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 (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.