Responses of pallidal neurons to striatal stimulation in intact waking monkeys.

Extracellular single-unit activity was recorded from neurons of the internal and external pallidal segments, and from 'border cells' at the periphery of the segments, in 3 waking cynomolgus monkeys during electrical stimulation of 3 sites bilaterally in the striatum: one in the caudate nucleus and two in the putamen. Nearly 90% of each of the 3 types of neurons responded to at least one ipsilateral stimulation site. Contralateral stimulation was much less effective, except for border neurons. Neurons responding exclusively to caudate stimulation were located in a dorsomedial zone of the pallidum, those responding exclusively to putamen were in a larger ventrolateral zone, and those responding to both nuclei were in an intermediate zone, larger at rostral than at caudal levels. The great majority of responses consisted of an initial inhibition, at a mean latency of 14 ms, followed by excitation, at a mean latency of 35 ms. Later components of weaker magnitude, often comprising inhibition, occurred in only 30% of the cases. Only border neurons displayed an initial excitation preceding the early inhibition. The responses were not different in the internal and external pallidal segments, except for the slightly more frequent occurrence of excitation in the latter segment. The early inhibition was always displayed by neurons located in the center of the pallidal zone of influence of each striatal stimulation site, and was ended and often curtailed by excitation. At the periphery of the zone, excitation occurred alone or as the initial component of responses. This topological arrangement suggests that excitation is used, temporally, to control the magnitude of the central striatopallidal inhibitory signal and, spatially, to focus and contrast it onto a restricted number of pallidal neurons.