Visual form perception

Abstract

Abstract 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. Human visual form perception has been extensively studied by psychophysical methods, some experiments being designed to determine visual performance criteria and others to obtain evidence regarding the functional organization of form perception. Visual performance has been expressed in terms of a spatial frequency response function and this can be used to calculate threshold detection characteristics. A number of psychophysical phenomena, such as Mach bands, which provide evidence of interactions between different areas of the visual field are described and a particular group of adaptation experiments, the results of which imply shape response selectivity similar to that observed in electrophysiological investigation of the primate cortex, are discussed. The complexity of neuronal connections in the visual cortex makes it improbable that these connections are completely specified genetically. Results of experiments with animals reared in controlled visual environments indicate that the pattern of neuronal connections established in the visual cortex depends on early visual experience, i.e. the visual system is self-organizing in its response to stimulus patterns. Finally, some analytical aspects of form perception are examined. These include Pitts and McCulloch's treatment of transformation invariance of form perception, the reliability of visual performance obtained from relatively unreliable neuronal components and Minsky and Papert's analysis of pattern recognition by parallel networks.