A large fraction of the cat's visual field is shared between the two eyes; correspondingly, the majority of neurons in the part of the visual cortex representing the binocular field have input from both eyes (Hubel and Wiesel 1962). Most of these cortical neurons have receptive fields that are highly selective for the orientation of a moving edge and they often have preferences for many other aspects of the visual stimulus, such as its direction, length and velocity of movement (Hubel and Wiesel 1962, 1965a; Pettigrew et al. 1968a; Rose 1974; Movshon 1975). Remarkably, in most cases the stimulus requirements of these binocular neurons are exactly matched in the two eyes. The preferred orientation, the optimal width and contrast of the stimulus and the best velocity of movement are usually similar for the two receptive fields of a single...