Representation of the ear in human primary somatosensory cortex

We studied 13 healthy subjects with a multichannel magnetoencephalography (MEG) system to investigate the somatotopic representation of the ear in the primary somatosensory cortex (SI). We stimulated three parts of the left ear: the helix, the lobulus, and the tragus. The somatosensory-evoked magnetic fields (SEFs) were successfully measured in 7 of 13 subjects. Short-latency responses were analyzed using both single dipole and multidipole models (brain electric source analysis, BESA). From the single dipole model, the equivalent current dipole (ECD) following the helix stimulation was estimated to be near the neck area of SI in all the subjects. In the lobulus stimulation, the ECDs were estimated around the neck area of SI in four subjects, in the face area in one subject, and in the deep white matter in two subjects. In the tragus stimulation, the ECDs were estimated around the neck area of SI in three subjects, in the hand area of SI in two subjects, and in the deep white matter in two subjects. When the ECDs were estimated to be located in unlikely sites (hand area and deep white matter), a two-dipole model, (1) the neck area of SI and (2) face area of SI, was found to be the most appropriate. Although this might be a preliminary study due to a relatively small number of subjects, it revealed that receptive fields of some part of the ear, such as the lobulus and tragus, might be present in both the neck and face areas of SI. These findings suggested that the "ear area" of SI has variability between subjects, unlike the other areas of SI, possibly because the ear is located on the border between the neck and face.