Dissociable contributions of prefrontal cortex and the hippocampus to short-term memory: Evidence for a 3-state model of memory

Behavioral research has yielded conflicting results regarding the architecture of short-term memory (STM). Whereas a consensus has emerged that within STM a single chunk within the focus of attention (FA) has a privileged status, it is unclear whether further distinctions exist. One proposal is that outside of FA, memory is all of one sort with a continuous progression from STM to long-term memory (LTM). On the other hand, sharp performance drop-offs when STM is loaded with more than 4±1 items suggest distinctions between STM and LTM. We use functional magnetic resonance imaging (fMRI) to adjudicate between these theories. A neural triple dissociation provided evidence for a 3-state model of memory. Critically, prefrontal cortex was selectively enhanced to retrieval from activated portions of LTM whereas the hippocampus was associated with retrieval of items within putative 4±1 capacity limits. We hypothesize that the associative properties of the hippocampus serve to inter-relate information actively maintained in STM which not only promotes strong STM, but also lays the foundations for subsequent LTM. By contrast, information not actively maintained in mind requires top-down retrieval processes mediated by the prefrontal cortex. These data provide key insights into the architecture of STM and its relationship to LTM.