An explanation for the presentation-order effect in the method of constant stimuli

The point of subjective equality obtained by the method of constant stimuli depends to a great extent on whether the standard (S) or the variable (V) stimulus occurs first. This presentation-order effect was studied using lines as stimuli. Successive S, V pairs were presented, with inter-stimulus and interpair intervals equal. Observers, who were not told which was S or V, reported whether a given line was longer or shorter than the immediately preceding line. Although the observers' subjective experience was of a train of lines that was not organized into pairs, the presentation-order effect still occurred. This implies that the effect does not depend on the order of presentation of the stimuli in an experienced pair. It was also shown that the observers could categorize line lengths, since they could identify stochastically the most frequent stimulus (S). We propose that the presentation-order effect depends on a decision process based on response probabilities inferred from length categories.