BACKGROUND
transcranial direct current stimulation  is a technique that stimulates the cerebral cortex with a weak constant electric current in a non-invasive and painless manner  <cit> .
the current flows from an active to a reference electrode, a part being shunted through the scalp and the rest being delivered to the brain tissue  <cit> , thereby inducing diminutions or enhancements of cortical excitability  <cit> .
the direction of the tdcs-induced effect depends on the current polarity: anodal tdcs typically has an excitatory effect on the local cerebral cortex, while cathodal tdcs decreases the cortical excitability in the region under the electrode  <cit> .
the mechanisms underlying these neuromodulatory effects are not well understood  <cit> .
animal studies suggest that anodal tdcs, via an extracellular negative sink, causes a depolarization of the resting-membrane potential and increases the firing rates of many perpendicularly oriented cortical neurons in the tissue under the electrode.
cathodal stimulation has the opposite effect, causing a hyperpolarisation of the resting-membrane potential and a decrease in firing rates  <cit> .
thus, tdcs seems to modify spontaneous neural excitability by tonic de- or hyperpolarization of the resting-membrane potential  <cit> .
however, the effects of tdcs are not limited to modulations in cortical excitability during stimulation, and may outlast the stimulation period by several minutes or even hours  <cit> .
these after effects of tdcs are associated with a number of different mechanisms, including local changes in ionic concentrations  and levels of cyclic adenosine monophosphate , alterations in protein synthesis, and modulation of n-methyl-d-aspartate  receptor efficacy  <cit> .
the neuromodulatory changes induced by tdcs have been associated with modifications of a variety of behavioural brain functions.
in animal studies, anodal tdcs of the cortical surface has been linked with facilitation of an unconditioned response  <cit>  and improved learning  <cit> .
in humans, the effects of tdcs have been demonstrated on various motor, visual, and somatosensory cortex functions .
in particular, previous studies have reported enhancements in motor  <cit>  and visuo-motor learning  <cit>  for anodal tdcs, while impairments in auditory learning have been observed for cathodal tdcs  <cit> .
similarly, anodal tdcs improves language learning  <cit> , picture-naming  <cit>  as well as implicit grammar learning  <cit> , whereas cathodal tdcs has been shown to impair verbal learning abilities  <cit> .
analogous polarity-specific effects of tdcs have been reported for working memory  functions, suggesting that anodal but not cathodal tdcs can improve wm performance  <cit> .
however, the nature of the neurophysiological mechanisms underlying this cognitive enhancement is not yet well understood, because modifications of wm functions by tdcs have never been studied in combination with neurophysiological methods.
in general, wm refers to a set of basic mental operations that define the ability to hold an item of information transiently in mind, in order to recall, manipulate and associate this information to incoming new information  <cit> .
wm is crucial to many higher-order strategic functions and has been linked to frontal  <cit>  and parietal lobe functioning  <cit> .
a commonly used wm paradigm is the n-back task which activates a fronto-parietal network, including the dorsolateral prefrontal cortex   <cit> , and the posterior parietal cortex  <cit> .
while the dlpfc is involved in the processing of stimulus information during retention times  <cit> , the parietal lobe participates in the storage of perceptual attributes  <cit> .
furthermore, the prefrontal cortex seems to be functionally lateralized, with the right hemisphere being recruited in particular during spatial wm tasks, and the left hemisphere being crucial for the processing of non-spatial  wm tasks  <cit> .
the critical role for the left dlpfc in verbal wm performance has been confirmed by lesion studies and studies using tms, showing that focal damage and temporary disruption of the left but not the right dlpfc is related to impairments in verbal wm task performance  <cit> .
the present study examined the impact of tdcs on wm performance and the underlying neural activity.
in particular, we explored the effect of tdcs applied over the left dlpfc on oscillatory brain activity during a letter n-back wm task.
based on previous findings we hypothesized tdcs-dependent alteration of wm performance  <cit> .
furthermore, we predicted tdcs-related modifications of the underlying rhythmic neural activity in the alpha and theta frequency range, given the view that alpha and theta oscillations play an important role in memory functions  <cit> .
to our knowledge this is the first study investigating the modulatory effects of tdcs on oscillatory brain activity in the context of a wm task.
the better understanding of the neuromodulatory effects of tdcs is also of clinical interest, since electrical brain stimulation seems to have potential as a therapeutic tool applied for several neurological and psychiatric disorders  <cit> , and particularly for the treatment of memory deficits in stroke patients  <cit> , patients with parkinson's disease  <cit> , and patients suffering from alzheimer's disease  <cit> .
RESULTS
behavioural data
pre-to-post measurements revealed that all participants improved their performance during the experiment.
table  <dig> shows the results of one-sample t-tests separately for the tdcs-treated group in each stimulation condition  and for the control group .
the behavioural improvement from sham to active tdcs was stronger after anodal than after cathodal stimulation .
this polarity effect was statistically significant for the comparison of Δa-d' >Δc-d' .
analysis of reaction times  showed that tdcs-treated participants responded faster after active tdcs compared to sham for hits, but not for false alarms .
similarly, pre-to-post measurements for the control group showed faster rt for hits but not for false alarms.
interestingly, the decrease of rt in the tdcs-treated group did not differ between anodal and cathodal stimulation , neither for hits  nor for false alarms .
table lists result of one-sample t-tests for d', rt-hits , and rt-fa .
results show a general improvement in behaviour from pre- to post measurements for anodal  and cathodal stimulation , and in the separate control group .
in order to separate tdcs-induced alterations of wm performance from repetition-related learning effects, we compared the behavioural pre-to-post measurements between the tdcs-treated group and the control group .
the comparison revealed for the tdcs-treated group a stronger, but statistically non-significant d' increase after anodal stimulation , and a weaker d' increase after cathodal stimulation  compared to the control group.
analysis of rt showed that pre to post decreases did not differ between the control group and the tdcs-treated group for the anodal or cathodal session, either for hits  or for false alarms .
event-related potentials
event-related spectral perturbation
discussion
we assessed the impact of transcranial direct current stimulation  on working memory  performance and the underlying neural activity.
in two experimental sessions participants performed a letter two-back wm task after sham and either anodal or cathodal tdcs.
the results showed that wm performance was generally enhanced from the sham to the active stimulation condition, indicating a repetition-related learning effect on wm task performance.
importantly, this regular improvement in wm performance was influenced by active tdcs in a polarity-specific way.
the improvement in wm performance was significantly stronger after the application of anodal than after cathodal tdcs over the left dorsolateral prefrontal cortex , while the wm improvement was intermediate when participants received no electrical stimulation.
thus, anodal tdcs improved the regular repetition-related increase in wm performance, whereas cathodal tdcs interfered with this effect.
these tdcs-induced effects were reflected in the neural oscillatory activity, showing polarity-specific alterations as a function of tdcs.
anodal tdcs increased whereas cathodal tdcs decreased the event-related oscillatory power in the theta and alpha range.
our results suggest that tdcs altered wm performance by modulating the underlying oscillatory brain activity in the theta and alpha frequency bands.
these results we consider an important step towards a better understanding of the mechanisms involved in the tdcs-induced modulations of wm performance, which is particularly relevant as electrical brain stimulation has been proposed as a useful therapeutic modality for the treatment of memory deficits in a clinical context  <cit> .
more specifically, the present results of tdcs-induced modulations of neural oscillations in specific frequency bands suggests that the combination of tdcs and eeg might provide a useful approach for the investigation of the functional significance of several oscillatory bands in human cognition.
furthermore, our results may provide additional explanations for tdcs-related therapeutic effects in patients suffering from parkinson's disease or alzheimer' disease.
based on our findings we predict that beneficial effects of tdcs in these patients may be associated with modulations in oscillatory brain activity, in particular in the alpha and theta frequency range.
in addition, our results may inspire future research on tdcs-related therapeutic applications, in particular with respect to pathologies that have been associated with alterations of oscillatory brain activity in specific frequency bands.
behavioural effects of tdcs
the effect of tdcs on wm performance is consistent with recent findings on the modulatory effects of tdcs on wm functions  <cit> .
similar to our results, anodal but not cathodal stimulation over the left dlpfc has been shown to increase performance in a sequential-letter wm task  <cit> .
likewise, a beneficial effect of anodal tdcs over the left dlpfc on wm has been observed in patients with parkinson's disease  <cit> , and in patients suffering from stroke  <cit> .
our study further revealed that the general decrease in response time  from sham to active tdcs did not vary with tdcs polarity.
similar rt insensitivity to tdcs has been previously reported  <cit> , although one former study has observed increased rt after both anodal and cathodal stimulation over fronto-cortical regions  <cit> .
this discrepancy between behavioural findings might be explained by the fact that the latter study evaluated the effects of bilateral and intermittent tdcs, while our study investigated unilateral and continuous tdcs.
furthermore, the difference in results between the studies may be a consequence of differences in electrode size, position and applied current intensities, and duration of the washout period between the active tdcs sessions.
computational approaches using spherical  <cit>  and realistic finite element head models  <cit>  have demonstrated that the size, location and shape of the stimulating electrode can influence the electric field in the underlying brain, thereby affecting the level of modulation of tdcs.
overall, our behavioural results, even though not novel per se, provide further evidence for a modulatory capability of tdcs on wm performance.
our results confirm previous findings by showing that tdcs can reliably induce alterations in wm performance in a polarity-dependent manner.
more specifically, our results suggest that repetition-related improvement in wm performance is elevated by anodal tdcs, but diminished by cathodal tdcs.
electrophysiological correlates of tdcs effects
in this study we assessed oscillatory brain activity during the wm task in order to investigate the underlying neural mechanisms mediating the tdcs-induced behavioural effects.
to date, reports of electrophysiological correlates of tdcs effects are sparse.
using visual event-related potentials , a previous study has demonstrated that the amplitude of the n <dig> erp component is increased by anodal tdcs, while it is decreased by cathodal tdcs  <cit> .
the opposite effect has been reported for the visual p <dig>  showing reduced amplitudes for anodal and increased amplitudes for cathodal stimulation  <cit> .
polarity-specific changes have also been observed for erps in the somatosensory modality  <cit> , and for motor cortex excitability  <cit> .
furthermore, previous studies have shown that cathodal tdcs over the visual cortex decreases beta and gamma activity in response to visual stimulation  <cit> , whereas cathodal tdcs over the motor cortex increases resting state theta and delta oscillation  <cit> .
however, the study of oscillatory brain activity in the context of tdcs-induced wm alterations addresses a research question that has been under-investigated so far.
given that electrical brain stimulation can be a useful therapeutic tool for the treatment of cognitive deficits  <cit> , a better understanding of tdcs-induced effects on the underlying brain activity seems to be of clinical relevance.
in the present study we show that tdcs over the left dlpfc modulates theta and alpha band activity during wm performance in a polarity-specific way.
oscillatory power in the theta and alpha range was increased after anodal stimulation, while it was decreased after cathodal tdcs.
oscillatory brain activity in the context of a wm task
changes in oscillatory brain activity play an important role in the formation of perception and memory and thus are essential for higher cognitive functions  <cit> .
accordingly, wm representations seem to be sustained by oscillatory brain activity  <cit> .
indeed, wm operations have been related to oscillatory brain activity in multiple frequency bands, including the theta , alpha , and beta  range  <cit> .
in particular the performance in visual n-back tasks has been specifically associated with alterations in event-related theta and alpha band activity  <cit> .
in this regard alpha band activity is assumed to reflect a general inhibition of non-task relevant areas  <cit>  and may index the degree of inhibition necessary during internally, as opposed to externally, directed attention  <cit> .
in contrast, theta band activity has been associated with memory encoding and retrieval  <cit>  and may thus be particularly related to the function of the central executive of the wm system  <cit> .
in the context of a wm task, theta band activity seems to reflect the continuous maintenance and manipulation of information required during the performance of an n-back task  <cit> .
interestingly, a power increase has been previously shown for both the alpha and theta band as a function of practice in a wm task  <cit> .
in sum, previous studies have shown that alterations of rhythmic activity, specifically in the theta and alpha range, are associated with proper wm performance.
these oscillations increase with behavioural enhancement.
in the present study we show that tdcs over the left dlpfc induces altered wm performance by modulating its underlying theta and alpha activity.
this result further highlights the importance of the left dlpfc and the specific role of theta and alpha activity during wm performance.
even though this interpretation implies that the modulatory effects of tdcs on wm are specifically related to the responsiveness of the left dlpfc, it may be assumed that altered local cortical excitability in one part of the responsible network influences the whole neural network associated with wm functions beyond the site of stimulation.
indeed, widespread tdcs-induced changes in cortical activity have been demonstrated by a previous neuroimaging study  <cit> .
thus, it is likely that by influencing one component of the wm network, the electrical stimulation of the left dlpfc had an influence on the functioning of the entire wm system.
CONCLUSIONS
the present investigation studied the impact of anodal, cathodal or sham tdcs over the left dlpfc on the oscillatory brain activity associated with higher-order cognitive processing.
our results show that tdcs can change the organized cortical activity associated with wm in concert with systematic alterations of wm performance.
to our knowledge, this is the first study investigating the effects of tdcs on oscillatory brain activity in the context of a wm task.
the results of the study will provide a better understanding of the neuromodulatory effects of tdcs and demonstrate its potential both at fostering knowledge on the functional significance of brain oscillations and for therapeutic application.

 transcranial direct current stimulation is a technique that stimulates the cerebral cortex with a weak constant electric current in a non-invasive and painless manner. the current flows from an active to a reference electrode, a part being shunted through the scalp and the rest being delivered to the brain tissue, thereby inducing diminutions or enhancements of cortical excitability. the direction of the tdcs-induced effect depends on the current polarity: anodal tdcs typically has an excitatory effect on the local cerebral cortex, while cathodal tdcs decreases the cortical excitability in the region under the electrode. the mechanisms underlying these neuromodulatory effects are not well understood. animal studies suggest that anodal tdcs, via an extracellular negative sink, causes a depolarization of the resting-membrane potential and increases the firing rates of many perpendicularly oriented cortical neurons in the tissue under the electrode. cathodal stimulation has the opposite effect, causing a hyperpolarisation of the resting-membrane potential and a decrease in firing rates. thus, tdcs seems to modify spontaneous neural excitability by tonic de- or hyperpolarization of the resting-membrane potential. however, the effects of tdcs are not limited to modulations in cortical excitability during stimulation, and may outlast the stimulation period by several minutes or even hours. these after effects of tdcs are associated with a number of different mechanisms, including local changes in ionic concentrations and levels of cyclic adenosine monophosphate, alterations in protein synthesis, and modulation of n-methyl-d-aspartate receptor efficacy. the neuromodulatory changes induced by tdcs have been associated with modifications of a variety of behavioural brain functions. in animal studies, anodal tdcs of the cortical surface has been linked with facilitation of an unconditioned response and improved learning. in humans, the effects of tdcs have been demonstrated on various motor, visual, and somatosensory cortex functions. in particular, previous studies have reported enhancements in motor and visuo-motor learning for anodal tdcs, while impairments in auditory learning have been observed for cathodal tdcs. similarly, anodal tdcs improves language learning, picture-naming as well as implicit grammar learning, whereas cathodal tdcs has been shown to impair verbal learning abilities. analogous polarity-specific effects of tdcs have been reported for working memory functions, suggesting that anodal but not cathodal tdcs can improve wm performance. however, the nature of the neurophysiological mechanisms underlying this cognitive enhancement is not yet well understood, because modifications of wm functions by tdcs have never been studied in combination with neurophysiological methods. in general, wm refers to a set of basic mental operations that define the ability to hold an item of information transiently in mind, in order to recall, manipulate and associate this information to incoming new information. wm is crucial to many higher-order strategic functions and has been linked to frontal and parietal lobe functioning. a commonly used wm paradigm is the n-back task which activates a fronto-parietal network, including the dorsolateral prefrontal cortex, and the posterior parietal cortex. while the dlpfc is involved in the processing of stimulus information during retention times, the parietal lobe participates in the storage of perceptual attributes. furthermore, the prefrontal cortex seems to be functionally lateralized, with the right hemisphere being recruited in particular during spatial wm tasks, and the left hemisphere being crucial for the processing of non-spatial wm tasks. the critical role for the left dlpfc in verbal wm performance has been confirmed by lesion studies and studies using tms, showing that focal damage and temporary disruption of the left but not the right dlpfc is related to impairments in verbal wm task performance. the present study examined the impact of tdcs on wm performance and the underlying neural activity. in particular, we explored the effect of tdcs applied over the left dlpfc on oscillatory brain activity during a letter n-back wm task. based on previous findings we hypothesized tdcs-dependent alteration of wm performance. furthermore, we predicted tdcs-related modifications of the underlying rhythmic neural activity in the alpha and theta frequency range, given the view that alpha and theta oscillations play an important role in memory functions. to our knowledge this is the first study investigating the modulatory effects of tdcs on oscillatory brain activity in the context of a wm task. the better understanding of the neuromodulatory effects of tdcs is also of clinical interest, since electrical brain stimulation seems to have potential as a therapeutic tool applied for several neurological and psychiatric disorders, and particularly for the treatment of memory deficits in stroke patients, patients with parkinson's disease, and patients suffering from alzheimer's disease.  behavioural data pre-to-post measurements revealed that all participants improved their performance during the experiment. table shows the results of one-sample t-tests separately for the tdcs-treated group in each stimulation condition and for the control group. the behavioural improvement from sham to active tdcs was stronger after anodal than after cathodal stimulation. this polarity effect was statistically significant for the comparison of Δa-d' >Δc-d'. analysis of reaction times showed that tdcs-treated participants responded faster after active tdcs compared to sham for hits, but not for false alarms. similarly, pre-to-post measurements for the control group showed faster rt for hits but not for false alarms. interestingly, the decrease of rt in the tdcs-treated group did not differ between anodal and cathodal stimulation, neither for hits nor for false alarms. table lists result of one-sample t-tests for d', rt-hits, and rt-fa. results show a general improvement in behaviour from pre- to post measurements for anodal and cathodal stimulation, and in the separate control group. in order to separate tdcs-induced alterations of wm performance from repetition-related learning effects, we compared the behavioural pre-to-post measurements between the tdcs-treated group and the control group. the comparison revealed for the tdcs-treated group a stronger, but statistically non-significant d' increase after anodal stimulation, and a weaker d' increase after cathodal stimulation compared to the control group. analysis of rt showed that pre to post decreases did not differ between the control group and the tdcs-treated group for the anodal or cathodal session, either for hits or for false alarms. event-related potentials event-related spectral perturbation discussion we assessed the impact of transcranial direct current stimulation on working memory performance and the underlying neural activity. in two experimental sessions participants performed a letter two-back wm task after sham and either anodal or cathodal tdcs. the results showed that wm performance was generally enhanced from the sham to the active stimulation condition, indicating a repetition-related learning effect on wm task performance. importantly, this regular improvement in wm performance was influenced by active tdcs in a polarity-specific way. the improvement in wm performance was significantly stronger after the application of anodal than after cathodal tdcs over the left dorsolateral prefrontal cortex, while the wm improvement was intermediate when participants received no electrical stimulation. thus, anodal tdcs improved the regular repetition-related increase in wm performance, whereas cathodal tdcs interfered with this effect. these tdcs-induced effects were reflected in the neural oscillatory activity, showing polarity-specific alterations as a function of tdcs. anodal tdcs increased whereas cathodal tdcs decreased the event-related oscillatory power in the theta and alpha range. our results suggest that tdcs altered wm performance by modulating the underlying oscillatory brain activity in the theta and alpha frequency bands. these results we consider an important step towards a better understanding of the mechanisms involved in the tdcs-induced modulations of wm performance, which is particularly relevant as electrical brain stimulation has been proposed as a useful therapeutic modality for the treatment of memory deficits in a clinical context. more specifically, the present results of tdcs-induced modulations of neural oscillations in specific frequency bands suggests that the combination of tdcs and eeg might provide a useful approach for the investigation of the functional significance of several oscillatory bands in human cognition. furthermore, our results may provide additional explanations for tdcs-related therapeutic effects in patients suffering from parkinson's disease or alzheimer' disease. based on our findings we predict that beneficial effects of tdcs in these patients may be associated with modulations in oscillatory brain activity, in particular in the alpha and theta frequency range. in addition, our results may inspire future research on tdcs-related therapeutic applications, in particular with respect to pathologies that have been associated with alterations of oscillatory brain activity in specific frequency bands. behavioural effects of tdcs the effect of tdcs on wm performance is consistent with recent findings on the modulatory effects of tdcs on wm functions. similar to our results, anodal but not cathodal stimulation over the left dlpfc has been shown to increase performance in a sequential-letter wm task. likewise, a beneficial effect of anodal tdcs over the left dlpfc on wm has been observed in patients with parkinson's disease, and in patients suffering from stroke. our study further revealed that the general decrease in response time from sham to active tdcs did not vary with tdcs polarity. similar rt insensitivity to tdcs has been previously reported, although one former study has observed increased rt after both anodal and cathodal stimulation over fronto-cortical regions. this discrepancy between behavioural findings might be explained by the fact that the latter study evaluated the effects of bilateral and intermittent tdcs, while our study investigated unilateral and continuous tdcs. furthermore, the difference in results between the studies may be a consequence of differences in electrode size, position and applied current intensities, and duration of the washout period between the active tdcs sessions. computational approaches using spherical and realistic finite element head models have demonstrated that the size, location and shape of the stimulating electrode can influence the electric field in the underlying brain, thereby affecting the level of modulation of tdcs. overall, our behavioural results, even though not novel per se, provide further evidence for a modulatory capability of tdcs on wm performance. our results confirm previous findings by showing that tdcs can reliably induce alterations in wm performance in a polarity-dependent manner. more specifically, our results suggest that repetition-related improvement in wm performance is elevated by anodal tdcs, but diminished by cathodal tdcs. electrophysiological correlates of tdcs effects in this study we assessed oscillatory brain activity during the wm task in order to investigate the underlying neural mechanisms mediating the tdcs-induced behavioural effects. to date, reports of electrophysiological correlates of tdcs effects are sparse. using visual event-related potentials, a previous study has demonstrated that the amplitude of the n erp component is increased by anodal tdcs, while it is decreased by cathodal tdcs. the opposite effect has been reported for the visual p showing reduced amplitudes for anodal and increased amplitudes for cathodal stimulation. polarity-specific changes have also been observed for erps in the somatosensory modality, and for motor cortex excitability. furthermore, previous studies have shown that cathodal tdcs over the visual cortex decreases beta and gamma activity in response to visual stimulation, whereas cathodal tdcs over the motor cortex increases resting state theta and delta oscillation. however, the study of oscillatory brain activity in the context of tdcs-induced wm alterations addresses a research question that has been under-investigated so far. given that electrical brain stimulation can be a useful therapeutic tool for the treatment of cognitive deficits, a better understanding of tdcs-induced effects on the underlying brain activity seems to be of clinical relevance. in the present study we show that tdcs over the left dlpfc modulates theta and alpha band activity during wm performance in a polarity-specific way. oscillatory power in the theta and alpha range was increased after anodal stimulation, while it was decreased after cathodal tdcs. oscillatory brain activity in the context of a wm task changes in oscillatory brain activity play an important role in the formation of perception and memory and thus are essential for higher cognitive functions. accordingly, wm representations seem to be sustained by oscillatory brain activity. indeed, wm operations have been related to oscillatory brain activity in multiple frequency bands, including the theta, alpha, and beta range. in particular the performance in visual n-back tasks has been specifically associated with alterations in event-related theta and alpha band activity. in this regard alpha band activity is assumed to reflect a general inhibition of non-task relevant areas and may index the degree of inhibition necessary during internally, as opposed to externally, directed attention. in contrast, theta band activity has been associated with memory encoding and retrieval and may thus be particularly related to the function of the central executive of the wm system. in the context of a wm task, theta band activity seems to reflect the continuous maintenance and manipulation of information required during the performance of an n-back task. interestingly, a power increase has been previously shown for both the alpha and theta band as a function of practice in a wm task. in sum, previous studies have shown that alterations of rhythmic activity, specifically in the theta and alpha range, are associated with proper wm performance. these oscillations increase with behavioural enhancement. in the present study we show that tdcs over the left dlpfc induces altered wm performance by modulating its underlying theta and alpha activity. this result further highlights the importance of the left dlpfc and the specific role of theta and alpha activity during wm performance. even though this interpretation implies that the modulatory effects of tdcs on wm are specifically related to the responsiveness of the left dlpfc, it may be assumed that altered local cortical excitability in one part of the responsible network influences the whole neural network associated with wm functions beyond the site of stimulation. indeed, widespread tdcs-induced changes in cortical activity have been demonstrated by a previous neuroimaging study. thus, it is likely that by influencing one component of the wm network, the electrical stimulation of the left dlpfc had an influence on the functioning of the entire wm system.  the present investigation studied the impact of anodal, cathodal or sham tdcs over the left dlpfc on the oscillatory brain activity associated with higher-order cognitive processing. our results show that tdcs can change the organized cortical activity associated with wm in concert with systematic alterations of wm performance. to our knowledge, this is the first study investigating the effects of tdcs on oscillatory brain activity in the context of a wm task. the results of the study will provide a better understanding of the neuromodulatory effects of tdcs and demonstrate its potential both at fostering knowledge on the functional significance of brain oscillations and for therapeutic application. 
the present study demonstrates that tdcs can alter wm performance by modulating the underlying neural oscillations. in order to better understand the neuromodulatory effect of tdcs, the present study examined the impact of tdcs on performance in a working memory task and its underlying neural activity. results showed that tdcs modulated wm performance by altering the underlying oscillatory brain activity in a polarity-specific way. we observed an increase in wm performance and amplified oscillatory power in the theta and alpha bands after anodal tdcs whereas cathodal tdcs interfered with wm performance and decreased oscillatory power in the theta and alpha bands under posterior electrode sides. this result can be considered an important step towards a better understanding of the mechanisms involved in tdcs-induced modulations of wm performance, which is of particular importance, given the proposal to use electrical brain stimulation for the therapeutic treatment of memory deficits in clinical settings. in two experimental sessions, participants performed a letter two-back wm task after sham and either anodal or cathodal tdcs over the left dorsolateral prefrontal cortex. transcranial direct current stimulation is a technique that can systematically modify behaviour by inducing changes in the underlying brain function.
 transcranial direct current stimulation is a technique that can systematically modify behaviour by inducing changes in the underlying brain function. in order to better understand the neuromodulatory effect of tdcs, the present study examined the impact of tdcs on performance in a working memory task and its underlying neural activity. in two experimental sessions, participants performed a letter two-back wm task after sham and either anodal or cathodal tdcs over the left dorsolateral prefrontal cortex.  results showed that tdcs modulated wm performance by altering the underlying oscillatory brain activity in a polarity-specific way. we observed an increase in wm performance and amplified oscillatory power in the theta and alpha bands after anodal tdcs whereas cathodal tdcs interfered with wm performance and decreased oscillatory power in the theta and alpha bands under posterior electrode sides.  the present study demonstrates that tdcs can alter wm performance by modulating the underlying neural oscillations. this result can be considered an important step towards a better understanding of the mechanisms involved in tdcs-induced modulations of wm performance, which is of particular importance, given the proposal to use electrical brain stimulation for the therapeutic treatment of memory deficits in clinical settings. 