Numerous neurophysiological studies have shown that when a region of primary visual cortex loses its normal input (e.g., due to a retinal lesion), the cells in that cortical region begin to respond to stimuli that normally activate adjacent cortical areas. This result is referred to as cortical reorganization, plasticity, or remapping. However, surprisingly little work has investigated how these cortical changes might affect perception. Over the past three years, we have been working with patient BL, who has damage to right-hemisphere inferior optic radiations. This damage caused a loss of sensory input to primary visual cortex representing the upper left visual field (LVF), producing a left superior homonymous quadrantanopia (i.e., blindness in the upper left quadrant). However, primary visual cortex itself is intact. Interestingly, BL exhibits dramatic distortion of perceived shape for stimuli presented in the lower LVF: The stimuli appear vertically elongated (toward and into the blind upper quadrant). For example, when shown a circle, he reports a “cigar” extending upward; when shown a square, he reports a vertically-elongated “rectangle”; and when shown an upside down triangle, he reports a “pencil” standing upright (see figure below). Psychophysical testing confirmed that the distortion selectively affects the vertical dimension of shapes (i.e., the height); judgments concerning the horizontal dimension (width) are intact. Additional experiments revealed that the deficit is selective to vision (i.e., tactile shape judgments are intact); that vertical distance as well as shape judgments are affected; that the vertical distortion arises in a retinocentric frame of reference; that the extent of vertical distortion monotonically decreases with distance from the blind quadrant; and that deficit affects not only vision-for-perception, but also vision-for-action (grip aperture).
In collaboration with colleagues Steve Yantis and John Serences in the Department of Psychological and Brain Sciences, we are using fMRI to seek evidence for cortical reorganization in BL (i.e., whether there are any changes in the visual cortical topographical map). Results revealed that there is activation in the deprived cortical area when a visual stimulus is presented below this area. Visual cortex deprived of input from the upper LVF has apparently become responsive to stimuli in the lower LVF. Taken together, these results support the hypothesis that BL’s perceptual distortions result from cortical reorganization in the early visual system. This work provides the first demonstration of a link between cortical reorganization and its perceptual consequences in the human adult visual system.
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