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 co mparison procedure, where two tones presented on each trial in fact had the same duration. In the conditions of particular interest, the first tone wa s 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 w ere shifted in both directions (i.e. first tone longer than second and vice versa) by the manipulation, consistent with relative speeding up and slowi ng down of the pacemaker. Experiment 2 used the popular bisection method, w ith 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 thei r similarity to one or the other of the standards. In one condition the com parison stimuli were preceded by clicks (thus timed 'fast') and the standar ds were presented without clicks (thus timed 'normally'); in another condit ion the clicks preceded the standards but not the comparisons. The psychoph ysical function obtained from the bisection procedure shifted in opposite d irections with the different manipulations, consistent with both relative ' speeding up' and 'slowing down' of the pacemaker of the internal clock. (C) 1999 Elsevier Science B.V. All rights reserved.