Input, Retention, and Output Factors Affecting Adult Age Differences in Visuospatial Short-Term Memory

AGE differences in verbal short-term or working memory have been studied extensively (for reviews, see Hoyer & Verhaeghen, 2006 and Zacks, Hasher, & Li, 2000). Age differences in visuospatial working memory have received less attention, although some studies have attempted to determine whether age differences are greater or lesser in one than the other (Allen, 1991; Allen, Kaufman, Smith, & Propper, 1998a; Jenkins, Myerson, Joerding, & Hale, 2000; Myerson, Emery, White, & Hale, 2003; Salthouse, 1995; Schear & Nebes, 1980; Shelton, Parsons, & Leber, 1982; Tubi & Calev, 1989; but also see Cattaneo, Bhatt, Merabet, Pece, & Vecchi, 2008). Other questions, too, have been asked and answered. Hartley, Speer, Jonides, Reuter-Lorenz, and Smith (2001) demonstrated that verbal and visual working memory, which are dissociated in younger adults, also appear to be largely dissociated in older adults. Rowe, Hasher, and Turcotte (2008) demonstrated that proactive interference in visual short-term memory was greater in older than in younger adults as had been found previously for verbal working memory (Lustig, May, & Hasher, 2001; May, Hasher, & Kane, 1999). What have not been fully addressed are the origins of age-related differences in visual short-term memory. Why do older adults perform less well than younger adults? We can identify at least three possible loci of age differences, corresponding to differences at the stages of input, retention, and recall. First, at input, one possibility is that the perceptual registration of the stimulus may be less precise. Second, during retention, older adults may lose precise information through simple decay or through failure to use maintenance mechanisms or operations employed by younger adults. Third and finally, at recall, older adults may have a different decision criterion for how close a probed location must be to a remembered location for a match response to be given. The three reasons are not mutually exclusive. Visual short-term or working memory has been studied extensively in younger adults (for recent summaries, see Luck, 2007 and Luck & Hollingworth, 2008). One of the most commonly used procedures involves sequential comparison (e.g., Luck & Vogel, 1997). A to-be-remembered visual display is presented, and then, after a retention interval (RI), a new display is presented and the individual is asked to determine whether the new display matches the original display in some particular way. Allen and colleagues (1998b) reported the results of one sequential comparison experiment that explored whether age-related differences in spatial memory reflected primarily early stimulus-encoding stages or later processing stages. In their Experiment 2, they manipulated the number of locations in which a stimulus whose location was to be remembered could appear. Older adults were more affected by an increase in the number of locations than younger adults, consistent with an age difference in the precision of initial encoding. This interaction was not qualified by higher order interactions with other manipulations, so the authors concluded that there were also age differences in later stages. Allen (1991) had previously reported that younger and older adults did not differ in response biases that would have been suggestive of age differences in output processes. In an attempt to provide a more systematic and definitive examination of the contributions of input, retention, and output factors, we revisited the question raised by Allen and colleagues (1998b). In the experiments reported here, we briefly displayed four dots simultaneously as a memory set; after a RI, we presented a probe that either matched one of the memory-set dots or mismatched by a manipulated distance. The stimuli were small dots that could appear over a wide area of the visual display rather than characters or symbols appearing in only seven possible locations as Allen had used (Allen, 1991; Allen et al., 1998a, 1998b). We selected a single set size of four, as did Allen, because it was a nontrivial memory load but also was likely to be within the short-term span of most younger and older adults. Our intention was to select a task load that would result neither in floor effects for older adults nor in ceiling effects for young adults. In our first experiment, we probed for age differences in input or output; in the second experiment, we probed for age differences during the retention period. The broader theoretical question we intended to address was whether any age differences we observed could be accounted for by a change in a single mechanism or whether changes in independent mechanisms seem more plausible. We will return to this issue in the Discussion.