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In the last decade, neuroscientific research has refocussed on the old observation, that identical stimuli can elicit different cortical responses and thus different percepts. What has for a long time been regarded as background noise in absence of external stimulation has since gained a lot of attention. There is now growing consensus, that power and phase of oscillatory activity significantly influence the fate of an upcoming percept. However, the question of whether multi-sensory information integration requires specific pre-stimulus brain states has rarely been assessed. Three studies were therefore designed to investigate the role of pre-stimulus activity in the perception of audiovisual stimuli.In the first study, I used the McGurk effect to elucidate the impact of ongoing brain oscillations - indexed by fluctuating local excitability and inter-areal synchronisation - on upcoming varying perception of identical stimuli. The McGurk effect demonstrates the influence of visual cues on auditory perception. Mismatching information from both modalities can fuse to a novel percept that neither matches the auditory nor visual stimulus while an illusion is reported in 60-80% of trials. Using magnetoencephalography (MEG), I found that the perception of the McGurk effect is preceded by high beta activity in parietal, frontal, and temporal areas and pronounced in the left superior temporal gyrus, considered to be a site of multimodal information integration. This area is functionally (de-)coupled to distributed frontal and temporal regions in illusion trials. The disposition to fuse multi-sensory information is enhanced as the left STG is more strongly coupled to frontoparietal regions. Illusory perception is accompanied by a decrease in post-stimulus theta band activity in the cuneus, precuneus and left superior frontal gyrus. Event-related activity in the left middle temporal gyrus is pronounced during illusory perception. Thus, the McGurk effect depends on fluctuating brain states suggesting that functional connectedness of left STG at a pre-stimulus stage is crucial for an audiovisual percept.In the second study, I investigated the Sound-Induced Flash Illusion (SIFI), which is an example for the influence of auditory information on visual perception. It consists of the perception of two visual stimuli upon presentation of only a single visual stimulus accompanied by two auditory stimuli. Again, I used MEG to assess the influence of ongoing pre-stimulus oscillatory activity and brain connectivity states on varying perception of invariant stimuli. I compared cortical activity from trials in which subjects perceived two visual stimuli (i.e., an illusion) with trials in which subjects perceived only one visual stimulus (i.e., no illusion), thus keeping the stimulation fixed. Subjects perceived the illusion in ~50% of trials. In trials containing an illusion, I found stronger pre-stimulus beta band activity in a left temporal sensor cluster and localised this to the left middle temporal gyrus (BA39). In addition to differences in local beta activity, illusory perceptions were preceded by increased beta band phase-synchrony with auditory areas as well as decreased phase synchrony with visual areas. Alpha band phase-synchrony between visual and temporal, parietal and frontal cortical as well as alpha band phase-synchrony between auditory and visual areas were found to be modulated. However, studies involving active external perturbations of the current brain state are needed in order to evaluate the role of oscillatory activity in multimodal cortical areas.I addressed this in the third study by replicating our work on the sound induced flash illusion (SIFI) and extending it with single pulse transcranial magnetic stimulation (TMS). The target for stimulation was in the bilateral BA39. I used electroencephalography (EEG) to assess the influence of ongoing oscillatory activity on varying perception of invariant stimuli in humans as well as the relationship between TMS and ongoing oscillatory cortical activity. I compared activity from trials in which subjects subsequently perceived an illusion with trials in which subjects perceived no illusion, thus keeping the stimulation fixed. I found a strong increase in gamma band power in right temporal cortex signalling an upcoming illusion. Regarding TMS, I split trials into strong and weak pre-TMS beta and gamma band power in order to evaluate the influence of the current brain state on the TMS effect. TMS reduces strong oscillatory power in the beta as well as gamma band at the site of stimulation, but also in inferior frontal and anterior temporal areas but increases weak power. TMS to the right BA39 differentially influences upcoming perception depending on the strength of pre-TMS beta band power.Based on these three studies, I suggest that ongoing pre-stimulus fluctuations of oscillatory activity in multimodal brain regions as well as its varying integration into a distributed network form predispositions whether different sensory streams will be integrated or not. These findings are consistent with and extend recent findings on the role in beta and gamma band activity in top-down and bottom-up network processes of multi-sensory perception. Identifying the spectral fingerprints of underlying cognitive processes can serve as a basis to exploration of the neural correlates of consciousness. |
