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VISUAL perception can be viewed as a transformation and a distortion of the spatial patterns which exist in the outside world. The transformation of external objects into neural images is accomplished by the summation of excitatory and inhibitory influences over the visual field of each neurone. Distortion occurs when the neural response is not simply proportional to the sum of excitation and inhibition because of nonlinear mechanisms in spatial summation. If the responses of all visual cells were distorted by such nonlinear summation, fine visual discriminations like those necessary for reading Nature would be impossible. Nonlinear mechanisms do however, seem to be important in one class of visual neurone, perhaps for signalling change or motion in the external world. We have attempted to understand the importance of the different kinds of spatial summation by studying single cells in the cat visual system.