Associative priming and explicit memory in aging and mild cognitive impairment

Advancing age is commonly associated with subjective memory complaints; these are usually substantiated by objective evidence of memory decline. Evidence of atrophy to relevant brain structures such as the hippocampus has supported the idea that memory is particularly vulnerable to the effects of aging. In individuals with amnestic mild cognitive impairment (MCI), memory is affected disproportionately to what would be expected in normal aging, and in some cases may progress to more severe diagnoses such as dementia or Alzheimer's disease. Behavioral and lesion-based studies of memory have provided experimental and clinical evidence of a distinction between explicit memory (the conscious recall of to-be-remembered information) and implicit memory (the unconscious recall of such material), and between implicit memory for novel versus preexisting (familiar) information. The primary purpose of the current study was to bridge data on these aspects of explicit and implicit memory with anatomical data in normal aging and in individuals with amnestic MCI.; Ninety participants completed an associative priming experiment which assessed implicit associative memory for novel and semantic word pairs. Following the priming experiment, all participants completed an incidental cued-recall test for the novel and semantic word pairs previously seen. In addition, each participant completed a brief neuropsychological screening battery which served the purpose of classification into different groups (amnestic MCI, healthy control). Lastly, structural magnetic resonance imaging (MRI) of hippocampal volumes was completed on the first consecutive forty-five participants. Results revealed that while MCI and control groups differed significantly on standardized, clinical measures of memory, there were no significant differences between groups on novel and semantic priming, or on incidental cued-recall. Additionally, within the entire sample, there was no association between measures of priming and standardized memory measures. Regression analyses revealed that for the forty-five participants who underwent an MRI, a standardized measure of explicit memory and novel priming accounted for a significant portion of variance in a ratio of hippocampal-to-hemisphere volume. Overall, these results suggest that while novel priming may depend in part on anatomical correlates of memory, at least in the current sample, amnestic MCI impairs explicit but not implicit associative memory.