Title : Substantia Nigra Output to Prefrontal Cortex via Thalamus in Monkeys

23 The present report investigated the involvement of primate nigro24 thalamo-cortical projections in discrimination of visual signals with behavioral 25 meaning. We tested the extracellular unit activity of mediodorsal (MD) and 26 ventral anterior (VA) thalamic neurons monosynaptically receiving inhibitory 27 input from the substantia nigra pars reticulata (SNr) and projecting to the 28 frontal cortex in Japanese monkeys performing a delayed conditional go/no-go 29 discrimination task. In the task two colored stimuli (S1, S2) intervened by 30 delay period required the monkeys lifting a lever (go) or not (no-go); the same 31 and different colored pairs of S1 and S2 meant go and no-go signals, 32 respectively. Prominent task-relevant responses were sustained activity with 33 color preference to S1 during delay period and S2-related activity with 34 different firing rate between go and no-go trials. Particularly, a high 35 proportion of such go/no-go differential S2-related activity was found in 36 thalamic relay neurons, receiving input from the caudolateral SNr and 37 projecting to the prefrontal area (PSv) ventral to the principal sulcus, in the 38 rostrolateral MD. The findings suggest that the caudolateral SNr-rostrolateral 39 MD-PSv pathways may be possible conduits of signals coding the behavioral 40 meaning of the visual stimuli, and thus may be responsible for generating 41