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Kathryn Murray, Geoffrey Boynton; FMRI responses in V1 represent the perceived rather than physical stimulus contrast. Journal of Vision 2007;7(9):242. doi: 10.1167/7.9.242.
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Introduction: The contrast of a stimulus appears lower in the presence of a high-contrast background. Electrophysiological measurements in monkeys and fMRI responses in humans are typically suppressed in the presence of a surrounding contrast background. This leads to the hypothesis that neuronal responses in early visual areas represent the perceived, and not the physical contrast of a stimulus. We tested this hypothesis by comparing contrast matching measurements with fMRI responses in early visual areas. Methods: We acquired fMRI-contrast-response functions and psychometric contrast matching data simultaneously using an event-related fMRI design. Subjects viewed checkerboard targets presented to the left and right of fixation, superimposed on a graded checkerboard background so that one target was surrounded by high contrast and the other by low contrast. Different combinations of target contrasts were presented on each trial while subjects indicated which target had the higher apparent contrast. Results: Psychophysically, more contrast was needed for a target imbedded in a high contrast background to perceptually match a target surrounded by a low contrast background. FMRI contrast-response functions in V1 for targets surrounded by high contrast were suppressed compared to those surrounded by low contrast. FMRI and psychophysics data were compared by assuming that an equal BOLD response to the two targets should result in a perceptual contrast match. Contrast response functions in V1 were affected by the background in a manner that predicts the effects of the background on perceived contrast. Conclusions: The neuronal basis of perceptual illusions are often associated with extrastriate visual areas, consistent with the idea that early visual areas maintain a veridical representation of the stimulus, whereas higher visual areas represent what is perceived. However, with surround suppression, it appears that perceived, rather than physical contrast may be represented by neuronal responses as early as the primary visual cortex.
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