Dissociable effects of schema congruency on item recognition and source memory across levels of expectedness and surprise

Prior knowledge can enhance memory for both schema-congruent and schema-incongruent events. According to the SLIMM framework, memory performance generally follows a U-shaped pattern, with both highly expected and highly surprising events being better remembered than neutral ones. We tested this prediction across four experiments in which participants viewed scenes containing congruent or incongruent objects, rating each as expected or surprising on a four-level scale. Memory was assessed using: (1) old/new item recognition categorized by recollection and familiarity, and (2) source memory, in which participants reported whether objects had been rated as expected or surprising at study. Schema effects on memory varied by test type. Item recognition consistently showed an advantage for expected trials only, primarily via recollection, while familiarity (if assumed independent of recollection) increased with surprise level-contrary to SLIMM's predictions. We hypothesize that schema-related retrieval strategies enhance recollection of expected objects. Source memory did produce a reliable U-shaped pattern, even though this particular task should only produce an advantage for surprising trials according to SLIMM. We suggest participants tagged events as "surprising" or "expected" during study and could later recall the tag. When the tag was 'weak' or unavailable-more likely for moderately categorized events-participants fell back on schema-related retrieval strategy, which benefits retrieval of expected events. Together, these mechanisms produce the observed U-shaped pattern for source memory. Our findings reveal the complex retrieval test dependent manner in which prior knowledge shapes episodic memory.