Mapping multisensory parietal face and body areas in humans

Abstract

Detection and avoidance of impending obstacles is crucial to preventing head and body injuries in daily life. To safely avoid obstacles, locations of objects approaching the body surface are usually detected via the visual system and then used by the motor system to guide defensive movements. Mediating between visual input and motor output, the posterior parietal cortex plays an important role in integrating multisensory information in peripersonal space. We used functional MRI to map parietal areas that see and feel multisensory stimuli near or on the face and body. Tactile experiments using full-body air-puff stimulation suits revealed somatotopic areas of the face and multiple body parts forming a higher-level homunculus in the superior posterior parietal cortex. Visual experiments using wide-field looming stimuli revealed retinotopic maps that overlap with the parietal face and body areas in the postcentral sulcus at the most anterior border of the dorsal visual pathway. Starting at the parietal face area and moving medially and posteriorly into the lower-body areas, the median of visual polar-angle representations in these somatotopic areas gradually shifts from near the horizontal meridian into the lower visual field. These results suggest the parietal face and body areas fuse multisensory information in peripersonal space to guard an individual from head to toe.

Fig. 1.

Tactile stimulation on multiple body parts. (A) Male and female tactile suits. A plastic tube ending with an elbow fitting delivers air puffs to the central hole of each white button attached to the suit. (B–F) Schematic diagrams of stimulation sites (black open circles) and traveling paths on the (B) face and lips, (C) shoulders (including part of the upper arms), (D) fingertips, (E) legs and ankles, and (F) toes.

Fig. 2.

Parietal face and body areas in a representative subject. (A) Anatomical location and Talairach coordinates of the parietal face area in structural images (Left three panels) and on an inflated cortical surface (Rightmost panel). The black square indicates the location of a close-up view of the superior posterior parietal region shown below. RH, right hemisphere. (B–E) Body-part ROIs and their average signal changes (Insets) for (B) face vs. fingers scans, (C) face vs. legs scans, (D) lips vs. shoulders scans, and (E) fingers vs. toes scans. (F) A summary of parietal face and body areas. Contours were redrawn from the ROIs in B–E. To reduce visual clutter, contours of face and finger ROIs from C and E were not shown in F.

Fig. 3.

A multisensory homunculus in the superior posterior parietal cortex. (A) Distributions of Talairach coordinates of body-part ROI centers across subjects. Color dots, cluster centers; radius of each outer circle, SD of Euclidean distance from each cluster center (Table S1). (B) Contours of group-average body-part ROIs (P = 0.05, uncorrected) overlaid on group-average retinotopic maps (P = 0.05, uncorrected) rendered on subject 1’s cortical surfaces. A cyan asterisk indicates the location of average lip ROI center from the left hemispheres of subjects (n = 7) showing significant activations (Fig. S2C). Color wheels, polar angle of the contralateral visual hemifield. (C) A model of the parietal homunculus overlaid on group-average retinotopic maps. (D and E) Percentage of V-T overlap in each body-part ROI defined on group-average (D) and single-subject (E) maps. Error bar, SD. (F and G) Box plots of the distribution of polar angle within each body-part ROI outlined on group-average (F) and single-subject (G) maps. Each box represents the interquartile range, the line within each box indicates the median, and whiskers cover 90% of the distribution. LVF, left visual field; RVF, right visual field. LH and L, left hemisphere; RH and R, right hemisphere. Fi, fingers; Li, lips; Fa, face; Sh, shoulders; Le, legs; To, toes.