The benefit of retrieval practice on cued recall under stress depends on item difficulty

Retrieval practice, the act of recalling previously studied information, yields greater memory retention than repeated studying (retrieval practice effect). Retrieval practice can also protect memories against the negative effects of stress at retrieval in free recall tests. In cued-recall tests, however, retrieval practice may not provide complete protection against stress-related memory impairments. Here we ask whether these conflicting results may be explained by the relative difficulty of the study materials. Participants (59 men) first studied 40 Swahili-Portuguese word pairs. Half of the pairs were then repeatedly studied while the other half were repeatedly recalled. In each condition, half of the pairs were easy to remember (high in memorability) while the other half were more difficult to remember (low in memorability). Participants returned 1 week later for a final cued-recall test. Half of the participants underwent a stress-induction protocol (modified Socially Evaluated Cold Pressor Test) 25 min before the final test for all 40-word pairs; the other half underwent a control condition. Salivary cortisol and questionnaire responses were used to assess the efficacy of stress induction. Participants recalled more word pairs learned via retrieval practice than via repeated study, replicating previous research. More importantly, the results revealed an interaction such that stress decreased recall for easy items, but increased recall for difficult items that had been successfully retrieved during encoding (conditional analysis). The results suggest that the impact of stress on cued recall depends both on the learning strategy and on the intrinsic difficulty of the to-be-learned materials.