THE reversal or displacement of the retinal image by prism spectacles leads to extreme disruption of visually guided behaviour, but after an extended period of visual transformation normal behaviour is gradually restored. It is unclear whether this adaptation involves a change in visual perception(1-5), the learning of new motor responses(6), a modification of the sensorimotor control system(7) or a proprioceptive change in the perceived positions of the body parts(8). Here I describe the effects of visual field reversal on neuronal activity in the monkey visual cortex After a few months of wearing reversing spectacles, some cells in the primary visual cortex began to respond to stimuli presented not only in the contralateral visual field but also in the ipsilateral field. These cells were not selective for orientation or direction of motion, but responded well to a light flash. This result suggests that adaptation to visual field reversal is mediated, at least in part, by a large-scale functional reorganization at an early stage the visual processing pathway.