The neurobiology of primate vision

Studies of the visual system of the primate have taken two directions. One group of neurobiologists have studied the oculomotor system, while an entirely separate group have analysed sensory processing in the retinogeniculo-cortical circuits. However, the versatility and adaptability of the primate visual system is only possible because sensory and oculomotor processing are highly integrated. More than any other species, primates have exploited the advantages of extensive and coordinated binocular eye movements to improve their visuomotor performance. This chapter describes the basic apparatus of primate vision. The integrated nature of visuomotor function is illustrated by two examples: smooth pursuit eye movements and stereovision. Both of these tasks require huge amounts of sensory processing in many different visual centres and, simultaneously, very precise control of binocular eye position. These examples illustrate the difficulty and artificiality of assigning unique functions to any component part of the system. Each part of the visual system, from the retina through the brain to the oculomotoneurones that drive the eye muscles, is involved in many different functions. This integration within the system raises the problem of how we represent and recognize our visual world in the brain. The traditional view from sensory psychology is that single nerve cells in the visual cortex signal significant percepts. As this chapter shows, the activity of single cortical neurones is influenced by multiple factors, including the qualities of the visual stimulus, the position of the eyes, and the attention being paid to the stimulus. Thus, the activity of single neurones is an ambiguous indicator of both perceptual and motor events. Only the collective action of many nerve cells, it seems, represents unique percepts and actions. Given this indivisibility of function, patients with localized brain lesions will invariably present with multiple visuomotor deficits.