Novel methods for neuronal entrainment [1-4] provide the unique opportunity to modulate perceptually relevant brain oscillations [5, 6] in a frequency-specific manner and to study their functional impact on distinct cognitive functions. Recently, evidence has emerged that tACS (transcranial alternating current stimulation) can modulate cortical oscillations [7-9]. However, the study of electrophysiological effects has been hampered so far by the absence of concurrent electroencephalogram (EEG) recordings. Here, we applied 10 Hz tACS to the parieto-occipital cortex and utilized simultaneous EEG recordings to study neuronal entrainment during stimulation. We pioneer a novel approach for simultaneous tACS-EEG recordings and successfully separate stimulation artifacts from ongoing and event-related cortical activity. Our results reveal that 10 Hz tACS increases parieto-occipital alpha activity and synchronizes cortical oscillators with similar intrinsic frequencies to the entrainment frequency. Additionally, we demonstrate that tACS modulates target detection performance in a phase-dependent fashion highlighting the causal role of alpha oscillations for visual perception.

When viewing ambiguous stimuli, conscious perception alternates spontaneously between competing interpretations of physically unchanged stimulus information. As one possible neural mechanism underlying the perceptual switches, it has been suggested that neurons dynamically change their pattern of synchronized oscillatory activity in the gamma band (30-80 Hz). In support of this hypothesis, there is correlative evidence from human electroencephalographic (EEG) studies for gamma band modulations during ambiguous perception. To establish a causal role of gamma band oscillations in the current study, we applied transcranial alternating current stimulation (tACS) at 40 Hz over occipital-parietal areas of both hemispheres during the presentation of bistable apparent motion stimuli that can be perceived as moving either horizontally or vertically. In this paradigm, the switch between horizontal and vertical apparent motion is likely to involve a change in interhemispheric functional coupling. We examined gamma tACS effects on the durations of perceived horizontal and vertical motion as well as on interhemispheric EEG coherence and found a decreased proportion of perceived horizontal motion together with an increase of interhemispheric gamma band coherence. In a control experiment using 6 Hz tACS, we did not observe any stimulation effects on behavior or coherence. Furthermore, external stimulation at 40 Hz was only effective when applied with 180° phase difference between hemispheres (anti-phase), as compared to in-phase stimulation with 0° phase difference. These findings suggest that externally desynchronizing gamma oscillations between hemispheres impairs interhemispheric motion integration and in turn biases conscious experience of bistable apparent motion.

Humans are social beings and often have to perceive and perform within groups. In conflict situations, this puts them under pressure to either adhere to the group opinion or to risk controversy with the group. Psychological experiments have demonstrated that study participants adapt to erroneous group opinions in visual perception tasks, which they can easily solve correctly when performing on their own. Until this point, however, it is unclear whether this phenomenon of social conformity influences early stages of perception that might not even reach awareness or later stages of conscious decision-making. Using electroencephalography, this study has revealed that social conformity to the wrong group opinion resulted in a decrease of the posterior-lateral P1 in line with a decrease of the later centro-parietal P3. These results suggest that group pressure situations impact early unconscious visual perceptual processing, which results in a later diminished stimulus discrimination and an adaptation even to the wrong group opinion. These findings might have important implications for understanding social behavior in group settings and are discussed within the framework of social influence on eyewitness testimony.

A recent functional magnetic resonance imaging (fMRI) study by our group demonstrated that dynamic emotional faces are more accurately recognized and evoked more widespread patterns of hemodynamic brain responses than static emotional faces. Based on this experimental design, the present study aimed at investigating the spatio-temporal processing of static and dynamic emotional facial expressions in 19 healthy women by means of multi-channel electroencephalography (EEG), event-related potentials (ERP) and fMRI-constrained regional source analyses. ERP analysis showed an increased amplitude of the LPP (late posterior positivity) over centro-parietal regions for static facial expressions of disgust compared to neutral faces. In addition, the LPP was more widespread and temporally prolonged for dynamic compared to static faces of disgust and happiness. fMRI constrained source analysis on static emotional face stimuli indicated the spatio-temporal modulation of predominantly posterior regional brain activation related to the visual processing stream for both emotional valences when compared to the neutral condition in the fusiform gyrus. The spatio-temporal processing of dynamic stimuli yielded enhanced source activity for emotional compared to neutral conditions in temporal (e.g., fusiform gyrus), and frontal regions (e.g., ventromedial prefrontal cortex, medial and inferior frontal cortex) in early and again in later time windows. The present data support the view that dynamic facial displays trigger more information reflected in complex neural networks, in particular because of their changing features potentially triggering sustained activation related to a continuing evaluation of those faces. A combined fMRI and EEG approach thus provides an advanced insight to the spatio-temporal characteristics of emotional face processing, by also revealing additional neural generators, not identifiable by the only use of an fMRI approach.

We examined interference effects of emotionally associated background colours during fast valence categorisations of negative, neutral and positive expressions. According to implicitly learned colour-emotion associations, facial expressions were presented with colours that either matched the valence of these expressions or not. Experiment 1 included infrequent non-matching trials and Experiment 2 a balanced ratio of matching and non-matching trials. Besides general modulatory effects of contextual features on the processing of facial expressions, we found differential effects depending on the valance of target facial expressions. Whereas performance accuracy was mainly affected for neutral expressions, performance speed was specifically modulated by emotional expressions indicating some susceptibility of emotional expressions to contextual features. Experiment 3 used two further colour-emotion combinations, but revealed only marginal interference effects most likely due to missing colour-emotion associations. The results are discussed with respect to inherent processing demands of emotional and neutral expressions and their susceptibility to contextual interference.

In a previous study, we showed that virtually simulated social group pressure could influence early stages of perception after only 100  ms. In the present EEG study, we investigated the influence of social pressure on visual perception in participants with high (HA) and low (LA) levels of autonomy. Ten HA and ten LA individuals were asked to accomplish a visual discrimination task in an adapted paradigm of Solomon Asch. Results indicate that LA participants adapted to the incorrect group opinion more often than HA participants (42% vs. 30% of the trials, respectively). LA participants showed a larger posterior P1 component contralateral to targets presented in the right visual field when conforming to the correct compared to conforming to the incorrect group decision. In conclusion, our ERP data suggest that the group context can have early effects on our perception rather than on conscious decision processes in LA, but not HA participants.

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