Mapping of individual somatosensory representations - comparison of fMRI and TMS

Invasive brain mapping, functional magnetic resonance imaging (fMRI) and navigated transcranial magnetic stimulation (nTMS) results indicate that somatosensory representations vary between individuals. However, it is unknown how well somatosensory representations determined using nTMS and fMRI correspond. Here, we used single-pulse nTMS and fMRI in 17 right-handed subjects to determine the S1 representation of the tip of the right index finger stimulated mechanically with a Braille device. In an nTMS mapping experiment, the S1 site at which tactile sensation was blocked by nTMS, was considered the S1 representation site (S1HS) of the fingertip. The location of S1HS varied up to 36 mm between subjects. In the fMRI experiment, passive and oddball tactile tasks were employed. The location of the somatosensory peak fMRI activation varied up to 39 mm between subjects in the passive condition and up to 84 mm in the oddball condition. Within subjects, the mean distance between the S1HS and the peak fMRI activation was 10 + 2 mm (S.E.M.) in the passive condition and 14 + 3 mm in the oddball task. Taken together, our findings underscore the importance of multimodal approaches to brain mapping in future studies.