Neural substrates for visual perceptual grouping in humans

Two experiments investigated the neural mechanisms of Gestalt grouping by r ecording high-density event-related brain potentials (ERPs) during discrimi nation tasks. In Experiment 1, stimulus arrays contained luminance-defined local elements that were either evenly spaced or grouped into columns or ro ws based on either proximity or similarity of shape. Proximity grouping was indexed by a short-latency positivity (110-120 ms) over the medial occipit al cortex and a subsequent right occipitoparietal negativity. Grouping by s imilarity was reflected only in a long-latency occipitotemporal negativity. In Experiment 2, proximity grouping was examined when local elements were defined by motion cues, and was again associated with a medial occipital po sitivity. However, the subsequent long-latency negativity was now enhanced over the left posterior areas. The implications of these results to the neu ral substrates subserving different grouping processes are discussed.