Functional architecture of the motor homunculus detected by electrostimulation

Key points: We performed a prospective electrostimulation study of the motor homunculus in 100 patients without motor deficit or brain lesion in the precentral gyrus in order to acquire accurate Montreal Neurological Institute (MNI) coordinates of the functional areas. The analysis of 248 body coordinates in the precentral gyrus showed rare inter-individual variations in the medial-to-lateral somatotopic movement organization with quite similar intensity thresholds. Electrostimulation only induced basic and stereotyped movements. We detected a relative medial-to-lateral somatotopy of the wrist/hand/global/individual fingers, with sometimes different sites for an individual muscle or movement. We found some similarities to, but also substantial differences from, the seminal work of Penfield and colleagues. We propose an updated version of the human motor homunculus and of its correlation with the somatosensory homunculus, previously defined in MNI space with a similar brain mapping technique. Abstract: In this prospective electrostimulation study, based on 100 operated patients without motor deficit or brain lesion in the precentral gyrus, we acquired coordinates of the functional areas of the motor homunculus and normalized them to standard MNI space. Among 608 sites stimulated in the precentral gyrus (and 1937 in gyri nearby), 248 positive points (40%) for motor response were detected – 245 in the precentral gyrus. Positive stimulations were detected through the ‘on/off’ outbreak effect, and only basic movements were detected. We found no significant difference in mean intensity threshold between the motor representations of the fingers (1.94 mA), tongue and lower limbs (both 2.0 mA), or face (2.25 mA). In the precentral gyrus, the evoked body movements displayed a medial-to-lateral somatotopy in very small (often <10 mm2) areas. The hand region displayed multiple areas for a specific movement, with areas inducing either global or single-finger movement (with a relative medial-to-lateral somatotopy). Among these tested patients, the somatotopic organization of the intact motor cortex showed little inter-individual variations. Unlike Penfield and collaborators, we evoked no sensations such as sense of movement or desire to move, and only 2% of motor responses outside the precentral gyrus. We propose a rationalization of the standard drawing of the motor homunculus according to MNI space. We found a somatotopic correlation perpendicular to the central sulcus when matching our motor data to those previously obtained for the somatosensory homunculus.