Responses of single unit in the caudate nucleus to thalamic stimulation

Abstract Activity of single units was recorded from the caudate nucleus in the cat anesthetized with barbiturate. Single-pulse stimulation of the thalamus induced the responses (either of excitatory or inhibitory response types) in 11.5% of the caudate units tested, and low-frequency stimulation in 47.4%. A different response type was encountered by a small advance of the microelectrode in the same stab. Effective regions for caudate unit responses were the nonspecific thalamic nuclei regardless of whether the stimulation was single or repetitive. When cortical recruiting response was induced by low-frequency stimulation of the nonspecific thalamic nuclei and the nucleus medialis dorsalis, the response of either of the two types was exhibited in 77.6% of caudate units tested. When cortical recruiting response was not induced, the response was observed only in 12.1%. In all caudate units responding to low-frequency stimulation of two different nuclei in the thalamus, the response type of each unit to the stimulation of both nuclei was the same. In all units responding to singlepulse stimulation of both the thalamus and the cortex, the response type was the same. In about two-thirds of caudate units responding both to single-pulse stimulation of the cortex and to low-frequency stimulation of the thalamus, the response type was the same. These results suggest that caudate unit response to low-frequency stimulation of the thalamus is mainly due to activation of the indirect thalamocaudate pathway through the cortex, and that the response type does not depend upon the stimulating site nor the region in which the units existed, but upon the units themselves.