Functional architecture of the somatosensory homunculus detected by electrostimulation.
Level 4 - case-series / case-control
Prospective case series of intraoperative direct cortical electrostimulation mapping without a separate comparison group.
PubMed 29285773 · doi:10.1113/JP275243
What was done
Prospective intraoperative direct cortical electrostimulation study in 50 neurosurgical patients with no sensory deficits and no structural lesions in the postcentral gyrus. Individual 3D brain volumes were normalized to standard Montreal Neurological Institute (MNI) space. Investigators tested 647 stimulation sites in Brodmann Area 1 (BA1) and 1,025 sites in adjacent gyri to map elicited somatosensory responses and determine stimulation thresholds.
What was found
Of 647 BA1 stimulation sites, 258 (40%) elicited positive somatosensory responses in the postcentral gyrus, presenting almost exclusively as discriminative touch. Contralateral BA1 hand representation demonstrated both medial-to-lateral (little finger to thumb) and rostral-to-caudal somatotopy, with fingertips located more caudally than proximal phalanges. Tongue representation showed medial-to-lateral (tip-to-base) but no rostral-to-caudal organization. Mean activation thresholds were significantly lower (P < 0.05) for the hand (2.39 mA) and tongue (2.60 mA) than for other body regions. No somatosensory responses were observed outside the postcentral gyrus, and no bilateral sensations or feelings of movement/desire to move were elicited.
Why it matters
This study provides an updated, stereotaxic MNI-space coordinate map of the human somatosensory homunculus, refining classic Penfield drawings by demonstrating intra-digit rostrocaudal somatotopy and localized threshold differences.
Limits
All participants were neurosurgical patients undergoing craniotomy, which may limit generalizability to healthy populations. Spatial resolution is constrained by bipolar stimulation probe spacing. Exact numerical threshold values and variance for non-hand/tongue body areas are omitted from the abstract.