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

Transcranial magnetic stimulation (TMS) over human primary somatosensory cortex (S1) does not produce immediate outputs. Researchers must therefore rely on indirect methods for TMS coil positioning. The 'gold standard' is to use individual functional and structural magnetic resonance imaging (MRI) data, but the majority of studies don't do this. The most common method to locate the hand area of S1 (S1-hand) is to move the coil posteriorly from the hand area of primary motor cortex (M1-hand). Yet, S1-hand is not directly posterior to M1-hand. We localised the index finger area of S1-hand experimentally in four ways. First, we re-analysed functional MRI data from 20 participants who received vibrotactile stimulation to their 10 digits. Second, to assist the localisation of S1-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 experiments mapping the effects of TMS across the scalp on tactile discrimination performance. Fourth, we examined all available neuronavigation data from our laboratory on the scalp location of S1-index. Contrary to the prevailing method, and consistent with systematic review evidence, S1-index is close to the C3/C4 electroencephalography (EEG) electrode locations on the scalp, approximately 7-8 cm lateral to the vertex, and approximately 2 cm lateral and 0.5 cm 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

45 required. The results of many TMS studies of S1-hand need reassessment.