'Subjective lengthening' during repeated testing of a simple temporal discrimination

Three experiments investigated behavioural changes, using a temporal generalization paradigm with human subjects, resulting from repeated testing without feedback. In Experiment 1, different groups received five initial presentations of a 400 ms 500 Hz tone, or a 400 ms 14×14 cm blue square, identified as having a standard duration, then received blocks of testing where stimuli with durations shorter than, longer than, or equal to the standard were presented. Subjects had to judge whether each presented stimulus was the standard duration, but no feedback was given. Temporal generalization gradients (proportion of identifications of a stimulus as being the standard, plotted against stimulus duration) shifted progressively to the right during the test phase (i.e. longer stimuli tended to be identified as the standard as testing proceeded) in the visual stimulus condition. Experiment 2 used a generalization procedure to examine, with different subject groups, behavioural changes when either the duration, or the length, of a blue bar presented on the computer screen was the basis of judgement. Across trials, both length and duration could vary, but for one group only duration was relevant whereas for the other group only length was. Generalization gradients shifted systematically to the right only in duration judgements. Experiment 3 replicated the rightward shift in generalization gradients when the duration of visual stimuli was measured, and in addition used a self-rating scale derived from Thayer [Thayer, R.E., 1967. Measurement of activation through self-report. Psych. Rep. 20, 663-678.) to measure subjects' arousal. This declined systematically as testing proceeded, suggesting that the shift in temporal generalization gradient was probably caused by an arousal-induced change in internal clock speed.