Retrieval aids the creation of a generalised memory trace and strengthens episode-unique information.

Generalised knowledge can adaptively guide our behaviour and help us navigate the world. In this study, we aim to test the role of memory retrieval in promoting such generalisation of memories. Retrieval is known to be a powerful memory enhancer. Both cognitive and neurobiological theories of retrieval-mediated learning propose that this benefit is due to the co-activation of related (semantic) information during retrieval, which strengthens this co-activated associative network. By doing so, retrieval might play an important role in the generalisation of the memory trace. Here, we used univariate and pattern fMRI analyses to investigate whether memory representations that undergo retrieval (vs. restudy) become generalised over time. Participants encoded scene-object pairs and either retrieved or restudied the objects over two sessions, two days apart. We analysed univariate and multivariate changes in brain activity specific to retrieval but not restudy, and tested whether predicted changes occur rapidly within a session, or evolve slowly, across the two days. Consistent with a role of retrieval in the semanticisation of memories, univariate analyses showed an increase in medial prefrontal cortex (mPFC) activation across consecutive retrieval attempts, and a multivariate increase in similarity between categorically related information. In addition to this semanticisation, we also observed that retrieval strengthened the patterns unique to the original study episodes. Semantic-categorical and episode-unique strengthening both evolved slowly, across two days, and were most pronounced in parietal areas. Our findings corroborate the hypothesis that retrieval supports the creation of a generalised memory trace, and show that this strengthening does not come at the expense of episode-unique information. Active remembering thus seems to promote a stable and adaptive memory that can be flexibly used to access both contextually specific and more abstract generalised information.

Spurious correlations in simultaneous EEG-fMRI driven by in-scanner movement.

Simultaneous EEG-fMRI provides an increasingly attractive research tool to investigate cognitive processes with high temporal and spatial resolution. However, artifacts in EEG data introduced by the MR scanner still remain a major obstacle. This study, employing commonly used artifact correction steps, shows that head motion, one overlooked major source of artifacts in EEG-fMRI data, can cause plausible EEG effects and EEG-BOLD correlations. Specifically, low-frequency EEG (<20Hz) is strongly correlated with in-scanner movement. Accordingly, minor head motion (<0.2mm) induces spurious effects in a twofold manner: Small differences in task-correlated motion elicit spurious low-frequency effects, and, as motion concurrently influences fMRI data, EEG-BOLD correlations closely match motion-fMRI correlations. We demonstrate these effects in a memory encoding experiment showing that obtained theta power (~3-7Hz) effects and channel-level theta-BOLD correlations reflect motion in the scanner. These findings highlight an important caveat that needs to be addressed by future EEG-fMRI studies.

Prefrontal dopamine and the dynamic control of human long-term memory.

Dopaminergic projections to the prefrontal cortex support higher-order cognitive functions, and are critically involved in many psychiatric disorders that involve memory deficits, including schizophrenia. The role of prefrontal dopamine in long-term memory, however, is still unclear. We used an imaging genetics approach to examine the hypothesis that dopamine availability in the prefrontal cortex selectively affects the ability to suppress interfering memories. Human participants were scanned via functional magnetic resonance imaging while practicing retrieval of previously studied target information in the face of interference from previously studied non-target information. This retrieval practice (RP) rendered the non-target information less retrievable on a later final test-a phenomenon known as retrieval-induced forgetting (RIF). In total, 54 participants were genotyped for the catechol-O-methyltransferase (COMT) Val(108/158)Met polymorphism. The COMT Val(108/158)Met genotype showed a selective and linear gene-dose effect on RIF, with the Met allele, which leads to higher prefrontal dopamine availability, being associated with greater RIF. Mirroring the behavioral pattern, the functional magnetic resonance imaging data revealed that Met allele carriers, compared with Val allele carriers, showed a greater response reduction in inhibitory control areas of the right inferior frontal cortex during RP, suggesting that they more efficiently reduced interference. These data support the hypothesis that the cortical dopaminergic system is centrally involved in the dynamic control of human long-term memory, supporting efficient remembering via the adaptive suppression of interfering memories.