Cognitive and Neural Correlates of Implicit Associative Memory in Young and Older Adults

Declines in episodic memory are a hallmark of cognitive aging. One explanation is an age-related deficit in binding, or associating, the separate contextual features of a memory event, a process that depends on the medial temporal lobes (MTL). An alternative viewpoint links the episodic memory declines to an impairment in strategic recollection of contextual features, a process that depends more on the prefrontal cortex (PFC). Assessing implicit memory of new associations is a way to distinguish between these viewpoints. To date, mixed findings have emerged from studies of implicit associative memory in aging. One factor that may account for the variability is whether the manipulations inadvertently involve explicit processes. In 6 experiments I present a novel paradigm of conceptual associative priming in which subjects make speeded associative judgments about unrelated objects. Using a size classification task, Experiment 1 showed equivalent associative priming between young and older adults. Experiment 2 generalized the results of Experiment 1 to an inside/outside classification task, while replicating the typical age-related impairment in associative recognition. Experiment 3 showed that associative priming in this task cannot be explained by explicit contamination. In Experiments 4 and 5, older adults showed preserved rapid response learning, a complementary form of incidental associative processing. Lastly, Experiment 6 used event-related fMRI to examine the neural basis of associative priming. During implicit associative retrieval, older adults showed under-recruitment of MTL regions coupled with over-recruitment of right dorsolateral PFC. Furthermore, activity in right dorsolateral prefrontal cortex correlated with behavioral priming in older but not young adults, consistent with the hypothesis that older adults may recruit prefrontal regions to compensate for MTL dysfunction. This study documents the first evidence that recruitment of right DLPFC operates during associative priming, on a task in which no age differences were found behaviorally. Taken together, the experiments provide an important example of a form of associative processing that is unimpaired in older adults. However, an absence of age differences in the behavioral measure did not map onto the same pattern of neural activations in the two age groups. This finding is consistent with patterns of structure-function reorganization in aging