Responses of monkey prefrontal neurons during a visual tracking task reinforced by substantia innominata self-stimulation.

The role of the substantia innominata (SI) in the generation of a behavior mediated by the prefrontal (PF) cortex was examined in two Japanese monkeys. PF neuronal activities related to a visual tracking task by wrist movement were recorded and neural responses to SI stimulation were analyzed. Sixty-six neurons showed task-related activity and were classified into 3 types. Type 1 (n = 31) showed transient activation during the movement. Type 2 (n = 26) showed gradually increasing activity before the reward presentation. Type 3 (n = 9) showed tonic activation from the GO signal to the reward presentation. Antidromic and orthodromic responses to SI stimulation were observed in every type. In Type 1, the percentage of antidromically activated neurons (26%) was similar to that of orthodromically activated ones (16%), but in Type 2, 83% of responding neurons showed antidromic responses, and 56% of Type 3 showed orthodromic responses. These results show that the different types of PF neurons have different anatomical relations to SI. Although orthodromically activated neurons were fewer than antidromically activated neurons, many orthodromically activated neurons showed movement-related activity. This suggests that ICSS at SI facilitates the generation of the behavior through the afferent pathway to PF.