Estimating the bandwidth of tuned normalization within human visual cortex

Abstract

Normalization within visual cortex is modulated by contextual influences; stimuli that share similar features tend to suppress each other more than those containing dissimilar features. This form of orientation tuned normalization is typically observed when collinearly oriented gratings arranged in a center-surround configuration evoke greater attenuation of each other than when they are oriented orthogonally to each other. While numerous studies have examined the nature of surround suppression at these two extremes (collinear vs orthogonal), much less is known about how the strength of tuned normalization varies as the orientation similarity between the center and surround is parametrically manipulated. Here, we set out to characterize the bandwidth of orientation-tuned suppression within human visual cortex using fMRI. Specifically, we examined how the strength of orientation-tuned surround suppression is attenuated when parametrically manipulating the orientation similarity between stimuli. A narrow bandwidth suggests that only a small deviation in orientation similarity leads to a large change in the BOLD response, while a broader bandwidth suggests smaller changes. Participants viewed a full-field circular stimulus composed of wedges filled with orientation-bandpass filtered noise, all presented at 50% contrast. This stimulus configuration allowed us to vary the orientation difference between neighboring wedges in 15° steps, spaced between collinear and orthogonal. We measured BOLD responses to these 7 different orientation similarity conditions in a blocked design, while participants performed a demanding task at fixation. Our results reveal a gradual decrease in the BOLD response as a function of orientation similarity, with lower BOLD responses for configurations that were perfectly matched in their orientation content and higher BOLD responses as the orientation differences increased. We then quantified the bandwidth of tuned normalization, and found that the bandwidth broadens along the visuocortical hierarchy, pointing to differences in the strength of tuned normalization across visual areas.

Acknowledgement: National Institutes of Health Grant EY028163 to S. Ling