Globus pallidus and motor initiation: the bilateral effects of unilateral quisqualic acid-induced lesion on reaction times in monkeys.

The results of many experimental studies have shown that the globus pallidus (GP) is involved in the control of motor activities, particularly during motor execution. Whether or not the GP is involved in the initiation phase is still a matter of controversy, however. This question was investigated in the present study in Papio papio monkeys after GP lesion using a simple reaction time (RT) task, focusing particularly on the initiation phase. The monkeys were trained to perform this task, which consisted of raising their hand as quickly as possible in response to a visual signal. The RT and its premotor and motor components were measured. In addition, the distribution of the RTs was analyzed in order to assess the number of long latency responses. After making unilateral GP cell lesions by locally injecting small amounts of the excitatory amino acid quisqualic acid, a bilateral increase was observed in RT. This lengthening involved both the premotor and the motor phases of the RT when the task was performed with the contralateral limb and only the premotor phase when it was performed with the ipsilateral one. A significant increase was observed in the percentage of long latency responses recorded in the contralateral limb after the GP lesion but not in the ipsilateral one. Increases in the RT and in the percentage of long latency responses are thought to constitute two indices of the akinesia observed in our task involving speed constraints, which suggests that the GP may participate in motor initiation. A complete recovery of the RT was observed within one month, whereas the increase in the percentage of long latency responses persisted. These two indices of akinesia seemed therefore to result from an impairment involving both motor and nonmotor processes. These data suggest that the GP may be involved in the control of postural adjustment, motivation, and/or the control of the initial isometric part of movements. The time course of the recovery from the deficits observed after GP lesion shows the existence of mechanisms which seem to have been operative particularly in the case of impairments affecting motor processes.