A warning signal that precedes an imperative stimulus by a certain length of time (the foreperiod) can accelerate responses (foreperiod effect). Plotting reaction time (RT) as a function of foreperiod results in a “U”-shape curve when the foreperiod is fixed in a trial block but manipulated across blocks. When the foreperiod is varied within a block, the foreperiod-RT function is usually negative, with the foreperiod effect modulated by both the current foreperiod and the foreperiod in the prior trial (sequential foreperiod effect). This sequential effect was found to be robust at the shorter foreperiod while diminished at the longer foreperiod. Capizzi et al. (2015) used a non-aging foreperiod distribution and found an increasing foreperiod-RT function (consistent with that in a fixed-foreperiod paradigm) and a sequential effect equal for different foreperiods. They thus proposed a repetition priming account for the sequential foreperiod effect. I conducted three experiments, aiming to test this repetition priming account and to rebuild the connection between the fixed- and variable-foreperiod paradigms. Experiment 1 attempted to replicate Capizzi et al. in a choice-reaction task scenario and found an increasing foreperiod-RT function but a larger sequential effect at the shorter foreperiod. Experiment 2 examined the priming account in a short-foreperiod context and found a decreasing foreperiod-RT function with a larger sequential effect at the shorter foreperiod. Experiment 3 detected a larger sequential effect in general by increasing the difference in duration between the foreperiods that were used in Experiment 2. The current study provided converging evidence that with a non-aging foreperiod distribution the foreperiod-RT function in a variable-foreperiod paradigm shares the same direction as that in a fixed-foreperiod paradigm. However, instead of following Capizzi et al.’s account, the size of the sequential foreperiod effect in general was found to be modulated by the difference in duration between the foreperiods while the relative sizes were determined by the proportions of different foreperiods.

Due to various shortcomings of the visual system, some visual stimuli can only be identified with 100% accuracy if they are shown for a certain amount of time. This time can be measured using the Inspection Time (IT) paradigm. In an IT task, a “pi” figure with differing leg lengths is typically presented briefly (e.g., 20-200 ms) and is then immediately masked to prevent retinal afterimages. Participants are subsequently required to choose which of the two legs was longer. The objective of this task is to determine the shortest amount of time the pi figure needs to be shown for it to be perceived with 80% accuracy. Given that visual processing has been shown to be altered during and /or prior to a movement, the present experiment sought to test how the requirement to perform a motor task affected IT. Twenty-eight participants took part in the experiment, which was comprised of three conditions: no-movement (NM), peak velocity (PV), and foreperiod (FP). In the NM condition, participants grasped a manipulandum and engaged in the IT paradigm. At the end of every trial, participants verbally stated which leg they believed was longest. In the PV condition participants made a rapid movement to a target, and the IT stimulus was presented when their limb reached peak velocity. Finally in the FP condition the IT stimulus was presented during foreperiod (FP). In all three conditions the IT stimulus was randomly presented from between 15-105 ms (in 15 ms increments) and masked for 400 ms. Results showed no significant differences on the IT task between the NM and FP conditions, suggesting no visual upregulation during foreperiod. However, IT performance was significantly poorer in the PV condition in comparison to both the NM and FP condition, suggesting a visual downregulation at that particular movement kinematic.

L’objectif de ce travail a été d’analyser les effets d’une préparation motrice sur l’activité des motoneurones (MN). Pour cela, des protocoles expérimentaux combinant l’enregistrement unitaire de l’activité des unités motrices des muscles extenseurs du poignet avec des paradigmes de préparation motrice de nature temporelle ont été utilisés chez l’Homme. L’analyse des caractéristiques de la décharge tonique des MN montre un allongement des intervalles inter-potentiels associé à une diminution de leur variabilité durant la période préparatoire, bien avant que la réponse motrice ne soit déclenchée. Ces changements démontrent clairement l’implication de mécanismes inhibiteurs spinaux au cours de la préparation motrice pouvant s’exercer au travers d’interneurones prémotoneuronaux. Il est montré que les modulations d’activité motoneuronales induites par la préparation motrice ne sont ni spécifiques au muscle effecteur de la réponse motrice ni prédictifs de la performance. Il est proposé que l’inhibition exercée sur les MN pendant la préparation motrice constitue un mécanisme généralisé de frein pour retenir le déclenchement prématuré de la réponse motrice, tandis que la diminution de variabilité dans la décharge serait un phénomène de compensation, permettant de produire des forces stables malgré la désactivation motoneuronale. L’implication du niveau motoneuronal dans la préparation motrice montre qu’une information au préalable influence l’état du système moteur jusqu’à son élément le plus périphérique, supportant ainsi le caractère hautement distribué des processus préparatoires. Ce travail a également conduit à proposer l’utilisation de l’entropie approximative pour l’analyse de l’activité motoneuronale, permettant d’éviter les écueils liés aux méthodes classiques d’analyse tout en respectant l’hypothèse d’un codage neuronal temporel. / The aim of this work was to analyze the effects of motor preparation on motoneuron (MN) activity. For this purpose, recordings of wrist extensor muscles motor unit activity were combined with time motor preparation paradigms in Human. Changes in the MN tonic discharge were found to occur during preparatory period, i.e. well before it is time to act. These changes were a lengthening of the mean inter-spike interval associated with a decrease of its variability. These data clearly demonstrate that spinal inhibitory mechanisms are activated during motor preparation and suggest the involvement of premotoneuronal interneurons. The modulations of motoneuronal activity induced by the motor preparation are neither specific to the agonist muscle involved in the motor response nor predictive of the performance. It is assumed that the inhibition acting on the MN during the motor preparation constitutes a general braking mechanism serving to prevent premature motor response, whereas the decrease of discharge variability would be a compensatory phenomenon, allowing to produce an efficient steady force in spite of lower motoneuronal activation. The involvement of the motoneuronal level in motor preparation demonstrates that advance information may influence the state of the motor system, including even the most peripheral motor neurons in the spinal cord, which supports the idea that motor preparation involves highly distributed functional processes. In addition, this work led us to argue in favor of the approximate entropy analysis as a suitable method for analyzing spike trains, allowing to detect changes in the regularity of the time-ordered inter-spikes intervals.