Introduction

Sensory systems have evolved to provide information that is particularly relevant to an animal's way of life. Sensory neurons have modalities and receptive fields that are strongly biased in favour of gathering information that is behaviourally significant for that species. Whatever their bias, sense organs can pick up large amounts of information about an animal's environment: for example, the photoreceptors of an insect's eye provide a point-by-point representation of light levels in the surrounding visual environment. Higher-order neurons in a sensory system cope with all this information by discarding much of it and keeping only the most significant aspects. These neurons act essentially as filters, and transmit only certain aspects of the signal they receive. A consequence of this is that much of the information present at the level of the sensory receptors is thrown away.

Filtering is largely achieved by circuits, in which neurons interact with each other through their synaptic connections. As a result of these interactions, some features of the signal are enhanced and others are discarded at each level in a sensory system. This progressive refinement of the sensory signal begins at the very first synapse, between a sensory receptor and a second-order neuron. Generally, lower-order neurons respond to fairly simple characteristics of stimuli, such as changes in brightness. Higher-order neurons, on the other hand, often respond to particular patterns of stimuli in which information coming from particular groups of sensory receptors is combined together.