SUBCORTICAL INPUT TO THE SMOOTH AND SACCADIC EYE-MOVEMENT SUBREGIONS OF THE FRONTAL EYE FIELD IN CEBUS MONKEY

We have recently identified two functional subregions in the frontal e ye field (FEF) of the Cebus monkey, a smooth eye movement subregion (F EFsem) and a saccadic subregion (FEFsac). The thalamic input to these two subregions was studied and quantified to gain more information abo ut the influence of the cerebellum and basal ganglia on the oculomotor control mechanisms of the cerebral cortex. A recent study using trans neuronal transport of virus demonstrated that there are neurons in the basal ganglia and cerebellum that project to the FEFsac with only a s ingle intervening synapse (Lynch et al., 1994). In the present study, we concentrated on the thalamic input to the FEFsem to define possible basal ganglia-thalamus-cortex and cerebellum-thalamus-cortex channels of information flow to the FEFsem. We localized the functional subreg ions using low threshold microstimulation, and retrogradely transporte d fluorescent tracers were then placed into the FEFsem and FEFsac. The neurons that project to the FEFsem are distributed in (1) the rostral portion of the ventral lateral nucleus, pars caudalis, (2) the caudal portion of the ventral lateral nucleus, pars caudalis, (3) the mediod orsal nucleus, (4) the ventral anterior nucleus, pars parvocellularis, and (5) the ventral anterior nucleus, pars magnocellularis. In contra st, the large majority of neurons that project to the FEFsac are locat ed in the paralaminar region of the mediodorsal nucleus. The FEFsac an d FEFsem thus each receive neural input from both basal ganglia-receiv ing and cerebellar-receiving cell groups in the thalamus, but each rec eives input from a unique combination of thalamic nuclei.