Electrophysiological Correlates of Visual Perceptual Grouping in Human

Abstract:
Abstract: Behavioral studies suggest that perceptual grouping based on Gestalt laws occurs early in vision to form the perceptual units that are subsequently analyzed by focal attention. However, little is known about the neural substrates of these early grouping processes. In three experiments we recorded high-density event related potentials (ERPs) (120 channels) as subjects discriminated perceptual groups defined either by the proximity or similarity of local elements. Difference waves were obtained by subtracting ERPs to the uniform stimuli from ERPs to the grouping stimuli to elucidate substrates of the grouping processes. Proximity grouping was indexed by a positive activity over the middle occipital cortex that occurred as early as 110 ms post-stimulus. This initial positivity was followed by an occipito-temporal negativity with a right hemisphere dominance when grouping was mediated by low spatial frequencies but a left hemisphere dominance when low spatial frequencies were removed. Grouping by similarity produced bilateral long-latency modulations of occipital cortex whose later phase showed a left hemisphere predominance. The results suggest that distinct neural substrates are involved in different grouping operations.