Interval timing and other areas of Cognitive Aging

Most of this chapter has focused on the ways in which age differences in attention affect older adults' interval timing performance relative to that of young adults, but interval timing research can also be useful for gaining a broader understanding of the cognitive changes that occur with advanced age. On a procedural level, the nonverbal and continuous nature of many interval timing tasks minimizes the con-founders that can be problematic for aging studies of attention and memory. In addition, age differences in timing performance may contribute to age differences in performance on other tasks, especially speeded tasks or those that require actions to be performed at a certain time or in a particular sequence. Finally, the great deal of research on the neurobiological underpinnings of interval timing performance may provide important insights for the relation between biological and cognitive changes that occur during aging.

For example, many standard memory experiments make use of verbal materials. This often presents a problem for studying age differences in memory, as older adults typically have richer vocabularies than do young adults, and may also suffer from breakdowns in the networks that connect semantic, phonological, and orthographic aspects of language (see reviews by Burke et al., 2000; Kemper and Mitzner, 2001; Wingfield and Stine-Morrow, 2000). Furthermore, older adults are often anxious about their memory performance, and the instructions used in most memory tests (e.g., "Remember the list of words you were shown earlier") can elicit this anxiety and distract them from the memory task, ironically reducing their performance. Older adults may do better in situations where the use of the word memory is minimized in the instructions (Rahhal et al., 2001). Timing tasks, which are nonverbal in nature and which typically use instructions that do not emphasize memory (e.g., "Is this duration shorter or longer than the standard?"), may help avoid both of these problems. However, some caution is required here, as older adults may show memory deficits that are specific to duration (McCormack et al., 2002; Rakitin et al., submitted).

Interval timing tasks may also be useful for researchers interested in sustained attention. One problem with many sustained attention or vigilance tasks is that their dependent measure is the detection of a rare, intermittent event, whereas the cognitive construct of interest is the ability to continuously maintain an attentional state. A person's attention may "flicker" or be temporarily diverted from the task, but this lapse will not be detected unless it coincides with the presentation of the rare target event. Thus, these experiments may underestimate age differences in the ability to maintain attentional control. In timing tasks, at least as characterized by pacemaker-accumulator models, the measurement of sustained attention is much more continuous (accumulation of pulses that is measurable by changes in clock speed). Timing procedures may thus be a more sensitive measure of how age and other variables affect the ability to sustain attention and vigilance.

Frequent attempts have been made to relate age differences in timing performance to age-related slowing, a topic of great interest in aging research (e.g., Cerella, 1985; Salthouse, 1996). The metaphor of an internal clock that may run at faster or slower speeds has led some investigators to propose that older adults' clocks may run more slowly because of reductions in processing speed, and others to propose that a slower clock may be a reason for cognitive slowing (e.g., Block et al., 1998; Craik and Hay, 1999; Rabbitt, 2000; Schroots and Birren, 1990; Vanneste et al., 2001).

There probably are strong relations between timing-related concepts such as clock speed or memory storage speed and cognitive slowing as measured by psychometric speed or reaction time tasks. However, this relationship is not likely to be as direct as would be expected from the shared use of the term speed. A number of factors complicate interpretation: depending on a particular investigator's background, processing speed may mean any of several different variables, including psychomotor speed, decision speed, or perceptual speed (Salthouse, 2000). The degree to which older adults are slower than young adults can vary across these domains, and change depending on response mode, task complexity, and whether verbal or nonverbal processing is involved (e.g., Jenkins et al., 2000; Oberauer and Kliegl, 2001; for a review, see Verhaegen, 2000). It is still unclear how these different aspects of slowing are related to each other and to other cognitive variables, including attention, or what biological changes underlie behavioral changes in processing speed (for discussion, see Park et al., 2001). However, in the context of the issues discussed in this chapter, it is interesting to note that the frontal-striatal brain circuits involved in attention and timing (e.g., Meck and Benson, 2002) have also been suggested as the locus of changes important for age reductions in processing speed (e.g., Backman et al., 2000; Rubin, 1999).

Some attempt to pit clock speed explanations of timing performance against attention and memory explanations, but as has been described here, attention is directly related to the speed of the clock, via its influence on the rate at which pulses accumulate. This problem might be circumvented by restricting the definition of the clock to the pacemaker. However, the idea that older adults have a slower clock than they did in their youth, when they learned the labels associated with particular clock readings (e.g., 3 sec), would predict that older adults should overproduce and underestimate durations on absolute time judgment tasks. The results of the Block et al. (1998) meta-analysis revealed the exact opposite pattern. It is also difficult to discern how a general slowing of the clock could account for the directional distortions older adults show on relative time judgment tasks.

Cognitive slowing could have an effect by reducing the attentional resources available to older adults (Salthouse, 1996). For example, Perbal et al. (2002) found a relation between processing speed as measured by a reaction time task and age differences on an interval production task performed under divided attention conditions. In this case, processing speed may have had its effects by influencing participants' ability to divide attention or switch rapidly between temporal and nontemporal processing. Processing speed's effect on clock speed would thus be indirect, mediated through its effect on attention. In general, age differences in attention functioning appear to provide more proximal and parsimonious explanations of age differences in interval timing than does the idea that older adults generally have a slower clock.

It has also been suggested that age differences in timing may influence performance on reaction time or speed tasks themselves. In such tasks, people tend to use timing information to prepare a response and to locate themselves on a speed-accuracy continuum such that they spend the smallest amount of time on each trial that will allow them to avoid making a mistake (Grosjean et al., 2001). If older adults have inaccurate or more variable representations of the time they take to complete each trial, they may be less able to optimally calibrate their performance on this continuum (Rabbitt, 2000). Age differences in timing may also affect performance on prospective memory tasks that require participants to remember to perform particular actions at particular times, or on executive control tasks that require the timing and sequencing of multiple actions (Krampe et al., 2002; Park et al., 1997).

Besides these relations between interval timing tasks and other measures of behavior, researchers interested in the neurobiological consequences of age may find important clues in both human and animal timing research. A variety of drugs and neurotransmitter systems have been examined in the context of interval timing, and in some cases, relatively clear distinctions have been drawn between their effects on attention (clock speed) and memory (for reviews, see Buhusi, this volume; Cevik, this volume; Mattell and Meck, 2000; Meck, 1996; Meck and Benson, 2002). In particular, dopamine changes have recently received a great deal of attention in human cognitive aging research (e.g., Backman et al., 2000; Braver et al., 2001; Li, Lindenberger, and Sikstrom, 2001; Park et al., 2001; Volkow et al., 1998, 2000). Dopamine functioning has been heavily investigated in interval timing experiments using both animals and human populations (e.g., schizophrenics, Parkinson's patients) with known dopamine dysfunctions. As the Rakitin et al. (submitted) experiments demonstrate, the findings from these populations are likely to be highly relevant for understanding the cognitive effects that occur as the result of neurobi-ological changes in healthy aging as well.

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