Cingulate input to the primary and supplementary motor cortices in the rhesus monkey: Evidence for somatotopy in areas 24c and 23c

We examined the distribution of cingulate projections to the somatotopically related parts of the primary (M1) and supplementary (M2) motor cortices of the monkey by using fluorescent dyes. Labeled neurons were found in layers 3, 5 and 6 of areas 24c and 23c and were heaviest following injections placed in M2. Projections to analogous somatotopic areas in M1 and M2 arose from similar cingulate regions. In area 24c, neurons projecting to the face area of M1 and M2 were located anteriorly, those to the hindlimb were located posteriorly, and neurons projecting to the forelimb area of M1 and M2 were located in between. In area 23c, neurons projecting to the forelimb area of M1 and M2 were located anteriorly and those to the hindlimb area of M1 and M2 were located posteriorly. The face area of M1 and M2 was not found to receive afferents from area 23c. In contrast to this discreteness, cingulate projections to Woolsey's axial representation of M1 were diffuse. The results support the presence of a separate and somatotopically organized cingulate motor cortex in area 24c. This is predicated on the facts that: (1) small injections of retrograde tracers placed in analogous somatotopic parts of M1 and M2 resulted in similar patterns of labeling within the electrophysiologically "excitable" portion of the anterior cingulate cortex, and (2) this organized topography infers somatotopy. Our data fail to support a somatotopically organized cingulate motor area in area 23c if the criterion of all body parts is demanded. By virtue of its anatomical location and its connectional relation to the spinal cord and isocortical motor fields on the one hand and to the limbic cortex on the other, area 24c may be considered as M3 and provide limbic influences at several levels of motor control.