Somatotopy and attentional modulation of the human parietal and opercular regions

Abstract

The somatotopical organization of the postcentral gyrus is well known, but less is known about the somatotopical organization of area 2, the somatosensory association areas in the postparietal cortex, and the parietal operculum. The extent to which these areas are modulated by attention is also poorly understood. For these reasons, we measured the BOLD signal when rectangular parallelepipeds of varying shape were presented to the immobile right hand or right foot of 10 subjects either discriminating these or just being stimulated. Activation areas in each subject were mapped against cytoarchitectural probability maps of area 2, IP1, and IP2 along the intraparietal sulcus and the parietal opercular areas OP1-OP4. In area 2, the somatotopical representation of the hand and foot were distinctly separate, whereas there was considerable overlap in IP1 and no clear evidence of separate representations in OP1, OP4, and IP2. The overlap of hand and foot representations increased in the following order: area 3a, 3b, 1, 2, IP1, OP4, IP2, and OP1. There were significant foot representations but no hand representations in right (ipsilateral) areas 3a, 3b, and 1. Shape discrimination using the foot as opposed to stimulation enhanced the signal in OP4 bilaterally, whereas discrimination with the hand enhanced the signal bilaterally in area 2, IP1, and IP2. These results indicate that somatosensory areas in humans are arranged from strong somatotopy into no somatotopy in the following order: 3a, 3b, 1, 2, IP1, OP4, IP2, and OP1. Higher order areas such as IP1, IP2, and OP4 showed task-related attentional enhancement.

Figure 1.

The set of six parallelepiped objects used for passive stimulation of the palmar surface on the hand (top set) and the planta surface on the foot (bottom set). For the discrimination tasks, all objects were compared with the most cubic object, object number 1.

Figure 6.

A ratio representing the common area shared by hand and foot representations for each cytoarchitecturally delineated area. The amount of overlap of the Boolean operations [(FD + FS-2 R) ∩ (HD + HS-2 R)]/[(FD + FS-2 R) ∪ (HD + HS - 2 R)] on the 6 mm filtered cytoarchitectural probability map for each area for each subject was calculated. Ratios are calculated for areas 6, 4a, 4p, 3a, 3b, 1, 2, OP, and IP in both hemispheres in each subject. An average of the ratio across 10 subjects is shown. A higher overlap between hand and foot activations in each area is reflected as a higher ratio.

Figure 2.

Psychophysical curve of the HD and FD tasks. The probability of choosing correctly the cubic object appears on the y-axis and the difference in object shape appears on the x-axis (calculated by where a₁ and b₁ are the short and long sides of the cubic object, and a₂ and b₂ are the short and long sides of the object for comparison). Both hand and foot psychophysics are represented in the figure.

Figure 3.

The locations of areas 3a, 3b, 1, 2, IP1, IP2, OP1, OP2, OP3, and OP4. The cytoarchitectural probability map of these areas was filtered with a 6 mm FWHM and overlaid on the standard brain. Areas 1, IP1, IP2, OP1, and OP4 can been seen in the surface render (A); in addition, OP1, OP2, OP3, and OP4 can be seen in the axial slice B and areas 3a, 3b, 1, 2, IP1, and IP2 in axial slice C.

Figure 4.

A, B, Hand and foot representations in the cortex for individual subjects 1(A) and 2(B). Significant clusters are shown for contrasts of all hand versus rest (yellow) and all foot versus rest (purple). The overlap between hand and foot clusters appears as a solid black color. The activations appear as solid colors and are overlaid on the cytoarchitectural probability map (translucent color coded) and standard brain (gray scale). Images are in Talariach space, and the level of the axial slice is given by the z-coordinate.

Figure 5.

The discrimination component of hand and foot tasks. The contrasts of HD-HS (yellow) and FD-FS (black) are illustrated. These contrasts are overlaid on the cytoarchitectural probability map (translucent color coded), which is overlaid on the standard brain (gray scale) in Talairach space.