Short-latency subliminal effects of transcranial magnetic stimulation on forearm motoneurones

The H-reflex technique has been used to evaluate the time-course of the effects evoked by transcranial clockwise magnetic stimuli in flexor or extensor carpi radialis motoneurones. In six subjects, magnetic stimulation was applied over the scalp in the focus for the motor response of those muscles. At intensities below motor threshold, a facilitation of the H-reflex started at a conditioning-test interval of -4 ms (i.e. when the magnetic stimulus lagged the test stimulus by 4 ms), reached a peak at about -2 ms and rapidly decayed. At about -1 ms, the decay attained a local minimum, which in three subjects had values indicating the presence of an inhibition. Thereafter, a second facilitatory phase peaked at about +1 ms. By matching the time course with the latency of the cortical muscle action potential (CMAP) evoked by suprathreshold magnetic stimulation, it is inferred that the motoneuronal discharge coincides with the second peak of facilitation and is preceded by 3-4 ms of subliminal excitation. This early effect could be brought to threshold by convergence of a subliminal Ia EPSP, leading to a reduction of the CMAP latency. The early excitatory effects reported above are as fast as those described as following transcranial electrical stimulation, and should likewise be considered as monosynaptic.