MEG alpha activity decrease reflects destabilization of multistable percepts.

Multistable stimuli offer the possibility to investigate visual awareness, since they evoke spontaneous alternations between different perceptual interpretations of the same stimulus and, therefore, allow to dissociate perceptual from stimulus-driven mechanisms. In the present study, we used an ambiguous motion paradigm and compared endogenous reversals of perceived motion direction which occur spontaneously during constant ambiguous stimulation with exogenous reversals that were induced externally by changes of stimulation. Contrasting the two conditions allowed to investigate processes that trigger endogenous reversals, since the related activity should be absent in the exogenous reversal condition. We employed ambiguous dot patterns which can easily be transformed to present two stable motion directions in order to induce exogenous pattern reversals. Whole-head MEG was recorded from 10 subjects. We analyzed event-related fields (ERFs) and oscillatory activity in the alpha and gamma ranges. The results showed P300-like slow waves in response to both endogenous and exogenous reversals reflecting the conscious recognition of pattern reversals. Analyses in the gamma-band did not reveal any significant modulations. The alpha activity showed different time courses for endogenous and exogenous reversals. While the exogenous alpha activity decreased in temporal relation to the pattern reversal, the endogenous alpha activity displayed a continuous decrease starting in the time interval preceding the reversal. This time course of the endogenous alpha activity is consistent with a bottom-up approach to figure reversals, since it reflects a process of destabilization of the actual percept until a switch of visual awareness occurs.