The time-course of preparatory spinal and cortico-spinal inhibition: an H-reflex and transcranial magnetic stimulation study in man.

In a previous study where reaction-time methods were combined with transcranial magnetic stimulation (TMS) of the motor cortex, cortico-spinal excitability was shown to reflect time preparation. Provided that subjects can accurately estimate time, the amplitude of motor-evoked potentials (MEPs) diminish progressively during the interval separating the warning signal from the response signal (i.e., the foreperiod). On the other hand, several experiments have demonstrated that the amplitude of the Hoffman (H) reflex elicited in prime movers diminishes during the foreperiod of reaction-time tasks. The aim of the present study was to compare the time course of the respective decrements of H-reflex and MEP amplitude during a constant 500-ms foreperiod. The subjects (n=8) participated in two experimental sessions. In one session, H-reflexes were induced in a tonically activated, responding hand muscle, the flexor pollicis brevis, at different times during the foreperiod of a visual-choice reaction-time task. In the other session, motor potentials were evoked in the same muscle by TMS of the motor cortex delivered in the same behavioral conditions and at the same times as in the first session. The results show that both H-reflexes and MEPs diminish in amplitude during the foreperiod, which replicates and extends previous findings. Interestingly, the time constants of the two decrements differed. There was a facilitatory effect of both electrical and magnetic stimulations on the subject's performance: reaction time was shorter for the trials during which a stimulation was delivered than for the no-stimulation trials. This facilitation was maximal when the stimulations were delivered simultaneously with the warning signal and vanished progressively with stimulation time.