Cortical Reorganization following Damage

In humans, the cortex is a large part of the brain that is so extensive that it covers most other parts. This cortex receives inputs over brainstem and thalamic relays from the sensory receptors in the eyes, skin, ears, and other parts of the body. This information is further processed to create a model of the external world that usefully directs our behavior via the outputs of a number of motor and motor-related cortical areas. Although cortical size and organization vary greatly across the various mammalian species, all have primary sensory areas, the subdivisions of the cortex at the start of the cortical processing systems that relay to variable numbers of additional sensory, multi-sensory, evaluative, motivating, motor, and memory related cortical areas and subcortical structures. When damage to the nervous system impairs the functions of some part or parts of the cortex, usually the person or animal is unable to perform all of the sensorimotor, or other functions. Impairments could result from direct damage to the cortex itself, such as that following a stroke, or after damage at any level of the incoming sensory relays, such as after spinal cord damage, so that cortical areas are deprived of some of the sensory information they use to do their job.

Clinical studies of patients with damage to the nervous system have shown that impairments vary with the parts of the nervous system that are damaged, and that the impairments can be severe or mild, depending partly on the extent of the damage. But, these studies also show that considerable recoveries of lost abilities can occur, and mild impairments can completely disappear after weeks to months of time. Furthermore, some behavioral therapies seem to potentiate these spontaneous recoveries. Why do such recoveries happen, and what can be done to maximize the return of functions?

To help answer these questions one needs to know what is happening to the brain during the recovery periods. The general assumption is that the damaged nervous system is, to some extent, repairing itself. To see if this is happening, and how, it is useful to consider the results of simple, controlled experiments on animals. In cases of human stroke, the damage often involves a number of cortical areas, and the extent of the damage and the areas impaired can be highly variable across individuals. Likewise, when sensory and motor losses occur as the result of accidental spinal cord damage, many systems may be impaired, and the types and extents of damage are variable. This entry considers the results of two types of experiments. In one type, there is no direct damage to the cortex itself. Instead, a sensory loss is created by damaging a peripheral sensory nerve or another part of a sensory pathway that leads to the cortex. Thus, part of the sensory input that is used by a primary sensory area and subsequent cortical processing areas is missing. The sensory and motor [Page 334]impairments that follow such damage usually diminish over time, but why? The second type of experiment is one where part of a cortical area is directly damaged. If behavioral improvements follow the initial impairments after such lesions of the cortex, do they depend on compensations based on other parts of the nervous system, or on internal alterations within the damaged areas? The results of controlled experiments of both types indicate that both the loss of activating inputs and direct damage to parts of cortical areas are followed by a sequence of changes in the internal organization of the affected cortical areas, and these structural and functional reorganizations are, at least partly, responsible for some of the recoveries that do occur.

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