Evidence for a subcortical contribution to intracortical facilitation

Intracortical facilitation (ICF) describes the facilitation of an EMG response (motor evoked potential) to a suprathreshold pulse (S2) of transcranial magnetic stimulation (TMS) by a preceding subthreshold pulse (S1) given 10-15 ms earlier. ICF is widely assumed to originate from intracortical mechanisms. In this study, we used spinal H-reflexes to test whether subcortical mechanisms can also contribute to the facilitation. Measurements were performed in the upper limb muscle flexor carpi radialis in 17 healthy volunteers, and in the lower limb muscle soleus in 16 healthy volunteers. S2 given alone facilitated the H-reflex. When S1 preceded S2 by 10 ms, the amount of facilitation increased, compatible with ICF. However, S1 given alone also facilitated the H-reflex, suggesting that it had evoked descending activity even though its intensity was well below resting motor threshold. Across participants, the amount of H-reflex facilitation from S1 alone was proportional to the degree of H-reflex facilitation with combined S1-S2. These results indicate that subcortical mechanisms can contribute to ICF and potentially add to the variability of the ICF measure reported in previous studies.