Fast ballistic arm movements triggered by visual, auditory, and somesthetic stimuli in the monkey. II. Effects of unilateral dentate lesion on discharge of precentral cortical neurons and reaction time

Abstract

Single-unit recordings from motor cortex (area 4) were obtained before and after dentate lesion in two monkeys executing fast elbow flexions and extensions in response to randomly presented visual, auditory, and somesthetic stimuli. There were no starting or ending reference points or preparatory signals. Monkeys were trained to perform movements larger than 15 degrees within 500 ms of the stimulus presentation. After electrolytic lesion of the dentate nucleus ipsilateral to the trained arm, changes in reaction time (RT) were observed. Mean daily RTs of movements triggered by light and sound were lengthened by 50-70 ms. RTs of movements triggered by somesthetic stimuli were not changed in one monkey, whereas a small increase of only 20 ms was observed in the other animal. Spontaneous firing of precentral neurons was about the same before and after dentate lesion. However, movement-related responses of cortical neurons were affected by the lesion. Whenever there was an increase in RT according to the triggering stimuli, a corresponding increase in the response time of neurons (RS) appeared. Both RS and RT increased by the same amount when movements were triggered by visual and auditory stimuli, whereas they remained about the same when somesthetic stimuli were used to trigger movements. In contrast, the time interval between the appearance of the change of neuronal firing and onset of arm displacement (RM) was not modified after the lesion. Gating of sensory conditioning inputs and modification of RT by the presentation of more than one stimulus were not abolished by dentate lesion. As a whole, the effects of dentate lesion on motor cortical neurons are consistent with the hypothesis that the neocerebellum controls the initiation of simple ballistic limb movements by controlling the discharge of motor cortex neurons. The effects could be attributed to the withdrawal of a facilitatory influence of dentate neurons on the motor cortical cells, particularly for movements triggered by teleceptive inputs.