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

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 (EF25mm) and at the individual depth of the motor cortex (EFcortex) determined from MRI-scans. Results: All excitability parameters (MT, EF25mm and EFcortex) correlated significantly with each other (P < 0.001). EFcortex 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 EFcortex (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 EFcortex 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. (C) 2011 Elsevier B.V. All rights reserved.