Neural Substrates of Ensemble Perception

Ensemble perception - the perception of numerous objects as a single group entity - is thought to be a basic visual process useful for surmounting known limitations of focused attention and individual object perception. While the speed of perception, resistance to distraction, and adaptation effects shown in behavioral studies suggest that ensemble perception may arise from processing in early visual areas, few studies have directly explored this by mapping activity specifically in striate and extrastriate regions of interest. We used a block design fMRI study to compare ensemble and individual object processing for different features using identical displays. Participants viewed two groups of phase shifting Gabor patches (750 ms stimulus duration, 8 trials per block) and were asked to compare either the average speed or orientation of the groups, or the individual speed or orientation of two cued individual objects, one from each group. These tasks were performed on separate runs and counterbalanced across participants. In order to distinguish task from difficulty effects, we varied the difficulty of each task on different blocks within runs. A region of interest analysis within visual cortex showed significantly increased activity in V2 and V3 for ensemble compared to individual feature processing. There were no main effects of feature (orientation, speed) or difficulty (easy, difficult) and no interactions within these areas. These results suggest that ensemble perception relies at least in part on distinct processing in extrastriate areas, and supports theories that ensemble perception constitutes a fundamental mechanism of visual information processing. Meeting abstract presented at VSS 2018