Effects of lateralized reaching and cerebellar stimulation on unit activity of motor cortex and caudate nucleus in rats.

Rats trained to reach into a narrow tubular feeder for food were implanted with bipolar stimulation electrodes in the dentate nucleus (DNT) ipsilateral to the preferred forepaw. Unit activity was recorded in freely moving animals (N = 15) with capillary microelectrodes inserted with a head-mounted microdrive into the contralateral motor cortex and caudate nucleus. Single-pulse stimulation of the DNT, subthreshold for eliciting overt movement and for interfering with reaching, evoked in both structures short-lasting excitatory responses sometimes preceded by brief inhibition. Reach-triggered stimulation changed the perireach histograms of the motor cortex neurons (N = 46) not only by interference with the movement-related and stimulation-elicited activities in the poststimulus interval but also considerably prolonged the prereach excitation. The perireach histograms of caudate neurons (N = 50) were not changed by the reach-triggered DNT stimulation in the prereach or in the poststimulus interval, probably because the movement-related activity overrode the electrical perturbation. The results indicated that the impairment of reaching caused by DNT stimulation was due to disorganization of the neuronal activity patterns in the motor cortex rather than in the caudate nucleus. The earlier onset of movement-related excitation in the motor cortex during stimulus-triggering reaching reflects attempts to cope with the disturbance by predictive reorganization of the motor discharge.