This article deals with the way in which vertebrate, and in particular primate, visual systems are organized for the detection of spatially distributed light stimuli, i.e. for form perception. The principles of this organization are of concern to physicists who design and employ pattern recognition machines for various purposes, as well as to those who are directly concerned with psychophysical studies of visual perception. A brief description of the anatomy and histology of the retina and the central visual pathways is given. The electrical responses of the nerve cells (neurones) which make up the neural networks of the visual system are then examined in two particular cases, namely, the frog and the primate. Maturana and his colleagues have shown that in the frog, the retinal ganglion cells are selectively responsive to a small number of geometric features of the retinal image, and that the responses of tho different clssses of ganglion cell are relayed to different layers of neurones in the mid-brain. ‘Feature extraction’ also appears to occur in primate vision, but in this case it is observed primarily in the responses of cortical neurones.