Characterisation of paired-pulse transcranial magnetic stimulation conditions yielding intracortical inhibition or I-wave facilitation using a threshold-hunting paradigm

Short-interval, paired-pulse transcranial magnetic stimulation (TMS) is usu ally used to demonstrate intracortical inhibition. It was shown recently th at with short-interval, paired-pulse TMS a facilitation - called intracorti cal I-wave facilitation - can also be demonstrated. It was the aim of this study to investigate which stimulus conditions lead to intracortical inhibi tion and what conditions yield an intracortical I-wave facilitation in a ha nd muscle of normal subjects. Paired-pulse TMS responses with an interstimu lus interval of 1.2 ms were obtained from the abductor digiti minimi muscle of four normal subjects. A threshold-hunting paradigm with hunting through first or second stimulus variation was used to obtain a curve of threshold -pair strengths. All subjects showed two branches of stimulus interaction o n this diagram. If the first stimulus of a threshold pair was below approxi mately 65% of resting motor threshold it modified the response primarily du e to the second stimulus through intracortical inhibition. However, if the first stimulus of a threshold pair exceeded approximately 65% of resting mo tor threshold it became responsible for the spinal action-potential initiat ion. The subsequent second stimulus served as a "booster" for the ongoing i ntracortical I-wave activity, making it impossible to observe the intracort ical inhibition evoked by the first stimulus.