Biology subjects

Imbach, L.

Publications and source records attributed to Imbach, L..

4 recordsLinked to original sources

bioRxiv2024

A hierarchical coordinate system for sequence memory in human entorhinal cortex.

The entorhinal cortex (EC) supports a coordinate system for spatial memories, organised in a hierarchy along the EC dorso-ventral axis. Recent theories suggest that a similar coordinate system could scaffold non-spatial memories. Here we show that an abstract hierarchical coordinate system supports arbitrary sequence memories in the human medial temporal lobe (MTL). In single-unit recordings from MTL, we find abstract, coordinate-like coding in a simple sequential memory task. In fMRI we find that abstract coordinate representations are arranged hierarchically along the entorhinal cortex, mirroring the anatomical gradient of grid cells in the rodent EC but now for non-spatial sequences. We replicate this finding in an independent cohort of participants. These data suggest that memories are scaffolded on a hierarchical coordinate system aligned to preserved anatomy across domains and species.

neuroscience↗

bioRxiv2024

Retrieval of human aversive memories involves reactivation of gamma activity patterns in the hippocampus that originate in the amygdala during encoding

Emotional memories require coordinated activity of the amygdala and hippocampus. Human intracranial recordings have shown that formation of aversive memories involves an amygdala theta-hippocampal gamma phase code. Yet, the mechanisms engaged during translation of aversive experiences into memories and subsequent retrieval remain unclear. Directly recording from human amygdala and hippocampus, here we show that hippocampal gamma activity increases for correctly remembered aversive scenes, while exerting unidirectional oscillatory influence within the theta/beta frequency range on the amygdala for previously seen aversive scenes. Crucially, patterns of amygdala high amplitude gamma activity at encoding are reactivated in the hippocampus, but not amygdala, during both aversive encoding and retrieval. Trial-specific hippocampal gamma patterns showing highest representational similarity with amygdala activity at encoding are replayed in the hippocampus during aversive retrieval. This reactivation process occurs against a background of gamma activity that is otherwise decorrelated between encoding and retrieval. Thus, retrieval of aversive memories is hippocampal-centered, with hippocampal activity patterns apparently entrained by the amygdala during encoding.

neuroscience↗

bioRxiv2023

Human hippocampal ripples tune cortical responses in uncertain visual contexts

To be able to encode information efficiently, our perceptual system should detect when situations are unpredictable (i.e., informative), and modulate brain dynamics to prepare for encoding. Here we show, with direct recordings from the human hippocampus and visual cortex, that after exposure to unpredictable visual stimulus streams, hippocampal ripple activity increases in frequency and duration prior to stimulus presentation, indicating context and experience-dependent prediction of predictability. Pre-stimulus hippocampal ripples suppress changes in visual (occipital) cortex gamma activity associated with uncertainty, and modulate post-stimulus prediction error gamma responses in higher-level visual (fusiform) cortex to surprising (i.e., unpredicted) stimuli. These results link hippocampal ripples with predictive coding accounts of neuronal message passing--and precision-weighted prediction errors--revealing a mechanism relevant for perceptual synthesis and subsequent memory encoding.

neuroscience↗

bioRxiv2022

Successful working memory linked to theta connectivity patterns in the hippocampal-entorhinal circuit

Working memory (WM) is the ability to actively maintain information for a short time and is central to human behavior. Rodent studies have proposed that hippocampal-entorhinal communication supports WM maintenance. However, the exact neural mechanisms of this interaction in WM remains unclear in humans. To address these questions, we combined machine learning analyses with intracranial electroencephalography (iEEG) recordings from the hippocampus and the entorhinal cortex (EC) in human participants, who maintained a set of letters in their WM. We found that WM maintenance was accompanied by elevated bidirectional hippocampal-EC information exchange via the theta band (2-8 Hz) and bidirectional cross-region theta-gamma phase-amplitude coupling (PAC). Further decoding analyses showed that the unidirectional inter-regional communication, with both theta oscillations in the hippocampus modulating EC gamma activity and theta band-coordinated information flow from the hippocampus, could decode correct performance at the level of participants. Taken together, our results demonstrate that theta functional coupling in the hippocampal-EC supports the maintenance of WM information via a specific pattern of frequency and direction. This connectivity-based coding could shed light on the neural mechanisms of WM processing. SignificanceRecent studies suggest a role for the hippocampus in working memory. How does the hippocampus coordinate with other brain regions to retain working memory information? The entorhinal cortex (EC) is the main gateway for information between the hippocampus and neocortex. To delineate whether (and how) the hippocampus and the entorhinal cortex interact during working memory and whether such interaction supports successful working memory, we used machine learning analyses of human intracranial EEG recordings while patients performed working memory tasks. Our results suggest that the human hippocampal-EC circuit supports working memory and is maintained in specific connectivity patterns, with a theta band (2-8 Hz)-coordinated unidirectional influence from the hippocampus to the EC. Our findings reveal that dynamic unidirectional interactions within the hippocampal-EC circuit underlie working memory and can contribute to a mechanistic circuit understanding of working memory.

neuroscience↗