How does context variability affect representational pattern similarity to support subsequent item memory?

Episodic memories are neurally coded records of personally experienced events across a lifetime. These records are encoded via medial temporal lobe structures in the brain, including the hippocampus, and are commonly called "representations" or "memory traces". Existing studies indicate that information about the neural signal corresponding to a memory representation can be found in functional magnetic resonance imaging (fMRI) data when the pattern across its smallest units (voxels, often 3mm3 sections of the brain) is measured. Many prior studies have measured these voxel patterns in response to stimuli as if they are a spontaneous brain function, regardless of cognitive factors. These studies sometimes find that similarity in the voxel patterns across repetition of a to-be-remembered event predicts later memory retrieval, but the results are inconsistent. The current fMRI study investigated the possibility that cognitive goals during encoding affect the type of neural representation (voxel pattern) that will later support memory retrieval. This seems likely because prior behavioral studies indicate that cognitive variability across repetitions of an event benefits later memory retrieval, which is difficult to reconcile with the common finding that voxel pattern variability across repetitions of an event harms later memory. We tested this hypothesis by comparing voxel patterns that support later memory retrieval to those associated with forgotten items in the medial temporal lobe, including the hippocampus, and lateral occipital cortex. Overall, as previously demonstrated, the behavioral results showed that exposure to variable cognitive goals across repetition of events during encoding benefited subsequent memory retrieval. Voxel patterns in the hippocampus indicated a significant interaction between cognitive goals (variable vs. consistent) and memory (remembered vs. forgotten) such that less voxel pattern similarity for the repeated events with variable cognitive goals, but not consistent cognitive goals, supported later memory success. In other words, variable hippocampal neural activations for the same events under different cognitive goals predicted better later memory performance. However, there was no significant interaction in neural pattern similarity between cognitive goals and memory success in medial temporal cortices or lateral occipital lobe. Instead, higher similarity in voxel patterns in right medial temporal cortices was associated with later memory retrieval, regardless of cognitive goals. In the lateral occipital lobe, the main effects of cognitive goals, hemisphere, and memory success were found but no interactions. In conclusion, we found that the relationship between pattern similarity and memory success in the hippocampus (but not the medial temporal lobe cortex) changes when the cognitive goal during encoding does or does not vary across repetitions of the event.