Prefrontal working memory activity predicts episodic memory performance: A neuroimaging study

Working memory (WM) and long term memory (LTM) are different neuropsychological processes, although distinction between these domains is an area of debate. LTM is thought to rely on hippocampal circuitry. Cognitive neuroscience models imply that WM processing may at least partially support LTM within regions of the prefrontal cortex (PFC). We sought to determine the association between PFC based WM processing and LTM in the visuospatial domain. In contrast to prior work, we aimed to query if WM was involved in learning and free recall trials as measured by standard neuropsychological tests of LTM. Forty-three older adults (24 with a diagnosis of amnestic Mild Cognitive Impairment and 19 elderly controls) were included in the analysis. Patients completed a fMRI task of visuospatial maintenance WM in which they were required to match a previously studied complex shape with one of two probes. Extent of activity in the right PFC during the WM task was tabulated for each patient. Hippocampal volume was quantified from T1 scans. On a separate day patients completed neuropsychological testing, including the Brief Visuospatial Memory Test- Revised (BVMT-R), which includes learning trials (total recall), delayed free recall, and recognition. Right PFC activity was associated with performance on BVMT-R total recall and delayed recall. Results from multiple regression showed that PFC activity explained an additional 9 % of the variance in memory performance above right hippocampal volume. These findings suggest that PFC processing that supports WM (including stimuli maintenance, retrieval, and selection) are also involved in LTM learning and recall.