Evidence of two separated processes in the production of hand/foot simultaneous responses

Jacques LaRue

Université de Caen, France

The reactive-projective model (Paillard, 1947, 1990) stated that a mental preparation is needed to get simultaneous responses of the hand and the foot. Indeed, it is mandatory to compensate for the longer peripheral motor conduction delay of the foot. In a reaction time situation, no compensation occurs; the hand responds before the foot; this is consistent with a single motor command released for both effectors. In a self-initiated condition, the foot tends to precede the hand suggesting that two distinct motor commands are issued, with the foot command first. Fully self-initiated movements are not usual. It is more usual to prepare a response in anticipation of the time occurrence of a stimulus (e.g. a musician following a conductor, synchronized swimmers emerging together with the music).

We first verified whether the model holds in an anticipation coincidence task. The task consisted in synchronizing the hand/foot responses with the passage of a mobile (constant speed) over a mark. Since the results fit the model, we used this set-up in a second study to provide new evidences of the timing of the motor commands. Anticipation-coincidence task were performed in three conditions: using the foot (1) or the hand (2) alone; the hand and the foot simultaneously (3). Following a constant stimuli protocol, short tones were randomly produced, prior the stimulus. The tones were presented in only 17% of the 1080 trials. There were eighteen predefined periods between the tone and the anticipated stimulus. When a tone occurred, the subjects had to inhibit their response. The frequencies of correct inhibition in each preset periods follows a logistic curve. The command release is assumed to occur at the bi-serial point (50% of inhibition). The results confirmed that the motor command of the foot is released sooner than the command of the hand. The hand/foot delay is lower in the simultaneous condition because the command of the hand is released 40 ms earlier; while the foot command is 10 ms earlier. These data confirm and extend the projective-reactive model to a new category of coordination behavior.