Locating primary somatosensory cortex in human brain stimulation studies: experimental evidence

Transcranial magnetic stimulation (TMS) over human primary somatosensory cortex (S1) does not produce a measurable output. Researchers must rely on indirect methods to position the TMS coil. The 'gold standard' is to use individual functional and structural magnetic resonance imaging (MRI) data, but this method has not been used by most studies. Instead, the most common method used to locate the hand area of S1 (S1-hand) is to move the coil posteriorly from the hand area of M1. Yet, S1-hand is not directly posterior to M1-hand. Here, we addressed the localisation of S1-hand, in four ways. First, we re-analysed functional MRI data from 20 participants who received vibrotactile stimulation to their 10 digits. Second, to assist localising S1-hand and M1-hand without MRI data, we constructed a probabilistic atlas of the central sulcus from 100 healthy adult MRIs, and measured the likely scalp location of S1-index. Third, we conducted two novel experiments mapping the effects of TMS across the scalp on tactile discrimination performance. Fourth, we examined all available MRI data from our laboratory on the scalp location of S1-index. Contrary to the prevailing method, and consistent with the systematic review, S1-index is close to the C3/C4 electroencephalography (EEG) electrode locations on the scalp, approximately 7-8cm lateral to the vertex, and approximately 2cm lateral and 0.5cm posterior to the M1-FDI scalp location. These results suggest that an immediate revision to the most commonly-used heuristic to locate S1-hand is required. The results of many TMS studies of S1-hand need reassessment.