Short-latency afferent-induced facilitation and inhibition as predictors of thermally induced variations in corticomotor excitability

Recently (Ansari et al., PeerJ 6:e6163, 2018a; Somatosens Mot Res 35:69–79, 2018b), we showed using transcranial magnetic stimulation (TMS) that focal application of innocuous thermal stimuli to the distal hand produced variable responses in terms of motor-evoked potential (MEP) suppression or enhancement. Here, we sought to investigate possible causes of this variability by examining circuits mediating sensorimotor integration and intra-cortical inhibition. Participants (n = 21) first underwent TMS to assess baseline corticomotor excitability by measuring MEPs at rest with the index finger wrapped in a gel pack at room temperature (24 °C). Then, conditioned protocols were applied to assess short-latency afferent inhibition (SAI), short-latency afferent facilitation (SAF) and short-interval intra-cortical inhibition (SICI). Following baseline measures, MEP modulation in response to distal cooling was recorded with the index finger wrapped in a gel pack at ~ 10 °C. At baseline, participants exhibited variable levels of SAI, SAF and SICI. Participant also exhibited variable responses to cooling with about half of them (11/21) showing suppressed excitability and one-third showing enhanced excitability (7/21). A linear regression analysis revealed that SAI and SAF proved to be good predictors of cooling-induced variations in corticomotor excitability but not SICI. These results provide novel evidence linking variations in SAI and SAF with those in corticomotor excitability elicited in response to focal thermal stimulation, suggesting that these markers could be used to predict responses to sensory stimulation protocols.