Influence of Task-Specific Processing Speed on Age Differences in Memory

Two studies were conducted to investigate the aspect(s) of processing involved in the hypothesized speed mediation of adult age differences in memory. Both studies involved a serial memory task in which information was to be recalled either in the original order of presentation, or in a reordered sequence. Results from both studies indicated that taskspecific processing durations were slower among older adults thon among loung adults, but that the attenuation of the age-related variance in memory was nearly as great after statistical control of a task-independent speed measure as after conlrol of nsk-specffic speed measures. These findings are consistent with the hypothesis that a substantial proportion of the adult age-related differences in memory is associated with a decrease with increased age in the speed of executing many cognitive processes, and not simply the speed of one or two specific processes. 'JHE goal of the studies described in this report was to r investigate how processing speed contributes to the relations between age and memory. The relations of interest have been documented. and some initial evidence relevant to this issue has been provided, with data from earlier research (Salthouse, 1983, in press). The memory tasks in the previous studies were free recall of two lists of, l2 unrelated nouns presented auditorily at a rate of -2 seconds/word, and paired associatc recall <lf six pairs of unrelated nouns also presented at -2 secondsiword. The speed measures were derived from three paper-and-pencil tests of perceptual speed; digit symbol substitution, letter comparison, and pattern comparison. The average of the z-scores in the three tests served as a composite index of the speed with which the individual could execute elementary operations. The results of these earlier studies are summarized in Table I . Notice that in the first study there was a large attenuation of the age-related variance in the memory measures when the variance associated with the processing speed measure was statistically controlled (i.e., 87o/o attenuation with free recall, from an R' for age of . 162-.O2l , and 857o attenuation with paired associates, from an R2 for age of .162-.024). An extreme-groups design was used in the second study, which resulted in larger age-related effects than those in the first study because of the omission of the variance associated with the intermediate ages. [The memory data were not reported in the Salthouse (1993) article, but most of the subjects from the second study in that project performed these tasks after completing the tasks reported in the article.l The speed and memory tasks were identical to those of the first study, but also included were measures of associational fluency that might be expected to be important in the performance of many memory tasks if effectiveness of elaborative encoding is related to the number of relevant associations that can be produced in a limited time. The measures were from tasks requiring the generation of as many words as possible, beginning with one of two stems (pro- and sub-) or ending with one of two terminations (-ay and -ow), and the average of the z-scores in the two tasks served as the measure of associational fl uency. It is apparent in the bottom panel of Table l that the major findings of the first study were replicated, as there was 9l7o attenuation of the age-related variance in free recall performance and 88o/o attenuation of the age-related variance in paired associates performance after control of the same composite speed measure. However, it was surprising that the attenuation of the age-related variance in memory was smaller after control of the presumably more specific fluency measure than after control of the more general speed measure; that is, only 44.5o/o attenuation with fiee recall performance and 37 .l%o attenuation with paired associates performance. Taken together, the results of these earlier studies suggest that processing speed, as indexed by various perceptual comparison tasks, is involved in the mediation of age differences in memory, and that the speed influence appears to be fairly general, in that it does not seem to be restricted to measures postulated to reflect memory-relevant processes. The existence of significant residual age-related variance even when the variation in speed is controlled indicates that other factors also contribute to the age differences evident in these tasks. Nevertheless, the discovery that influences independent of speed are associated with <157o of the total agerelated variance for free recall and paired associate measures of memory suggests that some aspect of speed plays a major role in the relations between age and memory. The purpose of the current project was to conduct a more detailed examination of the relation between speed and memory with a simpler task in which it might be possible to identify the mechanisms responsible for the apparent speed mediation of age-related differences in memory. The research strategy consisted of three distinct steps or phases. The first step was to determine the amount of age-related variance in the criterion measure of memory before and after controlling the variance in an index of processing speed derived from independent tasks. The goal of these analyses