Neurophysiological Mechanisms in the Visual Discrimination of Form

Abstract

Man, in common with many animals, has an extremely well developed ability to detect, discriminate and react to visual stimuli. The determinants of visual behaviour are many and include, for example, physical factors such as ambient illumination and the optical properties of the eye, behavioural or psychological factors such as the subject’s past experience, as well as the neurophysiological factors with which this chapter is particularly concerned. It is, of course, a basic assumption of neurophysiologists that the neurophysiological organisation of the visual pathway, and of the associative and efferent pathways involved in visually based behaviour, determines all measures of visual behaviour, from basic measures such as absolute sensitivity and visual acuity to the higher functions of visual memory, discrimination and perception. Yet, although psychophysicists and psychologists have investigated visual performance at all these levels, present understanding of the neurophysiological mechanisms of vision extends very little past the mechanisms involved in processing visual input. Our principal emphasis consequently is on the neurophysiological encoding of form information in the afferent visual pathway, and discussion of behavioural and psychophysical assessments of form vision is included only where relevant neurophysiological mechanisms have been described or sought. For the same reason the term “form discrimination” is taken to denote the behavioural demonstration by an experimental subject (whether animal or human) of the ability to distinguish one form or pattern or shape from another; this excludes from our scope one of the ultimate problems of form vision, viz. how neural activity gives rise to perception. Similarly, no distinction is made between the terms “pattern”, “shape” or “form”, although there may be grounds for doing so in other contexts.