Effects of Age, Domain, and Processing Demands on Memory Span: Evidence for Differential Decline

Analysis of cross-sectional data from the normative sample of the Wechsler Memory Scale – Third Edition (WMS-III) revealed different patterns of age-related differences in memory span measures depending on the type of memory item, processing demands, and the age of the older adult group. Regression of memory span on age revealed that the slope for Spatial Span raw scores was significantly more negative than the slope for Digit Span raw scores. There was no significant difference, however, either between the slopes for forward and backward Digit Span or between the slopes for forward and backward Spatial Span. Regression of Letter-Number Sequencing raw scores on age showed a distinctive, curvilinear pattern. Taken together, the present findings suggest that at least two mechanisms are involved in age-related differences in memory span. One mechanism, associated with a relatively linear decrease in memory span as a function of age, may differentially affect the storage of different types of information (e.g., sequences of digits vs. spatial locations). The other mechanism, evidenced by the curvilinear trend in Letter-Number Sequencing scores, may be tentatively attributed to a decline in executive aspects of working memory that becomes increasingly pronounced with age. It is well established that older adults perform more poorly on measures of working memory than do younger adults (Salthouse, 1994; Verhaeghen, Marcoen, & Goossens, 1993). There is no consensus, however, regarding which aspects of the working memory system are most affected by aging. Most models of working memory assume two types of components (e.g., Baddeley, 1986; Engle, Laughlin, Tuholski, & Conway, 1999): storage components that may be specific to the type of information (e.g., verbal and visuospatial) and processing or executive function components that are used for selecting, manipulating, and coordinating information in the storage components. Aging could have an effect on any or all of these components. For example, spatial storage may be more affected by aging than verbal storage (e.g., Jenkins, Myerson, Joerding, & Hale, 2000; Myerson, Hale, Rhee, & Jenkins, 1999), or executive aspects of working memory may be more affected than storage capacity (e.g., Dempster, 1992; Moscovitch & Winocur, 1992; West, 1996). It is commonly believed that older adults show a greater discrepancy between forward and backward digit span than young adults, and such a differential deficit could be a consequence of a specific age-related decline in executive or processing aspects of working memory. This is because forward digit span may primarily measure storage, whereas backward digit span may reflect both storage and processing because it requires that a person must maintain numbers in memory and also manipulate those numbers. Address correspondence to: Joel Myerson, Department of Psychology, Campus Box 1125, Washington University, Saint Louis, MO 63130, USA. E-mail: [email protected] Accepted for publication: August 20, 2002. D o w n l o a d e d B y : [ W a s h i n g t o n U n i v e r s i t y i n S t L o u i s ] A t : 2 3 : 3 7 2 1 A p r i l 2 0 1 0 Alternatively, if the additional processing required by backward span tasks is spatial in nature (Costa, 1975; Hoshi et al., 2000; Rapport, Webster, & Dutra, 1994; Weinberg, Diller, Gertsmann, & Schulman, 1972), then an age deficit in spatial processing could result in backward span being more age-sensitive than forward span. All of this, however, proceeds from the premise that backward span actually is more affected by aging than forward span. According to the technical manual for the Wechsler Adult Intelligence Scale – Third Edition (WAIS-III; Psychological Corporation, 1997a): The ability to repeat digits in the order of verbal (forward) presentation . . . tends to remain relatively stable with aging in normally functioning men and women . . .Digit Span Backward, however, is more affected by aging and by impairment . . . [N]ormally functioning adults over 70 years old show a greater discrepancy [than younger adults], with a significantly shortened backward span. (pp. 187–188) The experimental literature, however, presents conflicting evidence on this point. On the one hand, Babcock and Salthouse (1990) reported a meta-analysis that indicated a greater effect of age on backward than forward digit span. On the other hand, a meta-analysis by Verhaeghen et al. (1993) showed that the effect size for backward digit span did not differ from that for forward digit span. Moreover, Gregoire and Van der Linden (1997) analyzed data from the standardization sample for the French adaptation of the Wechsler Adult Intelligence Scale – Revised (WAIS-R) and found that the slope of the age-related decrease in forward digit span was the same as that for backward digit span. The evidence regarding a differential agerelated decline in verbal and visuospatial memory span is also conflicting. For example, a recent study by Myerson et al. (1999) showed that although young adults and older adults had similar digit spans, the older adults had much lower spatial spans than young adults. Other studies (e.g., Dolman, Roy, Dimeck, & Hall, 2000; Jenkins et al., 2000) have also reported greater age differences in memory span for visuospatial stimuli (e.g., locations and gestures) than for verbal stimuli (e.g., digits and words). In contrast, Salthouse, Kausler, and Saults (1988) reported evidence against a differential age-related decline in spatial memory span. Salthouse et al. presented participants with a matrix of letters, with certain letters highlighted, and instructed them to remember either the locations of these highlighted letters or the letters themselves. Regression analyses indicated that the rate of decrease in memory performance with age was similar for both verbal (letters) and spatial (locations) materials. A second study (Salthouse, 1995) examined parallel verbal and visuospatial versions of three different working memory tasks, and this study also failed to find evidence that the visuospatial memory abilities of older adults are differentially affected relative to their verbal memory abilities. Given the mixed evidence with respect to both forward versus backward span and verbal versus visuospatial memory span, a large scale study is needed to resolve these issues. The data from the standardization tables of the Wechsler Memory Scale – Third Edition (WMS-III) and the WAIS-III (Psychological Corporation, 1997b) afford the opportunity for such a study. The standardization sample included more than a thousand adults of various ages, and each age group was selected to be representative of the United States population with respect to sex, level of education, race=ethnicity, and geographic region. The WMS-III includes measures of forward and backward digit span as well as forward and backward spatial span (Psychological Corporation, 1997a). The WMS-III also includes a new measure of working memory, the Letter-Number Sequencing subtest, in which a person is presented with a series of numbers and letters, and then must recall the numbers in numerical order followed by the letters in alphabetical order. Because Letter-Number Sequencing presumably places an even higher load on the ability to manipulate information online than either forward or backward digit span, it may provide a more sensitive test of the effects of age on the processing or executive aspects of working memory. AGE DIFFERENCES IN MEMORY SPAN MEASURES 21 D o w n l o a d e d B y : [ W a s h i n g t o n U n i v e r s i t y i n S t L o u i s ] A t : 2 3 : 3 7 2 1 A p r i l 2 0 1 0