During phylogenesis, environmental constraints have lead several animal species to develop different abilities to cope with the immediate future. Among these abilities is preparation, which has been conceptualized as a cognitive mechanism for dealing with uncertainty. In the context of the reaction time (RT) paradigm, an efficient way to study preparation is to inform the subject about the upcoming signal and response. This is generally done by introducing a precue, which provides advance information about either the timing of the signal or identity of the signal-response (S-R) pair. The subject uses the temporal information to be ready at the right time and the identity information to prepare for specific events. The changes in RT, respectively related to time and event information allow the investigator to draw inferences about the corresponding preparatory processes. In what follows, we shall use the terms “time preparation” and “event preparation” to refer to each of these processes, respectively.

Time preparation has been extensively investigated through the study of the effect of foreperiod duration (i.e., the interval between the precue and the response signal) on RT. These studies have shown that this process is fatigue-sensitive. Because time preparation can be optimal for a few milliseconds only, it is set according to the subject’s expectation concerning the occurrence of the response signal. When the duration of the foreperiod is varied across blocks of trials, the subject times his (her) preparation so he (she) can be optimally ready at the onset of the response signal. However, RT lengthens as the duration of the foreperiod is increased, because the accuracy of this timing decreases as the time elapsed since the precue.

Studies combining reflex techniques with RT protocols have revealed neural correlates of time preparation. They have shown that the subjective state of readiness induces systematic variations in the excitability of peripheral and central motor structures. When the duration of the foreperiod is constant within a block of trials, the amplitudes of monosynaptic reflexes (Hoffmann (H) and tendinous (T)), triggered in a prime mover, decrease prior to the onset of the response signal. This reflex depression is thought to reflect the presynaptic inhibition of the motoneurons’ somesthetic afferents, which increases the sensitivity of the motoneuronal pool to supraspinal commands. In contrast, in similar experiments, the amplitude of the long latency reflex responses, which are often considered to reflect the excitability of the central structures, has been reported to increase during the foreperiod. Thus, time preparation seems to be simultaneously implemented by a progressive inhibition of the spinal structures and by a progressive increment of central excitability.

Event related potential (ERP) investigations have also documented the neural mechanisms of time preparation. Walter, Cooper, Aldridge, McCallum and Winter (1964) first described a sustained negative change in brain potentials during the foreperiod: the contingent negative variation (CNV). This composite wave, the amplitude of which is maximal at the vertex, has proved to be a meaningful electrophysiological index of time preparation. Indeed, as stressed by Macar and Bonnet (1997), the CNV develops as soon as the object attempts to associate two events, the second (e.g., the response signal) being expected because it follows the first one (e.g., the precue) after a definite delay. In terms of the threshold regulation theory, the development of the CNV during the foreperiod could manifest an increase of excitability of the underlying neural structures, an interpretation in line with the conclusions drawn from reflex investigations.

Event preparation is often studied by means of the movement-precuing procedure, in which a precue provides advance information about the identity of the upcoming movement (and signal) in a choice-RT task. Here, each possible event corresponds to one of the S-R alternatives and the precues reduce uncertainty about which event will next occur. For example, both Proctor & Reeve, 1986 and Osman et al., 1995 precued two of the four possible S-R alternatives in a task where the location of a visual signal specified which finger (FI) to respond with. The event preparation in this task presumably occurred during the foreperiod and could have speeded a variety of processes during the RT interval, e.g., location of the signal, translation between signal location and response FI or specification of anatomical parameters of the response. In the present paper, the emphasis will be on those effects of event preparation that involve motoric processes.

Several ERP studies performed in the framework of the movement-precuing procedure have demonstrated that the amplitude of the CNV is sensitive to event preparation. Thanks to topographical methods, they have further established that several cortical areas are differentially involved in the processing of the information conveyed by the precue. Bonnet & MacKay, 1989 and MacKay & Bonnet, 1990 presented evidence for an involvement of the parietal cortex in the specification of the direction of the to-be-completed response movement. Vidal, Bonnet and Macar (1995) demonstrated the role played by the supplementary motor area in the preparation of the duration of the forthcoming movement, whilst MacKay and Bonnet (1990) showed that the primary motor cortex is involved in the specification of the response force.