Perseverations in Alzheimer's disease: memory slips?

Previous studies of verbal fluency tasks reported higher rates of repeated responses in Alzheimer's disease (AD) compared to elderly controls. The present investigation aimed at determining if perseverations are caused by word retrieval deficits or working memory deficits, both of which are commonly observed in AD. Based on current theories of lexical processing and working memory, we derived specific predictions concerning the lag between the first occurrence of a word and its repetition. With word retrieval deficits, repetitions are expected to be progressively less frequent at greater lags; conversely, with working memory deficits, repetitions should occur especially after long lags. These predictions were tested analyzing the performance of 392 AD individuals in verbal fluency tasks. The finding of lags that were significantly longer than would be expected by chance is consistent with the hypothesis that perseverations are primarily caused by working memory deficits. Specifically, we propose that perseverations stem from an impairment affecting the working memory mechanisms that control response monitoring. We also investigated the relationship between perseverations and other cognitive deficits observed in AD. We discuss the implications of our findings for understanding the nature of perseverations, the effects of working memory deficits in AD, and the neural correlates of working memory components.