Effects of electric and magnetic transcranial stimulation on long latency reflexes

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Abstract
The interaction of transcranial electric and magnetic brain stimulation with electrically elicited shortand long latency reflexes (LLR) of hand and fore-arm flexor muscles has been investigated in normal subjects. In the first paradigm, the motor potential evoked in thenar muscles by transcranial stimulation was conditioned by median nerve stimulation at various conditioning-test intervals. At short intervals (electric: 5–12.5 ms, magnetic: 0–7.5 ms) facilitation occurred that corresponded to the H-reflex and at longer intervals (electric: 25–40 ms, magnetic: 22.5–35 ms) there was a facilitation corresponding to the LLR. Electric and magnetic stimulation resulted in a similar degree of facilitation. A second paradigm investigated the facilitation of the forearm flexor H-reflex by a cutaneo-muscular LLR elicited by radial superficial nerve stimulation and transcranial stimulation used separately or together. When electric and magnetic brain stimulation were compared, magnetic brain stimulation was followed by significant extrafacilitation but electric stimulation was not. This result favours an interaction between the afferent volley eliciting the LLR and transcranial magnetic stimulation most likely at supraspinal level.