T122. Does TMS primarily target premotor or motor cortex in precentral gyrus?

Abstract

Introduction Transcranial magnetic stimulation (TMS) can be used to map the corticomotor representations of hand muscles in the precentral gyrus. The spatial peak of the corticomotor representations is often not located in the primary motor hand area (M1HAND), but shows an anterior shift towards the caudal part of dorsal premotor cortex (PMd). Here we used magnetic resonance imaging (MRI) to test the hypothesis that the “hand knob” of the precentral gyrus shows different structural properties in individuals with a clear “premotor” representation compared to individuals with a preponderant “primary-motor” representation of hand muscles. Methods Twenty-four volunteers (mean age: 24.3 ± 0.9 SE, 12 women) underwent whole-brain structural MRI and sulcus-shape based TMS mapping of the right precentral gyrus. High-resolution T1-weighted MRIs were used for neuronavigation of TMS and to calculate regional cortical thickness of the precentral gyrus. We also performed quantitative multiparameter mapping of the longitudinal relaxation rate (R1 = 1/T1) as an index of cortical myelination and simulated the electric field strength induced by TMS at motor threshold (—E—) in the hand-knob region (SimNIBS software). Sulcus-shape based TMS mapping was used to obtain mediolateral and posterior-anterior corticomotor excitability profiles of the left abductor digiti minimi and first dorsal interosseus muscles. Results In 14 out of 24 individuals (58%), TMS mapping disclosed a clear spatial peak in the PMd (referred to as “PMd” subjects”), whereas the remaining 10 subjects (42%) showed maximal motor responses more posteriorly in M1HAND (referred to as M1HAND subjects). Motor threshold was not different between the two groups (p = 0.265). Compared “PMd” subjects, “M1” subjects displayed a higher electric field strength in the very rostral part of the M1HAND in the transition zone from the wall to the crown of the precentral gyrus (p = 0.01). In addition, “PMd” subjects showed an overall increase of regional myelination in the precentral gyrus relative to the “M1” subjects (mixed model ANOVA: F(1,18) = 5.362, p = 0.033). No difference between two groups was evident for curvature and cortical thickness. Conclusion Differences in location of the “motor hot spot” are associated with differences in microstructure in the precentral gyrus and the magnitude of the induced electrical field. The results support the notion of two distinct functional and microstructural phenotypes of corticomotor hand representations in human precentral gyrus. They also underscore the usefulness of electrical field modelling to account for inter-individual differences in the biophysiological effects of TMS in the human brain.