In this prospective electrostimulation study, based on 50 operated patients with no sensory deficit and no brain lesion in the postcentral gyrus, we acquired coordinates in the standard MNI space of the functional areas of the somatosensory homunculus. The 3D brain volume of each patient was normalized to that space to obtain the MNI coordinates of the stimulation site locations. For 647 sites stimulated on Brodmann Area 1 (and 1025 in gyri nearby), 258 positive points for somatosensory response (40%) were found in the postcentral gyrus. In the contralateral BA1, the hand representation displayed not only medial-to-lateral and little-finger-to-thumb somatotopy, but also rostral-to-caudal discrete somatotopy, with the tip of each finger located more caudally than the proximal phalanx. We detected a medial-to-lateral, tip-to-base tongue organization but no rostral-to-caudal functional organization. The analysis of the MNI body coordinates showed rare inter-individual variations in the medial-to-lateral somatotopic organization in these patients with intact somatosensory cortex. Positive stimulations were detected through the 'on/off' outbreak effect and discriminative touch sensations were the sensations reported almost exclusively by all patients during stimulation. Mean hand (2.39 mA) and tongue (2.60 mA) positive intensity thresholds were lower (P < 0.05) than the intensities required to elicit sensations in the other parts of the body. Unlike the previous, seminal works of Penfield and colleagues, we detected no sensations such as sense of movement or desire to move, no somatosensory responses outside the postcentral gyrus, and no bilateral responses for face/tongue stimulations. We propose a rationalization of the standard drawing of the somatosensory homunculus according to MNI space.

OBJECTIVE Electrostimulation in awake brain mapping is widely used to guide tumor removal, but methodologies can differ substantially across institutions. The authors studied electrostimulation brain mapping data to characterize the variability of the current intensity threshold across patients and the effect of its variations on the number, type, and surface area of the essential language areas detected. METHODS Over 7 years, the authors prospectively studied 100 adult patients who were undergoing intraoperative brain mapping during resection of left hemisphere tumors. In all 100 cases, the same protocol of electrostimulation brain mapping (a controlled naming task-bipolar stimulation with biphasic square wave pulses of 1-msec duration and 60-Hz trains, maximum train duration 6 sec) and electrocorticography was used to detect essential language areas. RESULTS The minimum positive thresholds of stimulation varied from patient to patient; the mean minimum intensity required to detect interference was 4.46 mA (range 1.5-9 mA), and in a substantial proportion of sites (13.5%) interference was detected only at intensities above 6 mA. The threshold varied within a given patient for different naming areas in 22% of cases. Stimulation of the same naming area with greater intensities led to slight changes in the type of response in 19% of cases and different types of responses in 4.5%. Naming sites detected were located in subcentimeter cortical areas (50% were less than 20 mm), but their extent varied with the intensity of stimulation. During a brain mapping session, the same intensity of stimulation reproduced the same type of interference in 94% of the cases. There was no statistically significant difference between the mean stimulation intensities required to produce interfereince in the left inferior frontal lobe (Broca's area), the supramarginal gyri, and the posterior temporal region. CONCLUSIONS Intrasubject and intersubject variations of the minimum thresholds of positive naming areas and changes in the type of response and in the size of these areas according to the intensity used may limit the interpretation of data from electrostimulation in awake brain mapping. To optimize the identification of language areas during electrostimulation brain mapping, it is important to use different intensities of stimulation at the maximum possible currents, avoiding afterdischarges. This could refine the clinical results and scientific data derived from these mapping sessions.