The somatotopic organization of area 2 in macaque monkeys

Abstract

Area 2 is a traditional architectonic subdivision of anterior parietal cortex in macaque monkeys, but its overall somatotopic organization and responsiveness to different types of somatic stimuli are poorly understood, and there are uncertainties concerning its rostral and caudal extent. The goals of the present study were to define the rostral and caudal borders of area 2 better, and to describe its overall organization and responsiveness. Somatic receptive fields were defined for hundreds of closely spaced microelectrode recording sites in postcentral parietal cortex of individual macaque monkeys anesthetized with ketamine. Electrophysiological and architectonic evidence suggested that a 3–4‐mm‐wide strip of cortex along the caudal border of area 1 includes all or most of area 2. The most lateral explored portion of area 2 adjoined the representation of the face in area 1. Much of this sector of area 2 was activated by cutaneous stimulation of the face, especially the chin, but more caudal parts of the head also were represented there. Medially, an adjacent sector of area 2 represented the hand. Rostrally, in the cortex within 1.5 mm from the area 1 border, the glabrous surfaces of digits 5 through 1 were represented in a mediolateral cortical sequence, and from tip to base in a rostrocaudal sequence, mirroring the organization in the adjacent portion of area 1. More caudally at this mediolateral level of area 2, digit tips and other phalanges were represented for a second time. The pads of the palm and the dorsal surfaces of the hand were represented laterally and medially within the portion of area 2 devoted to the hand. More medially, the wrist, forearm, and arm were represented in a lateromedial cortical sequence in area 2, roughly matching the mediolateral organization within the bordering area 1. However, immediately caudal to the representation of the occiput, neck, and shoulder in area 1, a rostrocaudal strip of cortex extending across area 2 represented the arm and forearm for a second time in area 2. This cutaneously activated strip of cortex extended into area 5, where the proximal portion of the hand was represented. More medially, next to the trunk representation in area 1, area 2 was devoted to the trunk and limbs. Next to the representations of the ankle, leg, and thigh in area 1, area 2 was activated from similar locations on the hindlimb. Most medially in area 2, the glabrous digits 1–5 of the foot were represented in a lateromedial sequence. From the results it is apparent that the somatotopic organization of area 2 only roughly parallels that of area 1. In addition, at least some body parts are represented at more than one location in area 2. Finally, the responsiveness of area 2 to cutaneous stimuli varies by location. In general, sites close to the area 1 border were much more responsive to cutaneous stimuli than more caudal sites, but clear exceptions were found. Rostral parts of area 2 representing the trunk and arm were often poorly activated by light cutaneous stimulation, while caudal parts of area 2 related to the arm and forearmwere highly responsive to cutaneous stimulation. Thus, as presently defined, area 2 is less homogeneous in responsiveness to somatic stimuli and more complex in somatotopic organization than more rostral somatosensory areas.