Human adult cortical reorganization and consequent visual distortion

In the visual system, cortical reorganization has principally been demonstrated in studies of adult animals: If a region of primary visual cortex (VI) loses its usual input (e.g., due to retinal damage), neurons in that region begin responding to stimuli that normally activate adjacent V1 cortex. This dissertation asks whether cortical reorganization occurs in the human adult visual system, and if so, what the perceptual consequences might be. These questions are addressed by studying an adult stroke patient, BL.; BL has damage to the right-hemisphere inferior optic radiations. This damage caused a loss of sensory input to the V1 region representing the upper left visual field (LVF), producing a left superior homonymous quadrantanopia (i.e., a scotoma or blind area 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, BL reports seeing a "cigar" extending upward; when shown a square, he reports seeing a vertically-elongated "rectangle"; and when shown an upside down triangle, he reports seeing a "pencil" standing upright. Psychophysical testing confirmed the perceptual distortion, and established that the vertical but not the horizontal dimension was affected. I hypothesized that the perceptual distortion in the lower LVF was a consequence of V1 reorganization. BL's stroke did not damage V1 directly, but the V1 region representing most of the upper LVF was deprived of its usual input (due to optic radiation damage). Neurons in the deafferented region may consequently have become responsive to inputs from the lower LVF, such that stimuli presented in the lower LVF activated not only the V1 region representing this area, but also the adjacent region that previously received input from the upper LVF. If activation of this latter region were still treated by BL's visual system as representing upper LVF stimulation, stimuli in the lower LVF might well appear vertically elongated.; Additional psychophysical experiments confirmed several predictions following from my hypothesis. Results 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 deficit affects not only vision-for-perception, but also vision-for-action (grip aperture); and that the extent of vertical distortion monotonically decreases with distance from the blind quadrant. Additionally, I used functional magnetic resonance imaging (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 studies show that BL's perceptual distortion results from V1 reorganization, providing the first clear demonstration of cortical reorganization in the adult human visual system, and the first evidence that reorganization affects visual perception. (Abstract shortened by UMI.)...