Visual cortex responses to different texture-defined boundaries: An fMRI study

Abstract

Variations of luminance and of texture have distinct causes in natural scenes, and several kinds of evidence indicate that the mammalian visual system employs distinct mechanisms for processing them. Human brain imaging studies have suggested that some visual cortical areas are differentially responsive to the two kinds of stimulus. However it is unclear whether different kinds of texture variation are processed in the same way, and whether they are represented uniformly across cortical areas. To address this issue we employed human fMRI to compare responses to four kinds of texture boundaries, using stimuli with equated global properties but varying in local luminance, contrast, spatial frequency, and orientation. Overall stimulus strength and attentional load were equated by psychophysical methods. Both stereotaxic and volume-of-interest analyses were performed. Percent BOLD signal change elicited by each stimulus type was determined in retinotopically mapped visual areas (V1 to V4) as well as data-driven VOIs. The early retinotopic areas responded to all four kinds of stimulus to a comparable extent. However higher-order visual occipito-parietal and -temporal regions typically responded more to texture than to luminance-defined boundaries. Modulations of contrast and of spatial frequency produced highly similar patterns of activation, which were markedly distinct from those to orientation variations. These findings suggest that different kinds of texture modulation might be processed, and perhaps utilized, in a distinct manner.

Footnotes

 Funded by NSERC grant to CLB (OPG0001978) and FK (OPG01217130).