The Organization of the Second Somatosensory Area

Microelectrode mapping methods were used to determine the organization of the second somatosensory representation, SmII, in grey squirrels. A systematic representation of the contralateral body surface was found in lateral parietal cortex adjoining the first somatosensory representation, SmI (Sur et al., ’78a). The representation of the body in SmII was found to be much less distorted than in SmI. Under our recording conditions, almost all recording sites were activated from strictly contralateral body locations. The most important finding was that the basic orientation of the body representation in SmII is “erect” rather than “inverted.” This orientation allows SmII and SmI to be adjoined along a common border representing the top of the head and face. This type of border has been called congruent (Allman and Kaas, ’75; Kaas, ’771, and it may have significance in the development of sensory representations. Recently, the basic orientation of the body surface representation in the second somatosensory area, SmII, has become a matter of controversy. The issue of disagreement is shown schematically in figure 1. In the early summary diagrams of SmII organization by Woolsey and Fairman (’461, the sequence of representation of the face, arm, and leg was portrayed, while the orientation of these body parts within SmII was not an issue (fig. 1A). Later, the organization of SmII was summarized by Woolsey (’52, ’58) and others (Hamuy et al., ’56; Lende and Woolsey, ’56; T. A. Woolsey, ’67; Campos and Welker, ’76) as an inverted “homunculus” adjoining the head representation of the primary somatosensory area, SmI (fig. 1B). In this conceptualization, SmII is a rotated serial replication of SmI. The “inverted” organization for SmII has been widely accepted, and it is commonly illustrated in reviews and textbooks (see Haight, ’72 for references). A quite different view was first expressed by Haight (‘721, and more recently by Johnson et al. (’74), Herron, (’75) and Campos and Welker (’76). These investigators suggest that the basic organization of the head, arm, and foot in SmII is the opposite of that commonly illustrated, and that when SmII organization is summarized as a “homunculus,” the orientation is erect rather than inverted (fig. 10 . In the erect pattern, J. COMP. NEUR. (19791 184: 473-490. SmII can be considered as a “mirror image” of SmI with a rotation from a parallel to a roughly perpendicular alignment. It is not completely clear from the published reports if the orientation of SmII is upright, inverted, or variable. Because of its relatively small size, the large size of receptive fields for SmII neurons, and the problem that SmII is often completely buried in fissures, the basic organization of SmII has been difficult to determine. In some of the early mapping studies with surface recordings, only part of SmII was explored and data relevant to the orientations of SmII were not collected (Adrian, ’40; Woolsey and Fairman, ’46; Berman, ’61). Some early investigations of SmII organization have been reported only in abstracts and reviews (Woolsey, ’43, ’52, ’58; Woolsey and Wang, ’45) without results supporting conclusions. Other studies in mammals have indicated that SmII is too small relative to the surface electrode for the method to indicate the orientation of SmII (Zeigler, ’64; Lende and Sadler, ‘67; Woolsey, ’67; Lende, ’70; Saraiva and Magalhaes-Castro, ’75). However, a t least two reports of SmII organization based on surface recordings illustrated receptive fields for progressions of recording sites that are clearly more consistent with the concept of an inverted orientation (Hamuy et al., ’56; Lende and Woolsey, ’56).