[Show abstract][Hide abstract] ABSTRACT: Introduction Studies of somatosensory evoked magnetic fields (SEF) by averaging the signal of magnetoencephalography (MEG) have since revealed the homunculus structure of SI : MEG is a noninvasive technique for the functional mapping of the human brain with high spatial and temporal resolution (in the order of mm and msec). This is accurate enough to estimate the location of the electrical source in the brain. The location of the face area in SI has been studied, mainly by stimulation of the lip, using electrocorticography in humans and monkeys. The location of the face area in SI has been also investigated by MEG [2,3], by positron emission tomography (PET) , and by functional magnetic resonance imaging (fMRI)  in humans. However, there have been no reports on stimulation of the ear. The objective of this study was to investigate the location of the ear area in SI. To our knowledge, this is the first systematic study of SEF following electrical stimulation of the ear .
[Show abstract][Hide abstract] ABSTRACT: 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.