Somatotopic organization of the white matter tracts underpinning motor control in humans: an electrical stimulation study

The somatotopic organization of the primary motor cortex is well documented. However, a possible somatotopy of the network involved in motor control, i.e., eliciting negative motor phenomena during electrostimulation, is unknown in humans, particularly at the subcortical level. Here, we performed electrical stimulation mapping in awake patients operated for gliomas, to study the distribution of the white matter tracts subserving movement control of the lower limb, upper limb(s), and speech. Eighteen patients underwent awake surgery for frontal low-grade gliomas, by using intraoperative subcortical electrostimulation mapping to search interference with movement of the leg, arm(s), and face. We assessed the negative motor responses and their distribution throughout the tracts located under premotor areas. The corresponding stimulation sites were reported on a standard brain template for visual analysis and between-subjects comparisons. During stimulation of the white matter underneath the dorsal premotor cortex and supplementary motor area, rostral to the corticospinal tracts, all patients experienced cessation of the movement of lower and upper limbs, of bimanual coordination, and/or speech. These subcortical sites were somatotopically distributed. Indeed, stimulation of the fibers from mesial to lateral directions and from posterior to anterior directions evoked arrest of movement of the lower limb (mesially and posteriorly), upper limb(s), and face/speech (laterally and anteriorly). There were no postoperative permanent deficits. This is the first evidence of a somatotopic organization of the white matter bundles underpinning movement control in humans. A better knowledge of the distribution of this motor control network may be helpful in neurosciences and neurosurgery.