Retrospective memory trace sustained by the human hippocampus during working memory task

Working memory is a subcategory of short-term memory that voluntarily maintains behaviourally relevant information to prepare for a subsequent action. An established theory is that working memory is supported by the prefrontal cortex (PFC) for executive control, while the hippocampus (HPC) is largely involved in long-term episodic memory. Recent studies suggest that the HPC is also involved in perception and short-term storage. However, it remains unclear whether the HPC supports active maintenance of short-term memory as working memory. To address this question, we devised a new delayed matching-to-sample task in which two visual items were presented at different locations one by one as samples. The sequential presentations of sample stimuli allowed us to dissociate the contents of working memory (i.e., identities and locations of two samples) from the constituent perceived information of single samples. By applying representational similarity analysis (RSA) to the blood-oxygen-level-dependent (BOLD) signals of human participants, we investigated the delay activity after the two sample presentations. The results of the RSA showed that the right HPC signalled only the second sample as a conjunctional representation of its item identity and location. In contrast, the right PFC, including both lateral and medial parts, represented the conjunctional information of both samples. These results suggested that the HPC may support short-term memory for retrospective coding to retain information of the last event rather than for prospective coding coupled with working memory.