Mapping of individual somatosensory representations - comparison of fMRI and TMS

Functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) results indicate that primary somatosensory cortical (S1) representations vary between individuals. Here we studied how well somatosensory representations determined using TMS and fMRI correspond.

We used single-pulse TMS and fMRI in 17 healthy subjects to map the S1 representation of the tip of the right index finger stimulated with a Braille device. In the TMS experiment, the S1 site at which tactile sensation was blocked by TMS was considered the S1 representation site (S1_HS) of the fingertip. In the fMRI experiment, passive and oddball tactile tasks were employed.

The locations of S1_HS varied up to 36 mm between subjects. The locations of fMRI peaks 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 S1_HS and the peak fMRI activation was 10 mm in the passive condition and 14 mm in the oddball task.

Both TMS and fMRI results showed considerable inter-individual variability in S1 representations, fMRI having larger spatial variability compared to TMS.

Our findings underscore the importance of multimodal approaches to brain mapping in future studies.