The sensory cortical representation of the human penis: revisiting somatotopy in the male homunculus

Abstract

Pioneering mapping studies of the human cortex have established the notion of somatotopy in sensory representation, which transpired into Penfield and Rasmussen's famous sensory homunculus diagram. However, regarding the primary cortical representation of the genitals, classical and modern findings appear to be at odds with the principle of somatotopy, often assigning it to the cortex on the mesial wall. Using functional neuroimaging, we established a mediolateral sequence of somatosensory foot, penis, and lower abdominal wall representation on the contralateral postcentral gyrus in primary sensory cortex and a bilateral secondary somatosensory representation in the parietal operculum.

Figure 1.

The sensory focus of the penis lies lateral to that of the toe. Significant fMRI activations (see Materials and Methods) in the right primary somatosensory cortex after stimulation of the contralateral hallux (red), prepuce or glans (green), penile shaft (blue), and lower abdominal wall (cyan in subjects 5-8) are overlaid onto each individual's anatomical scan. The square depicts the Rolandic template for analysis of primary sensory cortex, and dotted lines mark the central sulcus.

Figure 2.

Stimulation of the toe and penis leads to contralateral primary somatosensory and bilateral activation in the opercular secondary somatosensory cortex. Random effects group results were significant at p < 0.001 (uncorrected), but activations were thresholded at p < 0.006 for better visibility when projected onto a template-rendered brain. Red and green signify activations related to toe and prepuce. On the single-subject level, most activations reached a significance of p < 0.05, corrected for multiple comparisons within the delineated opercular template volume (dashed lines).

Figure 3.

A modified version of Penfield and Rasmussen's sensory homunculus.