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 transcra nial magnetic stimulation (TMS) of the motor cortex, cortico-spinal excitab ility was shown to reflect time preparation. Provided that subjects can acc urately estimate time, the amplitude of motor-evoked potentials (MEPs) dimi nish progressively during the interval separating the warning signal from t he response signal (i.e., the foreperiod). On the other hand, several exper iments have demonstrated that the amplitude of the Hoffman (H) reflex elici ted 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 respec tive decrements of II-reflex and MEP amplitude during a constant 500-ms for eperiod. The subjects (n=8) participated in two experimental sessions. In o ne session, H-reflexes were induced in a tonically activated, responding ha nd muscle, the flexor pollicis brevis, at different times during the forepe riod of a visual-choice reaction-time task. In the other session, motor pot entials 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 se ssion. The results show that both II-reflexes and MEPs diminish in amplitud e during the foreperiod, which replicates and extends previous findings. In terestingly, the time constants of the two decrements differed. There was a facilitatory effect of both electrical and magnetic stimulations on the su bject's performance: reaction time was shorter for the trials during which a stimulation was delivered than for the no-stimulation trials. This facili tation was maximal when the stimulations were delivered simultaneously with the warning signal and vanished progressively with stimulation time.