What causes the spacing effect? Some effects of repetition, duration, and spacing on memory for pictures.

Three experiments examined effects of the spacing of repetitions on memory for pictures. In Experiment I, the duration of the first presentation (P(1)) was manipulated, as was P(1)-P(2) spacing. The effect of spacing on judged frequency was independent of P(1) duration. In Experiment II, pictures were given M massed presentations just prior to the P(M)-P(M+1) spacing interval. The form of the spacing curve was independent of M. Neither experiment confirmed the prediction of "overhabituation," derived from the habituation-recovery explanation of the spacing effect. In Experiment III, subjects made both duration and frequency judgments. The duration judgment results were not consistent with the notion that subjects remember multiple massed presentations as single occurrences of especially long duration. Some evidence from Experiments I and III suggests that an interrupted stimulus is recognized better than one that is not interrupted-a finding that, if replicable, would support the habituation-recovery account of the spacing effect.