Probing the interaction of bottom-up and top-down processing in bistable perception: on the causal role of brain oscillations

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Understanding the neural mechanisms underlying the bistability of reversible figures may provide valuable insights into the normal functioning of our visual system. The proposed factors that control the perceptual alternations of reversible figures can be classified into sensory bottom-up processes like adaptation and cognitive top-down processes like attention. Current theories of ambiguous figures argue for hybrid approaches, which conceive bistable perception as arising from interactions between bottom-up and top-down processes. However, direct evidence from empirical neuroscience for such an interaction is still lacking. Here, we propose brain oscillations as a candidate mechanism that is capable of reflecting both local bottom-up processes and more global large-scale interactions within widely distributed cortical networks. More specifically, we aim to test the model that frontal gamma band activity reflects attentional top-down processes, whereas posterior alpha activity reflects adaptation-like bottom-up processes that destabilize the current percept. To establish a causal role for gamma and alpha oscillations in bistable perception, we capitalize on a new technique of noninvasive human brain stimulation, transcranial alternating current stimulation (tACS), which directly modulates the ongoing rhythmic brain activity by the application of oscillatory currents to the human scalp. Parallel tACS and EEG recordings allow for analyzing the interrelation of gamma and alpha frequencies and their impact on figure reversal.