TEMPORAL STRUCTURE OF PERCEPTUAL GROUPING: EEG ANALYSIS
We studied pre- and post-stimulus scalp-recorded electrical brain activity during perceptual grouping. Observers reported the orientations of perceived dot groupings in multistable dot lattices. We varied lattice ambiguity and orientation. Observer reports showed a bias for vertical orientation. This bias was reflected in the cortical alpha activity before stimulus onset: the tendency to see vertical groupings corresponded to low pre-stimulus alpha power. In the post-stimulus evoked activity we found that observersâ€™ grouping sensitivity was positively correlated with the amplitude of the earliest event-related peak, C1 (about 60 ms after stimulus onset) and negatively correlated with the amplitude of the following peak P1 (about 110 ms). The dissociation of two early brain activities is evidence of two separate early mechanisms serving perceptual grouping: one mechanism reflects individual grouping sensitivity, while the other reflects the modulation of this sensitivity by top-down influences.