Abstract
Recent neurophysiological studies suggest that primary visual cortex (V1) may play an important role in the perceptual grouping. For example, V1 neurons exhibit collinear facilitation when a bar placed outside the receptive field is congruently aligned with a bar lying within the receptive field (Kapadia, Gilbert, & Westheimer, 1995). We used fMRI to investigate the effects of perceptual grouping in human visual cortex. Observers were presented with two vertical bars positioned to the right of fixation, above and below the horizontal meridian (vertical separation = 4?). The bars oscillated from left to right within a 4? horizontal window (4-8? to the right of fixation), and either moved in phase (grouped condition) or out of phase (separated condition). We identified the retinotopic regions corresponding to the location of the 4x4? gap between the bars by presenting a reference stimulus in this location on separate scans. Preliminary data from three subjects suggested evidence of early perceptual grouping. Greater activity occurred in the V1 region corresponding to the gap when the bars moved in phase as opposed to out of phase. Some effect of grouping was also found in higher visual areas. These results support the notion that human primary visual cortex may play an important functional role in perceptual grouping. Further analyses will compare the magnitude of grouping effects for V1 in relationship to these other areas.