Functional architecture of the somatosensory homunculus detected by electrostimulation

• Published in: The Journal of physiology 2018-03, Vol.596 (5), p.941-956
• Main Authors: Roux, Franck‐Emmanuel, Djidjeli, Imène, Durand, Jean‐Baptiste
• Format: Article
• Language: English
• Subjects: Brain architecture; Brodmann's area; Cognitive science; Cortex (somatosensory); electrical stimulation; Finger; Functional morphology; Homunculus; Neuroscience; Outbreaks; Postcentral gyrus; Research Paper; somatosensory systems; Tongue
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/JP275243

Key points We performed a prospective electrostimulation study, based on 50 operated intact patients, to acquire accurate MNI coordinates of the functional areas of the somatosensory homunculus. In the contralateral BA1, the hand representation displayed not only medial‐to‐lateral, little‐finger‐to‐thumb, but also rostral‐to‐caudal discrete somatotopy, with the tip of each finger located more caudally than the proximal phalanx. The analysis of the MNI body coordinates showed rare inter‐individual variations in the medial‐to‐lateral somatotopic organization in these patients with rather different intensity thresholds needed to elicit sensations in different body parts. We found some similarities but also substantial differences with the previous, seminal works of Penfield and his colleagues. We propose a new drawing of the human somatosensory homunculus according to MNI space. 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