Executive dysfunction can explain word-list learning disability in very mild Alzheimer's disease: the Tajiri project

Elderly people with questionable dementia (i.e. a Clinical Dementia Rating (CDR) of 0.5) have been focused on as representing the borderline zone condition between healthy people and dementia patients. Many of them are known to have pathologic traits of very mild Alzheimer's disease (AD). Although they present mild memory disorder, the underlying mechanism has not been fully investigated. Herein is reported the mechanism of learning disability in very mild AD. Eighty-six CDR 0.5 participants and 101 age- and education-matched healthy controls (CDR 0) were randomly selected from a community in the town of Tajiri, Miyagi Prefecture. The word-recall task of the Alzheimer Disease Assessment Scale-Japanese (i.e. learning and recall of 10 words) was administered. The numbers of words recalled in each trial and those never recalled throughout the trials were compared for the two CDR groups. The serial-position function was depicted for three parts (i.e. primary, middle, and recency). The CDR 0.5 group recalled significantly fewer words than the CDR 0 group. The number of never-recalled words was greater in the CDR 0.5 group. A remarkable difference was found in the middle part of the word list. The number of never-recalled words of the CDR 0.5 group was greater in the middle part. The large number of never-recalled words accounted for the poor learning performance of very mild AD participants. The results suggested that very mild AD participants have difficulty in learning and retaining words in the middle part of the word-list because of a functional decline of the central executive system.