Thalamic connections of the primary motor cortex (M1) of owl monkeys

To determine the relative contributions of transthalamic cerebellar and pallidal projections to the primary motor cortex (M1) of owl monkeys, we examined the thalamic labeling resulting from injections of fluorescent tracers and wheatgerm agglutinin/horseradish peroxidase conjugate (WGA‐HRP) into regions of M1 identified by intracortical microstimulation. Injections were placed in the major somatotopic divisions of M1 (the hindlimb, trunk, forelimb, and face representations) and in the caudal and rostral M1 subareas. In most cases, we injected several differentiable tracers into different parts of M1. Our results indicate that the strongest connections of M1 are with subdivisions of the ventral lateral thalamus (VL); other connections are mainly with intralaminar nuclei (the central lateral, paracentral, and center median nuclei) and the reticular nucleus. Most projections are reciprocal and topographically organized. M1 is strongly connected with the principal (VLp), medial (VLx), and anterior (VLa) subdivisions of the VL complex but has at most weak connections with the dorsal division (VLd). Each of the major somatotopic divisions of M1 is connected with an anteroposteriorly elongated territory within the VL complex. The connections are somatotopically organized such that the M1 hindlimb representation is connected with a band of cells in the lateral and anterior portions of the VL complex (spanning VLa and VLp), whereas the trunk, forelimb, and face representations are connected with successively more medially and posteriorly situated cell bands (spanning VLa, VLp, and VLx). There is some degree of overlap between the somatotopic territories within VL, although the absence of double‐labeled cells in cases with injections of adjacent somatotopic divisions of M1 suggests that individual thalamic neurons project to single somatotopic regions.