Assessing TMS-induced D- and I-waves with spinal H-reflexes

Transcranial magnetic stimulation (TMS) of motor cortex produces a series of descending volleys known as D- (direct) and I- (indirect) waves. In the present study, we questioned whether spinal H-reflexes can be used to dissect D-waves, early and late I-waves from TMS. We therefore probed H-reflex facilitation at arrival times of D- and I-waves at the spinal level and thereby changed TMS parameters that have previously been shown to have selective effects on evoked D- and different I-waves. We changed TMS intensity and current direction, and applied a double-pulse paradigm known as short-interval intracortical inhibition (SICI). Experiments were conducted in flexor carpi radialis (FCR) in the arm and soleus (SOL) in the leg. There were two major findings: I) In FCR, H-reflex facilitation showed characteristic modulations with altered TMS-parameters that correspond to the changes of evoked D- and I-waves. II) H-reflexes in SOL did not, possibly because of increased interference from other spinal circuits. Therefore, the most significant outcome of this study is that in FCR, H-reflexes combined with TMS seem to be a useful technique to dissect TMS-induced D- and I-waves.

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