Reaction time, memory strength, and fMRI activity during memory retrieval: Hippocampus and default network are differentially responsive during recollection and familiarity judgments

Retrieval is often subdivided into recollection and familiarity. Memory strength and reaction time (RT) differ for each, complicating functional magnetic resonance imaging (fMRI) studies of these processes. Recollection leads to greater activity in the hippocampus and default network (DN). Increased DN activity with recollection is thought to reflect self-referential processes, but prior studies have not accounted for varying RT, which modulates DN activity and is consistently faster for recollection than for familiarity. This study examined the influence of RT and memory strength on recollection and familiarity activity. The results show the hippocampus functionally dissociated from DN during retrieval. DN was generally influenced by RT and signal was suppressed when subjects were task-engaged in recollection or familiarity; suppression was greater for slower trials of either type. The hippocatnpus showed a positive deflection of fMRI activity only for recollection trials; activation was greater for slower recollection trials, but RT did not influence hippocampal activity during familiarity trials.