Corticocortical connections of area 2 of somatosensory cortex in macaque monkeys: A correlative anatomical and electrophysiological study
Authors: Pons T. P. , Kaas J. H. 
Published: January 2005

The cortical connections of electrophysiologically identified locations in the body representations in somatosensory cortex of macaque monkeys were investigated after injections of horseradish peroxidase, wheat germ agglutinin (WGA) conjugated with horseradish peroxidase, tritiated WGA, or tritiated proline. After extensive microelectrode mapping of portions of the body representations in areas 3b, 1, 2, and 5 and careful determinations of electrophysiological borders between areas, restricted injections of tracers were placed, usually into the representation of the hand in area 2. Other injections were placed in the foot representation in area 2 or in area 1, in the wrist representation in area 1, and in the forearm and wrist representation in area 5. Connection patterns were related to the physiological mapping results and to cortical cytoarchitecture.

Injections confined to a lateral portion of area 2 representing the glabrous digits of the hand revealed reciprocal connections with the digit representations in areas 1 and 3b. Projections to area 2 were largely from layer III neurons in both of these fields, and return projections terminated largely in supragranular layers. Other inputs were from layer III cells in one or more separate locations in area 5 and in one or more closely spaced foci in the expected location of S‐II in the lateral sulcus. These connections were also reciprocal with terminations apparent in layers IV and III. A few neurons in area 4 were labeled in some of these cases. Results were similar after an injection in the foot representation in area 2 with the differences that infragranular neurons, in addition to supragranular neurons, formed a substantial part of the projection to area 2, terminations as well as projections were noted from area 4, interconnections were found more rostrally in area 6, and a dense focus of label was apparent in the dorsal bank of cingulate sulcus in the apparent location of the supplementary motor area. Injections in the foot representation in area 1 revealed dense layer IV terminations in the foot representation in area 2, as well as connections with area 3b, the S‐H region, and areas 5 and 7. The injection in the wrist representation in area 1 resulted in dense terminations in the portion of area 5 responsive to the distal forearm and hand, sparser connections with a lateral location in part of area 2 related to the hand, and interconnections with 3b and S‐11. Finally, an injection in the forearm representation of a part of area 5 that is highly responsive to cutaneous stimuli revealed connections with area 1, area 7, and S‐II. The results suggest that (1) somatotopically matched locations in area 3b and 1 provide much of the cutaneous receptor information for processing in area 2, (2) the highly cutaneous zone of area 5 receives cutaneous receptor information directly from area 1, (3) area 2 relays to area 5, and (4) major outputs of areas 1, 2, and at least a specialized cutaneous portion of area 5 are to the S‐II region.