Interaction of Working Memory and Long-Term Memory in the Medial Temporal Lobe

Abstract

Recent findings indicate that regions in the medial temporal lobe (MTL) do not only play a crucial role in long-term memory (LTM) encoding, but contribute to working memory (WM) as well. However, very few studies investigated the interaction between these processes so far. In a new functional magnetic resonance imaging paradigm comprising both a complex WM task and an LTM recognition task, we found not only that some items were successfully processed in WM but later forgotten, but also that a significant number of items which were not successfully processed in the WM task were subsequently recognized. Activation in the parahippocampal cortex (PHC) during successful WM was predictive of subsequent LTM, but was correlated with subsequent forgetting if the WM task was not successfully solved. The contribution of the PHC to LTM encoding thus crucially depends on whether an item was successfully processed in the WM task. Functional connectivity analysis revealed that across-trial fluctuations in PHC activity were correlated with activation in extensive regions if WM and LTM tasks were correctly solved, whereas connectivity broke down during unsuccessful attempts to do the task, suggesting that activity in the PHC during WM has to be well controlled to support LTM formation.

Regional activation of the human medial temporal lobe during intentional encoding of objects and positions

“…However, no significant effect of content was found during preparation or rehearsal stages. These results are inconsistent with previous studies that have implicated distinct roles for MTL regions object and spatial working memory (Stern et al, 2001;Petersson et al, 2006;Olson et al, 2006a,b;Nichols et al, 2006;Axmacher et al, 2007Axmacher et al, , 2008. Such studies have demonstrated a role of the MTL and inferior temporal cortex in active maintenance of novel information.…”

Working Memory: Maintenance, Updating, and the Realization of Intentions

“…There is imaging evidence that the hippocampus is engaged during working-memory maintenance under certain circumstances, such as during maintenance of novel information (e.g., Ranganath et al 2001;Cabeza 2004), and a recent high-resolution fMRI study of mediotemporal lobe (MTL) subregions provided evidence that neurons in these regions may act as a workingmemory buffer for novel information (Schon et al 2015). In the introductory hypothetical scenario, the first code was retained hours later, and some studies have linked workingmemory-related brain activity to subsequent long-term memory and found that parahippocampal-sustained fMRI activity during active maintenance was correlated with later memory performance (Schon et al 2004;Axmacher et al 2008; see also Rudner et al 2007; for related intracranial EEG evidence, see Axmacher et al 2007). Relatedly, findings by Ben-Yakov and Dudai (2011) show that poststimulus hippocampus activity contributes to the registration into long-term memory of complex real-life information.…”

Working Memory-Related Hippocampal Deactivation Interferes with Long-Term Memory Formation

“…Therefore, the effect of unsuccessful attempts to execute WM processes on LTM encoding could not be investigated. Using a WM task with a complex item manipulation, we recently found that parahippocampal activity predicted LTM formation only if the WM manipulation was executed successfully, whereas it was detrimental for LTM formation in incorrectly solved WM trials (Axmacher et al, 2008). This indicates that medial temporal regions may also be activated by unsuccessful attempts to perform a WM task, but that this activity actually deteriorates LTM formation.…”

“…Therefore, we do not know for sure how LTM is actually affected by the WM performance for specific items, i.e., it is unknown which items are actually maintained in WM. This is a problem which can only be addressed in tasks with both a WM and a LTM test (Schon et al, 2004;Ranganath et al, 2005;Axmacher et al, 2008). It would be interesting to extend these studies to lists with several items, although this would require a complex experimental design: To identify subsequent memory effects, items need to be presented individually.…”