1,261 research outputs found

    The spectro-contextual encoding and retrieval theory of episodic memory.

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    The spectral fingerprint hypothesis, which posits that different frequencies of oscillations underlie different cognitive operations, provides one account for how interactions between brain regions support perceptual and attentive processes (Siegel etal., 2012). Here, we explore and extend this idea to the domain of human episodic memory encoding and retrieval. Incorporating findings from the synaptic to cognitive levels of organization, we argue that spectrally precise cross-frequency coupling and phase-synchronization promote the formation of hippocampal-neocortical cell assemblies that form the basis for episodic memory. We suggest that both cell assembly firing patterns as well as the global pattern of brain oscillatory activity within hippocampal-neocortical networks represents the contents of a particular memory. Drawing upon the ideas of context reinstatement and multiple trace theory, we argue that memory retrieval is driven by internal and/or external factors which recreate these frequency-specific oscillatory patterns which occur during episodic encoding. These ideas are synthesized into a novel model of episodic memory (the spectro-contextual encoding and retrieval theory, or "SCERT") that provides several testable predictions for future research

    Modulation of Oscillatory Power and Connectivity in the Human Posterior Cingulate Cortex Supports the Encoding and Retrieval of Episodic Memories

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    Existing data from noninvasive studies have led researchers to posit that the posterior cingulate cortex (PCC) supports mnemonic processes: It exhibits degeneration in memory disorders, and fMRI investigations have demonstrated memory-related activation principally during the retrieval of memory items. Despite these data, the role of the PCC in episodic memory has received only limited treatment using the spatial and temporal precision of intracranial EEG, with previous analyses focused on item retrieval. Using data gathered from 21 human participants who underwent stereo-EEG for seizure localization, we characterized oscillatory patterns in the PCC during the encoding and retrieval of episodic memories. We identified a subsequent memory effect during item encoding characterized by increased gamma band oscillatory power and a low-frequency power desynchronization. Fourteen participants had stereotactic electrodes located simultaneously in the hippocampus and PCC, and with these unique data, we describe connectivity changes between these structures that predict successful item encoding and that precede item retrieval. Oscillatory power during retrieval matched the pattern we observed during encoding, with low-frequency (below 15 Hz) desynchronization and a gamma band (especially high gamma, 70–180 Hz) power increase. Encoding is characterized by synchrony between the hippocampus and PCC, centered at 3 Hz, consistent with other observations of properties of this oscillation akin to those for rodent theta activity. We discuss our findings in light of existing theories of episodic memory processing, including the information via desynchronization hypothesis and retrieved context theory, and examine how our data fit with existing theories for the functional role of the PCC. These include a postulated role for the PCC in modulating internally directed attention and for representing or integrating contextual information for memory items

    Improving episodic memory: frontal-midline theta neurofeedback training increases source memory performance

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    Cognitive and neurofeedback training (NFT) studies have demonstrated that training-induced alterations of frontal-midline (FM) theta activity (4-8 Hz) transfer to cognitive control processes. Given that FM theta oscillations are assumed to provide top-down control for episodic memory retrieval, especially for source retrieval, that is, accurate recollection of contextual details of prior episodes, the present study investigated whether FM theta NFT transfers to memory control processes. It was assessed (1) whether FM theta NFT improves source retrieval and modulates its underlying EEG characteristics and (2) whether this transfer extends over two posttests. Over seven NFT sessions, thetraining group who trained individual FM theta activity showed greater FM theta increase than an active control group who trained randomly chosen frequency bands. The training group showed better source retrieval in a posttraining session performed 13 days after NFT and their performance increasesfrom pre- to both posttraining sessions were predicted by NFT theta increases. Thus, training-induced enhancement of memory control processes seems to protect newly formed memories from proactive interference of previously learned information. EEG analyses revealed that during pretest both groups showed source memory specific theta activity at frontal and parietal sites. Surprisingly, training-induced improvements in source retrieval tended to be accompanied by less prestimulus FM theta activity, which was predicted by NFT theta change for the training but not the control group, suggesting a more efficient use of memory control processes after training. The present findings provide unique evidence for the enhancement of memory control processes by FM theta NFT

    Characterization And Perturbation Of Functional Networks That Support Human Memory

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    Episodic memory is essential to our daily lives, as it attaches meaning to the constant stream of sensory inputs to the brain. However, episodic memory often fails in a number of common neurocognitive disorders. Effective therapies remain elusive, owing to the complexity of brain networks and neural processes that support episodic encoding and retrieval. In particular, it is not understood how inter-regional communication within the brain supports memory function, though such communication may be critical to the highly integrative nature of episodic memory. To uncover the patterns of memory-related functional connectivity, we asked a large cohort of neurosurgical patients with indwelling electrodes to perform a verbal free-recall task, in which patients viewed lists of simple nouns and recalled them a short time later. As patients performed this task, we collected intracranial EEG (iEEG) from electrodes placed on the cortical surface and within the medial temporal lobe (MTL). First, we examined whole-brain functional networks that emerged during the encoding and retrieval phases of this task, using spectral methods to correlate frequency-specific signals between brain regions. We identified a dynamic network of regions that exhibited enhanced theta (3-8 Hz) connectivity during successful memory processing, whereas regions tended to desynchronize at high frequencies (30-100 Hz). Next, using only electrodes placed within the MTL, we asked whether functional coupling was also observed among this mesoscale subnetwork of highly specialized regions that play an outsize role in memory. Recapitulating our earlier findings, we noted broadly enhanced theta connectivity within the MTL, centering on the left entorhinal cortex during successful encoding operations. Finally, to determine whether such low-frequency functional connections reflect correlative or causal relations in the brain, we applied direct electrical stimulation via electrodes placed within the MTL. We found that low-frequency connections (5-13 Hz) predicted the emergence of theta activity at distant regions in the brain – particularly when stimulation occurred near white matter – indicating the potential causal relevance of iEEG-based functional connections. Taken together, these studies underscore the importance of low-frequency functional coupling to memory across spatial scales, and suggest this form of coupling indicates a causal relation between brain regions

    Electrophysiological signatures of memory reactivation in humans

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    Exploring memory impairment and post-traumatic amnesia following traumatic brain injury

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    Memory disturbances are among the most common and significant consequences of traumatic brain injury (TBI). The severity of these deficits can vary widely across the trajectory of recovery from TBI and can be highly heterogenous across individuals. In the acute stages memory disturbance can occur in the form of post-traumatic amnesia (PTA), but deficits are also present into the chronic stages of recovery. I present four studies that aim to understand the characteristics and underlying mechanisms of memory impairment following TBI. I investigated the cognitive profile of acute TBI patients with and without PTA. I found PTA patients show a transient deficit in working memory binding. I then assessed electrophysiological abnormalities to test the hypothesis that the binding deficit is underpinned by pathological low frequency slow-wave activity. PTA patients showed a significantly higher delta to alpha power ratio that correlated with binding impairment. To understand how this disruption to cortical communication impacts upon large-scale networks I performed a dynamic functional connectivity analysis on the resting state fMRI of acute TBI patients. I found four independent brain states that showed striking anti-correlation between core cognitive control networks. Patients in a more profound period of PTA spent more time in fewer states than those with less cognitive impairment. These findings suggest that PTA is likely underpinned by disruption to communication required for integration of features in working memory. Finally, I examined enduring memory failures in chronic TBI patients and found that patients with episodic memory impairment showed differential activation of key networks required for memory and attention. Memory impairment related to the white matter integrity directly underpinning the task-derived encoding networks. These findings suggest that in chronic TBI memory impairment may be associated with failed control of attentional resources.Open Acces

    Brain oscillations track the formation of episodic memories in the real world

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    Despite the well-known influence of environmental context on episodic memory, little has been done to increase contextual richness within the lab. This leaves a blind spot lingering over the neuronal correlates of episodic memory formation in day-to-day life. To address this, we presented participants with a series of words to memorise along a pre-designated route across campus while a mobile EEG system acquired ongoing neural activity. Replicating lab-based subsequent memory effects (SMEs), we identified significant low to mid frequency power decreases (<30 Hz), including beta power decreases over the left inferior frontal gyrus. When investigating the oscillatory correlates of temporal and spatial context binding, we found that items strongly bound to spatial context exhibited significantly greater theta power decreases than items strongly bound to temporal context. These findings expand upon lab-based studies by demonstrating the influence of real world contextual factors that underpin memory formation

    Closed-Loop Brain-Computer Interfaces for Memory Restoration Using Deep Brain Stimulation

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    The past two decades have witnessed the rapid growth of therapeutic brain-computer interfaces (BCI) targeting a diversity of brain dysfunctions. Among many neurosurgical procedures, deep brain stimulation (DBS) with neuromodulation technique has emerged as a fruitful treatment for neurodegenerative disorders such as epilepsy, Parkinson\u27s disease, post-traumatic amnesia, and Alzheimer\u27s disease, as well as neuropsychiatric disorders such as depression, obsessive-compulsive disorder, and schizophrenia. In parallel to the open-loop neuromodulation strategies for neuromotor disorders, recent investigations have demonstrated the superior performance of closed-loop neuromodulation systems for memory-relevant disorders due to the more sophisticated underlying brain circuitry during cognitive processes. Our efforts are focused on discovering unique neurophysiological patterns associated with episodic memories then applying control theoretical principles to achieve closed-loop neuromodulation of such memory-relevant oscillatory activity, especially, theta and gamma oscillations. First, we use a unique dataset with intracranial electrodes inserted simultaneously into the hippocampus and seven cortical regions across 40 human subjects to test for the presence of a pattern that the phase of hippocampal theta oscillation modulates gamma oscillations in the cortex, termed cross-regional phase-amplitude coupling (xPAC), representing a key neurophysiological mechanism that promotes the temporal organization of interregional oscillatory activities, which has not previously been observed in human subjects. We then establish that the magnitude of xPAC predicts memory encoding success along with other properties of xPAC. We find that strong functional xPAC occurs principally between the hippocampus and other mesial temporal structures, namely entorhinal and parahippocampal cortices, and that xPAC is overall stronger for posterior hippocampal connections. Next, we focus on hippocampal gamma power as a `biomarker\u27 and use a novel dataset in which open-loop DBS was applied to the posterior cingulate cortex (PCC) during the encoding of episodic memories. We evaluate the feasibility of modulating hippocampal power by a precise control of stimulation via a linear quadratic integral (LQI) controller based on autoregressive with exogenous input (ARX) modeling for in-vivo use. In the simulation framework, we demonstrate proposed BCI system achieves effective control of hippocampal gamma power in 15 out of 17 human subjects and we show our DBS pattern is physiologically safe with realistic time scales. Last, we further develop the PCC-applied binary-noise (BN) DBS paradigm targeting the neuromodulation of both hippocampal theta and gamma oscillatory power in 12 human subjects. We utilize a novel nonlinear autoregressive with exogenous input neural network (NARXNN) as the plant paired with a proportional–integral–derivative (PID) controller (NARXNN-PID) for delivering a precise stimulation pattern to achieve desired oscillatory power level. Compared to a benchmark consisted of a linear state-space model (LSSM) with a PID controller, we not only demonstrate that the superior performance of our NARXNN plant model but also show the greater capacity of NARXNN-PID architecture in controlling both hippocampal theta and gamma power. We outline further experimentation to test our BCI system and compare our findings to emerging closed-loop neuromodulation strategies

    How event-based memories change as a function of forgetting and consolidation

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    Episodic memories are composed of multiple elements, from the people we encounter, the locations we visit, and the objects we interact with. These ‘episodes’ are thought to be stored in memory as coherent event representations and are associated with holistic recollection at retrieval, such that the retrieval of one element is dependent on the retrieval of all other elements from the same event. Evidence for this ‘dependency’ has been shown to emerge from the association between the event elements themselves. Critically, dependency is seen when participants learn three overlapping pairwise associations in a ‘closed-loop’, but not when participants learn only two out of the three possible associations in an ‘open-loop’, suggesting that all pairwise associations between event-elements need to be explicitly encoded for a coherent event representation to emerge. Here I asked whether the associative structure formed at encoding affects how event-based memories are retained over a period of forgetting and consolidation. Recently formed representations are susceptible to forgetting via interference and/or decay, but also undergo memory consolidation; becoming less susceptible to interference and/or decay. As such, retention for an event-based representation will reflect an interaction between forgetting and consolidation. This thesis presents evidence that closed-loops tend to be forgotten in an all-or-none manner, such that closed-loops are more likely to either be retained or forgotten in their entirety. In contrast, open-loops are associated with a more asymmetrical pattern of forgetting as a function of memory reactivation during sleep. Further, the thesis presents fMRI evidence that closed-loops continue to be retrieved in a coherent manner following a period of forgetting and consolidation. These findings suggest that the associative structure formed at encoding has a lasting impact on the coherence of the underlying memory representation
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