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

Abstract

Previous findings indicate that the hippocampus does not only play a role in long-term memory (LTM) encoding, but is important for working memory (WM) as well, in particular when multiple items are being processed. A recent study showed that maintenance of multiple items was associated with hippocampal activation (hippocampus-dependent WM), while maintenance of individual items induced hippocampal deactivations (hippocampus-independent WM). Here, we used two complimentary approaches to investigate whether WM-related activity patterns occur also during LTM encoding of sequentially presented items and whether they are related to the primacy effect, i.e., improved LTM encoding of items presented at the beginning of the list. Intracranial electroencephalogram in epilepsy patients and functional MRI in healthy subjects were recorded during a word-list learning task. As expected, the proportion of successfully encoded items was higher at the beginning of the list than at later list positions. Items at the beginning of the list which were subsequently forgotten were associated with negative blood oxygen level dependent responses and positive direct current slopes, corresponding to hippocampal deactivations, suggesting that they were not processed in hippocampus-dependent WM. These deactivations were absent for items later in the list and for subsequently remembered items. These data show that if processing of items at the beginning of the list is accompanied by hippocampal activity patterns previously observed during hippocampus-dependent WM, these items are subsequently remembered. However, deactivations of the hippocampus as previously observed during WM maintenance of individual items predicts failure of LTM encoding.

Complex span tasks and hippocampal recruitment during working memory

“…Recent WM neuroimaging studies haves indicated the involvement of the hippocampus during WM encoding, maintenance and, retrieval (e.g., Axmacher et al, 2009aAxmacher et al, , 2009bFletcher et al, 2003;Öztekin et al, 2009;Schon et al, 2009). Using intracranial EEG, Axmacher et al (2007) demonstrated reduced hippocampus/MTL gamma power during maintenance of a single item, but increased power during maintenance of multiple items; fMRI data concurred with their intracranial EEG recordings.…”

“…Consequently, hippocampal activity during complex arithmetic operations may signal a role for the hippocampus as part of a temporary WM storage buffer for items retrieved from LTM not currently in the focus of attention. Axmacher et al (2009aAxmacher et al ( , 2009b recently arrived at a similar conclusion and this idea is akin to Cowan's embedded processes model where items retrieved from LTM not currently in the focus reside in an activated, easily accessible state within LTM. Support for the hypothesis that the hippocampus is involved in the maintenance of items during a CWMS task to a greater extent than during a TNWM task came from our OSPAN N Arithmetic contrast.…”

“…Using intracranial EEG, Axmacher et al (2007) demonstrated reduced hippocampus/MTL gamma power during maintenance of a single item, but increased power during maintenance of multiple items; fMRI data concurred with their intracranial EEG recordings. Later, Axmacher et al (2009aAxmacher et al ( , 2009b also found MTL activation under low load visual WM conditions. However, one must take into account that even under "low load" conditions, complex visual stimuli, such as the faces used in the latter study, likely incur a high WM/attentional load due to the numerous features present in such stimuli.…”

Working memory and left medial temporal cortical thickness

“…Traditionally, the frontal lobes are recognized as responsible for the control of complex cognitive processes such . However, more recently, there has been strong evidence indicating the contribution of the medial temporal lobe to WM 24,25,26,27 . In a less robust manner, the inferior temporal lobe's role in WM has also been demonstrated…”

Phase-Coupled Oscillations in the Brain: Nonlinear Phenomena in Cellular Signalling

“…The hippocampus employs these quadruplet signals (cf. Figure 4) when the brain handles more than one task [16]. In computational analyses, PCOs (doublets and quadruplets) are represented by stable limit cycles of longer periods (complex periodic orbits) in the phase space of the relevant dynamical system.…”