Speeding up and (…relatively…) slowing down an internal clock in humans.

Two experiments used click-trains to manipulate the subjective duration of stimuli they preceded, in attempts to demonstrate relative slowing down of the pacemaker of a hypothesized internal clock. Experiment 1 used a pair comparison procedure, where two tones presented on each trial in fact had the same duration. In the conditions of particular interest, the first tone was preceded by clicks (thus putatively timed with a faster clock), the other presented without (thus timed normally). The reverse condition (no-clicks/clicks) was also used. Judgements of the relative duration of the stimuli were shifted in both directions (i.e. first tone longer than second and vice versa) by the manipulation, consistent with relative speeding up and slowing down of the pacemaker. Experiment 2 used the popular bisection method, with 200- and 800-ms tones used as the Short and Long standards for the task. After standard presentations, subjects were required to classify a range of comparison stimuli (from 200 to 800 ms in 100-ms steps) in terms of their similarity to one or the other of the standards. In one condition the comparison stimuli were preceded by clicks (thus timed 'fast') and the standards were presented without clicks (thus timed 'normally'); in another condition the clicks preceded the standards but not the comparisons. The psychophysical function obtained from the bisection procedure shifted in opposite directions with the different manipulations, consistent with both relative 'speeding up' and 'slowing down' of the pacemaker of the internal clock.