High‐definition transcranial direct current stimulation of the occipital cortices induces polarity dependent effects within the brain regions serving attentional reorientation

Numerous brain stimulation studies have targeted the posterior parietal cortex, a key hub of the attention network, to manipulate attentional reorientation. However, the impact of stimulating brain regions earlier in the pathway, including early visual regions, is poorly understood. In this study, 28 healthy adults underwent three high‐definition transcranial direct current stimulation (HD‐tDCS) visits (i.e., anodal, cathodal, and sham). During each visit, they completed 20 min of occipital HD‐tDCS and then a modified Posner task during magnetoencephalography (MEG). MEG data were transformed into the time‐frequency domain and significant oscillatory events were imaged using a beamformer. Oscillatory response amplitude values were extracted from peak voxels in the whole‐brain maps and were statistically compared. Behaviorally, we found that the participants responded slowly when attention reallocation was needed (i.e., the validity effect), irrespective of the stimulation condition. Our neural findings indicated that cathodal HD‐tDCS was associated with significantly reduced theta validity effects in the occipital cortices, as well as reduced alpha validity effects in the left occipital and parietal cortices relative to anodal HD‐tDCS. Additionally, anodal occipital stimulation significantly increased gamma amplitude in right occipital regions relative to cathodal and sham stimulation. Finally, we also found a negative correlation between the alpha validity effect and reaction time following anodal stimulation. Our findings suggest that HD‐tDCS of the occipital cortices has a polarity dependent impact on the multispectral neural oscillations serving attentional reorientation in healthy adults, and that such effects may reflect altered local GABA concentrations in the neural circuitry serving attentional reorientation.     

Enhanced resting‐state oscillations in schizophrenia are associated with decreased synchronization during inattentional blindness

Patients suffering from schizophrenia have been characterized by an apparent lack of theta (around 6 Hz) and gamma (>40 Hz) brain oscillatory activity during task execution. The neurocognitive reasons for these abnormal synchronization patterns, however, remain elusive. Recording the electroencephalogramm (EEG) during a selective visual attention task, the current study investigates whether abnormal brain oscillatory resting‐state activity in the theta band might account for a lack of task‐related brain oscillatory activity in schizophrenia. EEGs were recorded from 26 patients with schizophrenia and 26 healthy matched controls during rest and during the execution of a selective visual attention task, in which an unexpected object (monkey) appeared on the screen. On a behavioral level, patients were less likely to report perceiving the unexpected event than controls. Controls showed a stronger increase in task‐related theta power than patients in prefrontal, parietal, and occipital brain regions. Task‐related theta power change differed between patients who perceived, and patients who did not perceive the unexpected event. Moreover, patients showed higher levels of theta power during rest than controls, whereas the absolute theta power values during the selective attention task did not differ between groups. These results suggest that the failure to increase oscillatory activity during a cognitive task can be accounted for by abnormally high oscillatory activity in a resting state. This finding has important implications for future studies examining abnormal brain oscillatory activity in schizophrenia, which usually treat resting‐state activity as a baseline for task‐related activity. Hum Brain Mapp 34:2266–2275, 2013. © 2012 Wiley Periodicals, Inc.     

Neural oscillations underlying selective attention follow sexually divergent developmental trajectories during adolescence

Selective attention processes are critical to everyday functioning and are known to develop through at least young adulthood. Although numerous investigations have studied the maturation of attention systems in the brain, these studies have largely focused on the spatial configuration of these systems; there is a paucity of research on the neural oscillatory dynamics serving selective attention, particularly among youth. Herein, we examined the developmental trajectory of neural oscillatory activity serving selective attention in 53 typically developing youth age 9-to-16 years-old. Participants completed the classic arrow-based flanker task during magnetoencephalography, and the resulting data were imaged in the time-frequency domain. Flanker interference significantly modulated theta and alpha/beta oscillations within prefrontal, mid-cingulate, cuneus, and occipital regions. Interference-related neural activity also increased with age in the temporoparietal junction and the rostral anterior cingulate. Sex-specific effects indicated that females had greater theta interference activity in the anterior insula, whereas males showed differential effects in theta and alpha/beta oscillations across frontoparietal regions. Finally, males showed age-related changes in alpha/beta interference in the cuneus and middle frontal gyrus, which predicted improved behavioral performance. Taken together, these data suggest sexually-divergent developmental trajectories underlying selective attention in youth.     

Behavioural and electrophysiological modulations induced by transcranial direct current stimulation in healthy elderly and Alzheimer’s disease patients: A pilot study

ObjectiveTo investigate whether anodal and cathodal transcranial direct current stimulation (tDCS) can modify cognitive performance and neural activity in healthy elderly and Alzheimer’s disease (AD) patients.MethodsFourteen healthy elderly and twelve AD patients performed a working memory task during an electroencephalogram recording before and after receiving anodal, cathodal, and sham tDCS over the left dorsolateral prefrontal cortex. Behavioural performance, event-related potentials (P200, P300) and evoked cortical oscillations were studied as correlates of working memory.ResultsAnodal tDCS increased P200 and P300 amplitudes in healthy elderly. Cathodal tDCS increased P200 amplitude and frontal theta activity between 150 and 300 ms in AD patients. Improved working memory after anodal tDCS correlated with increased P300 in healthy elderly. In AD patients, slight tendencies between enhanced working memory and increased P200 after cathodal tDCS were observed.ConclusionsFunctional neural modulations were promoted by anodal tDCS in healthy elderly and by cathodal tDCS in AD patients.SignificanceInteraction between tDCS polarity and the neural state (e.g., hyper-excitability exhibited by AD patients) suggests that appropriate tDCS parameters (in terms of tDCS polarity) to induce behavioural improvements should be chosen based on the participant’s characteristics. Future studies using higher sample sizes should confirm and extend the present findings.     

The rhythm of the executive gate of speech: subthalamic low‐frequency oscillations increase during verbal generation

We investigated neurophysiological mechanisms of subthalamic nucleus involvement in verbal fluency through a verbal generation task. The subthalamic nucleus is thought to act as a behavioural go/no‐go instance by means of oscillatory communication in the theta band with the prefrontal cortex. Because subthalamic alpha‐theta frequency stimulation has been shown to exert beneficial effects on verbal fluency in Parkinson′s disease, we hypothesized that an alpha‐theta oscillatory network involving the subthalamic nucleus underlies verbal generation task performance as a gating instance for speech execution. Postoperative subthalamic local field potential recordings were performed during a verbal generation compared to a control task. Time‐frequency analysis revealed a significant alpha‐theta power increase and enhanced alpha‐theta coherence between the subthalamic nucleus and the frontal surface EEG during the verbal generation task. Beta and gamma oscillations were not significantly modulated by the task. Power increase significantly correlated with verbal generation performance. Our results provide experimental evidence for local alpha‐theta oscillatory activity in the subthalamic nucleus and coherence to frontal associative areas as a neurophysiological mechanism underlying a verbal generation task. Thus, verbal fluency improvement during subthalamic alpha‐theta stimulation in Parkinson′s disease is likely due to an enhancement of alpha‐theta oscillatory network activity. Alpha‐theta oscillations can be interpreted as the rhythmic gating signature in a speech executing subthalamic‐prefrontal network.     

Aging modulates the oscillatory dynamics underlying successful working memory encoding and maintenance

Working memory is central to the execution of many daily functions and is typically divided into three phases: encoding, maintenance, and retrieval. While working memory performance has been repeatedly shown to decline with age, less is known regarding the underlying neural processes. We examined age‐related differences in the neural dynamics that serve working memory by recording high‐density magnetoencephalography (MEG) in younger and older adults while they performed a modified, high‐load Sternberg working memory task with letters as stimuli. MEG data were evaluated in the time‐frequency domain and significant oscillatory responses were imaged using a beamformer. A hierarchical regression was performed to investigate whether age moderated the relationship between oscillatory activity and accuracy on the working memory task. Our results indicated that the spatiotemporal dynamics of oscillatory activity in language‐related areas of the left fronto‐temporal cortices were similar across groups. Age‐related differences emerged during early encoding in the right‐hemispheric homologue of Wernicke's area. Slightly later, group differences emerged in the homologue of Broca's area and these persisted throughout memory maintenance. Additionally, occipital alpha activity during maintenance was stronger, occurred earlier, and involved more cortical tissue in older adults. Finally, age significantly moderated the relationship between accuracy and neural activity in the prefrontal cortices. In younger adults, as prefrontal activity decreased, accuracy tended to increase. Our results are consistent with predictions of the compensation‐related utilization of neural circuits hypothesis (CRUNCH). Such differences in the oscillatory dynamics could reflect compensatory mechanisms, which would aid working memory performance in older age. Hum Brain Mapp 37:2348–2361, 2016. © 2016 Wiley Periodicals, Inc.     

Theta dynamics reveal domain-specific control over stimulus and response conflict

Cognitive control allows us to adjust to environmental changes. The medial frontal cortex (MFC) is thought to detect conflicts and recruit additional resources from other brain areas including the lateral prefrontal cortices. Here we investigated how the MFC acts in concert with visual, motor, and lateral prefrontal cortices to support adaptations of goal-directed behavior. Physiologically, these interactions may occur through local and long-range synchronized oscillation dynamics, particularly in the theta range (4-8 Hz). A speeded flanker task allowed us to investigate conflict-type-specific control networks for perceptual and response conflicts. Theta power over MFC was sensitive to both perceptual and response conflict. Interareal theta phase synchrony, however, indicated a selective enhancement specific for response conflicts between MFC and left frontal cortex as well as between MFC and the presumed motor cortex contralateral to the response hand. These findings suggest that MFC theta-band activity is both generally involved in conflict processing and specifically involved in linking a neural network controlling response conflict.     

Cannabis use impacts pre‐stimulus neural activity in the visual cortices of people with HIV

People with HIV (PWH) use cannabis at a higher rate than the general population, but the influence on neural activity is not well characterized. Cannabis use among PWH may have a beneficial effect, as neuroinflammation is known to be a critical problem in PWH and cannabis use has been associated with a reduction in proinflammatory markers. Thus, it is important to understand the net impact of cannabis use on brain and cognitive function in PWH. In this study, we collected magnetoencephalographic (MEG) brain imaging data on 81 participants split across four demographically matched groups (i.e., PWH using cannabis, controls using cannabis, non‐using PWH, and non‐using controls). Participants completed a visuospatial processing task during MEG. Time–frequency resolved voxel time series were extracted to identify the dynamics of oscillatory and pre‐stimulus baseline neural activity. Our results indicated strong theta (4–8 Hz), alpha (10–16 Hz), and gamma (62–72 Hz) visual oscillations in parietal–occipital brain regions across all participants. PWH exhibited significant behavioral deficits in visuospatial processing, as well as reduced theta oscillations and elevated pre‐stimulus gamma activity in visual cortices, all of which replicate prior work. Strikingly, chronic cannabis use was associated with a significant reduction in pre‐stimulus gamma activity in the visual cortices, such that PWH no longer statistically differed from controls. These results provide initial evidence that cannabis use may normalize some neural aberrations in PWH. This study fills an important gap in understanding the impact of cannabis use on brain and cognitive function in PWH.     

Bifrontal transcranial direct current stimulation modulates tinnitus intensity and tinnitus-distress-related brain activity

Bifrontal transcranial direct current stimulation (tDCS), with the anodal electrode overlying the right and the cathodal electrode overlying the left dorsolateral prefrontal cortex, has been shown to suppress tinnitus significantly in 30% of patients. The source localized resting‐state electrical activity is recorded before and after bifrontal tDCS in patients who respond to tDCS to unravel the mechanism by which tDCS suppresses tinnitus. The present electroencephalography study (N = 12) provides support for the ability of bifrontal tDCS to suppress tinnitus intensity and tinnitus‐related distress by modulation of the pregenual anterior cingulate cortex, parahippocampal area and right primary auditory cortex in resting‐state spontaneous brain activity. These findings provide direct support for tDCS having an impact not only directly on the underlying dorsolateral prefrontal cortex but also indirectly on functionally connected brain areas relevant for tinnitus distress and tinnitus intensity, respectively.     

Modulation of decision‐making in a gambling task in older adults with transcranial direct current stimulation

Cognitive performance usually declines in older adults as a result of neurodegenerative processes. One of the cognitive domains usually affected is decision‐making. Based on our recent findings suggesting that non‐invasive brain stimulation can improve decision‐making in young participants, we studied whether bifrontal transcranial direct current stimulation (tDCS) applied over the right and left prefrontal cortex of older adult subjects can change balance of risky and safe responses as it can in younger individuals. Twenty‐eight subjects (age range from 50 to 85 years) performed a gambling risk task while receiving either anodal tDCS over the right and cathodal tDCS over the left dorsolateral prefrontal cortex (DLPFC), anodal tDCS over the left with cathodal tDCS over the right DLPFC, or sham stimulation. Our main finding was a significant group effect showing that participants receiving left anodal/right cathodal stimulation chose more often high‐risk prospects as compared with participants receiving sham or those receiving right anodal/left cathodal stimulation. This result is contrary to previous findings in young subjects, suggesting that modulation of cortical activity in young and elderly results in opposite behavioral effects; thus supporting fundamental changes in cognitive processing in the elderly.     

Transcranial random noise stimulation is more effective than transcranial direct current stimulation for enhancing working memory in healthy individuals: Behavioural and electrophysiological evidence

BackgroundTranscranial direct current stimulation (tDCS) has been shown to improve working memory (WM) performance in healthy individuals, however effects tend to be modest and variable. Transcranial random noise stimulation (tRNS) can be delivered with a direct-current offset (DC-offset) to induce equal or even greater effects on cortical excitability than tDCS. To-date, no research has directly compared the effects of these techniques on WM performance or underlying neurophysiological activity.ObjectiveTo compare the effects of anodal tDCS, tRNS + DC-offset, or sham stimulation over the left dorsolateral prefrontal cortex (DLPFC) on WM performance and task-related EEG oscillatory activity in healthy adults.MethodsUsing a between-subjects design, 49 participants were allocated to receive either anodal tDCS (N = 16), high-frequency tRNS + DC-offset (N = 16), or sham stimulation (N = 17) to the left DLPFC. Changes in WM performance were assessed using the Sternberg WM task completed before and 5- and 25-min post-stimulation. Event-related synchronisation/desynchronisation (ERS/ERD) of oscillatory activity was analysed from EEG recorded during WM encoding and maintenance.ResultstRNS induced more pronounced and consistent enhancements in WM accuracy when compared to both tDCS and sham stimulation. Improvements in WM performance following tRNS were accompanied by increased theta ERS and diminished gamma ERD during WM encoding, which were significantly greater than those observed following anodal tDCS or sham stimulation.ConclusionsThese findings demonstrate the potential of tRNS + DC-offset to modulate cognitive and electrophysiological measures of WM and raise the possibility that tRNS + DC-offset may be more effective and reliable than tDCS for enhancing WM performance in healthy individuals.     

Distinction between perceptual and attentional processing in working memory tasks: a study of phase-locked and induced oscillatory brain dynamics

Working memory involves the short-term storage and manipulation of information necessary for cognitive performance, including comprehension, learning, reasoning and planning. Although electroencephalogram (EEG) rhythms are modulated during working memory, the temporal relationship of EEG oscillations with the eliciting event has not been well studied. In particular, the dynamics of the neural network supporting memory processes may be best captured in induced oscillations, characterized by a loose temporal link with the stimulus. In order to differentiate induced from evoked functional processes, the present study proposes a time-frequency analysis of the 3 to 30 Hz EEG oscillatory activity in a verbal n-back working memory paradigm. Control tasks were designed to identify oscillatory activity related to stimulus presentation (passive task) and focused attention to the stimulus (detection task). Evoked theta activity (4-8 Hz) phase-locked to the visual stimulus was evidenced in the parieto-occipital region for all tasks. In parallel, induced theta activity was recorded in the frontal region for detection and n-back memory tasks, but not for the passive task, suggesting its dependency on focused attention to the stimulus. Sustained induced oscillatory activity was identified in relation to working memory in the theta and beta (15-25 Hz) frequency bands, larger for the highest memory load. Its late occurrence limited to nonmatched items suggests that it could be related to item retention and active maintenance for further task requirements. Induced theta and beta activities displayed respectively a frontal and parietal topographical distribution, providing further functional information on the fronto-posterior network supporting working memory.     

Losing the struggle to stay awake: Divergent thalamic and cortical activity during microsleeps

Maintaining alertness is critical for safe and successful performance of most human activities. Consequently, microsleeps during continuous visuomotor tasks, such as driving, can be very serious, not only disrupting performance but sometimes leading to injury or death due to accidents. We have investigated the neural activity underlying behavioral microsleeps – brief (0.5–15 s) episodes of complete failure to respond accompanied by slow eye‐closures – and EEG theta activity during drowsiness in a continuous task. Twenty healthy normally‐rested participants performed a 50‐min continuous tracking task while fMRI, EEG, eye‐video, and responses were simultaneously recorded. Visual rating of performance and eye‐video revealed that 70% of the participants had frequent microsleeps. fMRI analysis revealed a transient decrease in thalamic, posterior cingulate, and occipital cortex activity and an increase in frontal, posterior parietal, and parahippocampal activity during microsleeps. The transient activity was modulated by the duration of the microsleep. In subjects with frequent microsleeps, power in the post‐central EEG theta was positively correlated with the BOLD signal in the thalamus, basal forebrain, and visual, posterior parietal, and prefrontal cortices. These results provide evidence for distinct neural changes associated with microsleeps and with EEG theta activity during drowsiness in a continuous task. They also suggest that the occurrence of microsleeps during an active task is not a global deactivation process but involves localized activation of fronto‐parietal cortex, which, despite a transient loss of arousal, may constitute a mechanism by which these regions try to restore responsiveness. Hum Brain Mapp 35:257–269, 2014. © 2012 Wiley Periodicals, Inc.     

Cortical oscillations that underlie working memory are altered in adults with cerebral palsy

ObjectiveThis investigation used magnetoencephalography (MEG) to identify the neurophysiological mechanisms contributing to the altered cognition seen in adults with cerebral palsy (CP).MethodsAdults with CP (GMFCS levels I-IV) and demographically-matched controls completed a Sternberg-type working memory task during MEG. Secondarily, they completed the National Institutes of Health (NIH) cognitive toolbox. Beamforming was used to image the significant MEG oscillatory responses and the resulting images were examined using statistical parametric mapping to identify cortical activity that differed between groups.ResultsBoth groups had a left-lateralized decrease in alpha–beta (11–16 Hz) power across the occipital, temporal, and prefrontal cortices during encoding, as well as an increase in alpha (9–13 Hz) power across the occipital cortices during maintenance. The strength of alpha–beta oscillations in the prefrontal cortices were weaker in those with CP during encoding. Weaker alpha–beta oscillation within the prefrontal cortex was associated with poorer performance on the NIH toolbox and a higher GMFCS level.ConclusionsAlpha-beta aberrations may impact the basic encoding of information in adults with CP, which impacts their overall cognition. Altered alpha–beta oscillation might be connected with gross motor function.SignificanceThis experimental work highlights the aberrant alpha–beta during encoding as possible neurophysiological mechanism of the cognitive deficiencies.     

Effects of transcranial direct current stimulation on working memory in patients with Parkinson's disease

OBJECTIVES: Cognitive impairment is a common feature in Parkinson's disease (PD) and is an important predictor of quality of life. Past studies showed that some aspects of cognition, such as working memory, can be enhanced following dopaminergic therapy and transcranial magnetic stimulation. The aim of our study was to investigate whether another form of noninvasive brain stimulation, anodal transcranial direct current stimulation (tDCS), which increases cortical excitability, is associated with a change in a working memory task performance in PD patients. METHODS: We studied 18 patients (12 men and 6 women) with idiopathic PD. The patients performed a three-back working memory task during active anodal tDCS of the left dorsolateral prefrontal cortex (LDLPFC), anodal tDCS of the primary motor cortex (M1) or sham tDCS. In addition, patients underwent two different types of stimulation with different intensities: 1 and 2 mA. RESULTS: The results of this study show a significant improvement in working memory as indexed by task accuracy, after active anodal tDCS of the LDLPFC with 2 mA. The other conditions of stimulation: sham tDCS, anodal tDCS of LDLPFC with 1 mA or anodal tDCS of M1 did not result in a significant task performance change. CONCLUSION: tDCS may exert a beneficial effect on working memory in PD patients that depends on the intensity and site of stimulation. This effect might be explained by the local increase in the excitability of the dorsolateral prefrontal cortex.     

Facilitation of probabilistic classification learning by transcranial direct current stimulation of the prefrontal cortex in the human

The aim of our study was to test if the electrical stimulation of the prefrontal cortex (PFC) could modify probabilistic classification learning (PCL). Transcranial direct current stimulation (tDCS) was administered to the left prefrontal and to the primary visual cortex of 22 healthy subjects while they performed a PCL task. In this task subjects learned which of two outcomes would occur on each trial after presentation of a particular combination of cues. Ten minutes of anodal, but not cathodal, stimulation improved implicit learning only when the left PFC was stimulated. Our results show that implicit PLC can be modified by weak anodal tDCS, which probably increases neural excitability, as has been shown in the motor and visual cortices previously. Our results suggest that further studies on the facilitation of learning and memory processes by tDCS are warranted.     

The COMT Val158Met polymorphism does not modulate the after‐effect of tDCS on working memory

Transcranial direct current stimulation (tDCS) can alter cortical excitability, neural plasticity, and cognitive‐behavioral performance; however, its effects are known to vary across studies. A partial account of this variability relates to individual differences in dopamine function. Indeed, dopaminergic manipulations alter the physiological and cognitive‐behavioral effects of tDCS, and gene polymorphisms related to dopamine have predicted individual response to online tDCS (i.e., stimulation overlapping with the critical task). Notably, the role of individual differences in dopamine has not yet been properly assessed in the effect of offline tDCS (i.e., stimulation prior to the critical task). We investigated if and how the COMT Val¹⁵⁸Met polymorphism (rs4680) modulates the after‐effect of prefrontal tDCS on verbal working memory (WM). One hundred and thirty‐nine participants were genotyped for the COMT Val¹⁵⁸Met polymorphism and received anodal‐over‐left, cathodal‐over‐right (AL‐CR), cathodal‐over‐left, anodal‐over‐right (CL‐AR), or sham stimulation over the dorsolateral prefrontal cortex in a between‐subjects, pretest–posttest study design. WM was assessed using the N‐back task. The results provide no evidence that the COMT polymorphism impacts the after‐effect of prefrontal tDCS on WM. Taken together with previous findings on dopamine and tDCS interactions, the results of the present study suggest that (a) indirect markers of dopamine (such as COMT) are differently related to online and offline effects of tDCS, and (b) findings from studies involving pharmacological manipulation should be generalized with caution to findings of inter‐individual differences. In sum, we argue that state (i.e., a manipulation of) and trait (i.e., baseline) differences in dopamine may exert different effects on online and offline tDCS.     

Right parietal cortex mediates recognition memory for melodies

Functional brain imaging studies have highlighted the significance of right‐lateralized temporal, frontal and parietal brain areas for memory for melodies. The present study investigated the involvement of bilateral posterior parietal cortices (PPCs) for the recognition memory of melodies using transcranial direct current stimulation (tDCS). Participants performed a recognition task before and after tDCS. The task included an encoding phase (12 melodies), a retention period, as well as a recognition phase (24 melodies). Experiment 1 revealed that anodal tDCS over the right PPC led to a deterioration of overall memory performance compared with sham. Experiment 2 confirmed the results of Experiment 1 and further showed that anodal tDCS over the left PPC did not show a modulatory effect on memory task performance, indicating a right lateralization for musical memory. Furthermore, both experiments revealed that the decline in memory for melodies can be traced back to an interference of anodal stimulation on the recollection process (remember judgements) rather than to familiarity judgements. Taken together, this study revealed a causal involvement of the right PPC for memory for melodies and demonstrated a key role for this brain region in the recollection process of the memory task.     

Wisconsin Card Sorting Test synchronizes the prefrontal,temporal and posterior association cortex in different frequency ranges and extensions

Current findings show some brain regions consistently related to performance of the Wisconsin Card Sorting Test (WCST). An increase of local cerebral blood flow or metabolic demands has been detected in those regions. Functional integration of the neuronal circuits that subserve the task performance, based upon the identification of the oscillations and their distributed cerebral sources, has not been accomplished previously. The event-related tonic oscillations within a period of 2,000 msec after the stimulus onset and the probable neural substrate were evaluated in healthy volunteers by variable-resolution brain electrical tomography (VARETA). The WCST induced a significant increase of delta, theta, beta-2, and gamma oscillations, but decrease of alpha. Areas such as the frontal subregions, temporal, cingulate, parahippocampal, parietal, occipitotemporal cortex, and occipital poles showed modified activity during the task, with EEG spectral band selectivity as well as some overlapping among them. Frontal and temporal regions generated the delta/theta oscillations. Additionally, the occipitotemporal and parietal regions were the source of the delta activity, lacking theta activation. The parietal region also showed tonic alpha, beta-2 and gamma changes. These data imply that different processes have been simultaneously mediated during task performance. Relationships among the individual bands, the neural substrata and the specific cognitive process that support the task were established. The selectively distributed delta, theta, alpha, beta-2 and gamma oscillations reflect communication networks through variable populations of neurons, with functional relations to the working memory functions and the information processing that subserve the WCST performance.     

Neuromodulation of multisensory perception: a tDCS study of the sound-induced flash illusion

This study explores whether brain polarization could be effective in modulating multisensory audiovisual interactions in the human brain, as measured by the ‘sound-induced flash illusion’ (Shams et al., 2000). In different sessions, healthy participants performed the task while receiving anodal, cathodal, or sham tDCS (2 mA, 8 min) to the occipital, temporal, or posterior parietal cortices. We found that up- or down-regulating cortical excitability by tDCS can facilitate or reduce audiovisual illusions, depending on the current polarity, the targeted area, and the illusory percept. Specifically, the perceptual ‘fission’ of a single flash, due to multiple beeps, was increased after anodal tDCS of the temporal cortex, and decreased after anodal stimulation of the occipital cortex. A reversal of such effects was induced by cathodal tDCS. Conversely, the perceptual ‘fusion’ of multiple flashes due to a single beep was unaffected by tDCS.This evidence adds novel clues on the cortical substrate of the generation of the sound-flash illusion, and opens new attractive possibilities for modulating multisensory perception in humans: tDCS appears to be an effective tool to modulate the conscious visual experience associated with multisensory interactions, by noninvasively shifting cortical excitability within occipital or temporal areas.