Ipsilateral cortical projections to areas 3a, 3b, and 4 in the macaque monkey

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Abstract
In the macaque monkey area 3a of the cerebral cortex separates area 4, a primary motor cortical field, from somatosensory area 3b, which has a subcortical input mainly from cutaneous mechanoreceptive neurons. That each of these cortical areas has a unique thalamic input was illustrated in the preceding paper. In the present experiments the cortical afferent projections to these 3 areas of the sensorimotor cortex monkey were visualized and compared, using 4 differentiable fluorescent dyes as axonal retrogradely transported labels. The cortical projection patterns to areas 3a, 3b, and 4 were similar in that they each consisted of (a) a “halo” of input from the immediately surrounding cortex, and (b) discrete projections from one or more remote cortical areas. However, the pattern of remote inputs from precentral, mesial, and posterior parietal cortex was different for each of the 3 cortical target areas. The cortical input configuration was least complex for area 3b, its remote input projecting mainly from insular cortex. The pattern of discrete cortical inputs to the motor area 4, however, was more complex, with projections from the cingulate motor area (24c/d), the supplementary motor area, postarcuate cortex, insular cortex, and postcentral areas 2/5. Area 3a, in addition to the proximal projections from the immediately surrounding cortex, also received input from the supplementary motor area, cingulate motor cortex, insular cortex, and areas 2/5. Thus, this pattern of cortical input to area 3a resembled more closely that of the adjacent motor rather than that of the somatosensory area 3b. Contrasting with this, however, the thalamic input to area 3a was largely from somatosensory VPLc (abbreviations from Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger) and not from VPLo (with input from cerebellum, and projecting to precentral motor areas).