Use-dependent structural events in recovery of function.

We described research suggesting that forelimb use is essential for marked neural growth in the intact cortex after unilateral forelimb-cortical lesions. Although unilateral brain injury can cause severe functional impairment, the injury may be capable of mobilizing potent resources for compensatory changes such as dendritic arborization in the noninjured hemisphere, but only for a limited period of time and only with appropriate behavioral pressure. Unexpectedly, surviving tissue in the injured hemisphere may be fatally vulnerable to excessive behavioral demand. If the impaired limb is overused because the nonimpaired limb is restricted by a one-sleeve cast, injury size is greatly increased and recovery of function is severely disrupted. It is hypothesized that behaviorally driven neurotransmitter release relating to forced use of the forelimb may be toxic to surviving tissue that has been partially traumatized by the lesion. These data and hypotheses are summarized in Fig. 7. The "use-it-or-lose-it" rehabilitative approach is popular, but perhaps a less aggressive strategy should be adopted for optimal restoration of function in the injured hemisphere. Whereas traditional experiments on mechanisms of recovery of function are designed specifically to determine potentially compensatory neural changes that might mediate behavioral outcome, these experiments support a quite different view of the interplay between neural and behavioral events: behavioral changes may directly alter anatomical events.