Predictions and declarative memory encoding: two fMRI paradigms provide slim pickings for SLIMM

Memory is often better for both highly unexpected and highly expected information. To explain this U-shape relationship between memory and expectancy, the SLIMM framework appeals to two memory systems: one involving the hippocampus and another involving the medial prefrontal cortex (MPFC). Specifically, SLIMM predicts that these two brain regions support opposite ends of the Ushape, with the hippocampus supporting memory for unexpected events, and MPFC underpinning memory for expected events. We tested these predictions in two fMRI paradigms: one examined memory for the location of objects within scenes (e.g., a toothbrush in a bathroom, either on a sink, as expected, or on top of a bin, as not expected); the second tested memory for words that completed well-known idioms either in an expected or unexpected fashion (e.g., “Don’t count your chickens before they… hatch/cook”). Both paradigms replicated the predicted U-shaped relationship between expectancy and memory performance. However, neither study found the predicted interaction between expectancy and subsequent memory (remembered vs forgotten trials) in terms of fMRI activation at encoding in the a priori defined hippocampal and MPFC regions-of-interest. We discuss the implications of these findings for SLIMM, and how our results fit within the broader literature on expectancy and memory.