PRESETTING AND PREPROGRAMMING IN DONDERS TYPE-C REACTION TASKS

The purpose of the present experiment war to determine what type of pr eparatory adjustment is used by subjects in a Donders' (1868/1969) typ e C reaction task. In accordance with Lepine et al.'s (1989) suggestio n, two preparation mechanisms can be distinguished. The first, called preprogramming, refers to a structural interpretation of motor prepara tion. The second, called presetting, refers to a functional interpreta tion of motor preparation, i.e., to the energy supplied to the informa tion processing stages. To infer the type of preparatory adjustment th at subjects use in a Donders' type C task, we analyzed reaction time ( RT) on slow and rapid movements as a function of the probability of re sponding. In addition, we attempted to determine whether the differenc e usually observed between the RTs of slow and rapid movements persist ed or disappeared. Subjects took part in a GO versus NO-GO reaction ti me task and carried out hitting movements that differed in mean veloci ty (due to variations in amplitude at a constant duration) and in resp onse probability (.5,.7,.9, and 1.0). Such a task enables subjects to preprogram the required movement velocity. Response probability expres ses the degree of certainty that the preprogrammed response will be pe rformed. The results showed that increasing response probability decre ases the RT of both slow and rapid movements. For response probabiliti es less than 1.0, a persistent difference between slow and rapid movem ents was also observed. This indicates that subjects increased the ext ent to which they preset motor programming processes. Despite the fact that subjects could preprogram the response in all probability condit ions, they did not do so. In the condition in which the response proba bility was 1.0, we did not find any difference between the RTs of slow and rapid movements, showing that subjects fully preprogrammed the re sponse. These results are consistent with a presetting interpretation of response preparation when subjects are uncertain about whether a re sponse should be produced. Response probability seems to be a critical variable in the subjects' choice of which preparation mechanism to us e. The basis for this suggestion is our view that subjects do not rely exclusively on one or the other strategy, but predominantly use one o f the two.