Review Lecture - Behavioural analysis of the monkey’s visual nervous system

Abstract

1. Introduction Of the three million or so nerve fibres that stream into the primate brain, about two million originate in the eyes. Of these fibres, about one-and-a-half million are in the geniculo-striate system, so named because it connects the eyes with a region of the thalamus known as the dorsal lateral geniculate nucleus and that nucleus with the striate cortex (also known as area 17 or area OC, figure 1) in the occipital lobes. About half, therefore, of all the inputs to the brain are fibres of retinal origin having relatively direct and concentrated access to the cerebral cortex. One may be allowed some surprise, therefore, to find that David Ferrier claimed in 1886 that monkeys subjected to large occipital lobectomies (figures 2, 3) were unaffected by this drastic interruption of such a massive afferent channel. He said 'I removed the greater portion of both occipital lobes at the same time without causing the slightest appreciable impairment of vision. One of these animals within 2 h of the operation was able to run about freely, avoiding obstacles, to pick up such a minute object as a raisin without the slightest hesitation or want of precision, and to act in accordance with its visual experience in a perfectly normal manner’ (Ferrier 1886, p. 273). Ferrier went on to say that ‘Horseley and Schäfer inform me that their results of removal of the occipital lobes entirely harmonize with mine as to the completely negative effect of this operation’ (p. 276), which is a curious claim because two years later Schäfer was locked in a most bitter dispute with Ferrier over just this point, and their argument is merely the most extreme example of the lack of agreement about the functions of the visual cortex in animals that has persisted over the years. We now know, with the benefit of hindsight, that there may have been an uninteresting explanation of these early results of Ferrier’s, because not all of the fibres from the lateral geniculate nucleus project to the lateral surface of the brain. Some of the striate cortex ─ that part which responds to stimulation of the most peripheral parts of the retinae ─ is buried in the calcarine fissure on the medial aspect of the brain, and the most anterior portion of this may be spared even after a complete occipital lobectomy (figure 1). Were Ferrier’s animals using an intact part of their visual space?