Reynolds, Chelazzi, and Desimone (J. Neurosci. 19:1736, 1999) showed that neurons in areas V2 and V4 of macaque visual cortex exhibit stimulus competition: the response when two stimuli are presented simultaneously is a weighted average of the response to each stimulus presented alone. Attention directed to one stimulus changes the response to the combined image towards the response for the attended stimulus alone. We previously proposed a model for the mechanisms underlying both phenomena (Archie and Mel, SFN Abstracts 25, 1999). The receptive field (RF) of a V2/V4 neuron is divided into individually attendable subregions. Sensory input to a subregion drives excitatory projections directed to one dendritic branch of the V2/V4 cell and inhibitory projections scattered over the entire cell. Each branch, therefore, corresponds to a single RF subregion. Attention is directed to a particular RF subregion through increased excitation of the corresponding dendritic branch and increased inhibition across the enti...