Two experiments with human subjects, using short-duration tones as stimuli to be judged, investigated the effect of the range of the stimulus set on temporal bisection performance. In Experiment 1, six groups of subjects were tested on a temporal bisection task, where each stimulus had to be classified as "short" or "long". For three groups, the difference between the longest (L) and shortest (S) durations in the to-be-bisected stimulus set was kept constant at 400 msec, and the L/S ratio was varied over values of 5:1 and 2:1. For three other groups, the L/S ratio was kept constant at 4:1 but the L-S difference varied from 300 to 600 msec. The bisection point (the stimulus value resulting in 50% "long" responses) was located closer to the arithmetic mean of L and S than the geometric mean for all groups except that for which the L/S ratio was 2:1, in which case geometric mean bisection was found. In Experiment 2, stimuli were spaced between L and S either linearly or logarithmically, and the L/S ratio took values of either 2:1 or 19:1. Geometric mean bisection was found in both cases when the L/S ratio was 2:1, but effects of stimulus spacing were found only when the L/S ratio was 19:1. Overall, the results supported a previous conjecture that the L/S ratio used in a bisection task played a critical role in determining the behaviour obtained. A theoretical model of bisection advanced by Wearden (1991) dealt appropriately with bisection point shifts discussed above but encountered difficulties with stimulus spacing effects.

Four experiments investigated the effect of trains of clicks (usually 5 s long and at 5 or 25 Hz) on subjective duration in humans, as previous research had suggested that such a manipulation would speed up the pacemaker of an internal clock by increasing participants' arousal. The four experiments used temporal generalization, pair comparison of duration, verbal estimation, and production of short durations. In all cases, preceding the durations to be judged by clicks changed their subjective length in a manner broadly consistent with the idea that pacemaker speed was increased, by an average of about 10%.

Data from studies of timing in human participants were reviewed with respect to their conformity to the two scalar properties of timing: mean accuracy and the scalar property of variance. Results reviewed were taken from studies of temporal generalization, temporal bisection, discrimination methods, and "classical" timing tasks such as the reproduction, production, and verbal estimation of duration. Evidence for one or both scalar properties was found in many studies, including those using children and elderly participants, but systematic violations were sometimes noted. These violations occurred (a) when very short durations (< 100 ms) were timed, (b) in situations in which timing tasks varying in difficulty were compared, (c) when classical timing tasks were employed, and (d) in situations where highly practised observers exhibited unusual patterns of variance. A later section attempted to reconcile some of these violations with an underlying scalar-consistent timing mechanism.

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In 3 experiments, the authors compared duration judgments of filled stimuli (tones) with unfilled ones (intervals defined by clicks or gaps in tones). Temporal generalization procedures (Experiment 1) and verbal estimation procedures (Experiments 2 and 3) all showed that subjective durations of the tones were considerably longer than those of unfilled intervals defined either by clicks or gaps, with the unfilled intervals being judged as approximately 55%-65% of the duration of the filled ones when real duration was the same. Analyses derived from the pacemaker-switch-accumulator clock model incorporated into scalar timing theory suggested that the filled/unfilled difference in mean estimates was due to higher pacemaker speed in the former case, although conclusively ruling out alternative interpretations in terms of attention remains difficult.

Children of 3, 5, and 8 years of age were trained on a temporal bisection task where visual stimuli in the form of blue circles of 200 and 800 ms or 400 and 1600 ms duration, preceded by a 5-s white circle, served as the short and long standards. Following discrimination training between the standards, stimuli in the ranges 20-800 ms or 400-1,600 ms were presented with the white circle either constant or flickering. Relative to the constant white circle, the flicker (1) increased the proportion of "long" responses (responses appropriate to the long standard), (2) shifted the psychophysical functions to the left, (3) decreased bisection point values, at all ages, and (4) did not systematically affect measures of temporal sensitivity, such as difference limen and Weber ratio. The results were consistent with the idea that the repetitive flicker had increased the speed of the pacemaker of an internal clock in children as young as 3 years. The "pacemaker speed" interpretation of the results was further strengthened by a greater effect of flicker in the 400/ 1,600-ms condition than in the 200/800-ms condition.

Normal older participants (aged 60-79 years), with known scores on the Culture Fair Intelligence Test, were tested on 4 timing tasks (i.e., temporal generalization, bisection, differential threshold, and interval production). The data were related to the theoretical framework of scalar timing theory and ideas about information processing and aging. In general, increasing age and decreasing IQ tended to be associated with increasing variability of judgments of duration, although in all groups events could be timed on average accurately. In some eases (e.g., bisection), performance differences between the older participants and students nearly 50 years younger used in other studies were negligible. Anecdotes and some experimental findings suggest that time experience and behavior may change with increasing age from adulthood. As usual, in the psychology of time an initial problem is to decide on interrelations between diverse findings that may have no common theoretical basis (Wearden, 1994). For example, various authors have reported an apparent speeding up of phenomenological time experience with age (e.g., "Christmas seems to come round quicker every year"), a change claimed to be sufficiently orderly to warrant mathematical description and reification into "laws" (e.g., Doob, 1971; Lejeune & Pouthas, 1991; Lemlich, 1975), but it is unclear what relation this effect may have to judgments that people of different ages make about the duration of events in laboratory experiments (e.g., those that require the production of time intervals with specific durations). Laboratory studies are few and inconsistent. For example, Feifel (1957) required older and younger participants to produce intervals of 30, 60, 180, and 300 s and found that although productions from both groups underestimated the real-time duration, older participants underestimated it more, a finding not replicated in a similar study by Surwillo (1964), in which no clear age differences were found. McGrath and O'Hanlon (1968) studied the production of longer durations (1-8 minutes) and again found that older participants produced shorter durations, particularly at the shorter time values, and Kline, Holleran, and Orme-Rogers (1980) found the shortest durations produced by older participants when they counted to produce intervals from 30 to 90 s. In one of the few studies to report the variability of