
 among electrical stimulation methods , transcranial direct current stimulation ( tdcs ) is the most well - known noninvasive technique for shaping neuroplasticity in humans [ 15 ] .
much of the evidence for the neuronal effects of tdcs can be based on physiological studies both in man and in animals . in particular , measuring the amplitude of motor evoked potentials ( meps ) allows easy quantification of excitability changes induced by tdcs , however , only at the primary motor cortex ( m1 ) .
besides tdcs , it was shown that transcranial random noise stimulation ( trns ) , in a range either between 0.1  hz and 640  hz or between 100 and 640  hz , is a similarly effective technique for increasing cortical excitability , however , circumventing the directional sensitivity of standard tdcs . using magnetic stimulation , apart from high frequency repetitive transcranial magnetic stimulation ( rtms ) , intermittent theta burst stimulation ( itbs )   is regarded as the most time - efficient effective technique in order to increase cortical excitability .
so far in all paradigms the duration and the magnitude of the after - effects were controlled by varying stimulation duration , type , or intensity [ 2 , 3 , 6 , 811 ] , nevertheless always in different subjects populations .
while the subject - to - subject variability using different methods can be high , the aim of the present study was to compare the efficacy of different transcranial stimulation methods that are supposed to increase cortical excitability , in the same subject population .
twelve subjects ( age : 25.7  4.1 years ; range : 2338 years ) participated in this study .
all subjects were right - handed , according to the short version of the edinburgh handedness inventory , and they were naive with regard to the aim of the study .
those who were ill , were pregnant , were suffering from drug abuse , or had metallic implants / implanted electrical devices were excluded by an interview .
none of them took any medication acting on the central nervous system or had a history of a neurological and psychiatric disease .
the experimental procedures conformed to the declaration of helsinki and were all approved by the ethics committee of the university of gttingen .
subjects and the investigator , who made the mep measurements , were blinded for stimulation conditions in all of the studies .
atdcs and trns at 1  ma intensity were delivered by a battery - driven electrical stimulator ( version dc - stimulator - plus , neuroconn gmbh , ilmenau , germany ) through conductive - rubber electrodes placed in two saline - soaked sponges . with regard to tdcs
the random numbers were normally distributed ; the probability density function followed a bell - shaped curve . in the frequency spectrum ,
the noise signal contained all frequencies up to half of the sampling rate , that is , a maximum of 640  hz .
for both atdcs and trns , the current was ramped up and down over the first and last 5  s of stimulation .
the size of the stimulation electrode over the left m1 was 4  4  cm and of the reference electrode 6  14  cm , which was placed over the contralateral orbit ; both were fixed on the head by elastic bands .
the position of the stimulation electrode was determined prior to stimulation by single pulses of transcranial magnetic stimulation ( tms ) . for sham stimulation ,
tbs was applied over the cortical representation field of the right first dorsal interosseous ( fdi ) muscle and was delivered using a magstim super rapid stimulator . the tbs pattern consisted of bursts containing 3 pulses at 50  hz repeated at 5  hz and an intensity of 80% the active motor threshold ( amt ) .
for itbs , a 2  s train of tbs was repeated every 10  s for a total of 190  s ( 600 pulses ) .
amt was the minimum intensity needed to elicit an mep response of 200300 v during moderate spontaneous background muscle activity ( 15% of the maximum muscle strength ) in at least three of six consecutive trials . to examine changes in corticospinal excitability ,
meps of the right fdi were recorded following stimulation of its motor - cortical representation field by single - pulse tms .
these were induced using a magstim 200 magnetic stimulator ( magstim company , whiteland , wales , uk ) with a figure - of - eight standard double magnetic coil ( diameter of one winding , 70  mm ; peak magnetic field , 2.2  t ; average inductance , 16.35  h ) .
surface electromyogram ( emg ) was recorded from the right fdi through a pair of ag - agcl surface electrodes in a belly - tendon montage .
raw signals were amplified , band - pass filtered ( 2  hz2  khz ; sampling rate , 5  khz ) , digitized with a micro 1401 ad converter ( cambridge electronic design , cambridge , uk ) controlled by signal software ( cambridge electronic design , version 2.13 ) , and stored on a personal computer for offline analysis .
complete relaxation was controlled through visual feedback of emg activity , and whenever it was necessary , the subject was instructed to relax .
the coil was held tangentially to the skull , with the handle pointing backwards and laterally at 45 from the midline , resulting in a posterior - anterior direction of current flow in the brain .
single - pulse meps were recorded with the tms intensity adjusted to elicit 1  mv peak - to - peak amplitude at baseline .
the site was marked with a skin marker to ensure that the coil was held in the correct position throughout the experiment .
the order of the stimulation conditions with regard to all experiments occurred in a counterbalanced fashion , with at least 5 days between two measurements .
stimulus intensities ( in percentage of maximal stimulator output ) of tms were determined at the beginning of each experiment . immediately following stimulation ,
40 single test - pulse meps were recorded at 0.25  hz , that is , approximately 0  min , 5  min , and 10  min after stimulation and then every 10 minutes up to 60  min and then again at 90  min . a repeated measure of analysis of variance ( anova ) ( a given current condition versus sham  time points of mep recordings ; dependent variable : mean amplitude of meps ) was calculated . if a significant main effect of stimulation or the interaction of time and stimulation occurred , a fisher lsd test was performed .
pearson correlation coefficients were calculated in order to see if there is a correlation with regard to the cortical excitability changes induced by the different stimulation methods . in each subject at every single time point , the effects of two methods ( atdcs versus trns , atdcs versus itbs , and trns versus itbs ) were pairwise compared .
no side effects were reported by the subjects after any of the experimental sessions . the average baseline mep values were 1.00  0.02  mv for atdcs , 0.96  0.02 for the trns condition , 1.01  0.02 for itbs , and 0.98  0.02 for the sham condition obtained by 44  2.78% , 43.6  2.65% , 44.6  2.85% , and 44  2.86% of maximum stimulator output , respectively ( mean  sem ) .
amt values ranged between 23 and 40% of mso ( mean  sd 29.7  4.9 ) .
there were no correlations between the amt values and the induced cortical excitability change with regard to any of the stimulation conditions . as expected , both atdcs and full spectrum trns applied over the m1 using 10  min stimulation duration and 1  ma intensity showed the classical behaviour and induced excitability increase .
a similar pattern was seen by itbs . with regard to atdcs , repeated measurement of anova revealed significant main effects of stimulation ( f1.11 = 32.5 ; p = 0.0001 ) .
time ( f9.99 = 0.83 ; p = 0.6 ) and the interaction between stimulation and time were not significant ( f9.99 = 0.96 ; p = 0.5 ) .
fisher lsd test showed significantly higher mep amplitudes at each time point , compared to sham ( p < 0.05 ) .
student 's t - test demonstrated significantly increased meps between 10 and 60 minutes compared to baseline ( p < 0.05 ) , except at 20 minutes ( figure 1 ) .
concerning trns , repeated measures anova revealed significant main effects of stimulation ( f1.11 = 10.7 ; p = 0.007 ) and time ( f9.99 = 3.13 ; p = 0.002 ) .
the interaction between stimulation and time was also significant ( f9.99 = 2.99 ; p = 0.003 ) .
fisher lsd test showed significantly higher mep amplitudes at each time point between 0 and 60  min , compared to sham ( p < 0.05 ) .
student 's t - test demonstrated significantly increased meps between 5 and 60 minutes compared to baseline ( p < 0.05 ) ( figure 1 ) . with regard to itbs
, repeated measures anova revealed significant main effects of stimulation ( f1.11 = 17.9 ; p = 0.001 ) and time ( f9.99 = 2.57 ; p = 0.01 ) .
the interaction between stimulation and time was also significant ( f9.99 = 2.87 ; p = 0.005 ) .
fisher lsd test showed significantly higher mep amplitudes between 0 and 40 minutes at each time point , compared to sham ( p < 0.05 ) .
student 's t - test demonstrated significantly increased meps between 0 and 40 minutes compared to baseline ( p < 0.05 ) ( figure 1 ) . comparing all of the active stimulation conditions
, there were no significant main effects of interactions ( table 1 , figure 2 ) .
comparing the three active and one sham stimulation conditions , there was a main effect of stimulation ( f3.33 = 6.44 ; p < 0.05 ) .
after the pairwise comparisons of the individual data at every single time point between two methods ( atdcs versus trns , atdcs versus itbs , and trns versus itbs ) , we found no significant correlations between the increases of cortical excitability induced by the different stimulation methods , except between atdcs and itbs at 10  min after stimulation ( pearson correlation coefficient = 0.71 ; p = 0.005 ) .
the aim of the present study was to compare the efficacy of three widely applied stimulation methods inducing excitability enhancement in the m1 .
we have used 10 min atdcs , 10  min trns at 1  ma intensity , and 3 min itbs to compare the efficacy of these methods , because for stimulating the m1 these are the most frequently used stimulation durations .
nevertheless , the final effects of noninvasive brain stimulation methods depend on a lengthy list of parameters ( e.g. , frequency , temporal characteristics , intensity , geometric configuration of the coil / electrode , and current direction ) , when it is delivered before ( offline ) or during ( online ) the task as part of the experimental procedure ( for reviews , see , e.g. , [ 5 , 18 ] ) .
there were significant augmentations of the excitability caused by all three types of active stimulations compared to sham stimulation , with slightly different time courses .
trns resulted in the strongest ( in mep amplitude increase ) and significantly longest mep increase compared to sham .
on the physiological level ( e.g. , mep measurements ) or on the behavioural level ( e.g. , reaction time and performance measures ) , different transcranial stimulation methods might show comparable effects with regard to the direction and the magnitude of the excitability or behavioral changes [ 11 , 19 ] .
for example , both atdcs and trns over m1 resulted in mep increase using the same stimulation parameters and electrode sizes in a partly overlapping population , both applied over the dorsolateral prefrontal cortex ( dlpfc ) impaired performance in a categorization task   in a parallel group design .
nevertheless , opposite results were also observed : tested in the same subjects , atdcs over the left dlpfc improved working memory performance , while trns had no effect . applied over the primary visual cortex , anodal stimulation had no effect on perceptual learning while high frequency trns improved it , again in a parallel group design .
similar results were observed in a visuomotor learning task when the stimulation was applied over the m1 .
the partly contradictory results are explained by methodological differences ( different stimulation parameters with regard to the duration and intensity of the intervention ; electrode montage that results in different current flow in the brain ; time of the stimulation : during versus before task ) and anatomical and physiological differences ( variances in the number of excitatory and inhibitory neurons in a given cortical area , orientation of the axons with regard to the current flow , and different concentration of neurotransmitters ) .
importantly , the results obtained within the motor system are not always equivalent to the results obtained in the visual system or other areas [ 25 , 26 ] .
furthermore , considering the varying time courses of the applied stimulation methods suggests that the assumption of different mechanisms of actions is likely .
long - term tdcs effects are not observed after administration of an nmda receptor antagonist or blocking na channels [ 27 , 28 ] .
trns might induce temporal summation of small depolarizing currents , which could interact with the activity of the engaged neurons .
originally ,   it was suggested that trns augments sodium channels activity . of course , after a short depolarization , repolarization of sodium channels would take time ; with repeated stimulation these channels can be reopened with an asymmetrical time course .
trns is a noisy stimulation and noise in the nonlinear systems enhances performance through stochastic resonance . in case of anodal tdcs membrane polarization
probably induces a sustained depolarization with inactivation of voltage - dependent channels . with regard to the technical application in healthy and clinical populations , trns
has the advantage of higher cutaneous perception thresholds and lower response rates when compared with atdcs .
concerning itbs , the mechanism of the pulsatile stimulation might be completely different although pharmacological studies have demonstrated that the after - effects of itbs at least partially also depend on nmda receptor activity .
the nmda receptor antagonist memantine blocked the after - effects of itbs in healthy subjects , whereas placebo did not show a significant effect .
nevertheless , the durations of the after - effects are definitely shorter induced by itbs , compared to atdcs .
furthermore , there is evidence to suggest that neuroplastic responses to different nbs protocols might be due to activation of different populations of synapses .
both atdcs and trns , as well as itbs , produce cortical excitability changes over time .
however , atdcs / trns application is distinctly cheaper since it can be performed with a small low - priced battery - driven portable stimulator , also suitable , for example , for home use .
furthermore , since it produces less acoustic noise , skin sensation , like itching or tingling , or muscle twitching , it is more suitable for double - blind , sham - controlled studies and for clinical applications . in summary , in this study , we compared the magnitude of after - effects of different excitatory transcranial stimulation methods tested in the same healthy subjects and found that trns resulted in the strongest and significantly longest mep increase compared to sham . between the types of active stimulation , there was no significant difference .
 the common aim of transcranial stimulation methods is the induction or alterations of cortical excitability in a controlled way . 
 significant effects of each individual stimulation method have been published ; however , conclusive direct comparisons of many of these methods are rare . 
 the aim of the present study was to compare the efficacy of three widely applied stimulation methods inducing excitability enhancement in the motor cortex : 1  ma anodal transcranial direct current stimulation ( atdcs ) , intermittent theta burst stimulation ( itbs ) , and 1  ma transcranial random noise stimulation ( trns ) within one subject group . 
 the effect of each stimulation condition was quantified by evaluating motor - evoked - potential amplitudes ( meps ) in a fixed time sequence after stimulation . 
 the analyses confirmed a significant enhancement of the m1 excitability caused by all three types of active stimulations compared to sham stimulation . 
 there was no significant difference between the types of active stimulations , although the time course of the excitatory effects slightly differed . among the stimulation methods 
 , trns resulted in the strongest and atdcs significantly longest mep increase compared to sham . 
 different time courses of the applied stimulation methods suggest different underlying mechanisms of action . 
 better understanding may be useful for better targeting of different transcranial stimulation techniques . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion