Encoding brightness in the human cortex

Surface perception is fundamental to human vision, yet most studies of visual cortex have focused on the processing of borders. We therefore investigated the responses of human visual cortex to parametric changes in the luminance of uniform surfaces by using functional MRI.

Early visual areas V1 and V2 V3 showed strong and reliable increases in signal for both increments and decrements in surface luminance. Responses were significantly larger for decrements than for increments, which was fully accounted for by differences in retinal illumination arising from asymmetric pupil dynamics. Responses to both sustained and transient changes of illumination were transient. Signals in early visual cortex scaled linearly with the magnitude of change in retinal illumination, as did subjects’ subjective ratings of the perceived brightness of the stimuli.

Our findings show that early visual cortex responds strongly to surfaces and that perception of surface brightness is compatible with brain responses at the earliest cortical stages of processing. Thus, there could be import ant interactions between regions representing the surface and those representing the border. To ensure that the responses we measured were caused by the local surface alone rather than remote contours, we studied the represent action of parts of a surface that were separated from the closest contour by at least 5°. This distance is beyond the influence of boundary processing that has been measured in human V1, V2, and V3.

Encoding brighness

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Responses of human visual cortex to uniform surfaces measured with fMRI.

Haynes, J., Lotto, R. B. and Rees, G. (2004)

Responses of human visual cortex to uniform surfaces measured with fMRI.

Proceedings of the National Academy of Science USA 101:4286-4291.
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