Overflow of Working Memory from an Easily Accessible Active State

Working memory is a system that realizes short-term memory retention and is essential to everyday activities. Recently, it has been suggested that working memory items can be maintained in both an active state with sustained neural activity and a latent state without any explicit neural activity. However, how easily we can retrieve memories in each state and how much information can be retained in the active state remains unknown. Here, we addressed these questions by adopting a novel experimental paradigm for measuring the reaction time required to recall a letter of the alphabet when presented with a corresponding color, based on memorized color/letter pairs. The results demonstrated that when participants retained only two pairs of items, they could recall both pairs similarly quickly. However, when there were two or more memorized pairs, the most recent or internally attended pairs were recalled more quickly than the other pairs. These results suggest that memories in the "active" state are accessed first and memories that have overflowed into the "latent" state are accessed afterward. Additionally, the capacity of the easily accessible active state is extremely limited (to approximately two chunks of information) compared with the traditionally considered working memory capacity (four to seven chunks). These results provide robust behavioral support for the co-existence of multiple states in working memory and underscore the need for a more detailed classification of the working memory capacity. SIGNIFICANCE STATEMENTWe sometimes cannot quickly retrieve what we are sure we remember. Regarding working memory retrieval, we observed a phenomenon that the most recently encoded memory was retrieved quickly whereas others were retrieved afterward, showing an "overflow" of memory items from an easily accessible active state. This observation has two critical implications: 1) when maintaining multiple items in working memory, not all items are functionally the same, and there is a dissociation between easily accessible active items and items that have overflowed to a latent state; 2) the number of items that can be maintained in the active state is two, which is remarkably smaller than the traditionally considered working memory capacity of four, necessitating the update of working memory models.