Introduction

Part of the function of the neuron is communication. Neurons must communicate voltage signals to one another through their connections (synapses) in order to coordinate their control of an animal's behaviour. It is for this reason that information theory (Shannon and Weaver, 1949) represents a promising framework in which to study the design of natural neural systems. Nowhere is this more so than in the early stages of vision, involving the retina, and in the vertebrate, the lateral geniculate nucleus and the primary visual cortex. Not only are early visual systems well characterised physiologically, but we are also able to identify the ultimate “signal” (the visual image) that is being transmitted and the constraints which are imposed on its transmission. This allows us to suggest sensible objectives for early vision which are open to direct testing. For example, in the vertebrate, the optic nerve may be thought of as a limited-capacity channel. The number of ganglion cells projecting axons in the optic nerve is many times less than the number of photoreceptors on the retina (Sterling, 1990). We might therefore propose that one goal of retinal processing is to package information as efficiently as possible so that as little as possible is lost (Barlow, 1961a).

Important to this argument is that we do not assume the retina is making judgements concerning the relative values of different image components to higher processing (Atick, 1992b). Information theory is a mathematical theory of communication. It considers the goal of faithful and efficient transmission of a defined signal within a set of data.