8. Developmental research
In a recent behavioral study,Nakata and Mitani (2005) demonstrated, that even infants between 6 and 8 months of age are able to distinguish between regular and irregular tone sequences. When testing the average looking times for colored flashes, combined either with regular or irregular sound sequences, infants paid more attention to the flashes when the onset-onset distance was constant (173 ms), but not when this distance was variable.
In the framework of scalar timing theory, usually considered to be the most completely developed model of timing (Allan, 1998; Droit-Volet & Wearden, 2003), recent studies have investigated how temporal discrimination in children changes with age by using the temporal bisection method initially used in animals (Church & Deluty, 1977) and later modified for human adults (Allan & Gibbon, 1991; Wearden, 1991). In the bisection procedure commonly used with human adults, participants receive initial presentations of two standard durations, a short and a long standard duration. Then they are presented with comparison durations including these two standards as well as intermediate stimulus durations. Their task is to classify each presented duration as being more similar to the short or to the long standard duration.
Furthermore, by using different duration ranges, Droit-Volet and Wearden (2001, 2002) succeeded in showing that young children’s bisection performance exhibits the scalar property—that is, the requirement that the standard deviation of time judgements increases in proportion to the mean of the duration judged. These different results demonstrate that young children possess a fundamental ability to represent time, similar to that found in animals and human adults. Therefore, Droit-Volet and her colleagues concluded that a pacemaker–accumulator clock mechanism underlying time representations, like that proposed by scalar timing theory, is functional at an early age (Droit-Volet, 2002; Droit-Volet, Clément, & Wearden, 2001; Droit-Volet & Wearden, 2001, 2002). However, although young children are able to represent time, some age-related changes have been observed in their temporal discrimination behaviour on bisection tasks. For example, the steepness of the slope of the psychophysical functions increased with age, the functions being flatter in the 3- and the 5-year-olds than in the 8-year-olds (for a review, see Droit-Volet, 2003). A measure of this steepness of the psychophysical function and, consequently, an index of temporal sensitivity is the Weber ratio.
In young children, the greater sensitivity to duration for the auditory than for the visual stimuli suggests a sort of a dominance of audition over vision in the processing of time.
In conclusion, our results showed that the auditory stimuli were judged longer than the visual stimuli whatever the age group tested and that this was probably due to the faster clock rate for the auditory stimuli. Thus, these results provided support for the idea that the modality differences in the pacemaker speed were present at an early age. The age-related changes in the modality effect on time judgements would be explained by the temporal sensitivity that was lower for the visual than for the auditory stimuli in young children, related to the development of attentional control that itself affects the variance of the switch latency of the internal clock.