Waiting for the moment to react to an impending stimulus is accompanied by electrophysiological processes in the central nervous system (CNS) that can be measured from the surface of the body. In this chapter we will discuss electroencephalographic (EEG) and electromyographic (EMG) changes related to preparation for a movement. Most of the results originate from fixed foreperiod reaction time (RT) experiments. When a subject is informed about the moment the imperative (reaction) stimulus (RS) will be presented, the RT is shorter than when the subject is not informed. This information is provided by a warning stimulus (WS), with which the foreperiod starts. The WS has to be detected by the subject. Its significance has to be evaluated and a decision has to be made to prepare for the response. This preparation for the response might imply the presence of processes such as time estimation, anticipation, and response programming. The information processing in the CNS which takes place during the foreperiod and which results in the final response is reflected in electrophysiological changes at two different levels of the CNS: the cortex and the spinal cord. Although all kinds of subcortical and brain stem structures play an important role in the preparation for and the execution of movements, the cortical and spinal electrophysiological changes are the only ones which can be recorded from the surface of the body.

Waiting to Respond: Electrophysiological Measurements in Man During Preparation for a Voluntary Movement

In a warned reaction time task blink reflexes were evoked electrically at different points in time during a 3-s foreperiod between the warning signal (WS) and the response signal (RS;. Two groups of 16 well-trained right-handed subjects participated. One group was informed by the warning signal as to whether or not a voluntary blink response was to be given following the response signal. For the other group of subjects, response-relevant information was delivered by the response signal. Reaction times were shorter when subjects were precued by the warning signal, which indicates a more efficient preparation in this condition. In the blink reflex an early ipsilateral component (R1) and a late bilateral component (R2) could be distinguished. Within 300 ms after the warning signal, an increase in R1 was seen which was more pronounced when the warning signal precued the required response. R2 was inhibited shortly after the warning signal. At the end of the foreperiod R1 increased more and R2 was more inhibited the higher the response certainty. The general conclusion is that the presentation of a warning stimulus has a facilitatory effect on R1 and an inhibitory effect on R2. A warning stimulus with more information results in an extra increase in R1 magnitude only, while preparation for a response is seen in both R1 and R2 magnitudes.

Blink reflex magnitude during the foreperiod of a warned reaction time task: effects of precueing.

The relationship between the size of the first electromyographic (EMG) component of the cutaneous blink reflex (R1) and onset of eyelid closure in human adults was determined in 4 experiments in which R1 size was varied by different means : change in stimulus intensity, paired stimulation, and warning. Two-phase lid movements were frequently seen, with an early small movement followed by a large rapid movement. All experiments showed that larger R1s were associated with shorter latencies of both movements. This covariation was general across participants and was independent of shifts in the excitability of the blink reflex pathways indexed by R1 latency, R2 latency, and R2 area (R2 is the more prolonged, later EMG component). The results indicate that R1 acts first to evoke an early lid movement and second to facilitate eyelid closure by the later R2 burst. Identification of this second behavioral function for R1 aids the interpretation of other findings and encourages its use as a model system.

Functional significance of the early component of the human blink reflex

Blink reflexes were evoked at distinct times during a 3 sec foreperiod (FP) between a warning stimulus (WS) and a reaction signal (RS). Sixteen well-trained, right-handed subjects participated in the experiment. Following RS, they reacted instantaneously with a blink of the right eye. In the electrically evoked blink reflex, an early ipsilateral component (R1) with a latency of $10 msec and a late bilateral component (R2) with a latency of $25-40 msec could be distinguished. The peak-to-peak amplitude of R1 and the integral of R2 were computed. In the beginning of FP, R1 was enlarged as compared to control values. Toward the end of FP, a continuous rise of R1 amplitudes was seen. This increment was most pronounced in the muscle involved in the reaction following RS. R2 was inhibited immediately after WS, while no significant inhibition could be seen near the end of FP. The conclusion is that the facilitation of R1 and the disinhibition of R2 towards the end of FP reflect the preparation on the reaction af...

A. J. W. Boelhouwer

Papers

Changes of human blink reflex magnitude during a three second fixed foreperiod.