ABSTRACT
In this chapter, the work on the simple reaction time relevant to the concepts of expectancy and preparation carried out at the University of Helsinki during the last ten years is reviewed. The two main lines of this research involve: (1) studies on the effect of the temporal uncertainty of the imperative stimulus on reaction time (the foreperiods were randomly varied, using either rectangular or Bernoulli distributions of foreperiods); and (2) studies on the effect of imperative-stimulus probability on the reaction time (using within-block constant foreperiods). In the light of the results obtained, expectancy or the momentary subjective probability of the immediate delivery of the stimulus appears to be the most important determinant of preparation and, hence, of the reaction time, but the results also point to the existence of other kinds of determinants. As to expectancy, it follows rather well the momentary objective probability but there also exist interesting dissociation phenomena between the objective and subjective probabilities.
