Effect of Smooth Shape on Lightness Processing in Human Visual Cortex

Abstract

Neurophysiological and neuroimaging studies have found localized retinotopic responses in low-level visual cortical areas to lightness change in the absence of changes in physical intensity. Lightness responses in human V1 have also been found to reflect perceptual grouping through occlusion (Boyaci et al., 2010), suggesting the involvement of non-local grouping mechanisms in low-level lightness processing. This led us to ask whether 3D shape could also influence lightness responses in low-level cortical areas. Knill and Kersten (1991) showed that the lightness difference in a version of the Craik-O'Brien stimulus largely diminishes when the luminance pattern is interpreted as shading variations due to surface curvature (Cylindrical), but not if it is interpreted as a combination of a flat surface and an abrupt change in two otherwise uniform reflectances (Rectangular). Our current study asked whether retinotopically localized cortical lightness responses would be reduced by contour cues consistent with the perceptual explanation of smoothly changing 3D surface orientation. We measured BOLD signals using fMRI at 3T from human visual cortex (n=6) in response to observation, with diverted attention, of Rectangular and Cylindrical variants of a Craik-O'Brien stimulus. We found an unexpectedly higher response to Cylindrical than Rectangular stimuli in localized, physically constant stimulus regions. Further analysis attributed this difference to confounding effects of stimulus geometry (distance from object contours) that, when controlled, indicated no significant difference in the response to the Cylindrical and Rectangular stimuli. Hence, our current measurements are consistent with a lack of influence of smooth shape cues on lightness responses in low-level visual cortex.

Meeting abstract presented at VSS 2013