Based upon neurophysiologic, neuroanatomic and neuroimaging studies conducted over the past two decades, the cerebral cortex can now be viewed as functionally and structurally dynamic. More specifically, the functional topography of the motor cortex (commonly called the motor homunculus or motor map), can be modified by a variety of experimental manipulations, including peripheral or central injury, electrical stimulation, pharmocologic treatment or behavioral experience. The specific types of behavioral experiences that induce long-term plasticity in motor maps appear to be limited to those that entail the development of new motor skills. Moreover, recent evidence demonstrates that functional alterations in motor cortex organization are accompanied by changes in dendritic and synaptic structure, as well as alterations in the regulation of cortical neurotransmitter systems.
These findings have strong clinical relevance as it has recently been shown that after injury to the motor cortex, as might occur in stroke, post-injury behavioral experience may play an adaptive role in modifying the functional organization of the remaining, intact cortical tissue. Evidence will be presented to show that after ischemic damage to the primary motor cortex, use-dependent modifications occur in the adjacent, intact cortex. In these studies in adult monkeys, microstimulation techniques were used to define the topographic representation of the upper extremity. Motor maps were derived in great detail by using interpenetration distances of approximately 250µm. Then microlesions were made affecting only about 30 percent of the hand representation. Hand movement representations in the adjacent, undamaged cortex (spared representations) were tracked for several months after the microlesion. If monkeys were allowed to recover spontaneously (i.e., without any post-lesion behavioral training or encouragement to use the affected limb), the remaining, undamaged hand representation decreased in size.
Because it has long been suggested that physical therapeutic interventions might improve recovery after injury to motor cortex, additional studies have examined the effects of post-lesion motor training on recovery of motor maps. In these experiments, monkeys were placed in restraint jackets that restricted the use of the unimpaired limb. Daily repetitive training procedures were employed to encourage improvement in manual skill. After manual skill had returned to normal levels, the motor cortex was re-examined with microstimulation techniques. In contrast to spontaneously recovering monkeys, the monkeys that received post-injury behavioral training showed retention of the undamaged hand representations. More recently, after larger ischemic infarcts in primary motor cortex (destroying 60 percent to 100 percent of the primary motor hand area), functional reorganization can occur in other motor regions in the damaged hemisphere, such as the premotor cortex.
These results raise the possibility that after damage to specific cortical regions, other regions may have the capacity to assume at least some of the damaged functions. It is likely that this capacity for vicarious function is modulated by post-injury behavioral experience.