Complementary roles of medial temporal lobes and mid-dorsolateral prefrontal cortex for working memory for novel and familiar trial-unique visual stimuli.

It has been suggested that working memory (WM) for novel information requires the medial temporal lobes (MTL), but is not necessary for WM for familiar stimuli. In previous studies that directly compared WM for novel and familiar stimuli, only the novel stimuli were trial-unique. Here, 16 young human subjects performed a Sternberg WM task with visual scenes while in a functional magnetic resonance imaging scanner. All task stimuli were trial-unique, but were either new (Novel condition) or previously learned (Familiar condition). This design allowed investigation of whether MTL and prefrontal cortex (PFC) activity is related specifically to the novelty/familiarity of the stimuli or to their trial-unique status during WM. We observed greater hippocampal and parahippocampal activity during encoding and maintenance for novel than for familiar stimuli. In contrast, right mid-dorsolateral PFC (dlPFC) activity was greater during encoding of familiar than novel stimuli. The mid-dlPFC was not recruited during maintenance or for retrieval when the Familiar condition was contrasted with the Novel condition. However, left mid-dlPFC activity was present at retrieval when correct Match trials (i.e. hits) were contrasted with correct Non-match trials (i.e. correct rejections) for the Novel condition. The results support the hypothesis that MTL regions are required for the encoding and maintenance of novel stimuli during WM, demonstrating that the observed MTL activity is not related to the trial-uniqueness of the stimuli per se. Furthermore, the observed activation pattern in mid-dlPFC suggests a role for the mid-dlPFC in executive control-associated processes related to monitoring of scene familiarity at encoding and retrieval during WM.