Abstracts

Rationale: Clinical evaluation of memory usually includes testing retention at some time after learning or encoding has taken place. The delay interval between completion of the learning phase and the subsequent memory test varies depending on the test and on the philosophy of the examiner. Whereas some argue that a short delay of 20 or 30 minutes is sufficient to elicit some forgetting, others prefer a longer interval that will approximate the demands of everyday life more closely. Further, different types of information are represented in different ways and in separate regions of the brain, so one can expect differences in the retention of material as a function of tests in various modalities. The purpose of the present study was to determine how different delay intervals affect memory for different types of material in healthy individuals. We examined performance on six learning and memory tasks in two separate experiments, each comparing a 30-minute to a 24-hour delay interval.Methods: In Experiment 1, 50 subjects were tested on learning and memory for lists of 13 abstract nouns, 13 abstract designs, and 10 odors of low to medium familiarity. The tasks used in Experiment 2 (a separate group of 50 subjects) were a list of 15 concrete nouns, 12 unfamiliar faces, and a story learning test. In both experiments half of the subjects were assigned to a 30-minute delay condition and half to a 24-hour delay, and all subjects were treated in exactly the same way except for the length of the delay interval.Results: In both experiments we calculated percent retained scores, comparing the amount remembered after a delay to the amount remembered in the final learning trial, for each subject. Performance of the 30-minute delay group was compared to that of the 24-hour delay group in two 2-way ANOVAs, one for the 3 tasks of Experiment 1 and one for the 3 tasks of Experiment 2. Subjects retained significantly less after the 24-hour than the 30-minute delay on five of the six tasks; only memory for odors showed no significant difference on this measure. Separate raw-score analyses for the individual tasks showed that the extent to which the delay-interval groups differed was not the same for all tasks; besides odors, memory for faces and designs was not worse after 24 hours in the raw-score analyses. Conclusions: These results indicate that although more is forgotten after 24 hours than after 30 minutes on most tasks, the optimal delay interval for assessment of long term memory is not the same for all tasks. The results from the story, concrete- and abstract-words tasks indicated significantly greater losses after 24 hours for verbal material, even in terms of raw performance scores, whereas the difference between a short and long delay is less important for nonverbal material. These findings have important practical implications for clinical evaluations, as an informed choice can be made when deciding upon the delay interval to be used in a given memory test.