Distinct effects of boundary detection and figure enhancement in the early visual cortex

Abstract

When an orientation difference defines a region as a distinct 'figure' from its surround, V1 responses to the region increase. Is this enhancement driven by a release from local feature-tuned suppression at the boundary between the target region and surrounding background, or the result of a more widespread figure enhancement? Here, we used fMRI at 7T to resolve the spatial extent of figure-ground modulation in the early human visual system. We measured responses to a central 4° annulus of bandpass-filtered oriented noise embedded in an oriented surround while manipulating the phase alignment and orientation of the surround relative to the center, as well as whether the surround directly abutted the center. To control for effects of covert attention, the observers' focus was directed away from the figure by a task at fixation. In the same session, we mapped population receptive fields (pRF), allowing us to deduce the visual field location to which each voxel responds. We could thus isolate voxels that responded specifically to the central figure, the surround, or to the boundary between them. Results in V1 revealed distinct effects of boundary detection and figure enhancement. Voxels with pRFs encompassing the boundary showed elevated responses, even when the target and surround were iso-oriented and differed only in their spatial phase. When the center and surround were orthogonally oriented, we observed an additional effect of figural enhancement throughout the target representation in V1, even in V1 voxels that received no direct stimulation from the surround. A similar trend of figure enhancement was observed in the lateral geniculate nucleus (LGN). Our results indicate robust figure-ground modulation in early visual system, and clarify the spatial extent of these effects. Meeting abstract presented at VSS 2017