Alerting attention and time perception in children

Transcription

1 J. Experimental Child Psychology 85 (2003) Journal of Experimental Child Psychology Alerting attention and time perception in children Sylvie Droit-Volet * Laboratoire de Psychologie Sociale de la Cognition, CNRS (UMR 6024), Blaise Pascal University, Clermont II, 34 avenue Carnot, Clermont-Ferrand Cedex FR-63000/63006, France Received 8 March 2003; revised 10 June 2003 Abstract This experiment investigated the effect of a signal (i.e., a click) warning of the arrival of a stimulus to be timed on temporal discrimination in children aged 3, 5, and 8 years (N ¼ 86), using a bisection task with visual stimuli ranging from 0.5 to 2 s or 1 to 4 s. In all groups, the psychophysical functions were orderly with the proportion of long responses increasing with the stimulus duration, although the steepness of functions increased with age. Stimulus durations were judged to be longer with than without the click in all age groups. With the click, time sensitivity also improved and more particularly in the younger children. The statistical results suggest, within the framework of the scalar timing theory, that the click reduces the closing latency of the switch that connects the pacemaker to the accumulator in the internal clock and also reduces its trial-by-trial variability. Ó 2003 Elsevier Science (USA). All rights reserved. Keywords: Timing; Time perception; Attention; Children Introduction For several decades, most developmental psychologists, influenced by the Piagetian theory, have investigated the ability to judge time in children as the result of the development of the capacity to reason logically about time. However at the same time, data collected from animals and human adults support the idea that a basic internal clock mechanism is involved in temporal judgments. In the attempt to achieve a unified theory, a few psychologists have therefore decided to attempt to reconsider * Fax: address: Droit@srvpsy.univ-bpclermont.fr /$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi: /s (03)

2 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) timing in children with the methods applied in animal research, within the framework of the internal-clock based models (e.g., Gibbon, 1977; Gibbon, Church, & Meck, 1984). One of the methods used with children is the bisection task. In this task, children are initially trained to discriminate a short from 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 the long standard duration. In bisection, whatever the age and the duration group tested (shorter or longer than 1 s), children have been observed to produce orderly psychophysical functions with a proportion of long responses (i.e., duration classified as more similar to the long than the short standard) that increases with the stimulus duration values (i.e., Droit-Volet, Clement, & Fayol, 2003; Droit-Volet & Wearden, 2001, 2002; McCormack, Brown, Maylor, Darby, & Green, 1999; Rattat & Droit-Volet, 2001). Furthermore, young childrenõs bisection performance exhibits the scalar property of timing, which is characteristic of timing in animals and human adults (Droit-Volet & Wearden, 2001, 2002). The scalar property of timing is a form of conformity to WeberÕs law, which is a linear relation between the standard deviation of subjective time and the mean subjective time (i.e., a constant coefficient of variation). Overall, these results demonstrate that young children possess a fundamental ability to represent time similar to that of animals and human adults. Therefore, in a recent review, Droit-Volet (2003) concluded that a pacemaker-accumulator clock system similar to that described in the internal-clock based models is functional at an early age. However, although young children are able to represent time, there are age-related changes in their bisection behavior. For instance, there is an increase in the slope of the psychophysical functions between 3 and 8 years, the age at which it becomes similar to adult values. This reveals that the sensitivity to duration increases with age. The Weber ratio is another measure of this steepness of the psychophysical function, and therefore, of time sensitivity. It is calculated by dividing the difference limen (half the difference between the stimulus duration giving rise to 75% long responses and that giving rise to 25% long responses) by the bisection point (stimulus duration giving rise to 50% long responses). High Weber ratios indicate low temporal sensitivity and low Weber ratio high temporal sensitivity. Consequently, the 3- and 5-year-olds obtain lower Weber ratios than the older children. According to the scalar timing theory, which is the most completely developed theory of animal and human timing (Allan, 1998; Droit-Volet & Wearden, 2003; Meck, 2003; Wearden, 1999), temporal judgment depends on an internal clock system as well as on memory and decisional processes. Consequently, to take account of the developmental changes in bisection performance, Droit-Volet and Wearden (2001) proposed a developmental version of the scalar timing models with 3 different parameters: (1) a coefficient of variation of the long term memory representation of the standard durations, (2) a decisional bias value toward making long responses in ambiguous cases, and (3) a proportion of random responses. This model captured childrenõs bisection performance reasonably well, and found that two out of these three parameters can explain age-related changes in bisection performance: the

3 374 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) proportion of random responses, and the coefficient of variation of the memory representation of the standard. Indeed, the 3- and the 5-year-olds produced more random responses regardless of the duration values than the 8-year-olds. Furthermore, the coefficient of variation of the memory representation of the standard durations was greater in the younger children. Recent experiments have suggested that this fuzzier representation of time in memory could, to a large extent, be attributed to limited control in young children of the attention necessary for the memory encoding of duration (e.g., Block, Zakay, & Hancock, 1999; Gautier & Droit-Volet, 2002a, 2002b). In particular, young children are easily distracted by the external environment and this might delay or/and disrupt the processing of temporal information. The scalar timing theory proposes that the internal-clock system consists of a pacemaker, a switch and an accumulator. The onset of the stimulus to be timed closes the switch connecting the pacemaker to the accumulator, thus allowing the pulses emitted by the pacemaker to enter the accumulator. However, the switch does not close as soon as the stimulus begins, but after a short period of latency (Meck, 1984). This switch opening latency can be modulated by attention (for a review see Lejeune, 1998). Some studies have shown that reduced attention delays switch closure, and thereby shortens the perceived duration (e.g., Fortin, 1999; Macar, Grondin, & Casini, 1994). Other studies have shown that reduced attention also increases the variability of switch latency, thus reducing time sensitivity (Allan, 1992; Whiterspoon & Allan, 1985). To be able to capture a visual stimulus as soon as it occurs in order to process its duration, participants have to orient their attention toward the physical source of presentation of this target stimulus (computer screen) and to sustain focused attention until it starts. This cognitive activity is an important function of attention, which is called alertness (Posner & Boies, 1971; Posner & Peterson, 1990). Although there are only a few studies of the characteristics of the different attentional systems between 1 and 7 years of age, it is generally accepted that the ability to develop and sustain alertness to process a priority event improves during childhood as the level of distractibility falls (Bjorklund & Harnishfeger, 1995; Ruff & Lawson, 1990; Ruff & Rothbart, 1996). Young childrenõs difficulties in maintaining constant control over alerting attention may therefore, increase the trial-by-trial variance of the latency of switch closure. Consequently, we may assume that young childrenõs lower sensitivity to duration in a bisection task might be in part due to the amount of noise produced by the switch closing of the internal clock. The purpose of the present study was to investigate the developmental changes in the amount of noise or variability in the switch mechanism at the onset of the stimulus to be timed. No studies have tried to directly manipulate the switch mechanism in children. However, recently, Droit-Volet, Tourret, and Wearden (2003) compared time bisection performance in children from 5 to 8 years of age with durations presented either in a visual or an auditory modality. They found that, in all age groups, the time estimates were shorter with the visual than with the auditory stimuli, as has also been found in human adults (e.g., Penney, Gibbon, & Meck, 2000; Wearden, Edwards, Fakhri, & Percival, 1998). Furthermore, in the youngest children, the time estimates were also more variable with the visual stimuli. Therefore, Droit-Volet et al.

4 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) explained this lower temporal sensitivity in terms of the greater variability in the switch latency with visual stimuli, because the alerting of visual stimuli requires more effort than that of auditory stimuli (Posner, Nissen, & Klein, 1976; Turatto, Benso, Galfano, & Umilta, 2002). In Droit-Volet et al.õs (2003) study, as in the other studies of children or adults that have used visual stimuli for bisection tasks, each comparison duration was presented on an individual trial, the beginning of which was clearly indicated by a press on the computer keyboard either by the experimenter (for the children) or the participant (for the adults) in order to start the trial (e.g., Allan & Gerhardt, 2001; Droit-Volet & Wearden, 2001; McCormack et al., 1999; Wearden, 1991; Wearden, Wearden, & Rabitt, 1997). Next, the comparison duration appeared after an interval chosen at random between 1, 2 or 3 s. Although this interval value was short, it may have affected the younger childrenõs time processing by requiring them to maintain alerting attention for the oncoming stimulus. Thus, we may assume that, in this experimental condition, an additional signal that warns of the imminent arrival of a stimulus duration by preceding it with a constant short time interval (i.e., 1.5 s) would improve time sensitivity, particularly in the younger children. This warning signal may also lengthen the time estimates by inducing an earlier closing of the switch. Thus, in the present experiment, children aged 3, 5, and 8 years were tested in a bisection task with two probe trial types, one with and the other without a warning signal (i.e., a click). Two duration ranges were used: one from 0.5 to 2 s and the other from 1 to 4 s. Method Participants Eighty-six children from nursery and primary schools in Clermont Ferrand, France participated: 27 3-year-olds (17 girls and 10 boys; mean age ¼ 3.29 years, SD ¼ 0:3), 27 5-year-olds (14 girls and 13 boys; mean age ¼ 5.23 years, SD ¼ 0:22), and 32 8-year-olds (14 girls and 18 boys; mean age ¼ 8.37 years, SD ¼ 0:27). The data for five 3-year-olds, and four 5-year-olds were not included in the final sample because they produced the same responses (i.e., long or short) for the different stimulus comparison duration values. Material The children were tested individually in a quiet room in their school using a PowerMacintosh computer that controlled the experimental events and recorded the data with Psyscope. The responses were made on the buttons of a Psyscope response box. The stimulus used was a blue circle (4.5 cm in diameter) presented in the center of the computer screen. The warning signal was a click (50 ms) produced by the computer speaker. Post-response feedback took the form of a clown presented for 2 s in the center of the screen. The clown was either smiling (correct responses) or frowning (incorrect ones).

5 376 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) Procedure In each age group, the children were randomly assigned to one of the two duration groups. For the 0.5/2 s group, the short and the long standard duration were 0.5 and 2 s, respectively. The stimulus comparison durations were 0.5, 0.75, 1, 1.25, 1.5, 1.75, and 2 s. For the 1/4 s group, the short and the long were 1 and 4 s, and the comparison durations 1, 1.5, 2, 2.5, 3, 3.5, and 4 s. In each duration group, children received two successive experimental phases: training and testing. In the training phase, participants were initially shown the two standard durations, each presented 5 times in alternation. The experimenter said Look, itõs the short/long circle, it stays on for a short/long time. They were then trained to press one button after the short standard and the other one after the long standard. The button-press order was counterbalanced. Each participant was given successive blocks of 8 trials, with each standard having a 50% probability of appearance on each trial. The experimenter controlled the stimulus presentation by pressing a key on the computer keyboard. Before each press, the experimenter said: Are you ready! Now, the circle is coming. Then, after a delay period randomly chosen between 2 and 3 s, the stimulus appeared in the center of the computer screen. In the training phase, a correct response resulted in the appearance of the smiling clown and an incorrect one in that of the frowning clown. Training terminated when the participant made no error during a block of 8 trials (i.e., 8 consecutive correct responses). The test phase maintained the training conditions, except that the feedback was discontinued. The experimenter said ItÕs the same game, but now the clown isnõt here to tell you whether or not youõre playing well. Each participant received 8 blocks of 14 trials, 7 with and 7 without a click (i.e., one of each of the 7 comparison durations). The 14 trials were presented in a random order within each block. The click appeared 1.5 s before the end of the 2 3 s delay period in the click trial (i.e., 1.5 s before the stimulus onset). The experimenter said: You must always pay attention to the circle that is arriving to be able to say whether itõs the short or the long circle. Sometimes you have a click, which tells you that the circle is arriving, sometimes not. In any case, you always have to pay attention to the arrival of the circle. Results Table 1 shows the number of blocks required by the children in the two duration conditions to meet the learning criterion in the training phase (8 consecutive correct responses). There was a significant effect of age, Kruskal Wallis test, H ð2þ ¼41:65, p ¼ :0001. Between-age comparisons revealed significant differences between the 3-year-olds and both the 5- and the 8-year-olds, Kolmogorov Smirnov test, Z ¼ 1:91, 2.92, respectively, all p <:001, whereas no difference was observed between these two latter ages. There was also a significant effect of duration, Mann Whitney U ¼ 623:50, p ¼ :005. The required number of training blocks was greater in the 0.5/2 s than in the 1/4 s duration condition at the age of 3 years but not at the

6 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) Table 1 Mean, minimum, and maximum number of training blocks required to meet the learning criterion (8 consecutive correct responses) in the 3 age groups for the 0.5/2 and 1/4 s duration groups Age Duration 0.5/2 s 1/4 s M Min Max M Min Max 3 years years years older ages, Kolmogorov Smirnov test, Z ¼ 1:63, p ¼ :01. Thus, the 3-year-olds took more time to discriminate the short from the long standard durations than the 5- and the 8-year-olds, particularly in the shortest duration range condition. Fig. 1 shows the mean proportion of long responses plotted against the stimulus comparison durations for the click and the no-click trials in the two duration groups (0.5/2 vs. 1/4 s). The top panel shows data from the 3-year-olds, the centre panel data from the 5-year-olds and the lowest one those from the 8-year-olds. An overall analysis of variance 1 was run on the proportion of long responses with two between-subject factors (age and duration range) and 2 within-subject factors (click and stimulus comparison duration). This ANOVA revealed neither a main effect of duration range, nor any significant interactions involving this factor. 2 These results indicated that the duration range did not affect the proportion of long responses and that the psychophysical functions for the different duration ranges did not differ in shape. They support the suggestion that, in all age groups, childrenõs temporal bisection performance conforms well to the scalar property of variance (i.e., the requirement that the standard deviation of time judgments varies as a constant fraction of the mean time judgment). There were significant main effects of click, F ð1; 80Þ ¼23:88, p ¼ :0001, comparison stimulus duration, F ð6; 480Þ ¼ 297:15, p ¼ :0001, and a marginally significant main effect of age, F ð2; 80Þ ¼2:80, p ¼ 0:7. The age stimulus comparison duration interaction was also significant, F ð12; 480Þ ¼ 16:91, p ¼ :0001. No other significant interaction was observed involving either click or age. Thus, these statistical results indicate that the proportion of long responses was greater for the click than for the no-click trials and that the steepness of the bisection functions increased with age. To explore childrenõs bisection responses further, we calculated the bisection point and the Weber ratio from the individual bisection functions. The bisection point is the stimulus value giving rise to 50% long responses (i.e., point of subjective 1 Because initial analyses revealed neither any main effect nor interactions involving sex and button order factors, these factors were not included in the statistical analyses. 2 Duration click, F ð1; 80Þ ¼1:01, duration click age, F ð2; 80Þ ¼0:56, duration stimulus comparison duration, F ð6; 480Þ ¼ 1:90, duration stimulus comparison duration age, F ð12; 480Þ ¼ 0:88, duration click stimulus comparison duration, F ð6; 480Þ ¼ 0:15, duration age click stimulus comparison duration, F ð12; 480Þ ¼1:35, all p >:05.

7 378 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) Fig. 1. Proportion of long responses as a function of stimulus duration, separately for the click and no click conditions. Upper panel, data for 3-year-olds; middle panel, data for 5-year-olds; bottom panel, data for 8-year-olds.

8 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) equality). The Weber ratio is the difference limen divided by the bisection point (see introduction). As stated previously, the Weber ratio is a measure of the steepness of the psychophysical function curve, which is in turn an index of temporal sensitivity. A smaller Weber ratio indicates a steeper psychophysical function and greater time sensitivity, and a larger Weber ratio, a flatter bisection function, and lower time sensitivity. To calculate the bisection point and the Weber ratio, we used the linear regression method first employed by Church and Deluty (1977). Data from three 3-year-olds and one 5-year-old were omitted from the calculation of the means and all subsequent analyses, because calculating their Weber ratio required a datatransformation (for more details see Droit-Volet & Wearden, 2002, page 10). The resulting bisection points and Weber ratios are shown in Table 2. For the bisection points, we calculated, for each child, the absolute difference between the bisection point obtained with click and that obtained with no-click, divided by the mean bisection point appropriate to their duration group. These individual difference values were taken as the dependent variable. First, we tested whether this difference value was significantly greater than 0. This was indeed the case for all the age groups: 3-year-olds, tð25þ ¼7:18, 5-year-olds, tð23þ ¼5:86, 8-year-olds, tð31þ ¼6:11, all p <:0001. The click therefore lowered the bisection point whatever the age group tested. We then ran an ANOVA on these difference values. This revealed a significant effect of age, F ð2; 76Þ ¼8:10, p ¼ :001, but neither a significant effect of duration nor a significant age duration interaction. t tests for independent samples revealed that the click/no-click difference was greater in the 3- (.31) and the 5-year-olds (.23) than in the 8-year-olds (.12), tð56þ ¼4:3, tð54þ ¼2:81, all p <:007. Although it was also larger for the 3- than for the 5-year-olds, this difference did not reach significance between these two ages. As for the bisection points, we also calculated the difference between the click and the no-click values for the Weber ratios. The obtained values were significantly different from zero in the 3 age groups, indicating that the click increased the slope of Table 2 For the 3 age groups, means of individual bisection points (in seconds) and Weber ratios as a function of the stimulus comparison durations for the click and the no click trials in the 0.5/2 s and the 1/4 s duration range conditions Duration Age 3 years 5 years 8 years BP WR BP WR BP WR 0.5/2 s No click Click /4 s No click Click Note. The arithmetic means for the 0.5/2 s and the 1/4 s were 1.25 and 2.5 s, respectively, and the geometric means 1 and 2 s.

9 380 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) the bisection function, and thus the time sensitivity (3-year-olds, tð25þ ¼5:49; 5-year-olds, tð23þ ¼3:85, 8-year-olds, tð31þ ¼2:71, all p <:01). The ANOVA conducted on the Weber ratio differences revealed a significant main effect of age, F ð2; 76Þ ¼13:12, p ¼ :0001. Neither the effect of duration, nor the interaction between age and duration were significant. This lack of a significant effect involving the duration factor was entirely consistent with the results described previously, suggesting a scalar property of timing in children bisection behavior. Between-age comparisons using t tests indicated that the click/no-click Weber ratio difference was significantly or nearly significantly larger for the 3-year-olds (.24) than for either the 5-year-olds (.09), tð48þ ¼ 2:89, p ¼ :006, or the 8-year-olds (.04), tð56þ ¼ 4:66, p ¼ :0001, and also larger for the 5-year-olds than for the 8-year-olds, tð54þ ¼1:91, p ¼ :06. Thus, the click effect in the improvement of time sensitivity decreased with increasing age. Discussion The present experiment provided a series of data that were entirely consistent with those provided by previous studies in children using a temporal bisection or generalization task (e.g., Droit-Volet, 2002; Droit-Volet, Clement, & Wearden, 2001; Droit-Volet & Wearden, 2001, 2002; McCormack et al., 1999). Indeed, our study shows that, as young as 3 years of age, children are able to discriminate different durations, and that their temporal discrimination conforms well to the scalar property of timing. Furthermore, it shows that sensitivity to duration increases with age, as indicated by the flatter bisection functions in the younger children. Thus, our study adds to the volume of data indicating an early ability in humans to represent time. However, the most interesting result of the present study was the finding of developmental changes in the click effect on childrenõs bisection performance. Indeed, whatever the age group tested, the stimulus durations were judged longer in the click than in the no-click trials, thus shifting the bisection functions toward the left. However, this lengthening effect was relatively greater in the 3- and the 5-year-olds than in the 8-year-olds. The click also increased the slope of the psychophysical functions and lowered the Weber ratio in all the age groups. However, this improvement in time sensitivity was relatively greater in the younger children. According to the scalar timing theory, there are two ways in which the clock can be affected by experimental manipulations which make stimuli seem to last longer than normal. The first is to increase the pacemaker speed. Indeed, because it preceded the stimulus to be timed, the click would have increased the speed of the pacemaker. In this case, the pacemaker would have produced more pulses per unit time in the click than in the no-click trials, and the time would have seemed longer. Alternatively, the click may affect the switch between the pacemaker and the accumulator, by making the switch close earlier. In this case, the number of pulses accumulated would also be greater in the click than in the no-click condition. However, the mathematics of the scalar timing models allow us to distinguish between these two possibilities (Burle & Casini, 2001; Droit-Volet & Wearden, 2002; Maricq, Roberts,

10 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) & Church, 1981). In the case of the clock-speed hypothesis, they predict a multiplicative effect with the duration values. If the pacemaker runs faster, the effect should be greater for longer than for shorter times. In contrast, for the case of the switch-latency hypothesis, they predict a purely additive effect (i.e., an effect independent of the judged duration), and which should therefore, be constant whatever the duration values used. Consequently, if we are to be able to dissociate these two possibilities, more than one duration value or range of values must be tested, as we have done in the present experiment. Our study provides clear support for the switch-latency interpretation. Indeed, we obtained no interaction effect between the click and the duration range on the proportion of long responses (i.e., an additive effect). In the same way, the click lowered the bisection point, but this lowering effect was not significantly greater with the longest duration range. In other words, the observed click lengthening effect was constant whatever the duration ranges tested. These results suggest that the click used in our bisection task did indeed affect the switch mechanism by reducing its closing time on the presentation of the visual stimulus. This is consistent with the data provided by the studies on attention. Indeed, there is evidence that when a warning signal producing a more highly alerted state is used, the responses are selected more quickly (Posner & Peterson, 1990). Consequently, in the younger children who have limited attentional control, the temporal performance was more greatly affected when the warning signal was omitted before the temporal stimulus, as suggested by the greater click/no-click difference in the bisection point in the 3- and the 5-year-olds than in the 8-year-olds. When children were at a higher level of alertness because the target stimulus occurred with a warning signal, we have imagined, within the framework of the scalar timing theory, that the switch closing time was reduced, thus lengthening the subjective time. However, our results suggest that the warning signal also reduced the variability of the closing time of the switch. Indeed, the Weber ratio, which is an index of this variability, was lower in the click than in the no click condition. The warning signal therefore improved the accuracy of time processing, and, consequently, the sensitivity to duration. Furthermore, our data indicate that the reduction in the variability of the switch closing latency with a warning signal was greater in the younger children. Processing the duration of a visual stimulus as soon as it starts requires participants to prepare alertness to a location and to maintain this alerted state until the stimulus appears at this location (i.e., to produce a constant attentional effort). That is particularly difficult for young children (for reviews see Dempster & Brainerd, 1995; Ruff & Rothbart, 1996). Thus, the switch closing latency was less constant in the younger children, and the warning signal effect greater. The attention-based switch system is therefore one of the main sources of variability in young childrenõs temporal discrimination. In their developmental version of the scalar timing theory, Droit-Volet and Wearden (2001) explained that young childrenõs lower time sensitivity was due to the greater variability in their memory representation of standard durations. The standard durations are assumed to be represented not as a single duration value, but as a distribution with a mean equal to the standard duration and a coefficient of

11 382 S. Droit-Volet / Journal of Experimental Child Psychology 85 (2003) variation, c. The higher the value of c, the fuzzier the long-term memory representation of the standard duration, and the lower the time sensitivity. This coefficient of variation of the long-term memory representation of standard duration is thus a critical parameter for the understanding of age-related changes in temporal discrimination. Our study suggests that this fuzzier memory representation of time in young children is to a large extent due to their limited attentional control, which is a source of variance in the memory encoding of time. Indeed, attentional distraction introduced trial by trial variance in time processing at the level of the switch closure and flattened the psychophysical bisection functions. Consequently, a warning signal that improved the attentional preparation for time processing, reduced this source of variance and, logically, did so to in a greater extent in the younger children. These findings are consistent with those provided by Droit-Volet et al.õs (2003) study showing that time estimates are less variable for auditory than for visual stimuli, because auditory stimuli require attention but in a less constrained way for young children. The present study suggests that children have a fundamental capacity to represent time because they possess an internal clock mechanism that serves to represent time. However, it also provides evidence that, despite this basic mechanism, there is an age-related increase in sensitivity to time. Our data suggested that one of the reasons for this developmental change in temporal sensitivity is the improvement in the efficiency of timing signal in capturing childrenõs attention. However, that is only one of the different dimensions of attention that affect timing in children. References Allan, L., & Gerhardt, K. (2001). Temporal bisection with trial referents. Perception and Psychophysics, 63, Allan, L. (1992). The internal clock revisited. In F. Macar, V. Pouthas, & W. Friedman (Eds.), Time, action, and cognition: Towards bridging the gap (pp ). Dordrecht: Kluwer. Allan, L. (1998). The influence of the scalar timing model on human timing research. Behavioural Processes, 44, Bjorklund, D., & Harnishfeger, K. (1995). The evolution of inhibition mechanisms and their role in human cognition and behavior. In F. Dempster, & C. Brainerd (Eds.), Interference and inhibition in cognition (pp. 3 26). San Diego: Academic Press. Block, R., Zakay, D., & Hancock, P. (1999). Developmental changes in human duration judgments: A meta-analytic review. Developmental Review, 19, Burle, B., & Casini, L. (2001). Dissociation between activation and attention effects in time estimation: Implication for internal clock models. Journal of Experimental Psychology: Human Perception and Performance, 27, Church, R. M., & Deluty, M. Z. (1977). Bisection of temporal intervals. Journal of Experimental Psychology: Animal Behavior Processes, 3, Dempster, F. N., & Brainerd, C. J. (1995). Interference and inhibition in cognition. New York: Academic Press. Droit-Volet, S. (2002). Scalar timing in temporal generalization in children with short and long stimulus durations. The Quarterly Journal of Experimental Psychology, 55A, Droit-Volet, S. (2003). Temporal experience and timing in children. In W. Meck (Ed.), Functional and neural mechanisms of interval timing (pp ). Boca Raton: CRC Press.

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