Primate supplementary eye field. II. Comparative aspects of connections with the thalamus, corpus striatum, and related forebrain nuclei.

The supplementary eye field (SEF) was defined electrophysiologically in behaving monkeys to study its connections with the diencephalon and corpus striatum. The specificity of SEF pathways was determined with horseradish peroxidase (HRP) histochemistry to compare its connections with those of the arcuate frontal eye field (FEF), contiguous dorsocaudal area 6 (6DC), and primary motor cortex (M1, arm/hand region). Results indicate that patterns of SEF connectivity were similar to the FEF and markedly different from areas 6DC and M1. Primary reciprocal thalamic pathways of the SEF were with the magnocellular ventral anterior (VA) nucleus, medial parvicellular VA, medial area X, and paralaminar medialis dorsalis (multiformis and parvicellularis). FEF showed similar connections but its most robust pathway was with MD rather than VA. In contrast, area 6DC showed the most extensive reciprocal connections with lateral VApc and lateral area X with only sparse connections with paralaminar MD. Area 6DC also exhibited reciprocal connections with the ventral lateral (VL) complex and the ventral posterior lateral nucleus, pars oralis (VPLo). M1 showed dense bidirectional connections with VPLo, and to a lesser extent, with VL. M1 pathways with the medial dorsal nucleus were negligible. All areas exhibited connections with the paracentral and central lateral nuclei and only M1 lacked connections with the central superior lateral nucleus. SEF and FEF exhibited similar efferent projections to the caudate and putamen. In the caudate, terminal fields were restricted to a central longitudinal core while those from area 6DC were more widely distributed. Eye field efferents were restricted to the putamen's face region while 6DC projections were more exuberant. The arm/hand region of M1 projected to the arm/hand region of the putamen. Pathways are discussed with respect to their significance in oculomotor control.