Modifying excitability of trigemino-facial reflex circuits with transcranial direct current stimulation

Abstract

s / Brain Stimulation 8 (2015) 378e394 384 Background: Responses to a number of different plasticityinducing brain stimulation protocols are highly variable. However there is little data available on the variability of response to 1 Hertz repetitive transcranial magnetic stimulation (1Hz rTMS). Objective: We tested the effects of 1Hz rTMS over the motor cortex on corticospinal excitability. We also examined whether an individual’s response could be predicted frommeasurements of onset latency of motor evoked potential (MEP) following stimulationwith different orientations of monophasic transcranial magnetic stimulation (TMS). Methods: Thirty-two healthy subjects (mean age 30.3 years, SD 11.0; 13 females) participated in a crossover-design. Baseline latency measurements with different coil orientations and MEPs were recorded from the first dorsal interosseous muscle prior to the application of 900 pulses of 1Hz rTMS with two different stimulus intensities, i.e. 90% resting motor threshold (RMT) and 115%RMT. Thirty MEPs were measured every 5 min for up to half an hour after the intervention to assess after-effects on corticospinal excitability. Results: At the group level, there were no significant aftereffects of 1Hz rTMS for both stimulus intensities: about 50% individuals had only a minor or no response to 1Hz rTMS whereas the remainders had a facilitatory effect to both forms of stimulation. There was no significant correlation between the latency difference of MEPs (anterior-posterior stimulation minus latero-medial stimulation) and the response to 1Hz rTMS. Conclusions: The large variability in response to 1Hz rTMS is in line with similar studies using other forms of non-invasive brain stimulation. These results highlight the importance of individual factors influencing the responsiveness to these protocols.