Gross-anatomy classification

The somatosensory system as found here lies over four gross-anatomical regions, including the anterior part of the parietal lobe [from Brodmann area (BA) 3a to the ventral and medial intraparietal areas (BA 7)], the posterior part of the frontal lobe (from BA 4 to the anterior end of BA 6), the superior part of the medial wall (from the medial end of the pre- and post-central gyri to the middle cingulate gyrus) and the operculum-insular cortex (from parietal and frontal operculum to temporal operculum through the posterior insula). In order to define these regions precisely, we utilized the above-mentioned parcellation. We classified each of the somatosensory parcellation areas according to the following criteria: the parietal region includes all areas on the lateral surface posterior to the central sulcus, the frontal region includes all areas on the lateral surface anterior to the central sulcus, the medial region includes all areas on the medial wall, and the operculum-insula region includes areas inferior to the parietal and frontal lobes of the operculum and insular cortex (Supplementary Table 1, ‘Neuroanatomical results for a multi-modal parcellation of human cerebral cortex’; Glasser et al., 2016). Note that areas 4, 6 mp and 7 Am (Glasser et al., 2016) on the lateral surface extend into the medial wall and that area 5L is somehow different from Brodmann’s original definition, part of the medial region. Additional parcellation areas that were not included in these anatomical regions as part of the spatial continuous representation were found in the area of the temporo-occipital parietal junction and in the inferior-temporal gyrus (eight in the right hemisphere and seven in the left hemisphere, see Supplementary Table 1). Relative proportions of each body part within the entire somatosensory system and in specific anatomical regions (gross and parcellation areas) were calculated according to the percentage of significant vertices with a given lag value (lips-1, upper limb-2,3, trunk-4,5, lower limb-6,7,8). To estimate the level of confidence, we applied bootstrapping by resampling participants to create 1000 cross-correlation maps. Confidence intervals (CI) correspond to 2.5% and 97.5% percentiles and errors are represented by the standard deviation of body-parts proportions. In addition, we created a schematic drawing of the relative proportions of body parts by modifying Penfield’s S1 homunculus drawn by ‘Cortical Homunculus’, The Homunculus mapper, license CC-BY-NC-SA 4.0 (https://creativecommons.org/licenses/by-nc-sa/4.0/; https://www.maxplanckfl.org/fitzpatricklab/homunculus/science/). Body parts of S1 schematic drawing were rescaled (using Adobe Illustrator^® CS6) following the results presented in Supplementary Fig. 2. The body parts were then reconnected into a modified homunculus schematic drawing. Note that not all body parts were stimulated (e.g. tongue and forehead). Therefore, these body parts are represented according to the schematic figure of S1 homunculus, similar to the illustration by Penfield and Boldrey (1937).