Forgetting Rate on the Recency Portion of a Word List Differentiates Mild to Moderate Alzheimer's Disease from Other Forms of Dementia

Patients with Alzheimer's disease (AD) demonstrate a disproportionately larger forgetting rate in episodic memory tasks. Previous studies documented that, in comparison with healthy controls, the increased forgetting manifested by AD patients in word list recall tasks is confined to the recency portion of the list with normal forgetting rates on the primacy and mid-list portions. In this study we compared the primacy, mid-list, and recency ratios, obtained by dividing the immediate and delayed recall of words in position 1-4,5-11, and 12-15 of a 15-word list, in different groups of demented patients, i.e., AD, frontal variant of frontotemporal dementia (fvFTD), Lewy body disease (LBD), subcortical ischemic vascular dementia (SIVD), and a group of normal controls (NC). The aim was to investigate whether the above reported forgetting pattern would differentiate AD performance from that of other dementia groups. Results of the statistical analysis showed that only the recency ratio differentiated AD from patients in the other dementia groups. Consistently, hierarchical logistic regression analyses demonstrated that the recency ratio discriminated between AD patients and individuals affected by other forms of dementia. In particular, the discrimination power was high in differentiating AD from fvFTD patients but was less accurate in differentiating AD from LBD and SIVD patients. We assume that the increased forgetting in AD patients is due to a deficit in memory consolidation mechanisms (specific to AD) that prevent the terminal items in a list from being transferred from a temporary short-term memory store to a stable long-term memory store.