Electroencephalography and Clinical Neurophysiology/Electromyography and Motor Control
Reduced excitability of the cortico-spinal system during the warning period of a reaction time task
Introduction
Reaction times can be shortened if a warning signal is given prior to the imperative stimulus. It is thought that the presence of the warning signal reduces the temporal uncertainty of the imperative signal. Thus, the longer and more variable the interval between warning and imperative signal, the smaller the effect on reaction time (Bock and Lincoln, 1988). Maximum shortening of reaction time occurs when the warning signal precedes the imperative signal by a regular interval of about 0.2–0.5 s. The heightened responsiveness of the reaction system may be due either to shortening of the processing time for identification of the imperative signal, or to increased preparation of the motor response, or to both. In either case, the nervous system has the task of maintaining heightened excitability whilst at the same time preventing escape of the response until the imperative signal is identified correctly.
The aim of the present experiments was to test whether increased excitability of the corticospinal outflow during the warning period contributes to the decrease in reaction time. Previous experiments (Evarts and Tanji, 1976; Tanji and Evarts, 1976) on monkeys suggest very strongly that this should be the case. During the interval between an instructional warning stimulus and a reaction signal, 61% of precentral pyramidal tract neurones changed their discharge according to the nature of the instruction. For example, neurones that usually discharged during the push movement also increased their firing rate if the warning signal indicated that a push movement was about to occur whereas neurones that discharged in relation to the pull movement would decrease their firing. The implication of these results is that the excitability of the motor cortical projection to agonist muscles may be increased during a warning period.
Work on spinal reflexes has extended these concepts. Brunia and colleagues (Brunia and Vuister, 1979; Brunia et al., 1982) examined spinal reflexes during a 4 s warning period, and proposed that three types of reflex changes occur. Immediately after the warning signal, reflexes in both involved and non-involved muscles increase perhaps reflecting arousal produced by the stimulus. Thereafter, the responses in the future agonist may decrease slightly, whereas those in other muscles continue to increase. After the imperative signal has been received there is a clear increase in agonist excitability over and above that seen in other muscles. The persistent increase in spinal reflexes in the warning period was thought to be due to facilitatory corticospinal influences. The late suppression of agonist reflexes in the latter part of the warning period was postulated to be due to presynaptic inhibition of Ia afferents.
There is little information in humans about the effect of a warning signal on excitability of cortical mechanisms. Most studies using transcranial magnetic stimulation (TMS) have not used a warning stimulus and have therefore been limited to the period between the imperative signal and the onset of the voluntary response (Pascual et al., 1992). In one recent study, TMS was delivered in the period between the warning signal and the imperative signal in a reaction time task (Hasbroucq et al., 1997). Responses, which were measured only in the agonist muscle, were decreased during the latter part of the warning period. This suggests a decrease in cortico-spinal excitability during preparation for movement but does not indicate if this change is specific or generalized, or whether it reflects an effect at a cortical or spinal level. Using a different approach, Bonnet (1983)showed that the M2 component of the stretch reflex in wrist muscles increased during a 2 s warning period, and interpreted this as evidence for increased cortical excitability. However, recent work has cast doubt on whether the M2 response in these muscles is a transcortical response as they had assumed (Meyer et al., 1992). It seems more likely that it is a long latency spinal response mediated by slow conducting group II afferents.
The present experiments used TMS to test corticospinal excitability directly during a warning period. We measured responses in agonist, antagonist and contralateral muscles and used H-reflexes to examine reflex excitability changes. We used a short (0.5 s) warning period in order to produce a clear reduction in reaction time, so that any preparatory processes would be functioning at maximum effectiveness. We found that the cortico-spinal excitability as tested by transcranial magnetic stimulation was decreased during the warning period, particularly the projections to the future agonist muscle. Part of this work has been published previously in abstract form (Touge et al., 1993).
Section snippets
Methods
Subjects were 15 normal volunteers (12 male and 3 female, age range 28–44 years) and were studied with informed consent and the approval of the local ethical committee. They sat in comfortable chair with their arm well supported. Every 8 s or so, an auditory warning signal was given (1 kHz, 50 ms duration) via an earphone, and this was followed 0.5 s later by a small cutaneous stimulus (200 μs duration 4 times perceptual threshold) given through electrodes over the back of the left hand. The
Reaction time effect
The reaction time to the imperative signal in conditions 1, 2, 4 and 5 of the simple wrist flexion task is shown in Fig. 2. The reaction time to the imperative stimulus alone was rather long, but this was probably because it was interspersed between a majority of trials in which warnings were given. As expected, the presence of the warning signal speeded up reaction time (compare condition 1 and 4: P<0.05). Interspersing magnetic test stimuli between warning and imperative signals had no effect
Discussion
In the present experiments, a warning signal was given 0.5 s before an imperative stimulus. This reduced the reaction time of a simple wrist flexion/extension task by almost half, from about 400–200 ms. Unexpectedly, magnetic stimulation suggested that during the warning period, the excitability of the corticospinal system to the future agonist muscle decreased rather than increased.
There have been several previous studies in which corticospinal excitability has been examined after the
Acknowledgements
We should like to thank Mr. R. Bedlington for maintaining and constructing much of the equipment used in these experiments.
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