Older Adults Performance on Relative Time Judgment Tasks

In relative time judgment tasks, participants are exposed to an unnamed critical target duration in a series of training trials, so that a distribution of accumulator values associated with the target duration is built up in reference memory. On test trials, participants make judgments about presented durations relative to that earlier-learned target. Because they do not require the use of language-based duration labels (e.g., 5 sec), these procedures are used for animal experiments on interval timing. Many human experiments build directly off standard procedures from the animal literature (e.g., peak-interval production or temporal bisection), whereas others use procedures specific to a particular study. The critical feature these procedures share in common, and that differentiates them from absolute time judgments, is that both the reference memory values and the current accumulator values are acquired during the experiment.

Under full attention conditions, age differences in interval timing on relative time judgment tasks are typically smaller than those found using absolute time judgment tasks. This is to be expected, as the controlled attention demands should be largely the same during both training (acquisition of reference memory values) and test trials (acquisition of current accumulator values). Because of this, the clock should run at approximately the same speed when the target duration is learned as when it is tested, resulting in very little distortion. However, small age differences are often found on relative time judgment tasks, usually in a direction that suggests that older adults' clocks may be running more slowly during test trials than they were during training.

These differences could occur if older adults' attention to time was greater during the training trials, when they are still "fresh" and under the watchful eye of the experimenter, than it is during the numerous test trials, when participants may be distracted by unrelated thoughts, boredom, or fatigue. While both young and older adults may be more prone to lapses of attention during test trials than during training trials, leading to a slower clock speed, this effect is likely to be greater for older adults because of their reduced attentional control. Consistent with this suggestion, age differences on relative time judgments are often more evident for long durations (when attention would have more opportunity to drift) than for short durations. Furthermore, both the proximity of training trials to the testing trials and the nature of feedback have an effect on age differences on relative time judgment tasks that is consistent with the idea that those differences are due to older adults' decreased control of attention during test trials as opposed to training.

McCormack et al. (1999) tested participants using a temporal generalization procedure in which participants were first trained on a tone of a standard duration (500 msec) and at test were asked to indicate whether presented durations (ranging between 125 and 875 msec) were the same length as the standard. Both young (18 to 22 years) and old (65 to 73 years) participants showed a bias to underestimate the lengths of the long test durations and to say that they were the same length as the standard. Although the differences between these two age groups were not statistically significant, this tendency to underestimate long durations was more pronounced for the older adults than for the young adults. A third group of old-old adults (age 75 to 99 years) did show a significantly greater underestimation of the long durations, replicating previous findings for older adults in a study by Wearden et al. (1997).

Both Wearden et al. (1997) and McCormack et al. (1999) explain their results in terms of memory problems associated with age, but older adults' underestimation of the long durations might also be explained as the result of problems in attention. Wearden et al. (1997) suggest that older adults might have a more variable memory representation of the standard, leading to errors when making decisions about a test duration relative to the standard. However, a memory representation that was simply more variable should lead to more errors for both short and long test durations, rather than a unidirectional bias toward underestimating the long durations. McCor-mack et al. (1999) modified this idea by suggesting that older adults had a systematic bias in memory such that they remembered the standard as being longer than it really was, whereas young children (who showed the opposite result — that is, overesti-

mation of durations shorter than the standard) had a bias to remember the standard as being shorter than it was. However, there is no a priori reason to predict this overremembering on the part of older adults.**

The approach taken in this chapter would locate the source of underestimation in the attentional switch and accumulator, rather than in reference memory. This attention-based account assumes that keeping attention on the timing task will be relatively easy during training, when the participant is under the careful watch of the experimenter, compared to the many test trials, when the participant must begin to deal with interference, boredom, or fatigue. Thus, attention to time is assumed to require more control during testing trials than during training.

If this is the case, then lapses in attention will reduce the number of pacemaker pulses that pass through the attentional switch during test trials, as compared to training trials. Such lapses in attention may be especially likely to occur during long test trials, reducing the accumulator values acquired in those trials. When compared to the reference memory values for the standard acquired during training, the reduced accumulator values will make the long test durations seem more like the standard. Because older adults are more vulnerable to lapses of attention than are young adults, they will be especially sensitive to this effect. The pattern found by McCormack et al. (1999) was in keeping with this prediction: both age groups showed a tendency toward underestimation of the long test trials, but the underestimation effect was larger for older adults because of their problems with attentional control.

Wearden et al. (1997) and McCormack et al. (1999) also tested participants using temporal bisection tasks. On bisection tasks, participants first learn a short (e.g., 200 msec) and a long (e.g., 800 msec) anchor duration. At test, they are presented with intermediate durations and asked to indicate whether the presented duration is closer to the short or the long anchor duration. The primary measure in such tasks is the bisection point, the duration for which participants respond "short" and "long" with equal probability. In these experiments, young and older adults showed equivalent bisection points. However, older adults were slightly less likely than young adults to call the longer test durations (e.g., 600 msec) long. Such a result might occur if older adults were more likely to lose pulses during the long test durations due to lapses in attention, reducing the accumulator values for those durations and thus their perceived similarity to the long anchor.

The results presented by McCormack et al. (1999) and Wearden et al. (1997) for the temporal generalization and temporal bisection tasks do not allow for a clear differentiation between the memory-based explanations proposed by those authors and the attention-based framework used in the current chapter. However, the attention-based framework offers several advantages. First, it can explain why young adults often show effects that are in the same direction as those shown by older adults, but much smaller. Second, it parsimoniously explains the findings for both

** Some rat studies show an overreproduction with increased age that is not corrected by feedback, indicating a memory effect (Lejeune et al., 1998; Meck et al., 1986; but see Meck, 2001). In humans, older adults' overproductions are typically corrected by feedback, more consistent with a clock speed effect caused by age differences in attention (e.g., Malapani et al., 1998, 2002b; Wearden et al., 1997). This discrepancy may occur because humans approach the task differently than do animals: humans are more likely to become bored or distracted during test trials, because they are not working for food rewards.

the relative time judgment experiments discussed here and the absolute time judgment studies discussed in the previous section. Third, it can be tested by examining the effects of feedback and dividing attention.

As described earlier, interval timing effects that are sensitive to feedback are usually considered to be located at the attentional switch or other components of the clock stage, rather than in memory or decision stages of temporal processing (Meck, 1983). Age differences on interval reproduction tasks are influenced by both the timing and nature of feedback. This point can be illustrated by comparing the results of a series of experiments in which participants attempted to reproduce a target duration under various feedback conditions. When there was a significant delay between training and test trials and the test trials occurred without feedback, older adults overreproduced the interval, suggesting reduced attention to time and thus slower clocks during testing than training (Malapani et al., 1998). In contrast, when each test trial occurred immediately after exposure to a target interval, minimizing attentional differences between training and test, no age differences in reproduction were found (Vanneste and Pouthas, 1999).

Feedback that consists of simple reexposure to the target duration does not ameliorate older adults' tendency to overreproduce that duration (Malapani et al., 2002b). This finding seems inconsistent with the idea that older adults' memories are simply more variable (e.g., Wearden et al., 1997) or that older adults overre-member the target (McCormack et al., 1999). If either of those possibilities were the case, re-presentation of the target duration would be expected to refresh or correct older adults' memories of the target and improve their performance. Instead, Malapani et al. (2002b) found that older adults' overproductions were remedied by giving them specific feedback about their performance, potentially alerting them to changes in their clock speed from training to test and the need to pay attention to time.

The impact of age differences in controlled attention on interval timing performance is especially evident in experiments that deliberately manipulate the atten-tional demands between training and test. By the framework used in this chapter, greater attentional demands at training than at test should lead to underreproduction of the target duration on test trials and overestimation of test-presented durations relative to the target. In contrast, greater attentional demands at test than at training should lead to overreproduction of the target duration on test trials and underestimation of test-presented durations relative to the target. Both of these effects are expected to be greater for older adults than for young adults because of older adults' reduced attentional control.

The existing data are consistent with these predictions. For example, Vanneste and Pouthas (1999) tested young and older adults using a procedure in which three letters were presented on the screen. The presentation time of each letter overlapped with that of the other two letters, but each had a different onset and duration. Participants received both full attention and divided attention trials. For the full attention encoding condition, participants were told which letter's duration would later be tested, and that they should ignore the other two. For the divided attention encoding conditions, participants had to attend to the duration of two of the letters or all three and did not know which one would be tested. Reproduction always occurred under full attention: after all letters were removed, participants were presented with one of the letters and asked to reproduce its just-presented duration.

In this experiment (Vanneste and Pouthas, 1999), attention effects would be revealed by underreproduction of the target interval for those trials on which attention was divided at encoding. In the full attention encoding condition, the amount of pulses accumulated during encoding should be equivalent to the number accumulated during the full attention testing conditions, minimizing distortions in time. Under full attention at encoding, young adults and older adults showed largely equivalent performance. As the number of to-be-timed durations, and thus the need to divide attention, increased, both age groups showed an increasing tendency to underrepro-duce the target duration. However, older adults were much more affected by the increasing number of to-be-timed durations than were young adults. It is important to note that increasing the number of durations led not only to a general increase in errors but also to an underestimation of encoded time. The directional nature of the errors is highly consistent with the idea that the divided attention manipulation led to a slower clock speed during training than during test trials. In contrast, when the divided attention manipulation occurs at test trials rather than at training, older adults' errors are in a direction that suggests a slower clock speed during test trials than during training (Lustig and Meck, 2002).

Overall, the results for older adults' performance on relative time judgment tasks fit well with the idea that age differences on timing tasks often result from age differences in attentional control. Under full attention conditions, age differences on relative time judgment tasks are usually smaller than on absolute judgment tasks, because current accumulator values are acquired under roughly the same attentional demands as were reference memory values. Small time distortions are often seen, however, usually in the direction that suggests a slower clock speed during test trials than during training. These distortions — and age differences in their degree — are most often seen under conditions (long test trials, long delays between training and test) conducive to lapses of attention during the test. Both young and older adults may be affected by divided attention manipulations that change the attention demands between training and test, but older adults are more vulnerable than young adults to these manipulations.

10.2.3 Maintaining Attention, Dividing Attention, and Orcadian Fluctuations: Distinguishing Aspects of Attention That Influence Older Adults' Perception of Time

The previous sections present evidence that strongly suggests that older adults' timing distortions relative to those of young adults are the result of age-related difficulties in maintaining attention to time (Block et al., 1998; Malapani et al., 1998, 2002b; McCormack et al., 1999; Wearden et al., 1997) and in dividing attention between multiple stimuli (Craik and Hay, 1999; Vanneste and Pouthas, 1999). As mentioned previously, automatic attention functions may be largely spared from age-related declines, and not all of the disparate cognitive functions that have been labeled "attention" will have equivalent effects on interval timing performance. Is it possible to distinguish the influence (or lack thereof) of different aspects of attention on age differences on interval timing?

The results of a recent series of experiments that divide participants' attention between to-be-timed stimuli of different modalities suggest that the answer may be "yes." These experiments build on a newly proposed explanation for the classic interval timing finding that sounds are judged longer than lights. That is, an auditory stimulus will be judged to have a longer duration than a visual stimulus of the same physical duration if both modalities are used in the same experimental session and refer to the same standards (e.g., Goldstone and Goldfarb, 1964; Walker and Scott, 1981; Wearden et al., 1998). Penney et al. (1998, 2000; see also Penney, this volume) proposed an explanation for this finding that is grounded in SET and draws upon psychophysical and chronometric studies showing that auditory stimuli tend to capture and hold attention automatically, whereas attending to visual stimuli requires greater control (Posner, 1976; see also Liu, 2001; Spence and Driver, 1997; Schmitt et al., 2000).

By this explanation, auditory stimuli more efficiently and automatically close the attentional switch that allows the accumulation of pulses that mark the passage of time. As a result, more pulses accumulate during an auditory stimulus than during a visual stimulus of the same physical duration (Penney et al., 1998, 2000). The distribution of pulses associated with that duration in reference memory will be mixed, containing both auditory (i.e., relatively large) and visual (i.e., relatively small) values. On test trials, distortions occur when a current accumulator value from one modality is compared to a reference memory value from the other modality. A test duration presented in the auditory modality and compared to a reference memory value formed during a visually presented training trial will seem inappropriately long. In contrast, a test duration presented in the visual modality and compared to a reference memory value formed during an auditorily presented training trial will seem inappropriately short.

This idea was tested in a series of temporal bisection experiments using both visual and auditory stimuli for the anchor durations during training (Penney et al., 1998, 2000). Participants were then presented with intermediate test durations of either modality and asked to judge whether each test duration was closer to the short or the long anchor. Consistent with the differential switch-closing/mixed-memory hypothesis, visual stimuli had to be of a longer duration before they were judged "long" with the same probability as auditory stimuli.

Testing also included divided attention trials that required participants to simultaneously time two stimuli of different modalities, durations, and onsets (e.g., a visual stimulus 3.57 sec in duration and an auditory stimulus that began 1 sec into the visual stimulus and lasted for 4.24 sec). Young adults were equally accurate and sensitive to time on these divided attention trials as on single-stimulus trials. Furthermore, the divided attention trials did not lead to an exaggerated modality effect (Penney et al., 1998, 2000). This pattern suggests that the controlled attention functions responsible for maintaining attention and keeping the switch closed during visual stimuli are different from those required for dividing attention among multiple stimuli.

Interval timing experiments with older adults support the idea that both modality effects and divided effects are related to controlled attention, but that they may tap different functions of attentional control. Older adults show greater modality effects and greater divided attention effects than do young adults, but these effects appear to be independent.

In a mixed-modality bisection experiment using only single-stimulus trials (Lustig and Meck, 2001b), both young and older adults showed a modality effect similar to that shown by Penney et al. (1998, 2000): visual stimuli had to be of a longer duration before they were judged long with the same probability as auditory stimuli. However, this effect was greater for older adults than for young adults, consistent with older adults' greater difficulties with attentional control.

Lustig and Meck (2002) used a peak-interval timing task to examine young and older adults' sensitivity to modality effects under full and divided attention conditions. Participants were first trained to associate a signal of one modality (visual or auditory) with a short duration (8 sec) and the signal of the other modality with a long duration (21 sec). Because one modality was assigned to "short" and the other to "long," there was no mixed modality memory distribution as in the Penney et al. (1998, 2000) experiments. Instead, each duration had a separate reference memory distribution made up of values from only one modality. At test, participants were presented with visual and auditory stimuli and asked to indicate when the signal had been on for the duration associated with it during training. On full attention trials, participants were presented with only a single stimulus of either modality. On divided attention trials, participants were presented with one signal from each modality. On these trials, the long signal always began first, and the short signal onset after a variable delay.

Young adults were highly accurate on all trials, though they were less sensitive to time (had more variable productions) for visual stimuli than for auditory stimuli. Older adults performed very similarly to young adults, with one dramatic exception: older adults overproduced the short stimulus by almost 2 sec on divided attention trials when it was presented in the visual modality. Older adults were also approximately twice as variable in their productions of short visual stimuli on divided attention trials vs. full attention trials. These results suggest that older adults had great difficulty allocating attention to a visual stimulus, which required controlled attention, when they were already timing another stimulus presented in the auditory modality. In contrast, both young and older adults remained accurate for short auditory stimuli on divided attention trials, where they were presented in the context of an ongoing visual long stimulus, though for both age groups variability increased slightly for divided attention trials. This relative preservation of accurate performance for auditory stimuli even in the presence of visual stimuli that already occupy attention is consistent with the idea that auditory stimuli can keep the attentional switch closed relatively automatically.

Another experiment (Lustig and Meck, 2001a) tested young and older adults on a mixed-modality bisection task with both full attention and divided attention trials, and also examined the influence of circadian arousal patterns on these attentional effects. Many physiological variables change over the course of the day, including blood pressure, temperature, and the levels of various hormones and neurotransmit-ters, and attention and memory variables that affect interval timing may change as well (e.g., Fujiwara et al., 1992; Portman, 2001; for reviews, see Hinton and Meck, 1997; Yoon et al., 1999). In particular, the ability to inhibit or ignore information that is irrelevant to the current task seems to be greatest for older adults in the morning and for young adults in the evening. However, this crossover may be specific to inhibitory aspects of attention; it is not shown by tasks that tap other aspects of attention (May and Hasher, 1998).

This experiment provided additional evidence for the idea that both modality and divided attention manipulations tap functions of controlled attention that are impaired in older adults, but separate from each other. Overall, older adults showed a larger modality effect than did young adults, requiring a visual stimulus of a longer duration before judging it long with the same probability as an auditory stimulus of the same physical duration. Older adults were also more affected by the divided attention manipulation: the two age groups were equally sensitive to time on single-stimulus trials, but older adults were much less sensitive to time on divided attention trials. Although age differences in controlled attention are evident in both modality differences and divided attention effects, these two variables did not interact: for both age groups, the modality effect was the same size on full attention and divided attention trials.

The pattern of circadian arousal effects also suggested a separation between different controlled attention functions. For both young and old adults, modality differences were smaller and sensitivity to time greater in the afternoon than in the morning, but the ability to divide attention among multiple to-be-timed stimuli did not fluctuate with time of day. However, older adults tested in the morning showed a peculiar pattern, displaying very high sensitivity to time for visual trials in the full attention condition. This pattern might have occurred if older adults attempted to compensate for poor controlled attention overall by devoting their efforts to this type of stimulus, which would be effortful but not as difficult as the divided attention trials, and relatively ignoring the other stimulus types. Older adults' ability to focus on one stimulus type and ignore others would be best in the morning, when their inhibitory abilities are relatively high.

Overall, the results of these interval timing experiments strongly suggest that maintaining attention and dividing attention are both controlled attention functions that decline with age, but that these functions are separable and distinct. Older adults' timing performance was more affected than young adults' by modality manipulations of the need to maintain controlled attention on a to-be-timed stimulus and by divided attention manipulations that required the timing of multiple stimuli. However, the effects of these manipulations did not interact and, furthermore, were affected differently by the time of day at which participants were tested. Circadian influences on attention and interval timing may impact the results of many experiments, especially those that include groups whose circadian arousal patterns may differ (e.g., young and older adults). In the Lustig and Meck (2001a) experiment, time of day influenced older adults' choice of one time-related stimulus over another; changes in inhibitory abilities over the course of the day may also impact the ability to prioritize temporal vs. nontemporal tasks in divided attention experiments.

These interval timing experiments illuminate distinct aspects of attention that can affect the accuracy of older adults' interval timing. Understanding the interactions — or lack thereof — between age differences in these different controlled attention functions is likely to be important for understanding older adults' performance on many cognitive tasks. As described in the next section, in addition to accuracy measures, measures of variability in timing performance can also provide insights on older adults' attentional functioning under different conditions.

10.2.4 Variability Measures

In addition to asking about the accuracy of older adults' interval timing performance, one can also ask about its precision or consistency. The question of whether older adults' timing performance is more variable than that of young adults has more often been investigated in studies of rhythmic timing. In these tasks, which typically require the repeated production of durations much shorter than those often used in interval timing (< 2 sec), age differences are very small or nonexistent if participants can synchronize with an external pacer, but older adults are more variable if they have to maintain multiple rhythms simultaneously or self-generate a rhythm (e.g., Krampe et al., 2001).

Variability measures are used much less often in age studies of interval timing, and those papers that do report variability have mixed results. Using very brief intervals (< 1 sec), Wearden et al. (1997) found age increases in variability for temporal generalization, but not for temporal bisection or interval production. Rammsayer (2001) did not find age differences in variability for discrimination or reproduction of short intervals (1 sec) or reproduction of a much longer (15 sec) interval. However, Rammsayer also did not replicate standard age differences in reproduction accuracy for the 15-sec reproduction task, suggesting that the experiment may have been insensitive to age differences in general. Vanneste et al. (1999) found that older adults had larger coefficients of variation than did young adults when reproducing intervals ranging in duration from 6 to 8 sec.

In general, age differences in variability seem more pronounced under divided attention conditions. Vanneste et al. (1999) found that while the requirement to divide attention among multiple to-be-timed stimuli increased the coefficient of variation for both young and older adults, this increase in variability was greater for older adults. The sensitivity measure used by Lustig and Meck (2001a) is inversely related to variability; young and older adults were equally sensitive to time on single trials, but older adults were much less sensitive to time on divided attention trials. Perbal et al. (2002) found that dividing participants' attention with a nontemporal task during production and reproduction increased coefficients of variation for both young and older adults, but the increased variability was greater for older adults' reproduction.

Taken together, these results suggest that old age does lead to increased variability on timing tasks, but that age effects on variability are smaller than those on accuracy. Dividing attention seems to increase variability, with older adults being more affected than are young adults. There is also some suggestion that the type of timing task and interval length influence an experiment's sensitivity to age and attention effects on variability, with reproduction tasks and longer intervals most likely to show effects. These are all very tentative conclusions, though, and the picture may change with increased reports of variability in experiments on older adults' timing.

10.2.5 Age Differences in Attention and Interval Timing: Summary and Conclusions

Attention plays an important role in our judgments of time. The experiments reviewed in the previous sections use a variety of methods to address the question of how age differences in attention might lead to age differences in interval timing. Each of these methods has its own advantages and disadvantages, but overall the results of these experiments show that age differences in interval timing can be explained by considering how age differences in controlled attention might influence the function of the internal clock central to information-processing models of interval timing, including SET.

Some duration judgment tasks allow a closer examination of attention's role in interval timing than do others. Absolute time judgments, which make use of our verbal labels for time (e.g., 3 sec), have the advantage of being easy to explain to participants and have an obvious connection to the way that we use duration information in everyday life. These features make absolute time judgment tasks very attractive and popular, but because the experimenter does not control the conditions under which durations are learned, they are not ideal for studying attention. Relative time judgments may often be better for asking questions about attention's influence on interval timing performance, because the target duration is both learned and tested within the context of the experiment. However, even on relative time judgment tasks, attentional "drift" between training and test trials can affect performance. This is especially the case for older adults, who may have difficulty maintaining attention at a high level over long periods of time.

Divided attention manipulations, especially on relative time judgment tasks, are among the most powerful ways of examining the effects of attention on interval timing. Older adults are typically more sensitive than young adults to these manipulations on both absolute (e.g., Craik and Hay, 1999) and relative (e.g., Perbal et al., 2002; Vanneste and Pouthas, 1999) time judgment tasks. Experiments with young adults (e.g., Fortin and Massé, 2000; see review by Fortin, this volume) suggest that simply expecting a break or the addition of another task (even if the break or addition is omitted on that trial) can divert attention and influence the perception of time; presumably, these effects would be even greater for older adults. Even with divided attention manipulations, though, care must be taken because participants may prioritize one stimulus over another (e.g., between timing and a nontemporal stimulus, or between multiple to-be-timed stimuli of different modalities), and the order of prioritization may not be the same for young and older adults.

Despite these complexities, the results of most experiments on age differences in interval timing are consistent with the framework described at the beginning of this chapter: older adults' clocks will run slower than young adults' in situations that place strong demands on controlled attention, and age differences in interval timing will appear when those demands are different when a target duration is learned vs. when it is tested.

In most cases, older adults' timing distortions have certain characteristics that mark them as being the result of differences in attention and clock speed, rather than general performance declines or problems with memory or some other function.

Attentional manipulations do not just increase older adults' timing errors overall, but instead lead to directional effects that indicate a slower and more variable clock in the high-demand situation. These effects increase with increasing attentional demands (e.g., Vanneste and Pouthas, 1999) and are much reduced when the need for controlled attention is minimized (e.g., Lustig and Meck, 2001a, 2001b, 2002). Feedback about their timing distortions can often improve older adults' performance (e.g., Malapani et al., 2002b; Rakitin et al., submitted; Wearden et al., 1997), and this sensitivity to feedback is considered a hallmark of effects on attention and clock speed (Meck, 1983).

Age differences in controlled attention clearly appear to play an important role in age differences in judgments about time. However, timing and attention are not identical: some aspects of attentional functioning that change with age may not affect timing performance, at least not directly, and other cognitive functions that change with age may also play a role in interval timing performance. The following section examines recent attempts to understand the impact of age differences in these other areas of cognition on older adults' interval timing.

10.3 AGE DIFFERENCES IN MEMORY, PROCESSING SPEED, AND INTERVAL TIMING

Attention is not the only aspect of cognitive function that changes with age; memory problems are a common complaint of older adults (Craik and Jennings, 1992; Light, 1991; Zacks et al., 2000), and an apparent age-related decline in processing speed is often suggested as an underlying reason for age differences on many cognitive tasks (e.g., Salthouse, 1996). As described above, reference memory is a central component of many information-processing models of interval timing. Processing speed plays a special role in mathematical formulations of SET, as substantiated in the K* parameter, the speed at which accumulator values are encoded into and decoded from reference memory (Gibbon et al., 1984; Meck, 1983).

The contributions of these components to age differences in interval timing have not been as heavily researched as the contribution of attention, in part because they are not as amenable to experimental manipulations that can be easily implemented in humans. Furthermore, at least some interval timing experiments using human participants are designed in a way that minimizes the contribution of reference memory as typically defined in information-processing models of interval timing and tested in animal experiments (see Wearden and Bray, 2001). However, recent attempts have been made to examine the relationship between age differences on interval timing tasks and neuropsychological tests designed to target specific cognitive functions (Perbal et al., 2002), and to ask whether older adults may show memory deficits that are specific for time (Rakitin et al., submitted).

Perbal et al. (2002) examined the relations between young and older adults' scores on neuropsychological tests and their performance on different interval timing tasks. The neuropsychological measures included were designed to differentially emphasize processing speed, passive short-term memory storage, working memory (online storage and processing), and long-term, delayed memory storage. The inter val timing tasks included a production task under conditions of divided attention (read-aloud digits presented at random intervals) and a reproduction task in which attention was divided during encoding of the standard, but not its reproduction.***

Standard age and divided attention effects were found on the timing tasks, with older adults overproducing and underreproducing the target intervals to a greater extent than did young adults. Performance on the neuropsychological measures of working memory and long-term memory was moderately correlated with accurate performance on the timing tasks, especially reproduction. There was some tendency for these correlations, especially for long-term memory, to be greater for longer (14 and 38 sec) than shorter (5 sec) intervals. Moreover, when age differences in working memory and long-term memory were statistically controlled for, age differences for longer durations in the reproduction task were no longer statistically significant. Much of the age-related variance in the production task was accounted for by processing speed.

Perbal et al. (2002) conclude that working memory and processing speed contributed to the ability to divide attention between the digit-reading and timing tasks. (It should be noted that this "executive function" of working memory is also often described as working attention (e.g., Baddeley, 1993; Engle, 2001).) They explain the correlations between long-term memory and interval timing performance in the longer durations by suggesting that for long durations, pulses collected early in the interval eventually pass out of the accumulator and are stored in long-term memory. If this were the case, older adults' overproduction and underreproduction would result from deficits in two aspects of cognition: First, age deficits in working memory (or attention) would cause them to miss more pulses during initial accumulation. Second, deficits in long-term memory would cause older adults to lose more pulses from storage for the longer durations than would young adults.

The Perbal et al. (2002) experiments represent an important step in relating interval timing performance to performance on standard measures of attention and memory, but several caveats should be kept in mind when interpreting their results. First, an earlier experiment by this group (Vanneste and Pouthas, 1999) using a task that divided attention between multiple to-be-timed stimuli and a different measure of working memory did not find any relation between timing performance and working memory for either young or older adults. Second, the function of long-term memory as described by Perbal et al. is more closely related to accumulator functioning than to reference memory as usually conceived in models of interval timing. Finally, it is important to note that the contributions of both working memory and processing speed as described by Perbal et al. act through attention's mediation of the switch that gates pulses through the accumulator, not the rate of pacemaker pulsation or K*.

The reference memory component of the interval timing model has been the focus of several recent investigations. McCormack et al. (2002) trained young and

*** Perbal. et al. (2002) also included a control counting condition, in which participants counted aloud while producing the target interval in the production task and while encoding and reproducing the interval in the reproduction task. Participants were extremely accurate in this condition, and there were no age differences, so it will not be discussed further here.

older adults on six tones of increasing duration. On test trials, participants were asked to identify which of the six durations had just been presented. Young adults were quite accurate, but older adults misclassified the tones as being shorter than they really were. In contrast, when asked to make judgments about the pitch of tones rather than their duration, older adults made more errors than did young adults but did not show the same systematic underestimation as for duration. McCormack et al. suggested that older adults had a duration-specific distortion in memory such that they remembered the standards as being longer than they really were. This suggestion receives support from previous human and animal studies suggesting an age-related distortion in duration memory (McCormack et al., 1999; Meck, 1986; Wearden et al., 1997), but the same results could also occur if older adults showed a greater drift of attention from training to test trials.

Rakitin et al. (submitted) found age differences on a reproduction task that they attributed to age differences in reference memory for the target durations. They used a peak-interval reproduction procedure in which participants attempted to reproduce a duration learned in an earlier training session. When a single target duration is used, older adults will overreproduce the target more than will young adults (Mal-apani et al., 1998). As described previously, this result likely occurs because older adults' deficits in controlled attention cause them to miss pulses during test trials, requiring more physical time before current accumulator values match those associated with the target duration in reference memory. Rakitin et al. (submitted) found a very different result when they used two durations in the same session. Under these conditions, older adults overreproduced the shorter of the two intervals, but underreproduced the longer interval. Interestingly, this "migration effect" appeared to be specific to memory for durations; no such effect was found for a line-length reproduction task that also used a short and long standard.

Rakitin et al. (submitted) suggested that older adults' vulnerability to the migration effect for durations may be attributed to age-related declines in dopamine function. A very similar effect is found for Parkinson's patients, whose disease stems from a dopamine depletion (Malapani et al., 1998, 2002a). However, the migration effect in non-Parkinson's older adults was not proportionate: the overestimation of the short duration was greater than the underestimation of the long duration. In fact, underestimation of the long duration was only significant for a subgroup of the older adults. Rakitin et al. (submitted) suggested that the migration effect may be a marker for older adults who have especially pronounced declines in dopamine function and are thus particularly vulnerable to the memory effects.

The idea of a vulnerable subgroup of older adults can explain why only some older adults showed a significant underestimation of the long interval, but it does not account for the uneven effects for the short and long targets. One possibility is that age effects on attention and memory compound each other for the short duration and counteract each other for the long duration. That is, for the short duration, attention problems would lead to a loss of pulses and a tendency to overproduce the interval to match the memory representation of the target, a representation that would be inappropriately large because of migration toward the longer duration. For the long duration, attention problems would likewise lead to a tendency to overproduce the interval to match the memory representation of the target, but in this case, migration would shorten that representation, causing the attention and memory effects to partially cancel each other out.

Patterns of scalar and nonscalar variability for the Parkinson's and non-Parkinson's older adults support the idea of a dopaminergic influence. In addition to its role in cognition, dopamine plays an important role in motor functioning; the motor problems caused by dopamine deficits are the primary reason that Parkinson's patients medicate the disease. As described in the introduction, manipulations that influence clock and memory components of the SET model maintain the scalar property of increasing variability with increasing duration (Gibbon et al., 1984, 1997). Manipulations that lead to nonscalar variability are thought to have their effect through nontemporal avenues, such as response criterion or motor output. In the experiments conducted by Rakitin et al. (submitted; Malapani et al., 1998, 2002a), older adults were accurate in training trials, which were conducted with feedback, and inaccurate on delayed test trials without feedback, but showed nonscalar variability under both conditions. Parkinson's patients tested off the medication that remediates their dopam-ine deficit were both inaccurate and nonscalar for both training and testing trials, but both the migration effect and nonscalar variability were remediated by medication. The conclusion suggested by this pattern is that nonscalar production results from problems with motor processes, whereas the migration effect is located at attention and memory processes, and that both are influenced by dopamine.

To summarize, most of the research on age differences in interval timing has focused on the role of attention, and much less is known about the influence of memory, processing speed, or other cognitive characteristics that change with age. The experiments described above exemplify methods that may be useful for examining these influences. Correlations between interval timing performance and performance on neuropsychological tests designed to tap specific cognitive abilities can help establish connections between group and individual differences in both domains. In addition, the analysis procedures provided by SET and other information-processing models of timing can help to separate the relative contributions of attention, memory, and nontemporal processes to performance on an interval timing task.

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