Fig. 1.

Locating primary somatosensory cortex in human brain stimulation studies. Evidence for the scalp location of the primary somatosensory cortex representation of the right index finger (S1-index). All coordinates are in centimeters (cm) lateral to (i.e., left of) and anterior to (i.e., forward of) the vertex (Cz), or in Montreal Neurological Institute (MNI) space. Color available at https://osf.io/c8nhj. A: mean motor evoked potential (MEP) amplitude during systematic mapping of the first dorsal interosseus muscle’s primary motor cortex representation (M1-FDI) on the scalp in experiment 2. Black square at Cz(−5.2,0.4): mean location of M1-FDI from all available studies conducted in the laboratory (N = 56); white circles: locations tested. B: probabilistic anatomy of the central sulcus, as estimated from 100 MRI scans. Shading on the brain scan represent the proportion of participants with central sulcus at that location. Dark gray square and cross-hairs: mean reported MNI coordinate for the location of S1-index across 54 functional MRI (fMRI) studies, MNI(−48,−21,50). The graph shows a cross-section along the MNI y-axis for the selected coordinate. The range of likely central sulcus distances along this axis, after transformation of either the whole head (black), or the brain (gray), is 2–3 cm. C: transcranial magnetic stimulation (TMS)-related interference with tactile intensity discrimination in experiment 2 (E2) [N = 12, Auditory d′ – Tactile d′, t(11)-scores] is highest 7 cm lateral and 0.76 cm anterior to the vertex. Thin black contour: uncorrected 1-tailed statistical significance (alpha) threshold (P ≤ 0.05); thick black contour: alpha threshold Bonferroni corrected for 7 locations (P ≤ 0.007); black “target”: maximum tactile interference; square: M1-FDI; white circles: locations tested. D: summary of all scalp locations (mean ± SD) studied in this report and those from a recent systematic review. Circle: S1-index based on individual fMRI-guided TMS neuronavigation; large triangle: mean C3 location on 101 participants’ heads; open squares: estimated mean scalp location targeted for 43 TMS studies using M1-FDI as a reference point (black); 16 TMS studies using M1-thenar as a reference point (mid gray); 21 TMS studies using hand movement as a reference point (light gray); small triangle: estimated mean scalp location targeted for 16 TMS studies using C3 as a reference point; diamond: estimated mean scalp location of M1-FDI according to a meta-analysis. Black cross: mean location of maximum intensity discrimination thresholds in experiment 1; black target: mean location of maximum difference between auditory and tactile intensity discrimination performance in experiment 2. E: fMRI data. Reanalysis of fMRI data from Tamè and Holmes (2016): shading shows the contrast between the right index finger versus all other fingers on the right hand. F: mean tactile intensity discrimination thresholds for 9 participants in experiment 1 (E1). White circles: locations tested. G: mean ± 95% confidence ellipsoids for M1-FDI (superior) and S1-index (inferior) locations, as used by Tamè and Holmes (2016). TMS, transcranial magnetic stimulation. H: scalp landmarks used in the 10-20 electrode positioning system. White circle: vertex; light gray circles: other scalp landmarks and electrode positions, including C3′, often positioned as indicated, at 2 cm posterior to C3, though likely located ~3.6 cm posterior to C3.