Neuromagnetic evidence of the foot primary somatosensory area in the frontal cortex.

OBJECTIVE

The location of the foot primary somatosensory area (SI) is puzzling in contrast to the parietal localization of the hand SI. Electrical cortical stimulation studies have revealed the foot somatosensory function in the paracentral lobule anterior and posterior to the central sulcus (CS). A recent study of intraoperative somatosensory evoked potentials (SEPs) using posterior tibial nerve (PTN) stimulation revealed frontal localization of the foot SI. However, most previous studies using scalp SEPs and somatosensory evoked magnetic fields (SEFs) for PTN found that the initial cortical components (P40/P40m) were localized in the parietal lobe. This study evaluated the location of the equivalent current dipole (ECD) for PTN-SEFs on individual magnetic resonance imaging (MRI) in normal adults.

METHODS

PTN-SEFs were recorded in 16 hemispheres of 8 normal subjects. The P40m ECD was superimposed on individual anatomical MRIs.

RESULT: Mean ECD location of the P40m was 3.1 ± 4.2 mm (mean ± standard error) anterior and 6.9 ± 4.2 mm inferior to the inferior end of the CS.

CONCLUSIONS

Our findings suggest frontal localization of the human foot SI.

SIGNIFICANCE: The functional anatomy of the human foot SI cannot be established by extrapolation of the hand SI.