The goal of this research is to understand how changes in the organization of the cerebral cortex mediate recovery of function after cortical injury. Lesions in the monkey middle temporal area (MT), medial superior temporal area (MST), and frontal eye field (FEF) produce clear deficits in smooth pursuit eye movements. Pursuit recovers within 5 days after small MT, and MST lesions, and within 6 weeks of larger FEF lesions. To date, there has been no systematic study of how the visual cortical areas interact during this recovery, or of identifying the underlying neuronal mechanisms. The following three experiments are aimed at investigating these issues: 1) Changes in visual receptive field (RF) characteristics will be studied from surviving cells adjacent to small lesions in MT, MST and FEF. Extracellular single units will be recorded before and after the lesion, and any changes in RF size, directional tuning curves, and preferred direction will be determined. The time course of the RF changes will be compared to the rate of recovery to test for correlations between the two. 2) The cortical areas responsible for the rapid phase of pursuit recovery will be determined. It has been shown that complete unilateral removal of MT and MST eliminates the rapid recovery seen after small lesions in these areas, but a gradual improvement does occur over 5 months. However, it may be that MT, and not MST, is responsible for the quick recovery, since it has been shown that a complete lesion of MT alone eliminates recovery of direction discrimination. For this reason, total unilateral MT and MST lesions will be made separately to see which of these 2 areas (or both) are required for the quick recovery. Also, lesions in the ventral intraparietal area (VIP) do not produce pursuit deficits despite a large percentage of directionally selective cells there. If VIP contributes to the recovery after small MT/MST lesions, a subsequent VIP lesion should then produce a deficit. 3) Determine whether a gradual removal of MT/MST results in faster recovery than that seen after a single large lesion in these areas (Adametz effect). This will be combined with the strategy of the first experimental series to study how the cortex deals with slowly developing lesions. It is hoped that understanding the mechanisms of pursuit recovery will provide insight to such human phenomena as functional recovery after stroke or head injuries, and the return of intractable pain after surgical intervention.
