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Organization of cerebral cortex for perception.

The pathways by which sensory information is relayed as far as the projection areas of cerebral cortex are relatively well established. We know much less about what happens to the information once it has reached the brain, though the process is important since it results in the perception of meaningful objects in the world around us. Classically a distinction is made between projection and association areas of the cerebral cortex. Sensory projection areas such as the calcarine and postcentral gyri receive their input from third-order sensory neurons, whereas association areas receive their input from other areas of the cortex or the association nuclei of the thalamus. Even though association cortex does not receive information directly from the sensory pathways, its importance in analysing sensory information is evident from the disturbances of perception which result when it is injured. Lesions in posterior association areas frequently impair the ability to make use of familiar patterns of sensory stimulation or to distinguish new ones. Such perceptual disturbances are distinct from the simple sensory losses which are characteristic of damage to the projection areas. In any attempt at understanding the organization of cerebral cortex it is tempting to look for an anatomical localization of function. In this way we might expect different aspects of perception to be carried out in different parts of association cortex. The evidence for such a view is not entirely favourable. Very small lesions often produce no symptoms and when the damage is more extensive it seldom affects abilities singly. Further, lesions in a given region may produce different effects in different individuals. It has been argued that perceptual processes may be diffusely represented in the association areas.1 Since no part would be irrevocably specialized other regions could take over and compensate for a limited amount of damage, and indeed there is often a partial recovery of function in the weeks after experimental cortical ablations. The recovery can be shown to be dependent on postoperative experience, as though the work of lost neurons were being redistributed among those remaining. The facility for this reorganization is referred to as equipotentiality. The controversy between localization of function and a diffuse, equipotential representation provided much of the excitement in the early years of neurobiology. As with so many debates the truth seems to lie somewhere between the