Neural mechanisms of perceptual grouping in humans as revealed by high density event related potentials.

Behavioral studies suggest that perceptual grouping by proximity occurs earlier than grouping by similarity. This notion is supported by recent electrophysiological evidence that proximity grouping generates earlier occipital activation relative to grouping by shape similarity. The current work further investigated neural substrates that differentiate grouping by proximity and grouping by colour similarity by recording high density event related potentials. Subjects discriminated perceptual groups defined by either proximity or colour similarity. Proximity grouping resulted in short-latency modulations of medial occipital activity followed by longer latency modulations in the occipito-parietal cortex. Grouping by colour similarity, however, produced only long-latency occipito-temporal modulations. The results support the proposal that grouping by proximity and grouping by similarity have neural substrates over distinct time courses and cortical areas.