Rates of forgetting in Alzheimer dementia.

Patients diagnosed with Alzheimer's disease (AD) were compared with healthy controls on a picture recognition task, a forced choice word recognition task, a forced choice design recognition task, a picture recall task and a stem completion task. Performance on recognition and word stem completion was assessed at 1, 10 and 20 min after exposure to experimental stimuli, as these are delays across which previous studies had suggested there might be differing forgetting rates. AD patients did not show significantly faster rates of forgetting relative to controls on picture recognition, design recognition, word recognition or stem completion, after levels of learning had been matched as closely as possible. Moreover, once initial learning was equated in a small number of subjects, there were no qualitative differences between AD patients and controls following inclusion and exclusion instructions on the stem completion task. In particular, those AD patients who were matched to controls for initial levels of "recollection" showed comparable forgetting rates in recollection (or cued recall). Although matching was more difficult for a picture recall task, both the main analysis and subgroup analysis indicated faster forgetting in the AD group than controls, suggesting a difference between "free recall" and recognition or cued recall measures, comparable with the finding in a parallel study of organic amnesia.