Faculty Bibliography

Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, the role of neural synchrony in cortical networks has been expanded to provide a general mechanism for the coordination of distributed neural activity patterns. In the current paper, we present an update of the status of this hypothesis through summarizing recent results from our laboratory that suggest important new insights regarding the mechanisms, function and relevance of this phenomenon. In the first part, we present recent results derived from animal experiments and mathematical simulations that provide novel explanations and mechanisms for zero and nero-zero phase lag synchronization. In the second part, we shall discuss the role of neural synchrony for expectancy during perceptual organization and its role in conscious experience. This will be followed by evidence that indicates that in addition to supporting conscious cognition, neural synchrony is abnormal in major brain disorders, such as schizophrenia and autism spectrum disorders. We conclude this paper with suggestions for further research as well as with critical issues that need to be addressed in future studies.

We explore the hypothesis that induction of holistic or analytic strategies influences comprehension and processing of highly contextualized expressions of ordinary language, such as irony. Twenty undergraduate students were asked to categorize as coherent or incoherent a group of sentences. Each sentence completed a previous story, so that they could be ironical, literal or nonsensical endings. Participants were asked to evaluate whether each sentence was coherent or incoherent. Half of them were initially instructed to consider whether the sentences made sense (holistic condition); the other half were instructed to consider whether the sentences were congruent or incongruent (analytic condition). Behavioral responses and Event Related Potentials were registered during the experiment. Both behavioral and electrophysiological results allow clearly distinguishing between the holistic and the analytic strategies. The fact that the same set of stimuli elicits different ERP waveforms, depending on the strategy with which they are analyzed, suggests that different cognitive processes and different areas of the brain are operating in each case.

Subliminal stimuli can be deeply processed and activate similar brain areas as consciously perceived stimuli. This raises the question which signatures of neural activity critically differentiate conscious from unconscious processing. Transient synchronization of neural activity has been proposed as a neural correlate of conscious perception. Here we test this proposal by comparing the electrophysiological responses related to the processing of visible and invisible words in a delayed matching to sample task. Both perceived and nonperceived words caused a similar increase of local (gamma) oscillations in the EEG, but only perceived words induced a transient long-distance synchronization of gamma oscillations across widely separated regions of the brain. After this transient period of temporal coordination, the electrographic signatures of conscious and unconscious processes continue to diverge. Only words reported as perceived induced (1) enhanced theta oscillations over frontal regions during the maintenance interval, (2) an increase of the P300 component of the event-related potential, and (3) an increase in power and phase synchrony of gamma oscillations before the anticipated presentation of the test word. We propose that the critical process mediating the access to conscious perception is the early transient global increase of phase synchrony of oscillatory activity in the gamma frequency range.