Effect of individual anatomy on resting motor threshold – Computed electric field as a measure of cortical excitability

Abstract

Introduction Transcranial magnetic stimulation (TMS) is used for assessing the excitability of cortical neurons and corticospinal pathways by determining the subject-specific motor threshold (MT). However, the MT is dependent on the TMS instrumentation and exhibits large variation. We hypothesized that between-subject differences in scalp-to-cortex distance could account for the variation in the MT. Computational electric field (EF) estimation could theoretically be applied to reduce the effect of anatomical differences, since it provides a more direct measure of corticospinal excitability. Methods The resting MT of the thenar musculature of 50 healthy subjects (24 male and 26 female, 22–69 years) was determined bilaterally at the primary motor cortex with MRI-navigated TMS using monophasic and biphasic stimulation. The TMS-induced maximum EF was computed at a depth of 25 mm from the scalp (EF 25 mm ) and at the individual depth of the motor cortex (EF cortex) determined from MRI-scans. Results All excitability parameters (MT, EF 25 mm and EF cortex) correlated significantly with each other ( p < 0.001). EF cortex at MT intensity was 95 ± 20 V/m for biphasic and 120 ± 24 V/m for monophasic stimulation. The MT did not correlate with the anatomical scalp-to-cortex distance, whereas the coil-to-cortex distance was found to correlate positively with the MT and negatively with EF cortex ( p < 0.05). Discussion In healthy subjects, the scalp-to-cortex distance is not a significant determinant of the MT, and thus the use of EF cortex does not offer substantial advantages. However, it provides a purposeful and promising tool for studying non-motor cortical areas or patient groups with possible disease-related anatomical alterations.