Cortical Microstimulation Thresholds Adjacent to Sensorimotor Cortex Injury

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Abstract
The initial severe contralateral impairment of motor function after unilateral damage to a portion of sensorimotor (SM) cortex lessens within a few weeks after injury. In this study, two hypotheses proposed to explain recovery of behavioral function after cortical injury were tested: (1) Intact cortex adjacent to the injury reorganizes to take over the function of the destroyed area. (2) Intact SM cortex adjacent or connected to the injured area undergoes a transient shock (diaschisis), and as this dissipates, some behavioral recovery occurs. Using microstimulation of the cortex of the adult rat, movements evoked from areas near cortical injuries were studied at various times after undercut laceration, contusion, or suction ablation of an area of SM cortex. Stimulation areas were compared to those obtained from uninjured control animals and to the contralateral uninjured hemisphere. No evidence was obtained for any reorganization of stimulated motor responses in the injured hemisphere even in animals followed for as long as 475 days postinjury, suggesting other mechanisms underlying functional recovery. In intact cortex at some distance from contusion and laceration injuries, there was a marked elevation of thresholds to evoke movements that returned to normal by 9–15 days postinjury. Some intact hindlimb responses were observed after contusion injury that were absent in animals after 15 days postinjury, indicating a slow-growing lesion after this type of trauma. Surprisingly, no elevation in thresholds was noted for ablation injuries up to the edge of the cavity at any time postinjury, indicating that threshold changes near the boundary may be uncorrelated with functional recovery.