Functional magnetic resonance imaging evidence for a representation of the ear in human primary somatosensory cortex: comparison with magnetoencephalography study.

Our previous study (T. Nihashi et al., 2001, Neuro- Image 13: 295-304), using magnetoencephalography (MEG), revealed somatotopy of the ear in the human primary somatosensory cortex (SI); that is, the signals following stimulation of the ear reach both the neck and face areas of the SI. However, since this was the first report on somatotopy of the ear in humans, we performed an fMRI activation study to confirm the somatotopic representation, and compared the electrical activity by MEG and the cerebral blood flow change by fMRI. We studied eight healthy subjects using 3-T MRI. We stimulated three parts of the left ear: the helix, the lobulus, and the tragus. First, we identified the location of the ear area in the SI based on our previous MEG study, in which equivalent current dipoles (ECDs) were located in the neck and/or face areas of the SI. Then, we determined the search volume as a sphere with a 15-mm radius, which was placed in the neck and/or face area. We analyzed whether or not fMRI activation occurred inside such spheres. Stimulation of the helix activated the neck area of the SI in four of eight subjects, and both the neck and face areas in two. No activation was observed in two subjects. Stimulation of the lobulus activated the neck area in one subject, the face area in two, both in four, and neither in one. Stimulation of the tragus activated the face in four, both in three, and neither in one. These fMRI findings confirm the result of MEG that the representation of the ear in the SI is separated into neck and face areas.