Studying how motor adaptation to visuomotor rotation for one reach direction generalizes to other reach directions can provide insight into how visuomotor maps are represented and learned in the brain. Previous psychophysical studies have concluded that postadaptation generalization is restricted to a narrow range of directions around the training direction. A population-coding model that updates the weights between narrow Gaussian-tuned visual units and motor units on each trial reproduced experimental trial-by-trial learning curves for rotation adaptation and the generalization function measured postadaptation. These results suggest that the neurons involved in rotation adaptation have a relatively narrow directional tuning width ( approximately 23 degrees ). Population coding models with units having broader tuning functions (such as cosine tuning in motor cortex and Gaussian sum in the cerebellum) could not reproduce the narrow single-peaked generalization pattern. Visually selective neurons with narrow Gaussian tuning curves have been identified in posterior parietal cortex, making it a possible site of adaptation to visuomotor rotation. We propose that rotation adaptation proceeds through changes in synaptic weights between neurons in posterior parietal cortex and motor cortex driven by a prediction error computed by the cerebellum.