Variations in corticomotor excitability in response to distal focal thermal stimulation

This study investigated the effects of thermal stimulation on corticomotor excitability with transcranial magnetic stimulation (TMS).

Participants consisted of healthy young adults (n = 20) and seniors (n = 15). Each experimental session consisted of a baseline (BL) assessment, followed by a warming and a cooling protocol. At BL, recordings of motor evoked potentials (MEPs) and skin temperature were performed with the index finger covered with a 'neutral' gel pack (24 °C). For warming, the same measurements were performed, but with the index covered with a warmed gel pack (45 °C). The gel pack was kept for 5 min, and the measurements were performed at 1 min during warming and 5 and 10 min post. After a break, participants were tested with the cooling protocol (gel pack 10 °C) by repeating the same sequence as in the warming.

The two thermal protocols induced the desired range of skin temperatures (warming = 35-45°; cooling = 13-24°). For MEP modulation, the primary analysis revealed no main effects or interactions, owing to the variability of responses to either warming or cooling stimulation. Further analysis of individual responses revealed that modulation, when present, was short-lasting and was characterized by a depression in about half of the participants. Facilitation was also observed, but only in smaller clusters, especially with cooling (13/35). Modulation in MEP amplitude did not correlate with changes in skin temperature.

These results are consistent with previous reports regarding variability in response to sensory stimulation protocols. In the case of thermal stimulation, such variability likely reflects individual differences in the influences exerted by thermal afferents centrally.