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Geraint Rees, John-Dylan Haynes, R. Beau Lotto; Responses of human visual cortex to the brightness of uniform surfaces. Journal of Vision 2004;4(8):346. doi: 10.1167/4.8.346.
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© ARVO (1962-2015); The Authors (2016-present)
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 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 were fully accounted for by differences in retinal illumination arising from asymmetric pupil dynamics. 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 stimuli. These data suggest that asymmetric pupil dynamics should be explicitly considered in studies that systematically vary stimulus luminance. More importantly, our findings provide evidence that early visual cortex responds strongly to surfaces, and show that perception of surface brightness is consistent with brain responses at the earliest cortical stages of processing.
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