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Objectives: Whether people with Alzheimer's Disease present with accelerated long term forgetting compared to healthy controls is still debated. Typically, accelerated long term forgetting implies testing the same participants repeatedly over several delays. This testing method raises the issue of confounding repetition effects with forgetting rates. We used a novel procedure to disentangle the two effects.
Methods: Four short stories were presented during an initial in-person assessment of 40 patients with Alzheimer's Disease and 42 age-matched healthy controls. Our aim was for participants to reach a score of 70% correct (9 out of 13 questions) at encoding. If this criterion was not achieved after the first trial, the four stories were presented again (in a different order); participants took the 1 min filler task again and were then retested. We repeated this process until participants reached the 70% criterion or to a maximum of four trials. Cued recall memory tests were completed during follow-up telephone call(s) at different delay intervals. Study material was presented only at encoding, then probed with different question sets on all other delays. Each question set tested different sub-parts of the material. The experiment employed a mixed design. Participants were randomly allocated to either a condition without retrieval practice or a condition with retrieval practice. Participants in the condition without retrieval practice were only tested at two delays: post encoding filled delay and at one month. Participants in the condition with retrieval practice were tested at four delays: post encoding filled delay, one day, one week and one month. Our methodological design allowed us to separate the effects of retesting from the effects of delay.
Results: Alzheimer's Disease patients showed a significant encoding deficit reflected in the higher number of trials required to reach criterion. Using Linear Mixed Models, we found no group by delay interactions between the post encoding filled delay retrieval and one month delays, with Alzheimer's Disease groups having a similar decline in performance to healthy controls, irrespective of testing condition. Significant condition by delay interactions were found for both groups (Alzheimer's Disease and healthy controls), with better performance at one month in the condition with retrieval practice.
Conclusions: Our data showed that Alzheimer's Disease is not characterised by accelerated long term forgetting, patients in our sample forgot at the same rate as healthy controls. Given the additional trials required by Alzheimer's patients to reach the 70% correct criterion, their memory impairment appears to be one of encoding. Moreover, Alzheimer's Disease patients benefited from repeated testing to the same extent as healthy controls. Due to our methodological design, we were also able to show that performance improved under repeated testing conditions, even with partial testing (sampling different features from each narrative on every test session/delay) in both healthy controls and Alzheimer's Disease.
