Encoding Variability: When Pattern Reactivation Does Not Benefit Context Memory

A growing body of evidence suggests that neural pattern reactivation supports successful memory formation across multiple study episodes. Previous studies investigating the beneficial effects of repeated encoding typically presented the same stimuli repeatedly under the same encoding task instructions. In contrast, repeating stimuli in different contexts is associated with superior item memory, but poorer memory for contextual features varying across repetitions. In the present functional magnetic-resonance imaging (fMRI) study, we predicted dissociable mechanisms to underlie the successful formation of context memory when the context in which stimuli are repeated is either held constant or varies at each stimulus presentation. Twenty participants studied names of famous people four times, either in the same task repeatedly, or in four different encoding tasks. This was followed by a surprise recognition memory test, including a source judgement about the encoding task. Behaviourally, different task encoding compared to same task encoding was associated with fewer correct context memory judgements but also better item memory, as reflected in fewer misses. Searchlight representational similarity analysis revealed fMRI pattern reactivation in the posterior cingulate cortex to be higher for correct compared to incorrect source memory judgements in the same task condition, with the opposite pattern being observed in the different task condition. It was concluded that higher levels of pattern reactivation in the posterior cingulate cortex index generalisation across context information, which in turn may improve item memory performance during encoding variability but at the cost of contextual features.