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Frans W Cornelissen, Alex R Wade, Robert F Dougherty, Brian A Wandell; fMRI of brightness perception. Journal of Vision 2003;3(9):57. doi: 10.1167/3.9.57.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: The perceived brightness of a region depends on the luminance in that the region and the spatial distribution of nearby luminance values. The brightness of a region can change dramatically even though its own luminance remains constant. To begin tracing the neural basis of this perceptual phenomenon, we made fMRI measurements in human V1 while subjects viewed disks that underwent perceptually similar brightness changes caused either by changes in disk luminance or by changes in the surround luminance.
Methods: Subjects viewed disk (14 deg diameter) and annulus (7–17 deg) stimuli while their brains were scanned. Varying the luminance of either the disk itself or the surrounding annulus (1 Hz), we modulated the brightness of the central disk. Periods of 12 s of stimulus modulation alternated wAith 12 s of a fixation point on a medium gray background. We measured the fMRI BOLD signal in primary visual cortex (V1; identified by retinotopic mapping) and compared activation caused by modulations of either the disk or annulus.
Results: Powerful responses were found at the V1 locations representing the boundary between the disk and annulus in both conditions. The activity fell with distance from the edge representation, eventually reaching zero response. Within the region representing the disk, modulating the luminance of the annulus resulted in lower activation than modulating the luminance of the disk itself.
Conclusion: fMRI signals in human V1 are greatest at the contrast edges between the disk and annulus with relatively little modulation over spatially uniform portions of the image even when the spatially uniform region modulates in time. Brightness changes in the central visual field are not accompanied by commensurate modulations of the BOLD signal in retinotopically corresponding cortex in V1. This suggests that the computation that associates brightness with a retinotopic location occurs at a later stage in human visual cortex.
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