Implicit Memory in Alzheimer's Disease: A Meta-Analysis

Meta-analysis was performed on 33 articles (36 experiments involving 564 Alzheimer's disease [AD] patients and 592 controls). Overall, AD patients were significantly impaired on implicit memory tests, r = .163 (a difference of .329 SD s from normal performance). They were impaired on nonverbal tests (e.g., fragmented pictures), word stem completion, classical conceptual tests (e.g., free association), and on word-based perceptual tests with long delays. However, they performed normally on word-based perceptual tests (e.g., perceptual identification of words) with short study test delays. They also performed normally on word stem completion if they were older than 75, possibly because their age-matched controls were also impaired. These results are neither qualified by study list length nor patient mental status. None of the theories in the field is compatible with the results.