Radial frequency tuning in human visual cortex

Abstract

Radial frequency (RF) patterns are shape stimuli defined by a sinusoidal modulation of a circle's radius. Low frequency RF patterns, with few modulations around the perimeter, are processed by global, mid-level shape mechanisms, however the neural locus of these mechanisms in humans is not well understood. We used fMRI to measure neural responses to a large range of RFs, and modeled neural tuning to RF in early, lateral and ventral visual cortex. Responses were modeled by a Gaussian neural model defined in RF space, where each voxel's tuning to RF was defined by the model which generated a response that best predicted the fMRI data. To quantify this pattern, we measured tuning profiles to RF for visual areas V1, V2, V3, V4, VO1, VO2, LO1, LO2 and object-selective LOC. Low, globally processed RF tuning was localised to lateral occipital cortex (LO) in all subjects. Specifically, tuning to global RFs first emerged in visual field maps LO1 and LO2, and persisted through LOC. In addition, we correlated RF tuning profiles from each area against stimulus contrast energy and shape defined by circularity. Only LO2 and LOC profiles were significantly better explained by sensitivity to shape over contrast energy. All early and ventral areas showed tuning to high, locally processed RFs and were more strongly correlated with stimulus contrast energy over shape. We replicated our results using a control stimulus set where all RFs were combined with the same high-frequency contour modulations to match stimuli for low-level differences, showing LO responses were driven by the global shape of low RFs which remained constant across both stimulus sets. Our results suggest a shape processing pathway through lateral occipital cortex, where global shape representations are formed in LO2, likely providing input to LOC where more complex representations of objects are formed.

Meeting abstract presented at VSS 2017