Biology subjects

Axmacher, N.

Publications and source records attributed to Axmacher, N..

29 records · Page 2Linked to original sources

bioRxiv2023

Maintenance and transformation of representational formats during working memory prioritization

Visual working memory depends on both material-specific brain areas in the ventral visual stream (VVS) that support the maintenance of stimulus representations and on regions in prefrontal cortex (PFC) that control these representations. Recent studies identified stimulus-specific working memory contents via representational similarity analysis (RSA) and analyzed their representational format using deep neural networks (DNNs) as models of the multi-layered hierarchy of information processing. How executive control prioritizes relevant working memory contents and whether this affects their representational formats remains an open question, however. Here, we addressed this issue using a multi-item working memory task involving a retro-cue that prompted participants to maintain one particular item. We exploited the excellent spatiotemporal resolution of intracranial EEG (iEEG) recordings in epilepsy patients and analyzed activity at electrodes in VVS (n=28 patients) and PFC (n=16 patients). During encoding, we identified category-specific information in both VVS and PFC. During maintenance, this information re-occurred in VVS but not in the PFC - suggesting a transformation of PFC representations from encoding to maintenance which putatively reflects the prioritization process. We thus applied RSA in combination with different DNN architectures to investigate the representational format of prioritized working memory contents. Representations during the maintenance period matched representations in deep layers of recurrent but not feedforward DNNs, in both VVS and PFC. While recurrent DNN representations matched PFC representations in the beta band following the retro-cue, they corresponded to VVS representations in a lower theta-alpha frequency range (3-14Hz) towards the end of the maintenance period. Findings could be replicated in recurrent DNNs with two different architectures and using two different training sets. Together, these results demonstrate that VWM relies on representational transformations in VVS and PFC that give rise to distinct coding schemes of prioritized contents.

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bioRxiv2022

Concurrent maintenance of both veridical and transformed working memory representations

In the dynamic environment we live in, the already limited information that human working memory can maintain needs to be constantly updated to optimally guide behaviour. Indeed, previous studies showed that leading up to a response, representations maintained in working memory representations are transformed continuously. This goes hand-in-hand with the removal of task-irrelevant items. However, does such removal also include the representations of stimuli as they were originally, prior to transformation? Here we assessed the neural representation of task-relevant transformed representations, and the no-longer-relevant veridical representations they originated from. We applied multivariate pattern analysis to electroencephalographic data during maintenance of orientation gratings with and without mental rotation. During maintenance, we perturbed the representational network by means of a visual impulse stimulus, and were thus able to successfully decode veridical as well as imaginary, transformed orientation gratings from impulse-driven activity. The impulse response reflected only task-relevant (cued), but not task-irrelevant (uncued) items, suggesting that the latter were quickly discarded from working memory. By contrast, even though the original cued orientation gratings were also no longer task-relevant after mental rotation, these items continued to be represented next to the rotated ones, in different representational formats. This seemingly inefficient use of scarce working memory capacity was associated with reduced probe response times and may thus serve to increase precision and flexibility in guiding behaviour in dynamic environments.

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bioRxiv2022

Representational dynamics of memories for real-life events

The continuous flow of experience that characterizes real-life events is not recorded as such in episodic memory but is condensed as a succession of event segments separated by temporal discontinuities. To unravel the neural basis of this representational structure, we recorded real-life events using wearable camera technology and used fMRI to investigate brain activity during their temporal unfolding in memory. We found that, compared to the representation of static scenes in memory, dynamically unfolding memory representations were associated with greater activation of the posterior medial episodic network. Strikingly, by analyzing the autocorrelation of brain activity patterns at successive time points throughout the retrieval period, we found that this network showed higher temporal dynamics when recalling events that included a higher density of event segments. These results reveal the key role of the posterior medial network in representing the dynamic unfolding of the event segments that constitute real-world memories.

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bioRxiv2021

A gradient of electrophysiological novelty responses along the human hippocampal long axis.

The hippocampus is implicated in novelty detection, thought to be important for regulating entry of information into long-term memory. Whether electrophysiological responses to novelty differ along the human hippocampal long axis is currently unknown. By recording from electrodes implanted longitudinally in the hippocampus of epilepsy patients, here we show a gradual increase of theta frequency oscillatory power from anterior to posterior in response to unexpected stimuli, superimposed on novelty responses common to all long axis portions. Intracranial event-related potentials (iERPs) were larger for unexpected vs. expected stimuli and demonstrated a polarity inversion between the hippocampal head (HH) and body (HB). We observed stronger theta coherence between HH and hippocampal tail (HT) than between HB and HT, similarly for expected and unexpected stimuli. This was accompanied by theta and alpha traveling waves with surprisingly variable direction of travel characterized by a [~]180{degrees} phase lag between hippocampal poles. Interestingly, this phase lag showed a pronounced phase offset between anterior and middle (HH-HB) hippocampal portions coinciding anatomically with a drop in theta coherence and the novelty iERP polarity inversion. Our findings indicate common response properties along the hippocampal long axis to unexpected stimuli, as well as a multifaceted, non-uniform engagement along the long axis for novelty processing.

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bioRxiv2020

Conditioned fear reactions are associated with gray matter density but not cortical microstructure

Research has shown that fear acquisition, in reaction to potentially harmful stimuli or situations, is characterized by pronounced interindividual differences. It is likely that such differences are evoked by variability in the macro- and microstructural properties of brain regions involved in the processing of threat or safety signals from the environment. Indeed, previous studies have shown that the strength of conditioned fear reactions is associated with the cortical thickness or volume of various brain regions. However, respective studies were exclusively targeted at single brain regions instead of whole brain networks. Here, we tested 60 young and healthy individuals in a differential fear conditioning paradigm while they underwent fMRI scanning. In addition, we acquired T1-weighted and multi-shell diffusion-weighted images prior to testing. We used task-based fMRI data to define global brain networks which exhibited increased BOLD responses towards CS+ or CS- presentations, respectively. From these networks, we obtained mean values of gray matter density, neurite density, and neurite orientation dispersion. We found that mean gray matter density averaged across the CS+ network was significantly correlated with the strength of conditioned fear reactions quantified via skin conductance response. Measures of neurite architecture were not associated with conditioned fear reaction in any of the two networks. Our results extend previous findings on the relationship between brain morphometry and fear learning. Most importantly, our study is the first to introduce neurite imaging to fear learning research and discusses how its implementation can be improved in future research.

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bioRxiv2020

Fear learning sculpts functional brain connectivity at rest beyond the traditional fear network in humans

Neuroscientific research has identified specific brain networks involved in the acquisition of fear memories. Using fMRI to assess changes in resting-state functional connectivity (RSFC) induced by fear acquisition, single brain regions from these networks have also been linked to fear memory consolidation. However, previous studies only examined RSFC changes within restricted sets of brain regions or without a proper control group, leaving our knowledge about fear consolidation outside of traditional fear networks incomplete. Here, we tested a group of 84 healthy participants in a differential fear conditioning paradigm and quantified RSFC changes between 358 cortical and 16 subcortical brain areas. Subsequent to fear learning, 21 functional connections exhibited significant RSFC changes. Importantly, these connections were not restricted to the traditional fear networks but also comprised various frontal and visual areas. Our findings indicate that fear memory consolidation is a complex process that integrates relevant information across the entire brain.

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bioRxiv2019

Tracking selective rehearsal and active inhibition of memory traces in directed forgetting

Selectively remembering or forgetting newly encountered information is essential for goal-directed behavior. It is still an open question, however, whether intentional forgetting is an active process based on the inhibition of unwanted memory traces or whether it occurs passively through reduced recruitment of selective rehearsal [1,2]. Here we show that intentional control of memory encoding relies on both, enhanced active inhibition and decreased selective rehearsal, and that these two processes can be separated in time and space. We applied representational similarity analysis (RSA [3]) and timefrequency analysis to EEG data during an item-method directed forgetting experiment [4]. We identified neural signatures of both the intentional suppression and the voluntary upregulation of item-specific representations. Successful active forgetting was associated with a downregulation of item-specific representations in an early time window, 500ms after the instruction. This process was initiated by an increase in oscillatory alpha (8-13 Hz) power, a well-established signature of neural inhibition [5,6], in occipital brain areas. During a later time window, 1500ms after the cue, intentional forgetting was associated with reduced employment of active rehearsal processes, as reflected by an attenuated upregulation of item-specific representations as compared to intentionally encoded items. Our data show that active inhibition and selective rehearsal are two separate mechanisms whose consecutive employment allows for a voluntary control of memory formation.

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bioRxiv2018

Coding advantage of grid cell orientation under noisy conditions

Grid cells constitute a crucial component of the \"GPS\" in the mammalian brain. Recent experiments revealed that grid cell activity is anchored to environmental boundaries. More specifically, these results revealed a slight yet consistent offset of 8 degrees relative to boundaries of a square environment. The causes and possible functional roles of this orientation are still unclear. Here we propose that this phenomenon maximizes the spatial information conveyed by grid cells. Computer simulations of the grid cell network reveal that the universal grid orientation at 8 degrees optimizes spatial coding specifically in the presence of noise. Our model also predicts the minimum number of grid cells in each module. In addition, analytical results and a dynamical reinforcement learning model reveal the mechanism underlying the noise-induced orientation preference at 8 degrees. Together, these results suggest that the experimentally observed common orientation of grid cells serves to maximize spatial information in the presence of noise.\n\nAuthor summarySpatial navigation depends on several specialized cell types including place and grid cells. Grid cells have multiple firing fields that are arranged in a regular hexagonal pattern. The axes of this pattern are anchored to environmental boundaries at a universal angle of 8{degrees}. Here, we combine computer simulations of the grid cell network with analytical derivations and a reinforcement learning model to explain the functional relevance of this universal grid cell orientation. We show that spatial information provided by grid cells is maximized at the experimentally observed grid orientation within a broad parameter range. This relationship occurs only in the presence of noise. The model allows for several experimentally testable predictions including the number of grid cells.

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bioRxiv2018

Spectral fingerprints or spectral tilt? Evidence for distinct oscillatory signatures of memory formation

Decreases in low frequency power (2-30 Hz) alongside high frequency power increases (>40 Hz) have been demonstrated to predict successful memory formation. Parsimoniously this change in the frequency spectrum can be explained by one factor, a change in the tilt of the power spectrum (from steep to flat) indicating engaged brain regions. A competing view is that the change in the power spectrum contains several distinct brain oscillatory fingerprints, each serving different computations. Here, we contrast these two theories in a parallel MEG-intracranial EEG study where healthy participants and epilepsy patients, respectively, studied either familiar verbal material, or unfamiliar faces. We investigated whether modulations in specific frequency bands can be dissociated in time, space and by experimental manipulation. Both, MEG and iEEG data, show that decreases in alpha/beta power specifically predicted the encoding of words, but not faces, whereas increases in gamma power and decreases in theta power predicted memory formation irrespective of material. Critically, these different oscillatory signatures of memory encoding were evident in different brain regions. Moreover, high frequency gamma power increases occurred significantly earlier compared to low frequency theta power decreases. These results speak against a \"spectral tilt\" and demonstrate that brain oscillations in different frequency bands serve different functions for memory encoding.

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bioRxiv2017

Alpha activity in the ventral and dorsal visual stream controls information flow during working memory

The electrophysiological mechanisms underlying working memory maintenance of information in the ventral and dorsal visual stream (VVS, DVS) remain elusive. Here we used electrocorticography recordings covering VVS, DVS and prefrontal cortex (PFC) in epilepsy patients while they were performing a delayed match-to-sample task. The experimental conditions (face identity, orientation) were designed to engage either the VVS or DVS. Alpha power was reduced in the VVS during maintenance of face identity and in the DVS during maintenance of spatial orientation of the very same stimuli. The phase of alpha oscillations modulated broadband high-frequency activity (BHA) in both regions. Interestingly, BHA occurred across broader alpha phase ranges when task-relevant information was maintained, putatively reflecting longer excitable \"duty cycles\". Our findings support a model in which the VVS and DVS are recruited by the PFC via selective reduction of alpha power. As a result, excitable duty cycles in the relevant area are extended.

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bioRxiv2017

Theta Phase Synchronization Between The Human Hippocampus And The Prefrontal Cortex Supports Learning Of Unexpected Information

Events that violate predictions are thought to not only modulate activity within the hippocampus and prefrontal cortex, but also to enhance communication between the two regions. Several studies in rodents have shown that synchronized theta oscillations facilitate communication between the prefrontal cortex and hippocampus during salient events, but it remains unclear whether similar oscillatory mechanisms support interactions between the two regions in humans. Here, we had the rare opportunity to conduct simultaneous electrophysiological recordings from the human hippocampus and prefrontal cortex from two patients undergoing presurgical evaluation for pharmaco-resistant epilepsy. Recordings were conducted during a task that involved encoding of contextually expected and unexpected visual stimuli. Across both patients, hippocampal-prefrontal theta phase synchronization was significantly higher during encoding of unexpected study items, compared to contextually expected study items. In contrast, we did not find increased theta synchronization between the prefrontal cortex and rhinal cortex. Our findings are consistent with the idea that theta oscillations orchestrate communication between the hippocampus and prefrontal cortex during the processing of contextually salient information.

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