noninvasive brain stimulation ( nibs ) finds increasingly more applications in clinical neuroscience . under the term nibs
, different techniques are summarized that allow a noninvasive stimulation of the brain but that are characterized by different modes of action .
recent studies in the field have made clear that the response capacity to any nibs protocol is subject of a significant interindividual variability [ 25 ] .
contrary to initial expectations , these studies displayed that a specified number of subjects will not show the expected effects following a given nibs protocol but showed that these subjects may show no responses or even the opposite effects .
taking into consideration that nibs are currently been applied in clinical practice to treat various neuropsychiatric disorders ( e.g. , depression , schizophrenia , and stroke ) , it is necessary to better understand the response variability following nibs .
thus , we decided to perform a comparative study of the efficacy and the response variability of two well - established nibs protocols with different physiological modes of action : transcranial direct current stimulation ( tdcs ) and paired - associative stimulation ( pas ) .
tdcs is one of the most widespread techniques in clinical trials and experimental settings , because the application is simple and the neurophysiological and behavioural effects are discussed to be rather strong .
animal studies showed that tdcs modulates spontaneous neuron activity by a tonic depolarisation ( anodal tdcs ) or hyperpolarisation ( cathodal tdcs ) of their membrane potentials resulting in consecutive long - lasting changes in the neuronal firing rates [ 68 ] .
a more recent animal study conducted on motor - cortex slices from the mouse brain slices confirmed the polarity - specific effects of anodal tdcs and showed that anodal tdcs induces long - lasting synaptic potentiation outlasting the duration of stimulation .
human studies confirmed these findings by showing similar polarity - dependent changes in motor - cortical excitability following anodal or cathodal tdcs .
anodal tdcs led to an increase of motor - evoked potentials ( meps ) and cathodal tdcs resulted in a decrease of meps following a stimulation of 13 minutes or respective 9 minutes [ 10 , 11 ] .
pharmacological challenges in healthy subject demonstrated that this modulation in motor - cortical excitability following tdcs is critically dependent on calcium homeostasis and the activity of glutamatergic nmda receptors [ 12 , 13 ] . in summary , these findings from animal and human studies suggest that the after - effects following tdcs are related to neural plasticity and to the molecular mechanisms of long - term potentiation and long - term depression . on the other hand ,
pas is mainly used in experimental settings because the application is more complicated than the application of tdcs .
pas fulfils several criteria of a plasticity protocol , as the after - effects are long - lasting , persist the duration of the intervention , and are input - specific   and nmda - dependent . however , the special characteristics are that the after - effects of pas seem to be synapse - specific following hebbian principles and that they are related to spike - dependent plasticity [ 3 , 14 , 16 ] .
human studies indicate that the repeated pairing of an electric stimulus of a peripheral nerve ( somatosensory afferent ) followed by a single magnetic pulse of the contralateral motor cortex with an interstimulus interval of 25  ms ( or adjusted to the individual n20-latency plus 2  ms ) results in a long - lasting mep increase in terms of ltp - like plasticity [ 1 , 14 , 16 ] . an adjustment of the interstimulus interval to 10  ms ( or adjustment to the n20-latency minus 5  ms ) causes a reduction of mep amplitudes ( ltd - like plasticity ) [ 1 , 14 , 16 ] . the aim of this study was to compare the efficacy and response variability of two ltp - like plasticity inducing nibs protocols using the most established stimulation configurations .
anodal tdcs was applied with 1  ma for 13 minutes [ 7 , 17 ] and pas25 consisted of 180 pairs of peripheral nerve stimulation followed by a magnetic pulse with an interstimulus interval of 25  ms [ 14 , 15 , 18 ] .
only one previous study has yet directly compared these two ltp - like plasticity inducing protocols .
this study showed the same level of excitability change and the same proportion of responders following anodal tdcs and pas25 but no mean change of motor - cortex excitability when all subjects were analysed .
these findings contrast earlier reports that investigated either anodal tdcs ( for review see ) or pas ( for review see ) and showed significant group - level changes in cortical excitability .
we aimed to either replicate or refute these initial findings and to extend the current knowledge of response variability following nibs .
we hypothesised that both plasticity protocols will result in a mean increase in cortical excitability but that a certain proportion of subjects will not show the expected results .
the study protocol was conducted in accordance with the declaration of helsinki and approved by the ethics committee of the ludwig maximilians university of munich . after giving written informed consent , 30 healthy participants , all aged between 19 and 42 years ,
all subjects were medication - free and screened by clinically experienced psychiatrists for psychiatric comorbidities .
subject received two experimental sessions ( anodal tdcs versus pas25 ) on two different days and sessions for each subject were 7 to 8 days apart .
subjects received on the first study day anodal tdcs and on the second study day pas25 .
subjects were examined in half - reclined sitting position with their arms resting passively supported .
electromyographic activity ( emg ) was recorded by surface electrodes on the right first dorsal interosseous muscle ( fdi ) .
raw signals were amplified and bandpass - filtered ( 3  hz2  khz range ) using a digitimer d-360 amplifier setup ( digitimer ltd . , uk ) .
, cambridge , uk ) controlled by signal software ( version 5 , cambridge electronic design , cambridge , uk ) .
motor - evoked potentials ( mep ) were induced by tms applied to the left primary motor cortex ( m1 ) with a standard figure - of - eight magnetic coil ( outer diameter 70  mm , the magstim company ltd . , uk ) and a monophasic magstim bistim stimulator ( the magstim company ltd . , uk ) . throughout all experiments ,
the coil was held tangentially to the skull , with the handle pointing backwards and in a 45 angle lateral to the midline .
the stimulation site that produced the largest motor - evoked potential ( mep ) at moderately suprathreshold stimulation intensities was defined as the hot spot and marked for further optimal coil positioning .
rmt was recorded in the resting fdi muscle and defined as the minimum stimulator intensity that resulted in an mep amplitude of 50  v in at least 5 of 10 measurements .
the stimulation intensity corresponding to mep amplitudes of 1  mv ( 0.3  mv ) ( s1mv ) was adjusted at baseline and kept unchanged throughout the experiments .
single - pulse mep measurements using the s1mv intensity were conducted at baseline ( 40 stimuli ) and after stimulation ( time points 0 , 5 , 10 , 20 , and 30 minutes ; 20 stimuli at each time point ) to monitor after - effects following both plasticity protocols ( pas and tdcs ) . to test for after - effects on cortical recruitment , input - output curves ( io )
were measured at baseline and 8 minutes after stimulation using an increasing stimulus intensity order ( 90% , 110% , and 130% of rmt ) with 7 stimuli for each intensity ( figure 1 ) .
single- and paired - pulse tms was applied at 0.2  hz . to assess the after - effects of the respective stimulation types on inhibitory and facilitatory intracortical networks ,
short - latency intracortical inhibition ( sici ) and intracortical facilitation ( icf ) were obtained at baseline and 15 minutes after stimulation using a standardized paired - pulse protocol .
the conditioning stimulus was set at 80% rmt intensity and the test stimulus at s1mv ( 0.3  mv ) in the resting fdi .
the intensity of the test pulse was not adjusted after the intervention for paired - pulse measures . in total ,
65 randomised stimuli were applied , with 15 stimuli using the test stimulus alone and 10 stimuli for each interstimulus interval ( isi ) ( sici : 2  ms and 3  ms ; icf : 7  ms , 9  ms , and 12  ms ) .
anodal tdcs was applied through saline soaked rectangular surface sponge - electrodes ( 35  cm ) using a ce - certified standard stimulator ( dc - stimulator - plus , neuroconn gmbh , ilmenau , germany ) .
the anodal electrode was positioned on the left side of the skull over the representational field of the right fdi as identified by tms ; the cathodal electrode was contralaterally above the right orbit .
the stimulation intensity of the tonic electrical field was set at 1  ma and applied for a total duration of 13 minutes [ 7 , 17 ] , which has been consistently shown in previous tdcs publications to be an optimal duration time for the induction of cortical excitability changes in terms of ltp - like plasticity lasting for approximately one hour following tdcs . according to foregoing publications [ 14 , 15 , 18 ] , the pas25 protocol consisted of 180 pairs of peripheral nerve stimuli followed by tms stimuli after an interstimulus interval ( isi ) of 25  ms .
the peripheral nerve stimulation was applied to the right ulnaris nerve at the level of the wrist using a ce - certified ds7a peripheral nerve stimulator ( digitimer ltd . , uk ) .
the stimulation intensity was set at 300% of the individual perceptual threshold , resulting in an average electrical intensity of 9.8  2.1  ma , which has been demonstrated to result in a reliable plasticity response .
the tms stimuli were applied to the motor - cortical representation of the right fdi as identified in the excitability measurements . to maintain a constant level of attention during the stimulation ,
subjects were asked to watch their right hand , silently count the number of stimuli delivered , and report the adding number to the examiners request every 2030 stimuli ( random choice by examiner ) .
all subjects mean count of the total number of paired stimuli was 176182 ( mean = 179.5  1.5 ) and did not significantly differ from the total count of 180 stimuli , indicating a sufficient level of attention [ 22 , 23 ] .
for statistical analysis , spss 22 for windows was used and the level of significance was set at alpha = 0.05 . to test for differences concerning baseline parameters between the two experimental sessions paired - samples t - tests were computed for all depending variables .
cortical excitability changes were expressed as increase or decrease in mean mep amplitudes before and after stimulation . as the assumption of normal data distribution
was violated for most depending variables ( kolmogorov - smirnov test , p between < 0.001 and 0.043 ) , square root transformations were applied to meet the requirements to conduct rm - anovas . to test the time course of plasticity changes , a rm - anova ( 6  2 ) with the factors  time course  ( baseline , 0  min , 5  min , 10  min , 20  min , and 30  min ) and  stimulation type 
( anodal tdcs and pas ) and another rm - anova ( 2  2 ) with the factors time ( baseline , mean post - meps averaged ) and again  stimulation type 
( anodal tdcs and pas ) were performed . in the next step , separate rm - anovas for each stimulation type alone
were conducted . to test for differences in the cortical recruitment , a rm - anova with the factors
( before and after stimulation ) and intensity ( 90% , 110% , and 130% rmt ) was conducted for both stimulation conditions separately .
changes in intracortical excitability over time were analysed with rm - anovas with the factors  isi 
( test pulse , 2  ms , 3  ms , 7  ms , 9  ms , and 12  ms ) and  time  ( baseline , 15 minutes after stimulation ) .
when appropriate , that is , significant interactions in the rm - anovas , student 's t - tests ( paired , two - tailed ) were performed to determine more specifically whether mep amplitudes differed before and after plasticity induction within and between conditions . in cases of lacking interactions ,
sphericity was tested with mauchly 's test and , if necessary ( mauchly 's test < 0.05 ) , the greenhouse - geisser correction was used . to test the individual response pattern ,
subjects were categorised in responders ( r ) and nonresponders ( nr ) to the respective ltp - protocol . to test for whether any of the obtained baseline parameters of cortical excitability showed a correlation with the excitability changes following anodal tdcs or pas ( relative mean post - meps )
subjects were aged between 19 and 42 years ( mean 27.4  4.8 ) , 14 were female ( 47% ) , with one exception all were right handed ( n = 29 , 94% ) , the average body - height was 176.5  9  cm , and 13 were smokers ( 43% ) with a mean fagerstrom - score of 3 ( table 1 ) . to compare baseline values in both experiments ,
paired - samples t - tests were computed for all depending variables : rmt , s1mv ( both single- and double - pulse ) , 1  mv mep , sici ( 2  ms , 3  ms , mean ) , icf ( 7  ms , 9  ms , and 12  ms , mean 912  ms ) , and recruitment curve ( 90% , 110% , and 130% rmt ) .
none of the tested variables showed significant differences between the first and the second experimental sessions ( all p > 0.129 , table 2 ) .
a repeated - measures analysis of variance ( rm - anova ) was conducted with the factors  time course  ( baseline , 0  min , 5  min , 10  min , 20  min , and 30  min ) and  stimulation 
this analysis revealed a significant main effect on  time course  ( f5,25 = 4.412 , p = 0.001 ) but neither an effect on  stimulation 
( f1,29 = 2.190 , p = 0.150 ) nor an effect on the  time course  stimulation  interaction ( f5,25 = 0.617 , p = 0.687 ) .
in addition , the overall rm - anova with the factors  time  ( baseline , mean post - meps averaged ) and again  stimulation 
( anodal tdcs , pas ) showed also a significant main effect on  time 
( f1,29 = 12.392 , p = 0.001 ) and no effect on  stimulation 
rm - anovas separately computed for both stimulation protocols separately showed a significant main effect on  time course  in the pas - group ( f5,25 = 3.963 , p = 0.002 ) but not in the tdcs - group ( f5,25 = 1.408 , p = 0.225 ) . to analyse the general excitability changes following both stimulation types ,
a mean value of all poststimulation time points was included into an additional rm - anova analysis , which showed a significant main effect on  time  for both anodal tdcs ( f1,29 = 4.534 , p = 0.042 ) and pas ( f1,29 = 16.041 , p < 0.001 ) . for anodal tdcs , paired - samples t - tests showed a significant difference comparing baseline to the mean of all time points following stimulation ( t1,29 = 2.13 , p = 0.042 ) . in the case of pas , a significant increase in mep size
was found comparing baseline to the mean of all time points following stimulation ( t1,29 = 4.01 , p < 0.001 ) and at all single time points after stimulation ( all t1,29 > 2.13 , all p < 0.042 ) with the exception of 0 minutes after stimulation ( t1,29 = 1.95 , p = 0.061 ) .
baseline meps did not differ between the anodal and the pas conditions ( t1,29 = 0.06 , p = 0.953 ) and also mean post - meps did not differ between the anodal and the pas conditions ( t1,29 = 1.575 , p = 0.126 ) ( figure 2 ) . to further explore the observed differences between the mep increase in tdcs and pas , we further conducted a rm - anova of the standard deviations .
this analysis revealed a trend - level effect on  time course  ( f5,25 = 1.91 , p = 0.097 ) and no effect on  stimulation 
( f1,29 = 0.74 , p = 0.397 ) and no  time course  stimulation  interaction ( f5,25 = 1.02 , p = 0.407 ) .
this finding can be explained by higher standard deviations after stimulation in both conditions . a rm - anova for the io - curves with the factors  time  ( baseline , after stimulation ) and
( 90% , 110% , and 130% rmt ) revealed a significant main effect on  intensity  for both anodal tdcs ( f2,28 = 179.87 , p < 0.001 ) and pas ( f2,28 = 112.60 , p
( tdcs : f1,29 = 2.60 , p = 0.118 ; pas : f1,29 = 3.07 , p = 0.090 ) and no  time  intensity  interaction ( tdcs : f2,28 = 0.679 , p = 0.515 ; pas : f2,28 = 2.02 , p = 0.151 ) . for paired - pulse measurements ,
two additional rm - anovas were conducted for both anodal tdcs and pas separately with the factors  time 
( test pulse , 2  ms , 3  ms , 7  ms , 9  ms , and 12  ms ) or 
( test pulse , mean sici ( 2  ms , 3  ms ) , and mean icf ( 9  ms , 12  ms ) ) .
in the case of anodal tdcs , the 2  6 analysis showed a significant main effect on 
( f4,26 = 101.34 , p < 0.001 ) but not on  time  ( f1,29 = 0.03 , p = 0.871 ) or a 
time  isi  interaction ( f4,26 = 0.49 , p = 0.787 ) . for pas
this rm - anova revealed both a significant  isi  effect ( f4,26 = 91.13 , p < 0.001 ) and a significant  time  effect ( f1,29 = 5.97 , p = 0.021 ) but no  time  isi  interaction ( f4,26 = 1.65 , p = 0.150 ) .
a similar pattern was obtained in the  time  and  mean isi  2  3 rm - anova .
for anodal tdcs , the analysis showed a significant  mean isi  effect ( f1,29 = 128.02 , p < 0.001 ) and no  time  effect ( f1,29 = 1.01 , p = 0.753 ) or  time  isi  interaction ( f1,29 = 1.06 , p = 0.352 ) . for pas
we found a significant  mean isi  effect ( f1,29 = 124.64 , p < 0.001 ) and  time  effect ( f1,29 = 7.768 , p = 0.009 ) and a  time  isi  interaction ( f1,29 = 3.99 , p = 0.024 ) . in the pas experiments ,
subsequent dependent samples t - tests were conducted to compare paired - pulse measures before and after stimulation .
these analyses showed an increase in all tested variables following pas with significant differences for sici at 2  ms isi ( t1,29 = 2.61 , p = 0.014 ) and mean sici ( t1,29 = 2.42 , p = 0.022 ) and the test pulse ( t1,29 = 3.96 , p < 0.001 ) .
thus , the significant effects reported from the rm - anova are very likely to be a consequence of the increase in the test pulse after stimulation . due to the lacking time effect
, no further t - tests were conducted for the anodal experiments . to obtain an overview over the individual response patterns of all subjects
these cut - offs defined response as an mep size increase following the respective stimulation types over a cut - off of > 100% , > 110% , and 150% relative to the individual baseline ( figures 3 and 4 ) .
chi - square ( chi ) tests were computed to compare the stimulation protocol and the respective individual response pattern .
these analyses revealed trend - level differences between anodal and pas responders at > 110% ( p = 0.052 ) and > 150% ( p = 0.058 ) cut - off ranges but not at > 100% ( 0.243 ) in favour of the pas stimulation . defining a decrease of sici and an increase in icf following ltp - protocols as response
, we again defined three different cut - off ranges : > 100% , > 110% , and > 150% increase of the respective relative mean values ( post / pre : sici(2 - 3  ms ) ; icf(912  ms ) ) .
chi tests were used to compare the distribution of responders between both experiments . for sici decrease no significant difference in the distribution of responders was found ( > 100% : p = 1.000 ; > 110% : p = 0.795 ; > 150% : p = 1.000 ) . in comparison ,
the analysis for icf increase revealed a significant difference in the distribution of responders in all of the three ranges ( > 100% : p = 0.009 ; > 110% : p = 0.002 ; > 150% : p = 0.005 ) in favour of anodal tdcs . in order to explore whether gender affected the mep increase following stimulation , chi tests were obtained from both experiments comparing distribution of response and gender .
for all cut - off ranges , this analysis did not reveal any significant differences between gender and anodal tdcs ( 100% : p = 0.796 ; 110% : p = 0.491 ; 150% : p = 0.273 ) or pas ( 100% : p = 0.855 ; 110% : p = 0.855 ; 150% : p = 0.261 ) .
pearson correlation coefficients were used to examine the relationship between relative baseline values ( age ; standard deviation of meps ; sici 2  ms , sici 3  ms , icf 7  ms , icf 9  ms , and icf 12  ms ) and the relative mean mep values following stimulation in both experiments . for pas these analyses revealed a positive trend - level correlation between age and relative mean poststimulation meps ( r = 0.345 , p = 0.062 ) , which was not observed after anodal tdcs ( r = 0.010 , p = 0.959 ) .
in addition , we observed for anodal tdcs a positive correlation between the relative icf values at baseline ( 12  ms isi ) and the relative mean poststimulation mep values ( r = 0.557 , p = 0.001 ) . concerning all other variables no significant correlations
to further investigate the impact of the observed icf - correlation in the anodal experiments , we compared the relative baseline icf values ( 12  ms ) between responders and nonresponders in the anodal condition .
these analyses revealed a trend - level difference in the case of the > 100% cut - off range ( t1,28 = 1.87 , p = 0.072 ) but significantly higher relative baseline 12  ms icf values for both > 110% ( t1,28 = 2.15 , p = 0.041 ) and > 150% ( t1,28 = 3.55 , p = 0.0014 ) in responders compared to nonresponders .
the present results reveal a differential response pattern following two well - established ltp - protocols in a large sample of healthy controls .
both ltp - protocols ( anodal tdcs and pas25 ) induced a significant increase of meps for the observation period of 30 minutes . in the pas25 experiments
this mep increase could be observed for nearly all poststimulation time points , whereas in the anodal tdcs experiments the increase of meps could only be shown when all time points were averaged .
the likelihood to develop a meaningful mep response was higher in the pas25 compared to the anodal tdcs experiments , but the likelihood for a response in terms of an icf - modulation showed the opposite pattern . in the anodal tdcs experiments ,
the baseline values for icf at 12  ms showed a positive correlation with the increase of meps after stimulation and this value differed significantly between responders and nonresponders . in summary ,
our results indicate a sufficient increase of cortical excitability following anodal tdcs and pas25 in terms but also demonstrate less individual variability than reported in previous studies . considering the particular importance of interindividual response differences surprisingly limited data comparing nibs protocols with different modes of action is available .
one study compared the efficacy of three different ltp - protocols ( intermittent theta - burst stimulation ( tbs ) , anodal tdcs , and pas25 ) in 56 healthy controls and the authors were not able to show significant effects on excitatory or inhibitory circuits when all subjects were analysed as a group .
this response pattern resembles the results of another study conducted on 18 subjects showing in the group - level analyses no increase in meps following intermittent tbs and pas25 but a decrease in mep amplitudes following continuous tbs . for tdcs , a recently published study could not observe a modulation of mep amplitudes following anodal or cathodal tdcs in 53 healthy subjects .
the fact that this study used an intensity of 2  ma for tdcs   whereas most tdcs studies used 1  ma [ 7 , 25 ] should be taken into account .
recent evidence indicates that the increase in the tdcs intensity is not related to the efficacy of stimulation and that homeostatic mechanisms could counteract the efficacy of high stimulation intensity . for tbs ,
one study showed in 56 healthy subjects that neither excitatory nor inhibitory tbs resulted in a change in mep sizes after stimulation and that only 25% of all subjects showed the expected responses .
the lacking group - level response following different tbs protocols has now been reported from different groups ( e.g. , [ 2730 ] ) . for pas25 ,
similar results with lacking increase in mep amplitudes and a response rate in 14 out of 27 subjects were shown in one study .
this high response variability following standard pas protocols was confirmed in other studies with limited sample sizes [ 31 , 32 ] .
the reason most frequently discussed for not being able to induce a general change in motor - cortical excitability in the presented studies with sufficient sample sizes is the high response variability across subjects [ 2 , 4 , 5 ] .
our findings deviate from the aforementioned reports . in our study , both plasticity protocols were effective on a group - level analysis and our results are in the range of the initial reports for pas   and tdcs [ 10 , 11 , 17 ] .
furthermore , the likelihood to develop a mep response was higher in the pas25 experiments . remarkably , only 23% responded to anodal tdcs , but 47% responded to pas using the 150% cut - off . comparing the mean mep amplitudes after stimulation , a numeric but not statistical significant difference between the pas25 and tdcs experiments could be observed .
thus , it may be assumed that pas25 was more effective to increase mep amplitudes . on the other hand ,
furthermore , baseline icf correlated positively with an increase in mep amplitudes and mep responders had significantly higher icf baseline values compared to the nonresponders in the anodal experiment . it should be noted that a decrease in sici and an increase in icf have been reported following anodal tdcs   but not following pas25 ( for review see [ 1 , 16 ] ) .
therefore , paired - pulse measures may not be suitable as outcome parameters for pas25 but may be helpful to detect responders to anodal tdcs .
in one study baseline sici correlated with the pas25 response   and another study showed that the pas25 response correlated with sici measured with a threshold tracking method .
differences between these studies and our work may be the timing of the paired - pulse assessment and the configuration of the conditioning pulse .
our novel observation of an icf - associated efficacy of anodal tdcs may allow hypothesizing that the after - effects of anodal tdcs appear to be located in facilitatory interneuron networks and may be dependent on synaptic modulation .
the reservation must be made that our design does not allow a mechanistic explanation of results due to the lack of pharmacological interventions .
we aimed to identify further baseline characteristics ( other paired - pulse measures , age , gender , and variability in baseline meps expressed by the standard deviation ) that may predict the response to anodal tdcs or pas25 , but no other factors were detected .
we found a trend for a positive correlation between age and the mep changes following pas25 .
this is in contrast to previous reports of a negative correlation between age and the response to pas .
however , our age range is outside the age range of an expected age - dependent decrease in cortical plasticity .
we report the plasticity effects following nibs in a single - session design and thus can not rule out that repetitive sessions ( as used in the clinical context ) would have resulted in another distribution of plasticity .
furthermore , we focussed on two established ltp - protocols and as it is possible to modulate various parameters using tdcs ( e.g. , current intensity , current density ) and pas ( e.g. , isi , individualised pas , and target peripheral nerve ) it may be possible that other configurations would have resulted in different response patterns . regarding the paired - pulse measures , the lacking assessment of a paired - pulse response curve at different time points limits the generalizability of our sici / icf discussion .
furthermore , we did not readjust the test pulse intensity after the intervention in the paired - pulse paradigms .
on the one hand evidence is available that the intensity of the test pulse affects the percent sici / icf , but on the other hand this is not found to be the case in the range of mep sizes presented in our experiments [ 36 , 37 ] .
the mep sizes following the test pulse of the paired - pulse paradigms were 1.04  0.31 before and 1.23  0.76 after anodal tdcs and , respectively , 1.11  0.43 before and 1.64  0.86 after pas25 .
further limitations are the lacking sham condition in our experiments and the fact that we did not use randomized ordered session of anodal tdcs and pas25 .
thus , we can not rule out that the order of sessions had an impact on our results . for pas
, we used the ulnar nerve for our peripheral nerve stimulation and the fdi as target muscle .
most published pas studies stimulated the median nerve and used the abductor pollicis brevis muscle as target ( for review see ) . however , different studies reported comparable changes in meps following both ulnar and median nerve stimulation , in the context of pas protocols ( for review see ) .
one important confounding factor could be attention . in our pas experiment we asked the subjects to count the tms pulse and the reported values are within a range ( 179.5  1.5 ) [ 22 , 23 ] indicating a sufficient level of attention in our experiments . during anodal
tdcs no count of stimuli is conducted , possibly leading to different attention states compared with those in the pas setup .
thus , the reported higher efficacy of pas25 could also be led back to attention rather than to physiological differences between the protocols . despite our large sample ,
thus , replication studies with larger sample sizes are needed to confirm some of our findings .
however , in line with previous research , our sample also comprised a certain degree of nonresponders .
the likelihood to be a nonresponder was dependent on the kind of stimulation protocol , on the defined thresholds and on the used target items .
our new observation that subject had a higher likelihood to be a responder using the high 150% mep threshold in the pas25 protocol may be explained by the fact that this protocol was more individualised ( electrical and motor thresholds ) and controlled for attention compared to anodal tdcs .
moreover , we were for the first time able to identify a new baseline parameter ( icf at 12  ms ) that may predict the response to anodal tdcs .
future prospective studies need to independently confirm this parameter before it can be used as individual response predictor .
it is reassuring that our response rates using the lowest threshold were much higher than those reported in other trials , but the response rates using the highest threshold are less optimistic .
 interindividual response variability to various motor - cortex stimulation protocols has been recently reported . 
 comparative data of stimulation protocols with different modes of action is lacking . 
 we aimed to compare the efficacy and response variability of two ltp - inducing stimulation protocols in the human motor cortex : anodal transcranial direct current stimulation ( a - tdcs ) and paired - associative stimulation ( pas25 ) . in two experiments 30 subjects received 1ma a - tdcs and pas25 . 
 data analysis focused on motor - cortex excitability change and response defined as increase in mep applying different cut - offs . 
 furthermore , the predictive pattern of baseline characteristics was explored . 
 both protocols induced a significant increase in motor - cortical excitability . in the pas25 experiments the likelihood to develop a mep response was higher compared to a - tdcs , 
 whereas for intracortical facilitation ( icf ) the likelihood for a response was higher in the a - tdcs experiments . 
 baseline icf ( 12  ms ) correlated positively with an increase in meps only following a - tdcs and responders had significantly higher icf baseline values . 
 contrary to recent studies , we showed significant group - level efficacy following both stimulation protocols confirming older studies . 
 however , we also observed a remarkable amount of nonresponders . 
 our findings highlight the need to define sufficient physiological read - outs for a given plasticity protocol and to develop predictive markers for targeted stimulation . 
1. Introduction
2. Methods
3. Statistics
4. Results
5. Discussion