How Healthy Aging and Dementia Impact Memory Search

How Healthy Aging and Dementia Impact Memory Search Ana Sofia Morais (morais@mpib-berlin.mpg.de) & Hansjorg Neth (neth@mpib-berlin.mpg.de) Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, Germany Thomas Hills (T.T.Hills@warwick.ac.uk) Dept. of Psychology, University of Warwick, Coventry CV4 7AL, UK Abstract We model the semantic recall sequences of 424 older adults aged between 69 to 103 years in the animal fluency task. Our results suggest that, under normal intellectual functioning, memory search in old age (69–84 years) is consistent with a dynamic process that switches between retrieval probes. With dementia and very old age (85–103 years), however, memory search seems to become more consistent with a static process that activates items in memory as a function of their frequency. The weight that probes have in determining the activation of items in memory seems to be an informative signature of the impact of healthy aging and dementia on memory search. Our results show that, with healthy aging and dementia, the activation of items in memory is increasingly more determined by the frequency of past experience with those items. Keywords: Search; semantic memory; modeling; aging. Introduction Ronald Reagan became the oldest president elected in American history, when he took office at age 69, in 1981. He was diagnosed with Alzheimer’s in 1993, the most common form of dementia, four years after he left office. Yet Reagan’s signs of memory decline while in office – like forgetting names and being at a loss for words – have led to much speculation about how early dementia had set in. The question was whether his memory slips were a sign of normal aging or rather the early symptoms of dementia. Studies using the animal fluency task (“name all the animals you can think of”; Thurstone, 1938) have shown that healthy older adults recall fewer items relative to younger adults within a limited time interval (e.g., Hills, Mata, Wilke, & Samanez-Larkin, 2013; Kozora & Cullum, 1995), in much the same way as older adults with dementia produce fewer items compared with healthy older adults (e.g., Beatty, Salmon, Testa, Hanisch, & Troster, 2000; Epker, Lacritz, & Cullum, 1999). In this paper, we examine how healthy aging and dementia impact search in semantic memory beyond the sheer reduction in the number of recalls. To this end, we formally model the recall sequences of 424 older adults aged between 69 to 103 years in the animal fluency task. We then examine individual differences in model fit and parameter estimates, as a way of identifying signatures of cognitive decline in memory search with healthy aging and dementia. Static and Dynamic Search in Semantic Memory Memory retrieval can be viewed as the result of probing a memory representation with one or more probes to activate a response (e.g., Gronlund & Shiffrin, 1986; Walker & Kintsch, 1985). We apply two classes of models based on prior work – static vs. dynamic – that make different assumptions about how retrieval probes are used to search memory in the fluency task (Hills et al., 2013). Consider the following two types of probes. One type of probe, the frequency probe, activates animal names in memory as a function of their frequency of past occurrence. A second type of probe, the similarity probe, activates each item in relation to its semantic similarity to the previously-recalled item. In a static model, search is guided by the same probe arrangement over the entire recall interval (i.e., by either probe alone or by a combination of the two). A dynamic model, on the contrary, switches between a frequency probe and a probe that combines frequency and similarity to traverse clusters of similar items in memory. When leaving a cluster, a dynamic model uses frequency alone to find a new cluster, and goes back to using a combination of frequency and similarity information as the new cluster is entered. Past work has found that, from early to late adulthood, search in memory is overall more consistent with a dynamic search model than with a static model that uses the same probe arrangement during the entire recall sequence (Hills et al., 2013). In this paper, we examine the relative fit of static and dynamic models in old and very old age, for healthy individuals and individuals diagnosed with dementia. In very old age, do people use memory retrieval probes more in accordance with a dynamic model than with a static model as younger cohorts do? Moreover, we test alternative mechanisms of decline in memory search by investigating individual differences in the use of retrieval probes. We next turn to a brief discussion of the alternative mechanisms of decline in memory search. Mechanisms of Decline in Memory Search Existing hypotheses proposed to account for age-related differences in the number of items produced in fluency tasks make different assumptions about how memory search declines with aging. The age invariance hypothesis proposes that aging is associated with unaffected semantic processing, and thus predicts no age differences in the use of retrieval probes (Mayr & Kliegl, 2000). Two alternative hypotheses argue that the impact of aging affects the ability to switch between probes. The cluster-switching hypothesis views memory retrieval as a dynamic process involving a search for semantic categories like “pets”, and a search for words within a category (e.g., “dog”) (Troyer, Moscovitch, & Winocur, 1997; Troyer, 2000). A common finding is that aging is associated with fewer total switches between categories, leading to the proposal that aging is associated