Whereas numerous findings support a distinction between episodic and semantic memory, it is now widely acknowledged that these two forms of memory interact during both encoding and retrieval. The precise nature of this interaction, however, remains poorly understood. To examine the role of semantic organization during episodic encoding and retrieval, we recorded intracranial encephalographic signals as 69 neurosurgical patients studied and subsequently recalled categorized and unrelated word lists. Applying multivariate classifiers to neural recordings, we were able to reliably predict encoding success, retrieval success, and temporal and categorical clustering during recall. By assessing how these classifiers generalized across list types, we identified specific retrieval processes that predicted recall of categorized lists and distinguished between recall transitions within and between category clusters. These results particularly implicate retrieval (rather than encoding) processes in the categorical organization of episodic memories.

Abstract The study of functional connectivity and declarative memory has lately been focused on finding biomarkers of neuropsychological diseases. However, little is known about its patterns in healthy brains. Thus, in this systematic review we analyze and integrate the findings of 81 publications regarding functional connectivity (measured by fMRI during both task and resting-state) and semantic and episodic memory in healthy adults. Moreover, we discriminate and analyze the main areas and links found in specific memory phases (encoding, storage or retrieval) based on several criteria, such as time length, depth of processing, rewarding value of the information, vividness and amount or kind of details retrieved. There is a certain degree of overlap between the networks of episodic and semantic memory and between the encoding and retrieval stages. Although several differences are pointed out during the article, this calls to attention the need for further empirical studies that actively compare both types of memory, particularly using other baseline conditions apart from the traditional resting state. Indeed, the active involvement of the default mode network in both declarative memory and resting condition suggests the possibility that during rest there is an on-going memory processing. We find support for the ‘attention to memory’ hypothesis, the memory differentiation model and the appropriate transfer hypothesis, but some evidence is inconsistent with the traditional hub-and-spoke model.

Abstract The primary aims were to (1) identify the factor structure of tests thought to measure semantic and episodic memory and (2) examine whether patterns of impairment would show a double dissociation between these two memory systems at an individual level in patients with temporal lobe epilepsy (TLE). The secondary aim was to explore the impact of epilepsy-related variables on performance. This retrospective study involved a cohort of 54 adults who had been diagnosed with TLE and had undergone a neuropsychological assessment that included four memory tests traditionally used to measure either semantic memory (picture naming, animal fluency) or episodic memory (story recall, word list recall) at a single epilepsy surgery center in Australia. Principal component analysis revealed two factors albeit with unexpected loadings. Picture naming and story recall loaded on one factor. Animal fluency and word list recall loaded on another factor. There was no evidence of a double dissociation between semantic and episodic memory at an individual level. Left hemisphere seizure focus and early age of seizure onset related to worse performance on word list recall, picture naming and animal fluency, respectively. Our study highlights the importance of caution when interpreting the results of neuropsychological assessments, as not all putative tests of semantic and episodic memory may necessarily be measuring the same construct. Future directions for research are also considered.

Summary Episodic memory requires the precise coordination between the hippocampus and distributed cortical regions. This may be facilitated by bursts of brain activity called high-frequency oscillations (HFOs). We hypothesized that HFOs activate specific networks during memory retrieval and aimed to describe the electrophysiological properties of HFO-associated activity. To study this, we recorded intracranial electroencephalography while human participants performed a list learning task. Hippocampal HFOs (hHFOs) increased during encoding and retrieval, and these increases correlated with memory performance. During retrieval, hHFOs demonstrated activation of semantic processing regions that were previously active during encoding. This consisted of broadband high-frequency activity (HFA) and cortical HFOs. HFOs in the anterior temporal lobe, a major semantic hub, co-occurred with hHFOs, particularly during retrieval. These coincident HFOs were associated with greater cortical HFA and cortical theta bursts. Hence, HFOs may support synchronization of activity across distributed nodes of the hippocampal-cortical memory network. pmc iScience iScience 3532 isci iScience 2589-0042 Elsevier PMC11999489 PMC11999489.1 11999489 11999489 40235588 10.1016/j.isci.2025.112171 S2589-0042(25)00432-8 112171 1 Article Hippocampal and cortical high-frequency oscillations orchestrate human semantic networks during word list memory Mishra Akash amishra4@northwell.edu 1 2 7 ∗ Akkol Serdar 1 3 4 Espinal Elizabeth 1 3 5 Markowitz Noah 1 3 Tostaeva Gelana 1 3 Freund Elisabeth 1 3 Mehta Ashesh D. 11 , 12 , 13 Knowledge derived from this study would clarify how HFOs coordinate brain activity during human memory function and how HFOs interact with other mechanisms (oscillatory and non-oscillatory; high and low frequency) to support these processes. Word list learning is a cognitive process that requires coordination of distributed brain regions to process and encode words. Because multi-regional synchrony is involved, HFOs may coordinate this process. Recent studies have identified a semantic processing network consisting of precisely-coordinated cortical regions that are activated during speech perception. 24 Therefore, the semantic network and the ATL represent brain areas that require precise coordination during word list processing and memory and hence, represent a unique medium to investigate the function of HFOs. We used an auditory word list learning task with verbal free recall to assess the relationship between hippocampal and cortical HFOs and the activation of semantic networks during encoding and recall. We recorded intracranial electroencephalography (iEEG) from individuals undergoing invasive neuromonitoring for the treatment of intractable epilepsy to gain rare neural recordings of memory and semantic networks with high spatial and temporal resolution. We show that hHFOs are associated with both cortical HFOs and broadband HFA in specific areas of the semantic processing network, and the profile of this activity is influenced by memory content. Further, a subset of hippocampal-ATL co-HFOs are coincident with 3–5 Hz ATL cortical oscillations. These results extend our understanding of HFOs to the dynamic activation of semantic networks and elucidate the mechanisms by which hHFOs, cortical HFOs, and low-frequency oscillations coordinate the hippocampal-cortical memory network. Results Behavioral results Nine participants performed a word list learning task with verbal free recall ( Figures 1 A and S1 ). Hence, hHFOs may reflect hippocampal activity that relates to memory function during word list encoding and retrieval. hHFOs reactivate functional semantic network regions during free recall hHFOs may facilitate the interaction between the hippocampus and memory-containing cortical regions. Hence, hHFOs during the recall portion of a word list learning task may activate cortical regions that were previously engaged during word list encoding. We first defined the cortical semantic network by assessing HFA during word list presentation in all cortical contacts ( Figure S1 ). The auditory presentation of a set of words engages an assembly of interconnected cortical nodes that includes lower-order primary auditory cortical regions and higher-order regions involved in extracting rich semantic information. 14 It is possible that these higher-order regions may contain word list memory representations, which hHFOs may engage. Here, we isolated contacts located in the semantic network based on their activity during encoding, including identifying sentence-preferring and non-sentence-preferring contacts. We found these contacts to be located in widespread Hence, these findings link the semantic processing and word list memory literatures; further, it suggests that HFOs may serve as a linking mechanism between the two. It is interesting to note that the cortical HFA increases precede the hHFO peak by 400 ms. This finding supports a hypothesis that the cortex initiates memory retrieval by biasing hippocampal activity to generate hHFOs, which then function to reciprocally activate specific cortical memory representations. 32 , 33 , 34 To this end, a limitation of our recording technique is that it was not feasible to establish causal relationships between brain regions in the context of information exchange. Taken together, although this may detract from the relation of our findings to, for example, gamma oscillations in rodents or “non-pathologic” HFOs in participants without epilepsy, we support the use of the broader “HFO” terminology. In summary, while human participants performed word list learning task, we identified hHFO-associated activation of specific cortical regions that were active during semantic processing during memory retrieval. We observed an interplay between the hippocampus and the ATL where, in times of hHFOs, the ATL exhibits synchronous increases in cortical HFOs and broadband HFA.

A large body of research in the study of memory has accumulated to date on the part-list cuing impairment in recall. This phenomenon refers to the lower recall of studied information in the presence of some studied words provided as retrieval cues compared to when no cues are provided. We review the current literature on the part-list cuing impairment in recall and report a meta-analysis utilizing the procedural and statistical information obtained from 109 samples (N = 5,605). In each experiment, participants studied a list of words and subsequently performed a recall task either in the presence or absence of part-list cues. The meta-analysis shows that the part-list cuing impairment is a robust, medium-sized impairment (Cohen, 1988). This recall impairment was not significantly sensitive to the number of study items provided, the relationship among study items, the number of part-list cues provided, the amount of time provided for recall, or certain other factors of interest. Our analyses also demonstrate that longer retention periods between study and retrieval mitigate the part-list cuing impairment in recall. We discuss the implications of meta-analysis results for elements of experimental design, the findings of past literature, as well as the underlying theoretical mechanisms proposed to account for this impairment in recall and the applied consequences of this recall impairment.

Prior episodic long-term memory (LTM) can enhance working memory (WM) by improving recall of WM representations that match pre-learnt information and by freeing up capacity for new information. In this study, we investigated the flexibility of WM in doing so. Specifically, we tested whether WM can make use of pre-learnt item-item associations in a serial recall task, which typically requires the formation of item-positional bindings. We examined whether any benefits arise from accessing full episodic representations or from item activation, and assess whether the observed benefits are best explained by compression accounts during encoding (e.g., chunking, offloading) or by redintegration at test. Furthermore, we tested whether the benefits for pre-learnt and novel words depended on the position within the lists. Across three experiments, we consistently found that incorporating pre-learnt word pairs into a serial recall task facilitated immediate memory for words that matched pre-learnt representations - speaking against an item activation account. However, the benefit on new words within lists that included pre-learnt pairs depended on whether the words could be easily submitted to encoding strategies, such as chunking or offloading, which was facilitated by providing matching grouping structures during encoding. Overall, our results expand our understanding of how prior experiences can benefit WM processes, demonstrating that such benefits mainly result from the retrieval of prior episodes, rather than enhanced item activation in episodic memory.