Human gamma band activity and perception of a gestalt

Neuronal oscillations in the gamma band (above 30 Hz) have been proposed to be a possible mechanism for the visual representation of objects. The pres ent study examined the topography of gamma band spectral power and event-re lated potentials in human EEG associated with perceptual switching effected by rotating ambiguous (bistable) figures. Eleven healthy human subjects we re presented two rotating bistable figures: first, a face figure that allow ed perception of a sad or happy face depending on orientation and therefore caused a perceptual switch at defined points in time when rotated, and, se cond, a modified version of the Rubin vase, allowing perception as a vase o r two faces whereby the switch was orientation-independent. Nonrotating fig ures served as further control stimuli. EEG was recorded using a high-densi ty array with 128 electrodes. We found a negative event-related potential a ssociated with the switching of the sad-happy figure, which was most pronou nced at central prefrontal sites. Gamma band activity (GBA) was enhanced at occipital electrode sites in the rotating bistable figures compared with t he standing stimuli, being maximal at vertical stimulus orientations that a llowed an easy recognition of the sad and happy face or the vase-faces, res pectively. At anterior electrodes, GBA showed a complementary pattern, bein g maximal when stimuli were oriented horizontally. The findings support the notion that formation of a visual percept may involve oscillations in a di stributed neuronal assembly.