The Impact of Stimulation Intensity and Coil Type on Reliability and Tolerability of Cerebellar Brain Inhibition (CBI) via Dual-Coil TMS.

Cerebellar brain inhibition (CBI) describes the inhibitory tone the cerebellum exerts on the primary motor cortex (M1). CBI can be indexed via a dual-coil transcranial magnetic stimulation protocol, whereby a conditioning stimulus (CS) is delivered to the cerebellum in advance of a test stimulus (TS) to M1. The CS is typically delivered at intensities over 60% maximum stimulus output (MSO) via a double-cone coil. This is reportedly uncomfortable for participants, reducing the reliability and validity of outcomes. This feasibility study investigates the reliability and tolerability of eliciting CBI across a range of CS intensities using both a double-cone and high-powered figure-of-8 coil, the D702. It was expected that the double-cone coil would elicit CBI at intensities upwards of 60%MSO. The range for the D702 coil was exploratory. The double-cone coil was expected to be less tolerable than the D702 coil. CBI was assessed in 13 participants (25.92 ± 5.42 years, six female) using each coil (randomized) over intensities 40, 50, 60, 70, 80%MSO. Tolerability was assessed via visual analog scales. Comparisons across intensities and tolerability were assessed non-parametrically and via a linear model. The double-cone coil elicited CBI at intensities 60, 70, and 80%MSO (p < .05), with suppression elicited at 60%MSO not significantly different to that at higher intensities. CBI was not reliably elicited by the D702 coil at any intensity. The double-cone coil was significantly less tolerable than the D702. A CS of 60%MSO with a double-cone coil provides a balance between the reliability and tolerability of CBI.