400/1600 ms

O Flicker • No Flicker

Repetitive Stimulation Flicker
0.4 0.8 1.2 1.6 Stimulus duration (s)

200/800 ms

400/1600 ms

FIGURE 7.6 Psychophysical functions obtained with repetitive stimulation (flicker condition) and without repetitive stimulation (no flicker condition) in a temporal bisection task in two anchor duration conditions (200/800 and 400/1600 msec) (Droit-Volet and Wearden, 2002). Top two panels: Data from the 3-year-olds. Center two panels: Data from the 5-year-olds. Lower two panels: Data from the 8-year-olds. (From Droit-Volet, S. and Wearden, J., Q. J. Exp. Psychol., B, 55B, 193-211.)

According to most internal clock models, the overestimation of times obtained in our flicker condition can be explained either by an increase in clock speed or by a decrease in the latency to start the clock. Indeed, in each case, more pulses would be accumulated in the counter and the subjective time would be longer with the repetitive stimulation than without. However, the mathematics of internal clock models predict distinct effects for the clock speed and latency interpretations. For the clock speed explanation, the models predict that the effect should be proportional to the duration being timed, i.e., the overestimation of duration in absolute numbers of seconds would be greater at longer times than at shorter times. For the latency-to-start-the-clock explanation, the models predict an additive effect that is independent of the duration judged; in other words, a constant number of seconds increase in the duration judgment whatever the duration values used. As indicated before, the clearest demonstration that some manipulations can affect the speed of an internal clock comes from pharmacological studies in which stimulant drugs (e.g., methamphetamine) are administrated to rats performing in interval timing tasks (e.g., Maricq et al., 1981; Meck, 1983, 1996). In particular, Maricq et al. (1981) reported data from a temporal bisection procedure in which three different short-long standard duration conditions were used: 1/4, 2/8, and 4/16 sec. In all cases, the administration of methamphetamine shifted psychophysical functions to the left and lowered the bisection points. Furthermore, these shifts were proportional to the intervals being timed with the dose of the drug held constant. This proportional horizontal shift is precisely that required by changes in pacemaker speed, and that we obtained in our experiment with children using Treisman's repetitive stimulation technique. Indeed, in our study, the magnitude of the leftward shift of the bisection points was greater in the 400- to 1600-msec than in the 200- to 800msec duration condition, and this was observed for all age groups. Thus, the flicker increased clock speed in children in a temporal bisection task just as a stimulant drug (methamphetamine) did when administered to rats (Maricq et al., 1981). Our study was then a clear demonstration that the pacemaker-accumulator system is functional at an early age. However, as we will see, another demonstration of changes in clock speed is provided by tests in children of the effect of sensory modality on time perception.

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