectivity as a potential prognostic biomarker, as it is possible theta is a carrier wave on which the information rich gamma activity is nested.

Another explanation can be related to the exactness of the electrode positioning. As gamma band activity is usually spatially restricted and only present in a small focal area, the exact positioning of the cathode and anode might be critically involved in the success of the tDCS stimulation. Since this study was not performed using neuronavigation, because of methodological and technical reasons [(1) sLORETA uses standard head model,

#### **REFERENCES**

(2) EEG cannot be read into the neuronavigation machine] it cannot be excluded that the electrodes were not spatially correctly positioned (Vanneste et al., 2011b).

Thus, whereas the results of the bifrontal tDCS seem to confirm previous studies (Vanneste et al., 2010c, 2011b; Faber et al., 2011; Frank et al., 2012), in that bifrontal tDCS with anode overlying the right DLPFC and the cathode the left DLPFC has a beneficial effect on tinnitus loudness and distress perception, the use of gamma band functional connectivity seems not bring any advantage to tDCS for tinnitus suppression. Using other potential biomarkers, such as gamma band activity, or theta functional connectivity could theoretically be better alternatives. Further studies will have to elucidate whether brain state based tDCS has any advantages over "blind" bifrontal stimulation.

#### **ACKNOWLEDGMENTS**

The authors thank Jan Ost, Bram Van Achteren, Bjorn De Vree, and Pieter Van Looy for their help in preparing this manuscript.

P., Sanchez, T. G., Nitsche, M. A., and Pascual-Leone, A. (2006). Transient tinnitus suppression induced by repetitive transcranial magnetic stimulation and transcranial direct current stimulation. *Eur. J. Neurol.* 13, 996–1001.

combined function of gamma and theta oscillations. *Schizophr. Bull.* 34, 974–980.

surface electrodes. *Anesth. Analg.* 47, 717–723.

(2011b). Different resting state brain activity and functional connectivity in patients who respond and not respond to bifrontal tDCS for tinnitus suppression. *Exp. Brain Res.* 210, 217–227.

current stimulation: a preliminary clinical study. *Exp. Brain Res.* 202, 779–785.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 01 March 2012; accepted: 04 September 2012; published online: 25 September 2012.*

*Citation: De Ridder D and Vanneste S (2012) EEG driven tDCS versus bifrontal tDCS for tinnitus. Front. Psychiatry 3:84. doi: 10.3389/fpsyt.2012.00084*

*This article was submitted to Frontiers in Neuropsychiatric Imaging and Stimulation, a specialty of Frontiers in Psychiatry. Copyright © 2012 De Ridder and Vanneste. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.*

#### *Niels Hansen\**

*Department of Neurophysiology, Ruhr University Bochum, Bochum, Germany*

#### *Edited by:*

*Andre R. Brunoni, Universidade de São Paulo, Brazil*

#### *Reviewed by:*

*Luiz Kobuti Ferreira, Universidade de Sao Paulo, Brazil Pedro Shiozawa, Santa Casa de Misericórdia de São Paulo, Brazil*

#### *\*Correspondence:*

*Niels Hansen, Department of Neurophysiology, Ruhr University Bochum, Universitätsstrasse 150, MA 4/150, 44780 Bochum, Germany. e-mail<EMAIL_ADDRESS>
The pharmacological treatment of Alzheimer's disease (AD) is often limited and accompanied by drug side effects. Thus alternative therapeutic strategies such as non-invasive brain stimulation are needed. Few studies have demonstrated that transcranial direct current stimulation (tDCS), a method of neuromodulation with consecutive robust excitability changes within the stimulated cortex area, is beneficial in AD. There is also evidence that tDCS enhances memory function in cognitive rehabilitation in depressive patients, Parkinson's disease, and stroke. tDCS improves working and visual recognition memory in humans and object-recognition learning in the elderly. AD's neurobiological mechanisms comprise changes in neuronal activity and the cerebral blood flow (CBF) caused by altered microvasculature, synaptic dysregulation from ß-amyloid peptide accumulation, altered neuromodulation via degenerated modulatory amine transmitter systems, altered brain oscillations, and changes in network connectivity. tDCS alters (i) neuronal activity and (ii) human CBF, (iii) has synaptic and non-synaptic after-effects (iv), can modify neurotransmitters polarity-dependently, (v) and alter oscillatory brain activity and (vi) functional connectivity patterns in the brain. It thus is reasonable to use tDCS as a therapeutic instrument in AD as it improves cognitive function in manner based on a disease mechanism. Moreover, it could prove valuable in other types of dementia. Future large-scale clinical and mechanism-oriented studies may enable us to identify its therapeutic validity in other types of demential disorders.

**Keywords: Alzheimer's disease, cerebral blood flow, frontotemporal dementia, memory loss, network connectivity, neurotransmitter modulation, synaptic and non-synaptic after-effects, transcranial direct current stimulation**

#### **INTRODUCTION**

As the pharmacological treatment in Alzheimer disease (AD) is limited (Bauer, 2006), alternative therapeutic approaches are worth pursuing, such as non-invasive brain stimulation with transcranial direct current.

Transcranial direct current stimulation (tDCS) is the application of weak electrical currents by saline-soaked surface sponge electrodes to different cortical areas. tDCS can polaritydependently modulate cortical excitability with prolonged aftereffects (Nitsche et al., 2005) and modify neuronal excitability by tonic de-or hyperpolarization of the resting membrane potential (Creutzfeld et al., 1962; Purpura and McMurtry, 1965). The electrode positioning is determined according to the EEG 10–20 system.

tDCS has demonstrated efficacy in improving recognition memory in AD (Boggio et al., 2009, 2011) and it is a useful tool in cognitive neurorehabilitation, as improvements in cognitive functions were described in patients with depression (Fregni et al., 2006), Parkinson's disease (Boggio et al., 2006) and stroke (Monti et al., 2008).

Alzheimer's disease is a progressive neurodegenerative disorder (Thies and Bleiler, 2011) presenting a decrease in acethylcholine activity resulting in cognitive impairment (Schliebs and Arendt, 2011) in many cognitive activities such as memory, language, and executive functions.

The concept of benefiting from modulating cortical excitability via tDCS with consecutive improvement in cognitive functions in AD is thus tempting. We describe tDCS application in clinical studies in patients with dementia (see **Table 1**) and studies on cognitive functions (see **Table 2**) as well as potential underlying mechanisms in this article.

#### **METHODOLOGICAL ASPECTS**

The review section about the action mechanisms of tDCS in AD is based on a non-systematic approach, whereas the review section on clinical studies with tDCS in AD and memory is based on a somewhat systematic approach based on the PubMed database. A literature search for original and review articles on tDCS in demential disorders was performed through December 2011 seeking clinical studies on tDCS in AD and for demential disorders, by screening the PubMed database. The keywords were used in combination with "Alzheimer disease" AND "tDCS," "dementia" AND "tDCS" as well as "memory" AND "tDCS." The studies were published between 2004 and 12/2011. The exclusion criteria of articles in the article titles searched were only "brain stimulation," "depression," only "memory" or "Alzheimer disease," or "demential disorder" without "tDCS," "motor learning," and only "tDCS" without "memory" or "Alzheimer disease" or "demential disorder." Thirty-eight papers were screened from 75 articles according to the aforementioned criteria and 33

#### **Table 1 | Clinical studies of tDCS in dementia.**

*AChEI, acetylcholine esterase inhibitors; Adas-Cog, Alzheimer's disease assessment scale-cognitive sub scale; ADAS, Alzheimer's disease assessment scale; DLPFC, dorsolateral prefrontal cortex; DSM- IV, Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV); FC, frontal cortex; MDRS, Mattis Dementia Rating Scale; MMSE, Mini Mental State Examination; NINCDS-ADRADA, National Institute of Neurological Communicative Disorders and Stroke-Alzheimer disease and Related Disorders Association; TC, temporal cortex; TPC, temporoparietal cortex; VAT, visual attention task; VRT, visual recognition task.*

thereof were used as the basis for the Section "tDCS in Demential Disorders."

#### **tDCS IN DEMENTIAL DISORDERS**

#### **ALZHEIMER'S DISEASE**

The effect of anodal tDCS (atDCS) over the left temporal cortex (TC) and dorsolateral prefrontal cortex (DLPFC) was investigated on recognition and working memory (WM) in 10 AD patients (Boggio et al.,2009), revealing enhancement in a visual recognition memory task after atDCS of the DLPFC and left TC (Boggio et al., 2009). In another study, an improvement in a word-recognition memory in 10 patients with probable AD was proven after atDCS of the temporoparietal areas (Ferrucci et al., 2008a). In contrast, cathodal tDCS (ctDCS) lead to decreased word-recognition memory. The effect of atDCS persisted up to 30 min after stimulation, indicating a long-lasting increase in brain excitability (Ferrucci et al., 2008a). Long-term enhancement of visual recognition memory for up to 4 weeks after therapy was found after atDCS in 15 AD patients (Boggio et al., 2011).

#### **FRONTOTEMPORAL DEMENTIA**

A study demonstrated that active tDCS does not result in a beneficial effect in verbal fluency in 10 patients with frontotemporal dementia presenting mainly behavioral (and in one patient language) symptoms (Huey et al., 2007). The lack of effect may be due to the small current that reaches the frontal cortex due to brain atrophy and neuronal loss with concomitant incapability of the affected cortex to respond to brain polarization (Huey et al., 2007).

#### **SAFETY AND SIDE EFFECTS OF tDCS** *General observations*

There is evidence that tDCS applied to the scalp over the prefrontal cortex over 20 min does not alter local and global cortical function (Iyer et al., 2005). The current intensity of 1 mA did not result in significant effects on cortical function, whereas verbal fluency increased with 2 mA-atDCS and decreased with 2 mActDCS (Iyer et al., 2005). In a systematic review, itching, tingling, headache, burning sensation, and discomfort were the most often reported adverse effects of active tDCS vs. sham tDCS (Brunoni et al., 2011). Skin irritation and skin burning can occur after tDCS application due to the electrochemical products' skin contact generated by the direct current (Durand et al., 2002; Palm et al., 2008). In addition, magnetic resonance spectroscopy (MRS) in normal subjects failed to detect changes in acetylaspartate, thus showing that atDCS induced no neurotoxic effects (Rango et al., 2008). Furthermore,in motor cortical areas, tDCS induced no relevant changes in serum neuron-specific enolase, a neuronal damage marker, indicating that tDCS induced no harmful effects (Nitsche et al., 2003).

#### *Observations in AD*

In studies of tDCS in AD, no adverse effects from tDCS application were noted (Boggio et al., 2009). Only an itching sensation, but no side effects were reported in the study of 10 AD patients (Ferrucci et al., 2008a). No adverse effects, nor tDCS effects on the Mini Mental State Examination (MMSE), Alzheimer's Disease Assessment Scale-Cognitive sub scale (Adas-Cog), or visual attention task (VAT) scores were observed (Boggio et al., 2011).

**Table 2 | Studies of tDCS on cognitive functions.**

*(Continued)*

#### **Table 2 | Continued**

*Abbreviations for electrode placement according to the 10–20 electrode system (Cp5, Cz, Fp3, C3/4: see Recommendations for the practice of clinical. Neurophysiology: guidelines of the International Federation Clinical Neurophysiology. Electroencephalogr. Clin. Neurophysiol. Suppl. 1999;52:1–304), atDCS, anodal transcranial direct current stimulation; min, minutes, ctDCS, cathodal transcranial direct current stimulation, DLPFC, dorsolateral prefrontal cortex; mA, milli Ampere; min, minutes, PD, Parkinson's disease; TPC, temporoparietal cortex.*

#### **tDCS IN LEARNING AND MEMORY**

#### **OBJECT-LOCATION LEARNING IN THE ELDERLY**

The ability to memorize the location of objects is known to worsen by aging and in neurodegenerative dementia. atDCS over the temporoparietal cortex in 20 elderly healthy subjects resulted in improved retention of object-location learning for up to 1 week after learning (Flöel et al., 2011). This finding has relevance concerning memory deficits in normal and pathological aging.

#### **ENHANCING DECLARATIVE MEMORY BY tDCS**

Anodal tDCS enhances slow oscillatory EEG activity that in turn can enhance declarative memories (Marshall et al., 2004). As shown in 32 human healthy subjects, declarative memory can be improved by anodal and impaired by ctDCS of the DLPFC (Javadi and Walsh, 2011).

#### **ENHANCING IMPLICIT MEMORY BY tDCS**

Both declarative and implicit memory are known to improve via tDCS. For instance, atDCS of Broca's area enhanced implicit learning of an artificial grammar in 38 healthy subjects (de Vries et al., 2011), an interesting finding supporting tDCS as a potential instrument in the rehabilitation of aphasic patients.

#### **MODULATION OF WM BY tDCS**

#### **ENHANCING WM BY tDCS**

Several studies address the physiological effects of tDCS in theWM as a part of declarative memory playing a pivotal role in long-term memory, language, and executive function (Baddeley, 1992).

In 10 patients with cognitive defects after a first-ever stroke, atDCS of the DLPFC led to enhanced WM performance (Jo et al., 2009). In a neurodegenerative disease like Parkinson's, atDCS of the left DLPFC was also shown to improve WM in 18 patients (Boggio et al., 2006). atDCS to the DLPFC lead to WM enhancement in healthy subjects (Fregni et al., 2005;Andrews et al., 2011) and rats, frontal ctDCS enhanced visual–spatial WM (Dockery et al., 2011). Interestingly, atDCS led to amplified oscillation in theta and alpha electroencephalography (EEG) bands and increased WM performance in humans (Zaehle et al., 2011). WM representations are supported by oscillatory brain activity (Lisman and Idiart, 1995). In particular, theta EEG band activity

has been associated with memory encoding and retrieval (Jensen and Tesche, 2002). Thus amplified theta band activity is related to WM's executive function, indicating the continuous information processing required during WM performance.

As neuroimaging studies revealed a widespread effect in cortical activity by tDCS (Lang et al., 2005), it is likely to imply a tDCS influence on the entire WM system, and not only on the DLPFC. Furthermore, there is some evidence that WM performance can be improved in a manner dependent on current strength in 14 healthy subjects (Teo et al., 2011). No current strength or time-course effect was observed in the accuracy of WM tasks. However, a significant current by time interaction was found in a WM task (Teo et al., 2011). However, the effectdependence on current intensity of tDCS in memory function is not proven by this single study of one WM task; it requires further examination in healthy subjects and those with diverse cognitive functions.

A time-dependent enhancement of verbal memory resulted after atDCS of the DLPFC (Ohn et al., 2008). Name recall can benefit from atDCS of the anterior temporal lobes (Ross et al., 2011), and word retrieval improved in healthy and non-fluent aphasic patients after atDCS (Fiori et al., 2011).

#### **IMPAIRMENT OF WM AFTER tDCS**

Bilateral prefrontal ctDCS and atDCS during a memory task can impair neuronal processes related to a WM paradigm (Marshall et al., 2005). Furthermore, cerebellar tDCS of both polarities impaired use-dependent improvement in a WM task (Ferrucci et al., 2008b). WM also revealed impairment by ctDCS to the right parietal lobe (Berryhill et al., 2010).

#### **POTENTIAL MECHANISMS OF ACTION OF tDCS IN AD EFFECTS OF tDCS ON NEURONAL ACTIVITY BY ALTERING THE MEMBRANE POTENTIAL**

In an AD mouse model, ß-amyloid peptide was shown to disturb the resting membrane potential in muscle fibers (Mukhamediarov et al., 2011). Furthermore, ß-amyloid 1–42 peptide caused membrane depolarization leading to hyperexcitability of affected neurons in a human neuronal cell model of AD (Blanchard et al., 2002).

atDCS might be an instrument to alter the neuronal depolarization frequently altered in AD according to *in vitro* studies, as atDCS leads to the increased cortical excitability promoting neuronal depolarization (Nitsche and Paulus, 2000). Increasing cortical excitability is a relevant tool in AD, as AD patients reveal temporoparietal hypoactivity (as characterized by focal slow wave activity in magnetoencephalography; Fernandez et al., 2002).

Motor cortex (Di Lazzaro et al., 2004) and global cortical hyperexcitability is found in AD (Rossini et al., 2007), correlating with cognitive severity in a TMS study (Alagona et al., 2001). As ctDCS led to reduced cortical excitability caused by neuronal hyperpolarization (Nitsche and Paulus, 2000), it might also be beneficial in AD by lowering its somewhat increased cortical excitability.

Non-synaptic mechanisms based on changes in the membrane potential underlying the after-effects of atDCS and ctDCS (Ardolino et al., 2005) might be responsible for modulating cognitive function in AD. The local changes in ionic concentrations could be due to alterations in transmembrane proteins and from changes in H+ ions induced by exposure to a constant electrical field (Ardolino et al., 2005).

#### **SYNAPTIC AFTER-EFFECTS OF tDCS**

#### *NMDA receptor-dependent after-effects*

tDCS induces prolonged after-effects sharing similarities with long-term potentiation (LTP)- and long-term depression (LTD) like changes in cortical excitability (Paulus, 2004). In an *in vitro* and *in vivo* AD mouse model, LTP as the putative mechanism of learning and memory is evidently impaired by ß-amyloid peptide (Gengler et al., 2010; Middei et al., 2010). ß-amyloid peptide disruption of LTP is *N*-methyl-d-aspartate (NMDA) receptordependent in the mouse hippocampus *in vivo* and *in vitro* (Yamin, 2009).

tDCS-induced after-effects are partly NMDA receptordependent (Liebetanz et al., 2002), suggesting that tDCS aftereffects may alter NMDA receptor-dependent cortical plasticity that may be disturbed in AD.

#### *GABAergic interneurons*

Anodal after-effects are probably mediated in part by gammaaminobutyric acid (GABA)Aergic interneurons as a reduction in short-interval intracortical inhibition and an increase in I-wave facilitation after tDCS intracortical facilitation (Nitsche et al., 2005; Stagg et al., 2009; Stagg and Nitsche, 2011). As in AD, GABAergic cortical inhibitory interneurons play a role in the disease's early stage (Koliatsos et al., 2006); modulation of these interneurons by tDCS is a possible disease-modifying mechanism. Hippocampus changes in GABA B receptor protein were found in 16 elderly subjects with AD, indicating alterations between the excitatory and inhibitory neurotransmitter systems with consecutively dysfunctional hippocampal circuitry (Iwakiri et al., 2005). A MRS study provides evidence that atDCS causes reduced GABA concentration within the stimulated cortex, whereas ctDCS leads to impaired glutamatergic neuronal activity with a correlated reduction in GABA concentration due to a relationship between these two neurotransmitters (Stagg et al., 2009). Thus tDCS might reduce the disequilibrium between excitatory and inhibitory neurotransmitters systems in AD.

#### *Glutamatergic synapses*

In AD, glutamate receptors may be dysregulated by ß-amyloid accumulation resulting in the disrupted glutamatergic activity that coincides with cognitive decline (Parameshwaran et al., 2008). The dysregulation of glutamatergic activity might be altered by atDCS, as there is evidence that glutamatergic synapses are involved in anodal after-effects (Stagg et al., 2009; Stagg and Nitsche, 2011), and MRS data support that glutamate and glutamine levels were elevated in the parietal cortex after atDCS (Clark et al., 2011).

Therefore, ctDCS may have the potential to affect cognitive functions in AD by modulating glutamatergic synapses.

#### **EFFECTS OF tDCS ON HUMAN REGIONAL CEREBRAL BLOOD FLOW**

There is evidence in AD that characteristics of the cerebral microvasculature have changed, leading to altered cerebral blood flow (CBF; van Beek et al., 2012). atDCS induced an increase in regional cerebral blood flow (rCBF), whereas ctDCS resulted in a decrease in rCBF during and after stimulation (Zheng et al., 2011). As tDCS modulates CBF in many cortical and subcortical regions with sustained and widespread changes in neuronal activity (Lang et al., 2005), it is an auspicious instrument in AD.

#### **MODULATING OSCILLATORY BRAIN ACTIVITY AND FUNCTIONAL CONNECTIVITY PATTERN VIA tDCS**

AD led to an altered temporal correlation in parietal and prefrontal oscillations (Montez et al., 2009), more severe deceleration of spontaneous oscillatory activity (Rossini et al., 2007; de Waal et al., 2011), a functional disconnection (Gili et al., 2011),in particular between the prefrontal cortex and hippocampus in AD (Grady et al., 2001), and network connectivity changes (Zhou et al., 2010). It therefore makes sense to use tDCS as a therapeutic tool in AD, as it can reconfigure cerebral networks (Peña-Gómez et al., 2011) and cause changes in functional cerebral connectivity patterns suggesting alterations in brain synchronization (Polanía et al., 2011). As the cognitive dysfunction in brain diseases like AD is based on abnormal neural synchronization (Polanía et al., 2011), it may be beneficial to cause changes in brain synchronization via tDCS.

More specifically, atDCS over the primary motor cortex combined with inhibitory ctDCS of the contralateral frontopolar cortex caused an increased functional connectivity pattern within the premotor, motor, and sensorimotor areas of stimulated hemispheres in 10 healthy human subjects. Furthermore, intra- and interhemispheric connectivity changes became apparent after atDCS, indicating changes in brain topological functional organization (Uhlhaas and Singer, 2006). Another study demonstrated that ctDCS decreased while atDCS augmented normalized beta and gamma frequency EEG bands, suggesting transient reorganization of cortical activity (Antal et al., 2004). As gamma activity is also part of high-level information processing, it is an adjuvant method to influence higher-order cognitive function (Antal et al., 2004).

#### **MODULATING CORTICAL NEUROTRANSMITTERS VIA tDCS**

Neuronal loss implicates the impairment of serotonergic neuromodulation as a basic mechanism of promoting dementia in AD (Yang et al., 1999). Furthermore, there is dopaminergic modulation of LTD-like plasticity in AD (Koch et al., 2011). Cholinergic systems with ascending projections are also degenerated in neurodegenerative dementia (Schmitt, 2005; Fregni et al., 2006). Modulating these neurotransmitter systems via tDCS would therefore seem to be a mechanism-based treatment of AD. Dopaminergic (Nitsche et al., 2006), serotonergic (Nitsche et al., 2009) and cholinergic (Kuo et al., 2007) neuromodulations have been demonstrated by atDCS and ctDCS (Stagg and Nitsche, 2011), indicating another disease-modifying treatment option of tDCS.

There are other mechanisms that determine the response of humans to tDCS, i.e., the BDNF polymorphism (Antal et al., 2004). BDNF modulation is an interesting target in AD, as ßamyloid processing is involved in the BDNF pathway and (Forero et al., 2006) the BDNF ValMet 66 polymorphism is a neural risk for AD (Voineskos et al., 2011), suggesting BDNF as a factor shaping the cortical excitability response to tDCS in AD patients.

#### **LIMITATIONS OF CURRENT KNOWLEDGE OF tDCS IN DEMENTIAL DISORDERS**

There are few studies on the effects of tDCS in demential disorders (AD and frontotemporal dementia, see **Table 1**). The efficacy of tDCS in other demential disorders (for instance vascular dementia or Lewy body dementia) has thus not yet been proven. Furthermore, only a small battery of cognitive functions, i.e., selective attention, WM, visual and word-recognition memory, instruction remembering and word recall has been evaluated so far (see tDCS in Demential Disorders). tDCS effects on other cognitive functions like calculating, cognitive flexibility, language, orientation, short- and long-term memory, and writing will have to be evaluated in studies with larger cohorts and longer control periods. The tDCS effects studied thus far are short-lived (maximum up to 1 month; Boggio et al., 2011) and there are no observations regarding longer-duration interventions. Nor have the long-term side effects of tDCS been assessed.This is particularly important, as

#### **REFERENCES**

direct current stimulation of the human brain. *J. Physiol. (Lond.)* 568, 653–663.

tDCS applied over longer periods might interact with mechanisms involved in neurodegeneration with either beneficial (delayed deterioration of cognition) or harmful effects (accelerated cognitive deterioration). The interaction of tDCS with pharmacological treatment has not yet been addressed systematically in studies. However, current data indicate there is no significant interaction between medication outcome and its interaction with tDCS (Boggio et al., 2011).

#### **PERSPECTIVES OF tDCS IN AD**

tDCS may enhance our understanding of the neurobiological substrates underlying the cognitive decline in AD. Factors such as cognitive reserve, genetic variants, learning capacity, volumetric studies of cortical thinning and white matter volume, and integrity will have to be thoroughly and systematically investigated in future studies of tDCS on cortical functions in AD. The therapeutic efficacy of tDCS must be examined by outcome scales commonly used in trials of pharmacological agents such as the ADAS-Cog (Freitas et al., 2011). Moreover, multiple target tDCS or tDCS targeting new brain areas must be developed to overcome multiple cognitive deficits in AD. A multi-electrode stimulation set-up was recently demonstrated that increased focalty and intensity at the brain target (Dmochowksi et al., 2011).

#### **CONCLUSIONS**

tDCS is an easy to perform and non-invasive alternative therapeutic tool for neurodegenerative diseases such as AD. Its effects comprise the enhancement of cognitive functions in explicit and implicit memory. The mechanisms of tDCS are based on changes in membrane polarization, cerebral blood flow, functional connectivity, and brain oscillatory activity that may be altered in AD and other demential disorders.

current stimulation in Alzheimer's disease. *Brain Stimul.* [Epub ahead of print].

brain stimulation improves objectlocation learning in the elderly. *Neurobiol. Aging* [Epub ahead of print].

and cognitive effect of frontal DC brain polarization in healthy individuals. *Neurology* 64, 872–875.

sleep improves declarative memory. *J. Neurosci.* 3, 9985–9992.

M., Kincses, Z. T., Morris, P. G., Matthews, P. M., and Johansen-Berg, H. (2009). Polarity-sensitive modulation of cortical neurotransmitters by transcranial stimulation. *J. Neurosci.* 29, 5202–5206.

graph method. *Anal. Cell. Pathol.* 19, 125–138.

**Conflict of Interest Statement:** The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 02 March 2012; accepted: 24 April 2012; published online: 15 May 2012.*

*Citation: Hansen N (2012) Action mechanisms of transcranial direct current stimulation in Alzheimer's disease and memory loss. Front. Psychiatry 3:48. doi: 10.3389/fpsyt.2012.00048*

*This article was submitted to Frontiers in Neuropsychiatric Imaging and Stimulation, a specialty of Frontiers in Psychiatry. Copyright © 2012 Hansen. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.*

## Using transcranial direct current stimulation to treat depression in HIV-infected persons: the outcomes of a feasibility study

#### **Helena Knotkova1,2\* † , Mary Rosedale1,3,4† , Shiela M. Strauss <sup>3</sup> , Jaclyn Horne<sup>5</sup> , Eliezer Soto<sup>1</sup> , Ricardo A. Cruciani 1,2, Dolores Malaspina<sup>4</sup> and Daniel Malamud<sup>6</sup>**

<sup>1</sup> Research Division, Department of Pain Medicine and Palliative Care, Institute for Non-Invasive Brain Stimulation, Beth Israel Medical Center, New York, NY, USA

<sup>2</sup> Department of Neurology, Albert Einstein College of Medicine, Bronx, NY, USA

<sup>3</sup> College of Nursing, New York University, New York, NY, USA

<sup>4</sup> Department of Psychiatry, Institute for Social and Psychiatric Initiatives, New York University, New York, NY, USA

<sup>5</sup> Duke University, Durham, NC, USA

<sup>6</sup> College of Dentistry, New York University, New York, NY, USA

#### **Edited by:**

Paulo Sérgio Boggio, Mackenzie Presbyterian University, Brazil

#### **Reviewed by:**

Paul Croarkin, Mayo Clinic, USA Michael A. Nitsche, Georg-August-University, Germany

#### **\*Correspondence:**

Helena Knotkova, Research Division, Department of Pain Medicine and Palliative Care, Institute for Non-Invasive Brain Stimulation, Beth Israel Medical Center, 120 East 16th Street, 12th floor, New York, NY, USA. e-mail<EMAIL_ADDRESS>
†Helena Knotkova and Mary Rosedale have contributed equally to this work.

Transcranial direct current stimulation (tDCS) is a novel non-invasive neuromodulatory method that influences neuronal firing rates and excitability of neuronal circuits in the brain. tDCS has been shown to relieve Major Depressive Disorder (MDD) in the general population, suggesting its potential for other vulnerable populations with high MDD prevalence.**Aims:**This study evaluated the feasibility, safety, acceptability, and clinical outcomes of a 2-week tDCS antidepressant treatment in HIV-MDD co-diagnosed patients, and the feasibility of collecting serum and saliva for analysis of immunity biomarkers. **Methods:**Ten enrolled patients underwent baseline evaluation and started the tDCS treatment (Monday– Friday for 2 weeks) delivered with Phoresor II 850 PM for 20 min at 2 mA at each visit, using two saline-soaked sponge electrodes placed over the F3 position of EEG 10–20 system and the contralateral supraorbital region. Outcome measures were collected at baseline, after the last tDCS and 2 weeks later. A quantitative microarray (Ray Bio Tech Inc.) for TH1/TH2 cytokines was used for saliva and plasma analysis. **Results:** Analyzable outcome-data were obtained from eight subjects. Depression scores significantly decreased (p < 0.0005) after the treatment. No serious adverse events occurred. Several transient minor AEs and occasional changes of blood pressure and heart rate were noted. Mini-mental state examination scores remained unchanged or increased after the treatment. All subjects were highly satisfied with the protocol and treatment results and described the desire to find new treatments for HIV-MDD as motivating participation. **Conclusion:** Findings support feasibility and clinical potential of tDCS for HIV-MDD patients, and justify larger-sample, sham-controlled trials.

**Keywords: transcranial direct current stimulation, HIV, major depressive disorder, neuromodulation**

#### **INTRODUCTION**

Transcranial direct current stimulation (tDCS) has been shown to be a powerful technique for non-invasive neuromodulation (Nitsche and Paulus, 2010). The primary mechanism of tDCS is a subthreshold modulation of neuronal resting membrane potential which induces a polarity-dependent modification of *N*-Methyl-daspartate (NMDA) receptor function (Antal et al., 2010; Nitsche and Paulus, 2010) that plays a role in neuroplasticity. Some of tDCS induced changes occurs immediately during the stimulation (so called intra-tDCS changes), while others occur later as shortlasting or long-lasting after-effects (Nitsche and Paulus, 2010). As suggested by pharmacological studies (Liebetanz et al., 2002; Nitsche et al., 2004), the intra-tDCS effects depend on the activity of sodium and calcium channels but not on efficacy changes of NMDA and gamma-aminobutyric acid (GABA) receptors, and thus are probably generated solely by polarity specific shifts of

resting membrane potential. However, the after-effects have been shown to be protein synthesis dependent (Gartside, 1968; Hattori et al., 1990; Nitsche and Paulus, 2010) and also involve modulations of NMDA receptors efficacy. (Liebetanz et al., 2002; Nitsche et al., 2005; Nitsche and Paulus, 2010).

In several randomized controlled studies utilizing 2 or 4 week tDCS treatment protocols, tDCS delivered over the dorsolateral prefrontal cortex (DLPFC) was shown to safely relieve Major Depressive Disorder (MDD) in the general population (Fregni et al., 2006; Boggio et al., 2008a; Rigonatti et al., 2008; Murphy et al., 2009; Nitsche et al., 2009; Kalu et al., 2012). Although the mechanisms of tDCS antidepressant effect are not fully understood, it is reasonable to assume that tDCS might have induced a change in the DLPFC activity which is highly relevant to alterations of mood-related neuronal networks (Boggio et al., 2008a).

This suggests the clinical potential of tDCS treatment for other vulnerable populations with high prevalence of MDD, such as persons with HIV infection.Various estimates (McHorney et al., 1994; Lyketsos, 1995; Boland, 1997; Stober et al., 1997; Chander et al., 2006; Hartzell et al., 2008; Rabkin, 2008) suggest that up to 48% of HIV-infected patients have MDD comorbidity. MDD accelerates HIV disease progression, jeopardizes the completion of antiretroviral treatment and is a potent risk factor for transmission of the virus to others (Wilson et al., 2007). Despite the fact that MDD is the most common psychiatric disorder in HIV populations after substance abuse (Rabkin, 2008), conventional antidepressant treatments, such as medication or psychotherapy, leave many patients with undertreated depressive symptoms (Lyketsos, 1995; Chander et al., 2006; Hartzell et al., 2008). For treatment-resistant depressed patients, depression is often lifelong and disabling and represents a significant source of suffering, disruption in role functioning, economic burden to society, and mortality (Gaynes et al., 2008).

Although tDCS treatment may greatly improve the quality of life of HIV patients suffering MDD, to our knowledge, no previous tDCS study has included such patients. Therefore, we carried out a quantitative and qualitative open-label pilot trial to evaluate the safety, tolerability, acceptability, and clinical outcomes of a 2-week tDCS treatment protocol to treat MDD in HIV-infected patients and explore the feasibility of collecting serum and saliva cytokines for future analysis. The purpose of the study was to determine overall feasibility of the study protocol in an HIV patient-sample and to provide initial data for sample size estimates and power for a future, sham-controlled randomized trial (RCT).

### **MATERIALS AND METHODS**

#### **SUBJECTS**

Ten adult subjects diagnosed with HIV and MDD who fully met the following inclusion criteria participated in the study.

Inclusion Criteria comprised of the following: (i) Diagnosed with HIV; (ii) MDD as measured by the Hamilton Depression Rating Scale score at least >17 at the time of enrollment as well as 1 week later at the time of the baseline; (iii) MDD as measured by the Montgomery-Asberg Depression Rating Scale (MADRAS) score at least>11 at the time of enrollment as well as at the baseline.

Subjects were excluded from participation if they met the following exclusion criteria.

Exclusion Criteria: (i) Diagnosed with AIDS; (ii) Active and/or history of schizophrenia, schizoaffective disorder, psychosis, mental retardation, substance dependence, or abuse within the past year (except nicotine), bipolar disorder, psychotic features, amnesic disorder, dementia, delirium, or obsessive-compulsive disorder; (iii) History of opportunistic infection affecting the brain; (iv) Methadone treatment; (v) History of neurological disorder or seizure (except induced by electroconvulsive therapy, ECT), increased intracranial pressure, brain surgery, or head trauma with loss of consciousness for >15 min, implanted electronic device, metal in the head; (vi) History of autoimmune, endocrine, or vascular disorder, unstable cardiac disease, uncontrolled hypertension, or sleep apnea; (vii) Active suicidal intent; (viii) Pregnancy; (ix) Unable to follow instructions or complete assessment tools in English.

Participation in this study did not require any changes in the patient's medication regimen.

#### **PROCEDURES**

#### **Overview**

The protocol consisted of 12 study visits. At the initial visit, the informed consent was obtained and patients were screened for eligibility. Baseline values of outcome measures were established at the initial visit and 1 week after the initial visit. The subjects who fully met the inclusion criteria then started a 2-week course of tDCS treatment (10 sessions, visits #2 – #11, Monday–Friday for 2 weeks, each session consisting of 20 min of tDCS). Two weeks after the last tDCS treatment, participants came for visit #12 (Follow-up/Completion). The primary outcome measure was safety; secondary outcomes were treatment benefits/clinical outcomes, tolerability, acceptability, and patients' satisfaction. The outcome measurement time points were before the first tDCS treatment, immediately after the last tDCS treatment and at the follow-up visit which was conducted 2 weeks after the treatment.

#### **Transcranial direct current stimulation**

Transcranial direct current stimulation was delivered by trained study personnel using the battery-operated device Phoresor II Auto (Model No. PM850) with two saline-soaked sponge electrodes of size 6 cm × 6 cm, with a rubber rim of 1 cm and spongeskin contact area of 5 cm × 5 cm. The anode was placed over the left DLPFC as determined by the international EEG 10–20 classification (point F3), and the cathode was over the contralateral supraorbital region. The current was delivered at the intensity of 2 mA for 20 min. These parameters of stimulation were successfully and safely used in many previous tDCS studies (Fregni et al., 2006; Rigonatti et al., 2008; Knotkova et al., 2009) and were well within safety limits. A detailed review of tDCS safety and parameters of stimulation-protocols in human subjects appear in Sundaram et al. (2009). The tDCS treatment course was considered complete if the subject received at least 8 of 10 treatment sessions, and if the two missed sessions were not on consecutive days.

#### **Assessment methods**

because of their sensitivity to change. The 10 selected items are rated on a scale of 0–6 with anchors at 2-point intervals. The interviewer is encouraged to use his or her observations of the patient's mental state as an additional source of information. Total scores on the MADRAS range from 0 to 60. It has been shown to have high inter-rater reliability (Spearman *r* = 0.94) and good concurrent validity (*r* with HamD between 0.83 and 0.94; Davidson, 1986). We have used this psychometric tool in addition to HamD because MADRAS offers an alternative view of depressive illness, and may be more sensitive than HamD to some depressive symptoms such as hypersomnia, increased appetite, and concentration/indecision.

#### **Others**

One of the study aims was to determine the feasibility of collecting serum and saliva samples for immunity-biomarkers assessment. At the baseline, after the last tDCS treatment, and at the 2-week follow-up visit, approximately 10 ml of plasma and 10 ml of whole stimulated saliva were collected from subjects. Fluid samples were pipetted in to.0 5 ml aliquots and immediately frozen at −80˚C, to be prepared for analysis using RayBiotech TH1/TH2 antibody arrays. Results of the analysis will be reported in a separate manuscript (in preparation).

#### **Statistical analysis**

To examine change in scores from pre- to post-tDCS administration to 2-week follow-up, we conducted a repeated measures analysis of variance using Predictive Analytics SoftWare (PASW) Statistics, formerly known as Statistical Package for the Social Sciences (SPSS) before it was acquired by IBM. First, analyses determined whether the sphericity assumption was violated for the analysis of the HamD and MADRAS scores. Helmert contrasts were used when comparing (1) the pre-tDCS with the average of the post-tDCS scores and the 2-week follow-up scores, and (2) the post-tDCS scores and the 2-week follow-up scores.

Enrollment and the study procedures took place at Beth Israel Medical Center, New York, NY. Biochemical analysis took place at New York University, New York, NY. The study was approved by the Institutional Review Boards at both institutions.

#### **RESULTS**

#### **DEMOGRAPHIC CHARACTERISTICS AND SUBJECTS' FLOW THROUGH THE STUDY**

The 10 study participants included 5 males and 5 females. Their mean age was 52.5 (s.d. = 6.3) years, ranging from 38 to 59 years. Almost all (9 of 10) were African American and one was non-Hispanic Caucasian. Five were living with partners/spouses and the others were separated, single, or never married, and living alone. Although five of the participants had attended college, only one was working (and on a part-time basis) and three of the remaining nine were disabled. Scores on the MMSE examination at the baseline were 28.0 (s.d. = 3.1), range 20–30, consistent with the possibility of some mild cognitive impairment for a few of the participants. Regarding their health, the 10 participants had been living, on average, with HIV for 19.7 (s.d. = 4.1) years, range10– 26 years. All had co-morbid conditions, including three with hepatitis C virus, three with hypertension, and five with a substance abuse history. All had MDD as reflected in their scores of 17 or greater on the HamD scale; 24.8 (5.8), range 17–34, and a score of 14 or greater on the MADRAS scale: 24.6 (7.4), range 14–34. Before the study, none of the participants had ever had ECT, transcranial magnetic stimulation (TMS), or tDCS treatment for their depression or other reasons. All subjects were taking HIV antiretroviral medication. Besides that, several subjects were on stable regimens of other medications, including the following CNS-acting agents: Antidepressants – selective serotonin reuptake inhibitors [Celexa, Zoloft] (subjects #3,6,9), tetracyclic antidepressants [Remeron] (subject #4), and serotonin-norepinephrine reuptake inhibitors [Effexor] (subject #4); Antipsychotics [Zyprexa, Seroquel, Risperidone] (subjects #4,9); Anticonvulsants [Depakote, Gabapentin] (subjects #4,7); Sedative [Ambien] (subject #9); and Histamine H2-receptor antagonists [Famotidine] (subject #3).

The 10 participants were recruited from a pool of 15 subjects referred to the study. Of these, five were interested in participation but could not participate for various reasons (e.g., could not make commitment to 10 consecutive days of tDCS, lived far away). Ten subjects provided informed consent and were screened according to the inclusion/exclusion criteria. All 10 subjects satisfied the criteria and began the study protocol. Analyzable outcome-data were obtained from 8 subjects; 7 of them completed all 10 treatments, and 1 subject completed 8 of 10 treatments (missed the last two treatments due to the death of a family member). Two remaining subjects received only two initial treatment sessions and did not continue due to conflicts with their personal schedules.

#### **CHANGES IN DEPRESSION SCALES BEFORE AND AFTER tDCS ADMINISTRATION**

#### **Hamilton Depression Rating Scale**

Hamilton Depression Rating Scale scores from the eight subjects at baseline, immediately after the last tDCS treatment and at 2-week follow-up averaged 26.3 (s.d. = 5.5), range 17–34; 9.9 (s.d. = 4.3), range 5–17; and 7.6 (s.d. = 6.7), range 1–19, respectively. The mean HamD scores differed significantly between time points according to the repeated measures ANOVA: *F*(2, 14) = 94.555; *p* < 0.0005. The sphericity assumption (see Materials and Methods) was not violatedfor the HamD scores (*p* = 0.926).As can be seen in**Table 1**, the pre-tDCS HamD scores were significantly higher than the average of the post-tDCS HamD scores and the 2-week follow-up HamD scores [*F*(1, 7) = 174.112; *p* < 0.0005], indicating a highly significant decrease in depression scores.

Notably, the post-tDCS HamD scores did not differ significantly from the 2-week follow-up HamD scores [*F*(1, 7) = 2.498; *p* = 0.158], indicating duration of the decrease of depressive symptoms in the follow-up period.

#### **MADRAS**

On MADRAS, scores for the eight patients at baseline,immediately after the last tDCS treatment and at 2-week follow-up averaged 26.8 (s.d. = 6.7), range 14–34; 11.3 (s.d. = 6.9), range 6–26; and 7.0 (s.d. = 7.4), range 0–20, respectively. According to the repeated measures ANOVA, the mean MADRAS scores differed significantly between time points: [*F*(2, 14) = 46.490; *p* < 0.0005]. The sphericity assumption was not violated for the MADRAS scores (*p* = 0.421). As can be seen in **Table 2**, the pre-tDCS MADRAS scores were significantly higher than the average of the post-tDCS HamD scores and the 2-week follow-up MADRAS scores [*F*(1,

The post-tDCS MADRAS scores differed significantly from the 2-week follow-up MADRAS scores [*F*(1, 7) = 7.692; *p* = 0.028], indicating a further decrease in depression scores in the follow-up period.

#### **SAFETY**

Safety measures included monitoring side effects throughout the study, systolic and diastolic blood pressure and heart rate before and after each tDCS session, and MMSE at baseline and at the completion of the study.

#### **Side effects**

No serious adverse events (SAEs) occurred. Several minor nonserious AEs that occurred during the treatment course or in the follow-up period can be seen in **Table 3**. All these minor events were transient and fully resolved.

#### **Systolic and diastolic blood pressure and heart rate**

For each participant, and for each of the 10-days of the tDCS administration, we examined changes in systolic and diastolic

**Table 3 | Adverse events presented in the table in italics were rated by the study physician as unlikely related or unrelated to the study procedure.**

#### **Table 2 | Tests of within-subjects contrasts for MADRAS.**

blood pressure and heart rate before and after tDCS administration. For systolic and diastolic blood pressure, we considered a change from pre- to post-tDCS administration to be clinically significant if it varied by 10 mmHg or more. For heart rate, we considered a change from pre- to post-tDCS administration to be clinically significant if the change was 10% or greater.

#### (a) **Clinically significant changes in systolic blood pressure**

For the eight participating patients, clinically significant change in systolic blood pressure varied from −27 to 20 mmHg. All patients had such a significant change on at least one of the days of tDCS administration, ranging from 1- to 5-days. As can be seen in **Table 4**, there were 6 clinically significant increases and 19 clinically significant decreases in systolic blood pressure.

Regarding the days on which the changes occurred, they included a change for one of the patients on each of days 6 and 9; for two patients on days 1, 3, 5, and 8; for three patients on days 2 and 10; for four patients on day 7; and for five patients on day 4.

#### (b) **Clinically significant changes in diastolic blood pressure**

For the eight participating patients, clinically significant change in diastolic blood pressure varied from −20 to 15 mmHg. Six of the eight patients had such a significant change on at least one of the days of tDCS administration, ranging from 1- to 5-days. As can be seen in **Table 4**, there were five clinically significant increases and 10 clinically significant decreases in diastolic blood pressure. Regarding the days on which the changes occurred, they included no changes for any patients on day 2; changes for one patient on days 5, 6, 9, and 10; for two patients on days 1, 3, 7, and 8; and for three patients on day 4.

#### (c) **Clinically significant changes in heart rate**

Each of the eight evaluated subjects had at least 1 day on which there was a clinically significant change in heart rate, ranging from 1- to 5-days. There were nine clinically significant increases and 14 clinically significant decreases in heart rate (**Table 4**). Regarding the days on which the changes occurred, they included change for one patient on days 2, 5, 9, and 10; for two patients on days 3 and 6; for three patients on days 7 and 8; for four patients on day 1; and for five patients on day 4.

#### **Mini-mental state evaluation before and after treatment**

Scores for the eight subjects on the MMSE ranged from 20 to 30 (mean = 28; s.d. = 3.02) before the tDCS treatment and from 25 to 30 (mean = 29.0; s.d. = 1.8) at the follow-up. The individual participant scores remained either unchanged or increased after the treatment. There was no case of the score-decrease between the baseline and the end of the study assessment.

#### **ADHERENCE TO THE TREATMENT**

Adherence to the treatment was determined as the percentage of participants who finished the 2-week tDCS treatment course, and the percentage who completed the entire study, including participation in the 2-week follow-up. Only two subjects (20%) did not receive the minimum of required eight tDCS treatment sessions. Each of the two subjects completed two treatment sessions at the beginning of the treatment course. Both subjects dropped out due to a personal-schedule conflict and did not come for the follow-up evaluation.

Eight subjects (80% of patients) received the required eight or more treatment sessions. Of these, six subjects did not miss any treatment sessions, one subject missed one treatment, and one subject missed two treatments. As the two missed treatments (last two treatments of the course) for this latter subject were on consecutive days, the subject as per protocol did not finish the treatment course and was not considered a completer. However, all other data and evaluations were collected from this subject and included in the analysis. Qualitative evaluation of this noncompleter revealed that the subject missed the two sessions due to the death of family member. The subject's frequent communication with the study personnel revealed high motivation to participate in the study procedures and exhibited behavior typical for treatment-adherent subjects. If this non-completer is eliminated from the analysis, there were very few substantial changes in study results. In particular, as was the case with the eight participants, the pre-tDCS HamD scores were significantly higher than the average of the post-tDCS HamD scores and the 2-week followup HamD scores for the seven completers [*F*(1, 6) = 137.388; *p* < 0.0005]. In addition, the pre-tDCS MADRAS scores were significantly higher for the seven completers than the average of the post-tDCS MADRAS scores and the 2-week follow-up MADRAS scores [*F*(1, 6) = 72.199; *p* < 0.0005]. For the seven completers,

<sup>a</sup>A change of 10 mmHg is considered clinically significant.

<sup>b</sup>A change of 10% is considered clinically significant.

<sup>c</sup>Data on systolic and diastolic blood pressure and heart rate were missing on Day 3.

<sup>d</sup>Data on systolic and diastolic blood pressure and heart rate were not collected on Days 9 and 10 as the subject did not come for the two study visits.

analyses also indicate duration of the decrease in depressive symptoms from immediately post-tDCS to the 2-week follow-up on both the HamD and the MADRAS. However, although there was a statistically significant further decline in depressive symptoms as measured by the MADRAS when the eight participants were included in the analysis [*F*(1, 7) = 7.692; *p* = 0.028], the decline on the MADRAS was only of borderline significance when only the seven completers were included [*F*(1, 6) = 5.250; *p* = 0.062].

#### **PATIENTS' SATISFACTION**

Patient's scores on the 4-point (0–3) Satisfaction numerical rating scale after the last tDCS session as well as at the follow-up ranged between 2 (satisfied a lot with the results of tDCS treatment) and 3 (fully satisfied). After the last tDCS, six of eight subjects were fully satisfied with the results, and two subjects were satisfied a lot,mean score 2.75, s.d. 0.46. At the 2-week follow-up, seven subjects were fully satisfied and one subject was satisfied a lot, mean score 2.88, s.d. 0.35, indicating that the level of satisfaction with the results of the tDCS treatment did not deteriorate during the post-treatment follow-up period.

#### **PRELIMINARY FINDINGS FROM QUALITATIVE INTERVIEWS**

Four themes emerged to describe participants' experiences with tDCS: (a) a narrative of frustration and helplessness with previous depression and medication treatment, (b) the sensory experience of tDCS, including tingling at site placement and desensitization to tingling or irritation after the first few treatments, (c) altruism, with particular attention to improving depression treatment for other HIV-infected persons, and (d) clearer thinking with reduced perseveration, rigidity of thinking, and drug cravings.

Subjects who did not complete the treatment indicated a high motivation to participate in the entire study but said that unanticipated scheduling conflicts prevented them from doing so. They each suggested that future tDCS treatment protocols be developed to maximize flexibility for participants (i.e., offering options of evening and weekend appointments so that subjects could "make up" a missed session). The subject who received eight sessions of tDCS said she was noting improvement in her mood and clarity in her thinking. The other two non-completers who received two treatment sessions did not perceive any changes in depressive symptoms. The interviews did not reveal any identifiable differences in the demographic or clinical characteristics of subjects who completed the treatment compared with non-completers. All 10 participants reported their willingness to participate in future tDCS clinical trials.

#### **FEASIBILITY OF BIOMARKER-COLLECTION**

All study participants agreed to provide samples of blood and saliva for biochemical analysis of immunity biomarkers. At the time of collection, all subjects fully cooperated and followed instruction of study personnel. Data collection of serum and saliva samples for cytokine assays was feasible in this patient population.

#### **DISCUSSION**

This was a pilot, open-label study to determine the feasibility of using novel tDCS antidepressant treatment in HIV-infected persons. The study evaluated clinical outcomes, safety, acceptability, and patient's satisfaction with the treatment, as well as the feasibility of collecting serum and saliva cytokine biomarkers in a 2-week tDCS treatment protocol.

To our knowledge, this is the first study to provide evidence about the feasibility of the tDCS treatment protocol in an HIV population, and the first tDCS study applying a mixed methods approach.

In accordance with the previous tDCS studies in the general population (Fregni et al., 2006; Boggio et al., 2008a;Rigonatti et al., 2008; Kalu et al., 2012), the results of our study showed a decrease of depressive symptoms after tDCS treatment. Although our study was not sham-controlled, the relief of depressive symptoms was well beyond effects attributable solely to placebo response observed in previous studies (Fregni et al., 2006; Boggio et al., 2008a; Rigonatti et al., 2008; Kalu et al., 2012). Further, it is of great clinical interest that the improvement of depressive symptoms after tDCS in our study did not deteriorate in the 2-week follow-up period, which provides a rationale for future studies of durability of tDCS effects.

Further, results of our study contributed to evidence on the safety of tDCS treatment, as none of our subjects experienced any SAE; the few non-SAEs that occurred in our study were minor and transient. Monitoring and evaluation of blood pressure and heart rate before and after each tDCS session showed occasional bi-directional changes in these parameters. The changes were not specific to the study participants with a history and/or present status of hypertension, and a decrease of the parameters was observed more often than an increase. Of note, numerous factors could contribute to the observed changes, including anticipation stress prior the first tDCS stimulation, physical activity immediately prior the study visit, or level of relaxation during the 20-min "quiet time" when receiving tDCS stimulation. The decreasing effect of tDCS on blood pressure has been previously noted and Cogiamanian et al. (2010) discussed the potential use of tDCS in the treatment of hypertension. However, our finding of bi-directional fluctuation suggests caution and monitoring of blood pressure and heart rate during tDCS stimulation, and further studies of tDCS effects.

A gratifying finding in our study was the high adherence of subjects to the study protocol. It could be partially because our study subjects were completers of an NIH-funded study of HIV patients (R01 AI070005, ACT 2, PI M.V. Gwadz; Gwadz et al., 2011) that educated patients on relevant issues concerning research participation to facilitate minorities' access to participation in suitable research trials. The high adherence in our study not only supports feasibility of the 2-week tDCS treatment protocol and its use in a future RCT, but also contributes to evidence of validity of the ACT 2 educational intervention.

The findings from the qualitative evaluation of the semistructured interviews at the end of the study revealed high satisfaction with the study and willingness of all subjects to participate in future tDCS studies. The findings were consistent with previously reported (Rosedale et al., 2009) patients' experiences with the repetitive transcranial magnetic stimulation (rTMS) treatment; participants described intense frustration and helplessness with previous depression and medication treatment, arguing for novel treatment approaches (Rosedale et al., 2009). The theme of electrode placement, tingling, and desensitization to tingling or irritation after the first few treatments has been previously described in the rTMS, but not in the tDCS literature (Anderson et al., 2006). Altruism has not been previously described as a motivating factor for participation in brain stimulation studies and may have been a particular feature of the ACT 2 group participants or a particular motivating factor for persons infected with HIV.

Clearer thinking with reduced perseveration, reduced rigidity of thinking, and reduced drug cravings have been previously described in the tDCS literature and suggest that tDCS may enhance acute attention and working memory in people with depression (Loo, 2012). Notably, attention-enhancement as well as reduced cravings for specific foods and alcohol has also been observed in patients treated with tDCS (Boggio et al., 2008b; Fregni et al., 2008; Kang et al., 2009). This may present enormous clinical opportunity for the HIV patients who present with high comorbidity of substance abuse.

Our results also support feasibility of saliva and blood sample collection for the immunity-biomarker analysis and indicate an applicability of this procedure in future RCT tDCS protocols involving HIV-infected subjects.

#### **REFERENCES**

brain stimulation for the management of arterial hypertension. *Med. Hypotheses* 74, 332–336.

The major limitations of the study and its findings are (i) the small sample size that limits generalizability of the results, (ii) the open-label design of the study, (iii) the trained clinical raters were not blinded to the study protocol, (iv) the study have not used parallel versions of MMSE at the post-treatment re-testing, and therefore an order effect might have contributed to the MMSE results. As the study was not sham-controlled, a future RCT is warranted before conclusions on effectiveness of tDCS antidepressant treatment in an HIV population can be drawn.

#### **CONCLUSION**

The study demonstrated feasibility of the 2-week tDCS treatment protocol in HIV-infected patients, as well as the feasibility of study enrollment and satisfactory retention of the subjects which supports future tDCS studies in this patient population.

#### **ACKNOWLEDGMENTS**

(1) Department of Pain medicine and Palliative Care, Beth Israel Medical Center, New York, NY, USA. (2) Muriel and Virginia Pless Center for Nursing Research, New York University, New York, NY, USA.

the mechanisms of transcranial DC-stimulation-induced aftereffects of human motor cortex excitability. *Brain* 125, 2238–2247.

direct stimulation, and fluoxetine for the treatment of depression. *Eur. Psychiatry* 23, 74–76.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 21 March 2012; accepted: 25 May 2012; published online: 18 June 2012.*

*Citation: Knotkova H, Rosedale M, Strauss SM, Horne J, Soto E, Cruciani RA, Malaspina D and Malamud D (2012) Using transcranial direct current stimulation to treat depression in HIV-infected persons: the outcomes of a feasibility study. Front. Psychiatry 3:59. doi: 10.3389/fpsyt.2012.00059*

*This article was submitted to Frontiers in Neuropsychiatric Imaging and Stimulation, a specialty of Frontiers in Psychiatry. Copyright © 2012 Knotkova, Rosedale, Strauss, Horne, Soto, Cruciani, Malaspina and Malamud. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.*

## A randomized double-blind sham-controlled study of transcranial direct current stimulation for treatment-resistant major depression

**Daniel M. Blumberger <sup>1</sup>† , Lisa C.Tran<sup>1</sup>† , Paul B. Fitzgerald<sup>2</sup> , Kate E. Hoy <sup>2</sup> and Zafiris J. Daskalakis <sup>1</sup>\***

<sup>1</sup> Department of Psychiatry, Centre for Addiction and Mental Health, University of Toronto, Toronto, ON, Canada <sup>2</sup> Monash Alfred Psychiatry Research Centre, The Alfred and Monash University Central Clinical School, Melbourne, VIC, Australia

#### **Edited by:**

Andre R. Brunoni, Universidade de São Paulo, Brazil

#### **Reviewed by:**

Pedro Shiozawa, Santa Casa de Misericórdia de São Paulo, Brazil Leandro Da Costa Lane Valiengo, Universidade de São Paulo, Brazil Veronica Gálvez-Ortiz, IDIBELL, Spain Ulrich Palm, Ludwig-Maximilians-University, Germany

#### **\*Correspondence:**

Zafiris J. Daskalakis, Schizophrenia Program, Centre for Addiction and Mental Health, Faculty of Medicine, University of Toronto, 250 College Street, 7th Floor, Toronto, ON, Canada M5T1R8.

e-mail<EMAIL_ADDRESS>†Daniel M. Blumberger and Lisa C. Tran are first authors.

**Objectives:**Transcranial direct current stimulation (tDCS) has demonstrated some efficacy in treatment-resistant major depression (TRD). The majority of previous controlled studies have used anodal stimulation to the left dorsolateral prefrontal cortex (DLPFC) and a control location such as the supraorbital region for the cathode. Several open-label studies have suggested effectiveness from anodal stimulation to the left DLPFC combined with cathodal stimulation to the right DLPFC. Thus, this study evaluated the efficacy of tDCS using anodal stimulation to the left DLPFC and cathodal stimulation to the right DLPFC compared to sham tDCS. **Methods:** Subjects between the ages of 18 and 65 were recruited from a tertiary care university hospital. Twenty-four subjects with TRD and a 17-item Hamilton Rating Scale for Depression greater than 21 were randomized to receive tDCS or sham tDCS.The rates of remission were compared between the two treatment groups. **Results:** The remission rates did not differ significantly between the two groups using an intention to treat analysis. More subjects in the active tDCS group had failed a course of electroconvulsive therapy in the current depressive episode. Side effects did not differ between the two groups and in general the treatment was very well tolerated. **Conclusion:**Anodal stimulation to the left DLPFC and cathodal stimulation to the right DLPFC was not efficacious in TRD. However, a number of methodological limitations warrant caution in generalizing from this study. Ongoing, controlled studies should provide further clarification on the efficacy of this stimulation configuration in TRD. ClinicalTrials.gov Identifier: NCT01078948.

 Despite the vast number of pharmacological and psychotherapeutic treatments that are available, as many as 50% of patients fail to respond to treatment (Pincus and Pettit, 2001; Sackeim, 2001; Fava, 2003). In addition, the pharmacological augmentation and combination strategies frequently used in treatment-resistant depression (TRD) often increase the risk of adverse events and drug interactions (Joo et al., 2002; Dew et al., 2007; Papakostas, 2008). Electroconvulsive therapy (ECT) has demonstrated superior efficacy outcomes in TRD (Eranti et al., 2007; Lisanby, 2007). However, many patients are reluctant to engage in a trial due to stigma and the risk of cognitive adverse effects (Lisanby, 2007). The need for alternative treatment strategies to optimize outcomes for patients who experience TRD has been recognized as one of the future directions for addressing this disorder (Insel, 2006).

Transcranial direct current stimulation (tDCS) is a noninvasive and non-convulsive form of brain stimulation in which a weak, direct current (typically 1–2 mA) is applied using two surface scalp electrodes. Initial studies in animals suggested that such stimulation could elicit polarity-dependent alterations in cortical excitability and activity, with anodal stimulation increasing cortical excitability and cathodal stimulation causing cortical inhibition (Bindman et al., 1964). Furthermore, these resultant changes were not limited solely to the period of stimulation, but endured for minutes to hours afterward (Bindman et al., 1964). More recently, Nitsche and Paulus (2001) demonstrated that comparable changes occurred following tDCS directed to the human motor cortex, providing further evidence of its neuromodulatory potential.

As a result of its capacity to alter cortical activity, investigators in the 1960s began to investigate tDCS as a possible treatment for depression (Costain et al., 1964; Lippold and Redfearn, 1964; Redfearn et al., 1964); however, results were mixed, methodological differences between studies confounded results, interest in pursuing tDCS waned and the development of pharmacological antidepressant agents dominated the ensuing decades. Since the 1990s, however, research in various forms of invasive and non-invasive brain stimulation such as deep brain stimulation (DBS) and repetitive transcranial magnetic stimulation (rTMS) has been re-invigorated. A resurgence of interest may be partially a consequence of the recognition that, despite advances in pharmacotherapy, treatment-resistance remained a persistent issue in the treatment of depression (Fava, 2003; Rush et al., 2006).

In spite of renewed interest in examining tDCS as a potential treatment for major depression, its efficacy, as well as its optimal stimulation parameters, have yet to be established. A recent metaanalysis that reviewed 10 studies (six of which were randomized controlled trials) reported that compared to sham tDCS, active tDCS was more effective in reducing symptoms of depression (Kalu et al., 2012). The authors caution, though, that the small number of studies hindered their meta-analysis, many of which had limited sample sizes, eligiblefor inclusion.A large, randomized sham-controlled trial that used anodal stimulation over the left dorsolateral prefrontal cortex (DLPFC) and cathodal stimulation over the contralateral supraorbital region showed a significantly greater improvement in depression scores in subjects receiving active tDCS compared to sham over a 3-week controlled phase, although differences in response or remission criteria were not demonstrated. However, after an additional 3 weeks in an openlabel extension phase, those subjects who had received active stimulation were significantly more likely to achieve a 50% reduction in symptoms (Loo et al., 2012).

Although the pathophysiology and etiology of major depression is complex, one hypothesis underlying a number of brain stimulation studies is that there exists a pathological aberration and imbalance in the activity of the left and right prefrontal cortices, with the left DLPFC hypoactive and right DLPFC overactive in those with depression (Baxter et al., 1989; Fitzgerald et al., 2008; Grimm et al., 2008). With the aim of ameliorating this putative imbalance between the two hemispheres, many brain stimulation studies attempt to enhance the excitability of the left DLPFC while dampening the activity of the right prefrontal cortex (Fitzgerald et al., 2006; Blumberger et al., 2011). Though there is much debate in the ECT literature regarding the efficacy of unilateral and bilateral treatment, it is clear that both forms of stimulation involve widely distributed neurobiological change as a consequence of seizure generalization (Nobler et al., 2001). A recent brain imaging study has demonstrated that tDCS can produce electrode dependent changes in regional brain activity in the prefrontal cortex (Merzagora et al., 2010). Thus, there is a rationale for directing anodal tDCS over the left DLPFC, while placing cathodal stimulation over the right DLPFC.

The optimal placement of the electrodes remains under investigation – several tDCS studies, using bilateral frontal stimulation that resulted in an improvement of depressive symptoms, have positioned the cathode over the right supraorbital region rather than over the right DLPFC (Fregni et al., 2006a; Boggio et al., 2008; Loo et al., 2010). Moreover, as regions other than the prefrontal cortices have also been implicated in depression, it may be prudent to explore the effects of alternative electrode montages on the efficacy of tDCS. Another recent open-label, pilot study used fronto-extracephalic stimulation, in which anodal stimulation was directed over the right DLPFC and cathodal stimulation was directed over the right, upper arm (Martin et al., 2011). The subjects had previously participated in a tDCS trial that delivered bifrontal stimulation and subjects experienced the two treatment groups consecutively. The authors reported a 43.8% reduction in depression scores with a more rapid response when compared to bilateral frontal stimulation.

The relationship between degree of symptom severity and treatment-resistance is intrinsic to the question of efficacy of tDCS treatment. Many earlier studies that demonstrated promising results, included individuals experiencing mild to moderate depression and did not necessitate that participants meet criteria for treatment-resistance (Fregni et al., 2006a; Boggio et al., 2008; Rigonatti et al., 2008). Several, open-label studies have suggested that left DLPFC cathodal and right DLPFC anodal tDCS may be an effective treatment configuration in more severely depressed patients (Ferrucci et al., 2009; Brunoni et al., 2011a; Dell'Osso et al., 2011). Thus, the current study was designed to determine the efficacy of tDCS providing both left and right DLPFC stimulation using anodal and cathodal stimulation respectively. We hypothesized that this electrode placement configuration would lead to greater improvement compared to sham, with a larger effect size than previous unilateral approaches with anodal stimulation of the left DLPFC and cathodal stimulation over the supraorbital region. In addition,we hypothesized that tDCS would be as tolerated as well as sham stimulation with minimal side effects.

#### **METHODS SUBJECTS**

Twenty-four outpatients (20 female, 4 male; mean age 47.3 years, range 24–62) were recruited from the Mood and Anxiety, Geriatric Mental Health, and Brain Stimulation Treatment and Research programs at the Centre for Addiction and Mental Health (a tertiary university teaching hospital) as well as via referrals from physicians in Ontario, Canada. All subjects had a diagnosis of unipolar Major Depressive Disorder without psychotic features and were experiencing a Major Depressive Episode, as confirmed by the Structured Clinical Interview for the DSM-IV (SCID-IV). Subjects were required to have a score of ≥21 on the 17-item Hamilton Rating Scale for Depression (HRSD-17). Subjects were required to meet stage II criteria on the Thase Scale for treatmentresistance (failure to achieve remission or inability to tolerate two trials of an antidepressant from separate classes; Thase and Rush, 1995). Concomitant medications, such as various classes of antidepressants (e.g., selective serotonin reuptake inhibitors, tricyclic antidepressants), benzodiazepines, and antipsychotics were permitted provided that subjects had been on a stable dose of their medications for at least 4 weeks prior to entering the study and were able to maintain those stable dosages for the duration of the protocol. Subjects taking anticonvulsants were ineligible for the study, as certain agents have been found to disrupt the effects of anodal tDCS (Nitsche et al., 2003). Moreover, individuals were not included in the study if they: (i) had a DSM-IV history of substance abuse or dependence in the 6-months prior to enrolling in the study; (ii) had a concomitant, major and unstable medical, or neurologic illness; (iii) had a history of seizures; (iv) were pregnant; and/or (v) met DSM-IV criteria for borderline personality disorder or antisocial personality disorder based on the SCID for DSM-IV Axis II Disorders (SCID-II). The research ethics board at the Centre for Addiction and Mental Health approved the study and all subjects provided written, informed consent prior to commencing their involvement in the trial.

#### **STUDY DESIGN AND TREATMENT**

Following completion of baseline clinical measures, subjects were randomly assigned using a computer-generated randomization list with the information stored on a centralized computer to receive either active or sham tDCS. Only the treating clinician was aware of subjects' treatment condition. Fifteen treatments, each lasting 20 min, were administered over the course of 3 weeks (one treatment per weekday) with clinical raters and subjects blind to treatment group allocation. After receiving seven treatment sessions, subjects were assessed using the Montgomery–Asberg Depression Rating Scale (MADRS; Montgomery and Asberg, 1979) and continued with the remainder of their treatment course, whereupon they were reassessed with the full clinical rating battery and the blind was broken. During the informed consent process, subjects were told that there were two treatment conditions (i.e., active or sham stimulation) and were instructed not to discuss their treatment experiences with the clinical rater.

At the time of the study design, there was no data on the bilateral electrode placement proposed. However, we postulated that 46 patients would be required to have a 80% chance of detecting, as significant at the 5% level, a decrease in the primary outcome measure from 8 in the sham group to 15 in the active tDCS group. We planned an interim analysis at the midpoint of the trial.

#### **TREATMENT PROTOCOL**

Transcranial direct current stimulation treatment was delivered using a battery-operated, constant current stimulator (CX-6650; Rolf Schneider Electronics, Germany) and transmitted by two rubber electrodes (7 cm × 5 cm = 35 cm<sup>2</sup> ), each covered by a salinesoaked sponge and affixed to the head with a headband. The anode was directed over the left DLFPC and the cathode was placed over the right DLPFC, corresponding to electrodes F3 and F4, respectively, according to the 10–20 EEG system. Neuronavigation studies (Herwig et al., 2001) have indicated that this is a reasonably accurate method of locating the DLPFC, and it has also been used in previous tDCS studies targeting the DLPFC (Fregni et al., 2005, 2006a). In the active treatment group, stimulation was delivered at 2 mA for 20 min; sham stimulation was delivered using parameters identical to those in the active condition with the exception of the stimulator being programmed to turn off after 30 s, allowing the investigators to mimic the initial somatic sensations experienced with active tDCS, but without providing putative therapeutic benefits (Gandiga et al., 2006; Ambrus et al., 2010). In both treatment arms, the stimulator was oriented in such a way that subjects were unable to view the settings of the treatment parameters on the front panel of the machine. Subjects were permitted to make up missed treatments; however, they were not allowed to miss more than four treatments over the duration of the study.

#### **CLINICAL ASSESSMENTS**

Experienced clinical raters blind to treatment assignment administered the following rating scales at baseline and post-treatment: the MADRS, HRSD-17 (Hamilton, 1967), the Brief Psychiatric Rating Scale (Overall and Gorharn, 1962), and the Beck Depression Inventory (BDI; Beck et al., 1961). Subjects underwent an abbreviated assessment at the trial midpoint (i.e., after seven treatments) consisting of the MADRS only.

#### **OUTCOME MEASURES**

The primary outcome for the study was change from baseline to endpoint on the HRSD-17. All subjects were assessed at baseline, at the point of early treatment termination, if possible, and after 15 treatments. Secondary outcomes included remission (score ≤7) and response (50% improvement). Other measures included change from baseline to endpoint, as well as response and remission on MADRS and BDI-II.

#### **DATA ANALYSIS**

All statistical analyses were conducted using statistical software (SPSS for Windows 15.0; SPSS Inc., Chicago, IL, USA) and the analysis was conducted on an intention to treat basis. Baseline differences in demographic and clinical variables were compared between treatment groups. Continuous variables were analyzed with one-way analysis of variance (ANOVA). Categorical variables were analyzed with a two-tailed Fisher's exact test (for dichotomous comparisons). All procedures were two-tailed and we used a significance level set at α = 0.05 for the primary outcome. Analysis of the primary outcome was performed using repeated measures ANOVA.

#### **RESULTS**

#### **PARTICIPANT FLOW, FOLLOW-UP, AND SAMPLE CHARACTERISTICS**

Of 47 patients screened, 4 did not meet eligibility criteria and 19 declined participation. A total of 24 patients were randomized (see **Figure 1**).

The subjects' baseline clinical and demographic characteristics are summarized in **Table 1**. There were no clinically important differences between groups. Nineteen subjects were taking antidepressant medication (with or without other agents) during the trial. There were no differences in the proportion of subjects taking any of the medication classes. Six subjects in the tDCS group and two subjects in the sham group had received treatment with ECT in previous depressive episodes. Three subjects in the tDCS group and one in the sham group had failed a course of ECT during the current depressive episode. Post-treatment (week 1) data on the primary outcome measure was available for *n* = 21 subjects (87.5%). Subjects who were lost to follow-up did not differ from retained subjects on any of the baseline clinical, cognitive, or demographic variables. Nineteen subjects received all 15 treatments, of the remaining five subjects the number of missed treatments were 14, 12, 4, 1, 7 respectively. Of a total of 19 subjects who were assessed for maintenance of the blind, 14 subjects (73.7%) correctly guessed whether they received active or sham treatment: 6 (60.0%) in the active tDCS group and 8 (88.9%) in the sham group. These proportions did not differ significantly between the two groups (*p* = 0.30). The blinding of clinical raters was not assessed.

#### **PRIMARY OUTCOME: CHANGE IN HRSD-17**

The mean post-HRSD score in the active tDCS group and sham stimulation group are shown in **Table 2**. There was no difference in HRSD change between the two groups (*F* = 0.063; df = 1; *p* = 0.80). The same analysis was run for all subjects who completed all 15 treatments. Similarly, there was no difference in HRSD change between the two groups (*F* = 0.30; df = 1; *p* = 0.59). None of the subjects in either group met criteria for remission on the

HRSD. One subject in each group met criteria for response on the HRSD (Fisher's exact *p* = 1.00).

#### **SECONDARY OUTCOME MEASURES**

#### **Montgomery-Asberg depression rating scale**

The mean post-MADRS score in the active tDCS group and sham stimulation group are shown in **Table 2**. There was no difference in MADRS change between the two groups (*F* = 0.38; df = 1; *p* = 0.55). One of the subjects in the active tDCS group achieved response (Fisher's exact *p* = 1.00) and remission criteria (Fisher's exact *p* = 1.00) while no subjects met response or remission criteria in the sham stimulation group.

#### **Beck depression inventory-II**

The mean post-BDI-II score in the active tDCS group and sham stimulation group are shown in **Table 2**. There was no difference in BDI-II change between the two groups (*F* = 1.1; df = 1; *p* = 0.38). Two of the subjects in the active tDCS group and one in

the sham stimulation group achieved remission criteria (Fisher's exact *p* = 1.00). Three subjects in the active tDCS group and one in the sham stimulation group met criteria for response (Fisher's exact *p* = 0.58).

#### **ADVERSE EFFECTS AND TOLERABILITY**

As indicated, 3/24 subjects (28%) did not complete an endpoint assessment for the primary outcome: 2/13 in the active tDCS group and 1/11 in the sham group (see **Figure 1**). Three subjects had an endpoint assessment but did not receive all 15 treatments as they missed too many sessions and were withdrawn. Four subjects in the sham group reported mild skin tingling. Two subjects in the active group reported mild skin tingling and two reported mild to moderate skin tingling. Three subjects in the active group reported mild headache while no subjects reported headache in the sham group. No serious adverse events were reported during the trial. One subject in the sham group withdrew due to scalp discomfort.

#### **DISCUSSION**

To our knowledge this is the first randomized sham-controlled trial comparing tDCS that employed anodal stimulation to the left DLPFC and cathodal stimulation to the right DLPFC. We did not find any differences between the efficacy of active and sham stimulation. Both treatment groups improved over the 3-weeks of the trial. Overall, the treatment was well tolerated with only one subject withdrawing due to scalp discomfort.

The strengths of this study included focus on inclusion of treatment-resistant subjects with stage II or higher treatmentresistance (Thase and Rush, 1995), the use of sham tDCS as a control, and an increase in the number of treatments to 15 over 3 weeks (longer than most previous treatment trials).

A number of potential limitations may explain the lack of efficacy from active tDCS in the current study. The most important limitation is the small sample size of the study. Despite the clear lack of separation between the two conditions, it is possible that differences may have been demonstrated had the study continued to its anticipated sample size of 46. Given the lack of **Table 2 | Primary and secondary outcome measures (mean, SD) at baseline and post-treatment.**

differences between groups and the interim analysis we felt that continuing the study would not be ethical. Though we sought to include patients with treatment-resistance, the level of treatmentresistance may have been too high to observe an effect. Indeed, a third of the sample in the active stimulation group had failed a course of ECT in the current episode and nearly half had ECT in previous episodes. Though the number of subjects who failed a course of ECT in the active tDCS group was not statistically different from the number that failed a course of ECT in the sham group, the active group may have been biased toward nonresponse due to the small numbers in the study. Failure of ECT has generally been an exclusion criterion in other brain stimulation trials (Fregni et al., 2006b; O'Reardon et al., 2007; George et al., 2010). Future controlled trials should ensure that subjects with excessively high levels of treatment-resistance are characterized and accounted for in the randomization by stratification or excluded from the eligibility criteria. Another major limitation of the study is the high overall correct guess of treatment condition in the study. The correct guess rate calls into question the adequacy of the blinding and thus the sham control in this study. However, the sham procedure in the current study followed the directions and recommendations of previous studies (Gandiga et al., 2006; Ambrus et al., 2010). Equal numbers of subjects in both the sham and active group reported skin tingling suggesting that the sham was effective at providing a somatic sensation. However, more subjects in the active group reported headache and more intense skin tingling. It is possible that the treating technician gave non-verbal cues to subjects indicating treatment condition, however, we have no way of assessing whether this occurred. Though medication initiation was controlled in this study, the possibility remains that subjects who started an antidepressant immediately before study entry may have experienced a delayed response (i.e., greater than 4 weeks) to their antidepressant during the trial (Rush et al., 2003). However, the majority of subjects, who were taking an antidepressant, had been on stable doses of medication for longer than 8 weeks. Furthermore, the variability in the use of any antidepressant may have impacted the effect of the treatment. The use of benzodiazepines by patients may have also limited the efficacy of the treatment as this class of medication has been shown to impair the neurophysiological effects of stimulation (Nitsche et al., 2004). A greater percentage of patients in the active stimulation group were taking benzodiazepines. In addition, the patients in the active group had a longer duration of illness and had failed more medication trials. Collectively, these

differences suggest that the active stimulation group were more treatment-resistant.

Notwithstanding these limitations, it is concerning that we did not demonstrate differences on any of the primary or secondary outcome measures. None of the subjects in the study met criteria for remission on the HRSD-17-item. A recent metaanalysis has also concluded that the effects of tDCS are somewhat muted (Kalu et al., 2012). A more recent randomized, doubleblind, sham-controlled study in patients who had failed to respond to at least two previous trials of antidepressants from different classes did not find a difference between active left DLFPC and right supraorbital stimulation and sham stimulation of 2 weeks duration (Palm et al., 2012). Subjective ratings on secondary outcome measures, such as the Positive and Negative Affect Scale, suggested that active tDCS was associated with an increase in positive emotions and also trended toward a decrease of negative emotions (Palm et al., 2012). The largest study of tDCS in depression has recently been reported and though there was a significant difference in the change of MADRS scores, there was no difference in responders and remitters between active and sham stimulation (Loo et al., 2012). The authors suggested that longer treatment durations up to 6 weeks might be necessary to achieve clinical response with tDCS (Loo et al., 2012). We hypothesized that providing excitatory stimulation (anode) to the left DLPFC and inhibitory stimulation (cathode) to the right DLPFC would lead to improved efficacy. The theoretical rationale for this comes from the ECT literature and some previous rTMS studies showing improved efficacy with excitatory stimulation to the left DLPFC and inhibitory stimulation to the right DLPFC (Fitzgerald et al., 2006; Blumberger et al., 2011). However, recent data has not replicated the finding of improved efficacy with this stimulation pattern (Fitzgerald et al., 2012). Furthermore, it is possible that the montage of left and right DLPFC were too close together leading to shunting over the scalp. While it may be theoretically advantageous to stimulate bilaterally, the

#### **REFERENCES**

after-effects. *J. Physiol. (Lond.)* 172, 369–382.

physical properties of tDCS may not be amenable to this electrode placement.

A series of three open-label trials have suggested that the stimulation technique used in this study is an effective form of tDCS in the treatment of depression (Ferrucci et al., 2009; Brunoni et al., 2011a; Dell'Osso et al., 2011). One study showed a 30% improvement in depression rating scale scores in 14 inpatients with a severe major depressive episode using twice daily treatments. Similarly, the other two studies found positive effects in both unipolar and bipolar depressed patients after 10 treatments over 5 days (Brunoni et al., 2011a; Dell'Osso et al., 2011). In contrast, we did not find this stimulation configuration to be more beneficial than sham stimulation when providing treatment once daily. However, we would caution generalizing from the current study due to the limitations identified. The design of the Sertraline vs. ELectrical Current Therapy (SELECT) tDCS trial will utilize the same stimulation parameters as the current study and should provide greater clarification regarding the efficacy of anodal stimulation to the left DLPFC and cathodal stimulation to the right DLPFC (Brunoni et al., 2011b).

#### **ACKNOWLEDGMENTS**

The authors gratefully acknowledge the assistance of all volunteers whose participation was essential in the successful completion of the study. Zafiris J. Daskalakis, Lisa C. Tran, and Daniel M. Blumberger had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. This work was funded, in part by the Canadian Institutes of Health Research (CIHR) Clinician Scientist Award (Zafiris J. Daskalakis), CIHR Fellowship (Daniel M. Blumberger), by a National Health and Medical Research Council (NHMRC) Practitioner Fellowship (Paul B. Fitzgerald), and by Constance and Stephen Lieber through a National Alliance for Research on Schizophrenia and Depression (NARSAD) Lieber Young Investigator award (Zafiris J. Daskalakis).

Transcranial direct current stimulation (tDCS) in unipolar vs. bipolar depressive disorder. *Prog. Neuropsychopharmacol. Biol. Psychiatry* 35, 96–101.

current stimulation for the outpatient treatment of poor-responder depressed patients. *Eur. Psychiatry*. PMID: 21621982. [Epub ahead of print.]

Nahas, Z., Bulow, P., Zarkowski, P., Holtzheimer, P. E. III, Schwartz, T., and Sackeim, H. A. (2010). Daily left prefrontal transcranial magnetic stimulation therapy for major depressive disorder: a shamcontrolled randomized trial. *Arch. Gen. Psychiatry* 67, 507–516.

sham-controlled trial. *Br. J. Psychiatry* 200, 52–59.

**Conflict of Interest Statement:** Daniel M. Blumberger receives research support for an investigator-initiated trial from Brainsway Ltd., Lisa C. Tran reports no biomedical conflicts of interest. Paul B. Fitzgerald is supported by a NHMRC Practitioner fellowship. He has received research funding from Neuronetics Inc., and equipment for investigator-initiated research from Magventure A/S and Brainsway Ltd., Zafiris J. Daskalakis receives research

support for an investigator-initiated trial from Brainsway Ltd., He has received research funding from Aspect Medical Inc., and Neuronetics Inc.; he has also received a travel allowance from Pfizer.

*Received: 06 May 2012; accepted: 27 July 2012; published online: 17 August 2012.*

*Citation: Blumberger DM, Tran LC, Fitzgerald PB, Hoy KE and Daskalakis ZJ (2012) A randomized double-blind sham-controlled study of transcranial direct current stimulation for treatmentresistant major depression. Front. Psychiatry 3:74. doi: 10.3389/fpsyt.2012.00074 This article was submitted to Frontiers in Neuropsychiatric Imaging and Stimulation, a specialty of Frontiers in Psychiatry.*

*Copyright © 2012 Blumberger, Tran, Fitzgerald, Hoy and Daskalakis. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.*

## Immediate effects of tDCS on the µ-opioid system of a chronic pain patient

#### **Marcos Fabio DosSantos 1,2,Tiffany M. Love<sup>3</sup> , Ilkka Kristian Martikainen1,3,Thiago Dias Nascimento<sup>1</sup> , Felipe Fregni <sup>4</sup> , Chelsea Cummiford<sup>3</sup> , Misty Dawn Deboer <sup>1</sup> , Jon-Kar Zubieta<sup>3</sup> and Alexandre F. M. DaSilva1,3\***

<sup>1</sup> Headache and Orofacial Pain Effort, Biologic and Materials Sciences Department and MCOHR, School of Dentistry, University of Michigan, Ann Arbor, MI, USA

<sup>2</sup> Faculdade de Medicina, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

<sup>3</sup> Translational Neuroimaging Laboratory, Molecular and Behavioral Neuroscience Institute, University of Michigan, Ann Arbor, MI, USA

<sup>4</sup> Laboratory of Neuromodulation, Spaulding Rehabilitation Hospital, Harvard University, Boston, MA, USA

#### **Edited by:**

Paulo Sérgio Boggio, Mackenzie Presbyterian University, Brazil

#### **Reviewed by:**

Rosana Lima Pagano, Hospital Sírio-Libanês, Brazil Wolnei Caumo, Universidade Federal do rio Granade do Sul, Brazil

#### **\*Correspondence:**

Alexandre F. M. DaSilva, Headache and Orofacial Pain Effort, The Molecular and Behavioral Neuroscience Institute, University of Michigan, 205 Zina Pitcher Pl, Room 1021, Ann Arbor, MI 48109-5720, USA; Biologic and Materials Sciences, School of Dentistry, 1011 North, University Avenue, Room 1014A, Ann Arbor, MI 48109-1078, USA. e-mail<EMAIL_ADDRESS>
We developed a unique protocol where transcranial direct current stimulation (tDCS) of the motor cortex is performed during positron emission tomography (PET) scan using a <sup>1</sup> µ-opioid receptor (µOR) selective radiotracer, [ <sup>1</sup>C]carfentanil. This is one of the most important central neuromechanisms associated with pain perception and regulation. We measured µOR non-displaceable binding potential (µOR BPND) in a trigeminal neuropathic pain patient (TNP) without creating artifacts, or posing risks to the patient (e.g., monitoring of resistance).The active session directly improved in 36.2% the threshold for experimental cold pain in the trigeminal allodynic area, mandibular branch, but not theTNP patient's clinical pain. Interestingly, the single active tDCS application considerably decreased µORBPND levels in (sub)cortical pain-matrix structures compared to sham tDCS, especially in the posterior thalamus. Suggesting that the µ-opioidergic effects of a single tDCS session are subclinical at immediate level, and repetitive sessions are necessary to revert ingrained neuroplastic changes related to the chronic pain. To our knowledge, we provide data for the first time in vivo that there is possibly an instant increase of endogenous µ-opioid release during acute motor cortex neuromodulation with tDCS.

**Keywords: tDCS, PET, opioid receptors, neuroplasticity, trigeminal neuropathic pain, post-herpetic neuralgia**

#### **BACKGROUND**

Pain is described as a complex experience affecting not only the sensory, but also the affective and cognitive systems (Merskey and Bogduk, 1994). Although the central mechanisms involved in pain perception and modulation have not been completely elucidated, recent years have seen significant advances in the understanding of the anti-nociceptive mechanisms controlling the pain experience in humans. One of the most important modulatory mechanisms is the endogenous opioidergic system, which is involved in the regulation of experimental and clinical pain, as well as in the effects of analgesic opiate drugs. Studies with positron emission tomography (PET) have shown decreased opioid receptor nondisplaceable binding potential (BPND) in patients with chronic pain disorders, including rheumatoid arthritis (Jones et al., 1994), neuropathic pain (Maarrawi et al., 2007a; DosSantos et al., 2012), and fibromyalgia (Harris et al., 2007) when examined with both selective for µ-opioid receptor (µOR; Harris et al., 2007; DosSantos et al., 2012) and non-selective (Jones et al., 1994; Maarrawi et al., 2007a) opioid receptor markers. The data available points to either or both endogenous opioid release, and down-regulation of opioid receptors. It has also been demonstrated that sustained pain activates µOR mediated neurotransmission in a complex network of brain areas related to pain, including the dorsolateral prefrontal cortex, anterior cingulate, anterior and posterior insula, thalamus, hypothalamus, amygdala, and periaqueductal gray matter. Furthermore, the magnitude of these regional activations was

related to the individual's capacity to suppress sensory and affective elements of the pain experience (Zubieta et al., 2001).

Therapies that directly modulate brain activity in specific neural networks might be particularly suited to relieve chronic pain. Interestingly, a novel method of non-invasive brain stimulation, namely transcranial direct current stimulation (tDCS), has been reported to produce lasting therapeutic effects, when applied to the motor cortex, in chronic pain disorders, including fibromyalgia (Fregni et al., 2006; Riberto et al., 2011), orofacial pain attributed to viral infection (Antal and Paulus, 2011), and chronic migraine (DaSilva et al., 2012). This technique is based on the application of a weak direct current to the scalp that flows between two electrodes (anode and cathode). Some studies have shown that the efficacy of tDCS depends critically on parameters such as electrode position and current strength (Nitsche et al., 2003). In fact, application of tDCS for 13 min to the motor cortex can modulate cortical excitability for several hours (Nitsche and Paulus, 2000, 2001). Two cortical areas have been explored in pain studies using tDCS: primary motor cortex and dorsolateral prefrontal cortex (Nitsche et al., 2008; DaSilva et al., 2011). In the most common setup for pain research the anode is positioned over the motor cortex (M1) and the cathode over the supra-orbital area (DaSilva et al., 2011). It has been described that the cortical excitability can be changed up to 40% with this method (DaSilva et al., 2011). Regarding the specific area stimulated in M1, studies with non-invasive brain stimulation have shown better results for

facial pain with the stimulation of the hand cortical area (medially located) and more significant improvement of hand pain when the cortical area representing the face (more laterally located) is stimulated. One possible explanation would be the direct effect of tDCS/TMS on the thalamus, which could lead to stimulation of the ventroposteromedial nucleus (VPM), responsible for the nociceptive input from the face (Lefaucheur et al., 2004, 2006; Lefaucheur, 2006).

#### **CASE PRESENTATION**

#### **SUBJECT**

A 62-year-old woman was recruited by the Headache and Orofacial Pain Effort (H.O.P.E.) laboratory at the University of Michigan to participate in an ongoing study investigating the effects of the tDCS in the µ-opioidergic system. She had a history of herpes zoster in 2008, with severe pain, affecting the distribution of the left ophthalmic (V1) and maxillary (V2) divisions of the trigeminal nerve. The pain persisted after the complete healing of the initial lesions, leading to a diagnosis of post-herpetic neuralgia. During the baseline evaluation, she described the pain as constant, spontaneous, throbbing, aching, heavy, and hot-burning. The average pain intensity was four out of ten and the average of the unpleasantness associated with the spontaneous pain was six out of ten. The pain was alleviated by sleep and massage and aggravated by sleepiness, stress, and alcohol. The patient reported eye dryness and nasal congestion related to her pain. The symptoms could not be triggered with heat, cold, touch, or chewing. Her pain was not associated with nausea, vomiting, photophobia, or headache. The patient rated the levels of social interaction (0 = isolation, 10 = social gathering), attention (0 = inattention, 10 = high awareness), and anxiety (0 = least, 10 = most) at two, three, and six out of ten, respectively, during the spontaneous pain. She had been treated with amitriptyline 10 mg once a day and pregabalin 50 mg twice a day, with only partial control of her pain. The scores of the McGill Pain questionnaire (MPQ) descriptors during the baseline evaluation were: 24 (sensory), 5 (affective), 2 (evaluative), and 7 (miscellaneous). The pain rating index (PRI) was 38 and the present pain intensity (PPI) was three (distressing). All procedures reported were carried out in accordance with the bioethical rules for studies involving human beings of the WMA (World Medical Association, 2012) – Declaration of Helsinki (2008). The protocol of this study was previously approved by the University of Michigan Investigational Review Board for Human Subject Use and by the Radioactive Drug Research Committee of the US Food and Drug Administration. The patient gave written informed consent prior to the participation in the study.

#### **NEUROIMAGING**

We used a radiotracer with specific affinity for µORs, [11C] carfentanil. The participant underwent one baseline and one tDCS90-min PET scan using a Siemens (Knoxville, TN, USA) HR + scanner in 3D mode (reconstructed images have a full-width at half maximum (FWHM) resolution of approximately 5.5 mmin-plane and 5.0 mm axially). Synthesis of high specific activity [ <sup>11</sup>C]carfentanil (>2000 Ci/mmol) was produced by the reaction of [11C]methyliodide and a non-methyl precursor (Dannals et al., 1985; Jewett, 2001). Each [11C]carfentanil dose (10–15 mCi, ≤0.03µg/kg) was administered at 50% as a bolus with the remnants constantly injected across the session to reach normalized tracer levels approximately 35 min after tracer administration.

Positron emission tomography images were reconstructed using interactive algorithms into a 128 × 128 pixel-matrix in a 28.8 cm diameter field of view (FOV). Twenty-eight image frames were obtained and co-registered to one another. They were corrected for motion and decay (Minoshima et al., 1993). Dynamic image data for each scan were converted on a voxel-by-voxel basis into two sets of parametric images: First, a tracer transport measure (K1 ratio) used for co-registration and normalization procedures; and second,a receptor-related measure,distribution volume ratio (DVR, equal to *B*max/*K*<sup>d</sup> + 1 or binding potential at equilibrium (BPND) + 1). These two measures were estimated using a modified Logan graphical analysis using the occipital cortex as the reference region (Logan et al., 1996).

A T1-weighted anatomical MRI scan was acquired on a 3 T scanner (General Electric, Milwaukee, WI, USA). The MRI acquisition utilized the following sequence parameters: axial spoiledgradient recalled (SPGR) 3D acquisition, 15.63 bandwidth, repetition time [TR] = 9.2 ms, echo time [TE] = 1.9 ms, inversion recovery preparation 500 ms, flip angle = 15˚, 25/26 FOV, number of excitations [NEX] = 1, 144 contiguous slices, 1.0 mm slice thickness, 256 × 256 matrix.

Images were anatomically standardized into template space using Statistical Parametric Mapping (SPM8) software by (A) coregistering the MR scan and K1 scans; (B) normalizing the MR scan to the Montreal Neurologic Institute (MNI) template brain using DARTEL; and (C) applying the resulting deformation matrix to the PET images. Co-registration and normalization accuracy was verified by comparing the transformed MR and PET images to the MNI atlas template.

#### **TRANSCRANIAL DIRECT CURRENT STIMULATION**

Both placebo and active tDCS were applied during the second PET scan. The placebo tDCS was applied during the early phase of the exam (15–35 min), while the active tDCS during the late phase (60–80 min). This sequence was adopted to avoid carry-over effects from the placebo tDCS. In active stimulation 2 mA of tDCS was applied for 20 min. The anode was placed over the area corresponding to the primary motor cortex (M1) while the cathode was positioned over the supra-orbital region. For placebo tDCS, the same method was used; however current was applied only for 30 s. This has been demonstrated to be a reliable method of sham stimulation (Gandiga et al., 2006) as sensations arising from tDCS treatment are observed usually at the beginning of application. The impedance was controlled under 5 kΩ during the whole period of active stimulation to avoid abnormal increase of the overall resistance and consequently heat that could potentially burn the patient. The tDCS protocol used in this study is fully explained in a stepwise manner by our scientific team in DaSilva et al. (2011). Due to the space restrictions, considering the stimulation inside the PET scanner, a special system was developed to add more solution to the sponges when needed. This system consisted of two syringes, each one connected to one sponge by two small tubes. Each electrode was positioned inside a 35 cm<sup>2</sup> sponge, that was soaked with approximately 12 mL of saline solution (6 mL per

side) before the PET and up to 12 mL during the procedure. We used saline solutions with lower concentrations of NaCl (15 mM) (DaSilva et al., 2011).

#### **QUANTITATIVE SENSORY TESTING**

In this study we controlled the effects of tDCS on the thermal perception as assessed by the Quantitative Sensory Testing (QST) in three moments during the second PET: before starting the scan, in the period between sham and active tDCS (approximately 40–60 min) and after the scan. For this purpose, a QST protocol, consisting of thermal pain thresholds for cold and hot stimuli, was performed using a Thermal Sensory Analyzer TSA 2001-II (Medoc, Israel) (Yarnitsky and Sprecher, 1994; Bachmann et al., 2010). The thermal stimuli were applied upon V3, bilaterally, and dorsal radial area of both hands. Each stimulus was applied for three consecutive times and the average was calculated.

#### **RESULTS**

Levels of µOR BPND in our trigeminal neuropathic pain patient (TNP) patient during a single tDCS application immediately induced significant decrease inµOR binding in many (sub)cortical pain-matrix structures, including nucleus accumbens (NAc), anterior cingulate cortex (ACC), insula (Ins), and thalamus (Thal; **Figures 1** and **2**). For instance, the M1-tDCS montage considerably decreased µOR binding in the posterior thalamus (R: 21.5%; L: 19.54%), compared to sham tDCS (R: 2.2%; L: 4.7%).

No significant changes were observed in the clinical pain levels related to tDCS. The pain as assessed by the visual analog scale (VAS) was four out of ten before the second PET, three after placebo tDCS, and returned to four after the PET. Regarding the QST, a significant increase in the temperature for heat threshold was observed in the left V3 after sham tDCS and when comparing baseline and active tDCS. On the hand, the temperature for cold threshold showed a significant decrease in the left V3 after active tDCS but not after placebo tDCS. When comparing the cold threshold after active tDCS to the baseline threshold (before starting the PET scan), there was a reduction of the temperature at which cold pain was detected in the left V3 of approximately 36.2%. Significant changes in the heat and cold thresholds associated with sham and active tDCS were also observed in other regions, such as right V3, and right hand. The QST results are presented in the **Table 1**.

#### **DISCUSSION**

To our knowledge, this is the first study showing an immediate reduction in theµOR binding in response to an acute motor cortex neuromodulation, suggesting that the analgesic effect of M1-tDCS is possibly due to direct increase of endogenous opioid release.

Endogenous opioid systems have long been implicated in regulating pain nociceptive signals, with µORs being the primary mediators of opiate analgesia, but also the rewarding and tolerance-producing effects of opiates (Sora et al., 1997). Both elements, endogenous opioid release and µOR concentrations, are therefore critical elements for the understanding of chronification and alleviation of pain in TNP patients. The first direct evidence of regional endogenous µ-opioid activation during sustained experimental trigeminal pain in healthy humans was published by

Zubieta et al. (2001) using PET, measured with external imaging as reductions in the *in vivo* availability of µORs BPND quantified with [11C]carfentanil. Acute reductions in µOR BPND were observed in the PAG, thalamus, hypothalamus, NAc, ventral pallidum, amygdala, insula, and dorsal anterior cingulate (dACC), correlating with suppression of sensory and affective qualities of the pain challenge.

The investigation of the response of the endogenous opioid system to TNP and its neuromodulation models is of importance to understand the mechanisms in place to regulate the pain experience. This information is key to better predict the varied responses of TNP patients to therapeutic interventions. Jones et al. (1994, 1999) utilized [11C]diprenorphine, a nonselective opioid radiotracer, to examine the *in vivo* availability of opioid receptors in a small group of patients diagnosed with rheumatoid arthritis and trigeminal neuralgia before and 3 weeks to 3 months after treatment and pain relief. Substantial reductions in cortical and subcortical opioid receptor availability were observed prior to treatment at resting state (baseline), which were reversed after pain relief. Similar results were obtained with [ <sup>11</sup>C]diprenorphine in four central post stroke pain patients and in a patient with a pontine infarction and pain (Willoch et al., 1999, 2004), suggesting a dysregulation of central opioid mechanisms at baseline in response to chronic pain, regardless of pain etiology. Interestingly, in a study with eight refractory neuropathic

**Table 1 | Variations in the heat and cold thresholds related to placebo and active tDCS.**

receptor binding potential in the right and left thalamus during the late phase of the first and second PET scans.

Statistical significance (in bold) was defined at p < 0.05 (one-way ANOVA followed by Tukey's test).

pain patients, postoperative (invasive) motor cortex stimulation induced decreases of [11C]diprenorphine binding in the anterior mid-cingulate cortex (MCC) and PAG, which were significantly correlated with pain relief (Maarrawi et al., 2007b). The authors suggested that the decrease in binding of the exogenous ligand was possibly due to receptor occupancy by enhanced release of endogenous opioids. This analgesic mechanism is highly associated with M1 cortex stimulation, at least with rTMS, since it is blocked with naloxone injection (Taylor et al., 2012). tDCS over M1 induces immediate changes in thermal sensory percepts in health subjects, especially cold (Bachmann et al., 2010). In addition, it produces long lasting pain relief in chronic pain patients, including TNP (Lima and Fregni, 2008). Recently, it was reported that acute tDCS modulates functional connectivity depending on its polarity (Polania et al., 2011). Anodal stimulation over M1 with contralateral frontocortical cathode placement (our protocol) immediately increases functional coupling between ipsilateral M1 and thalamus. On the contrary, cathodal tDCS over M1 decreases functional coupling between ipsilateral M1 and contralateral putamen.

The findings above hint why the anode M1/cathode orbitofrontal electrode montage results in optimal modulation of pain-matrix hyperactivity, specially the thalamus, which underlies chronic pain. Here, in our case report data with TNP, the same active M1-tDCS montage considerably decreased µOR binding in the posterior thalamus (**Figure 2**). Nonetheless, it is possible that an additional opioid release might have been prevented by a potential carry-over effect related to the sham stimulation.

Remarkably, the single tDCS application immediately improved 36.2% the threshold for experimental cold pain in the allodynic V3 area (baseline: 23.7˚C ± 2.3; placebo tDCS: 22.7˚C ± 2.5; active tDCS 15.1˚C ± 3.2), but not the TNP patient's clinical pain (baseline: 4, VAS 0-10); placebo tDCS: 3; active tDCS: 4). Suggesting that the immediate opioidergic effects of a single tDCS session are subclinical, and repetitive sessions are necessary to revert ingrained neuroplastic changes related to the chronic TNP suffering (see next paragraph). This is in agreement with the results from multiple clinical tDCS studies, showing a direct relationship of patients' clinical pain improvement with the number of tDCS sessions (Lima and Fregni, 2008).

#### **CONCLUDING REMARKS**

This case report represents a change of paradigm, as we directly modulated the same opioid mechanisms under study by applying novel neuroimaging and neuromodulatory tools. Future studies are necessary to confirm our results, and to investigate further the effects of tDCS on the endogenous opioid system in a larger cohort of patients.

#### **REFERENCES**

clinical studies in brain stimulation. *Clin. Neurophysiol.* 117, 845–850.

#### **ACKNOWLEDGMENTS**

This work was supported by the following grants: Dr. DaSilva, the principal investigator of the study, was supported by MICHR Clinical Trial Planning Program/CTSA high-tech funding UL1RR024986, University of Michigan. Dr. Santos was also supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil, and by the University of Michigan, Ann Arbor, USA. Dr. Martikainen was supported by the Swedish Cultural Foundation in Finland, Helsinki, Finland.

and meta-analysis of the literature. *Neurology* 70, 2329–2337.

DC motor cortex stimulation in humans. *Neurology* 57, 1899–1901.

**Conflict of Interest Statement:** Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 20 June 2012; accepted: 02 October 2012; published online: 02 November 2012.*

*Citation: DosSantos MF, Love TM, Martikainen IK, Nascimento TD, Fregni F, Cummiford C, Deboer MD, Zubieta J-K and DaSilva AFM (2012) Immediate effects of tDCS on the* µ*-opioid system of a chronic pain patient. Front. Psychiatry 3:93. doi: 10.3389/fpsyt.2012.00093*

*This article was submitted to Frontiers in Neuropsychiatric Imaging and Stimulation, a specialty of Frontiers in Psychiatry.*

*Copyright © 2012 DosSantos, Love, Martikainen, Nascimento, Fregni, Cummiford, Deboer, Zubieta and DaSilva. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.*

## Enhancing motor skill learning with transcranial direct current stimulation – a concise review with applications to stroke

### **Sangeetha Madhavan\* and Bhakti Shah**

Department of Physical Therapy, University of Illinois, Chicago, IL, USA

#### **Edited by:**

Andre R. Brunoni, Universidade de São Paulo, Brazil

#### **Reviewed by:**

Joseph M. Galea, University College London, UK Marcel Simis, Santa Casa de São Paulo, Brazil; Harvard Medical School, USA

#### **\*Correspondence:**

Sangeetha Madhavan, Department of Physical Therapy, University of Illinois, 1919 West Taylor Street, Chicago, IL 60612, USA. e-mail<EMAIL_ADDRESS>
In the past few years, there has been a rapid increase in the application of non-invasive brain stimulation to study brain-behavior relations in an effort to potentially increase the effectiveness of neuro-rehabilitation.Transcranial direct current stimulation (tDCS), an emerging technique of non-invasive brain stimulation, has shown to produce beneficial neural effects in consequence with improvements in motor behavior. tDCS has gained popularity as it is economical, simple to use, portable, and increases corticospinal excitability without producing any serious side effects. As tDCS has been increasingly investigated as an effective tool for various disorders, numerous improvements, and developments have been proposed with respect to this technique. tDCS has been widely used to identify the functional relevance of particular brain regions in motor skill learning and also to facilitate activity in specific cortical areas involved in motor learning, in turn improving motor function. Understanding the interaction between tDCS and motor learning can lead to important implications for developing various rehabilitation approaches. This paper provides a concise overview of tDCS as a neuromodulatory technique and its interaction with motor learning. The paper further briefly goes through the application of this priming technique in the stroke population.

 The past decade has seen a rapid increase in the application of non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). These neuromodulatory techniques have been widely studied in an effort to provide support for their use as therapeutic adjuvants to enhance functional recovery after impairment. Because of its relative ease of use, portability, and decreased safety risk compared to other neurostimulatory protocols, tDCS is emerging as an effective and versatile clinical tool to prime the neuromotor system prior to or during rehabilitation. As "functional improvement after injury is a relearning process" (Kleim and Jones, 2008), in this review we will provide a brief overview on the application of tDCS to enhance motor skill learning in healthy humans and the physiological mechanisms associated with it.We will also briefly review articles that have used tDCS to enhance motor performance in stroke survivors. Understanding the interaction between tDCS and motor learning can lead to important implications for optimizing neuro-rehabilitation.

#### **REVIEW CRITERIA**

A search of the literature through January 2012 was performed in the following databases: PubMed, Web of Science, and OVID. The keywords "tDCS and motor learning," "tDCS and motor performance," and "tDCS and stroke" were used. Peer reviewed studies were selected if they met the following inclusion criteria: (1) written in English, (2) involved more than one human participant, and (3) included MEP amplitude and/or at least one motor performance-based outcome measure. Because of the vast number of studies that have used tDCS and the presence of numerous review articles on tDCS, we focused on articles relevant to the context of this paper. Data on participants, study design, analysis, follow-up, and outcomes were abstracted. Only studies rated as good or fair by the first author were included. Results were then summarized for the review.

#### **PARAMETERS OF tDCS**

Transcranial direct current stimulation involves delivering a low intensity direct current between two sponge electrodes, which are typically moistened with NaCl solution and placed on the scalp (Priori et al., 1998; Nitsche and Paulus, 2000). NaCl solution is usually preferred as it minimizes discomfort (Dundas et al., 2007). The active electrode (size between 5 and 35 cm<sup>2</sup> ) is conventionally placed on the area of the brain to be stimulated. The other electrode (usually equal or larger size than the active electrode) is placed on a region contralateral to this placement; such as the forehead if the stimulated area is the primary motor cortex. Typically current intensities of 0.5–2 mA are applied for a duration of 5–20 min, yielding a current density of 0.02–1 mA/cm<sup>2</sup> , and a total charge between 15 and 100µC/cm<sup>2</sup> .

The efficacy of tDCS depends on current density which determines the induced electrical field strength (Parazzini et al., 2011). Depending on electrode dimensions, position, and the current density, approximately half of the current injected during tDCS is shunted through the scalp (Miranda et al., 2006; Sadleir et al., 2010). Low intensity currents when applied for even short periods of 10–13 min have shown to induce cortical excitability changes lasting up to 90 min (Nitsche and Paulus, 2000, 2001). These changes in corticospinal excitability, as measured by TMS evoked motor potentials, range from 40 to 150% above baseline values for anodal stimulation, and 20 to 50% below baseline values for cathodal stimulation (Nitsche and Paulus, 2000, 2001). In order to induce after effects, Nitsche and Paulus (2000) showed that a stimulus duration of at least 3 min at 1 mA or an intensity of 0.6 mA for 5 min is required. Anodal stimulation typical enhances cortical excitability while cathodal stimulation decreases excitability (Nitsche and Paulus, 2001; Nitsche et al., 2003b). These effects have been more robust in the upper limb representations compared to the lower limb motor representations.

#### **SAFETY AND ADVERSE EFFECTS OF tDCS**

Because tDCS does not involve direct brain-electrode interface and uses low currents, it can be used safely without adverse risks. The density and charge values that are conventionally used compare well with two safety studies that reported no neural damage or change in cognitive function with applied current charges less than 96µC/cm<sup>2</sup> (Nitsche and Paulus, 2001;Iyer et al., 2005). However special care should be taken if the patient has alteration of the skull, such as trepanation or fracture, or if the patient has decreased integrity to the skin surface, as it may result in tissue damage because of increased current densities.

During current application, neurally driven vasodilatation may result in transient mild redness below the electrode surface (Durand et al., 2002). A study by Poreisz et al. (2007), focusing on the safety aspects of tDCS, reported a mild tingling sensation as the most common adverse effect; observed by 71% of the subjects during and 8% after the stimulation. Moderate fatigue was the second frequent adverse effect and a light itching sensation under the electrodes occurred in 30% of the subjects during the stimulation and in 15% after the stimulation. Very few subjects felt a slight burning sensation or a mild pain sensation under the electrodes. About 11% reported difficulties in concentrating during tDCS whereas headache seemed to occur in 5% of cases during and 12% after stimulation.

Care should be taken that there are no metallic implants near the electrodes. None of the tDCS studies have reported any serious complications of stimulation, such as seizure or instance of psychotic symptoms. However, it is necessary to keep in mind that anodal tDCS increases cortical excitability and it may be prudent to exclude patients at risk for epileptic seizures. In addition, repetitive application and long durations of tDCS should be carefully monitored for adverse effects even though none have been reported so far. Thus, to the best of our knowledge, tDCS protocols that have been conventionally used appear to be safe and the side effects are commonly limited to focal tingling, itching, and a local erythema, making it a preferred technique of choice.

#### **MECHANISM OF ACTION**

#### **DURING STIMULATION**