Closed Eyes and Coil Size -- Effects on Motor Threshold and Intracortical Inhibition, measured with TMS

TL;DR

This study investigates how eye state and coil size affect TMS measurements of motor threshold and intracortical inhibition, finding that eye state does not influence these measures and coil size affects motor threshold but not inhibition.

Contribution

It provides evidence that eye state and coil size variations do not significantly impact TMS measures, supporting consistency in multi-center studies.

Findings

RMT is higher with smaller coils.

Eye state does not affect RMT or SICI.

Coil size affects RMT but not SICI.

Abstract

Rationale: Transcranial magnetic stimulation (TMS)-based measures such as resting motor threshold (RMT) and short interval intracortical inhibition (SICI) are widely employed to study motor cortical and corticospinal tract function, and effects of diseases and drug therapies thereon. However, the effect of key experimental factors, including as eye state (open or closed) or stimulating coil size, remain unclear. As such, it is unknown whether these factors must be kept consistent across multi-center studies, and whether differences in such factors may underpin contradictory findings in existing literature. Materials and Methods: Threshold tracking TMS was employed to measure RMT and SICI (3ms interstimulus interval, conditioning at 70% of RMT) in 21 alert and awake, healthy controls. Motor evoked potentials were recorded from abductor pollicis brevis. Both RMT and SICI were measured under 6 conditions, while eyes were open or closed, using 3 figure-of-eight coils of differing winding diameter. Mixed effects modelling was employed to investigate effects of eye state and coil size on each measure. Results: RMT was found to be significantly higher for the smallest (30BFT) coil compared to both larger (50BFT and 70BF) coils. No difference in SICI was identified across coil sizes. Eye state was not found to affect either RMT or SICI measurements. Conclusions: Measurements of RMT and SICI can be considered comparable if recorded with eyes open or closed, provided the individual is awake and alert. Measurements of SICI recorded with figure-of-eight coils of different size can be considered comparable.