A 64 year old woman with posterior cortical atrophy secondaryto probable Alzheimer's disease is described. Her presenting symptomwas of seeing objects as abnormally coloured after prior exposure to acoloured stimulus. Formal testing disclosed that the patientexperienced colour after-images of abnormal latency, duration, and amplitude. The demonstration of prolonged colour after-images in a patientwith a cortical disease process provides strong evidence that thegeneration of colour after-images is mediated at least in part by thevisual cortex. A mechanism for the generation of colour after-images isproposed in which abnormal prolongation of the images results fromexcessive rebound inhibition of previously excited wavelength selectiveneurons in V1. This may occur as a consequence of the relative sparingof inhibitory interneurons in V1 in the context of the degeneration ofexcitatory neurons that occurs in Alzheimer's disease.
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