Cortical and subcortical afferent connections of the squirrel monkey's (lateral) premotor cortex: evidence for visual cortical afferents.

The afferent connections of the squirrel monkey's lateral premotor cortex were investigated with the horseradish peroxidase (HRP) technique. Seven monkeys received small, unilateral injections of HRP spaced between the most dorsal and the most lateral region of the premotor cortex. Results are based on six brains, all of which showed a considerable number of labeled neurons both cortically and subcortically. At the cortical level, labeled cells were found both ipsi- and contralateral to the injection site. Heavy labeling occurred in the fields surrounding the prefrontal, central, and precentral cortex, while the temporal regions usually innervated the premotor cortex with a moderate number of cells, and the parietal fields with only a minor number. Surprisingly, in all brains, but especially in those with the more lateral injections, we also found a considerable number of afferents originating from occipital fields, including a few neurons from the striate cortex. Subcortically, a few labeled cells were seen in basal forebrain regions, the amygdala, and the claustrum; a considerable number of thalamic nuclei and also some hypothalamic nuclei contained labeled cells; and within the brainstem, labeling was found most consistently in the locus ceruleus and in the tegmental fields. It is concluded that the (lateral) premotor cortex is involved in sensorimotor integration to a higher degree than has been recognized up to now.