scholarly journals Functional wiring of the human medial temporal lobe

2018 ◽  
Author(s):  
E. A. Solomon ◽  
J. M. Stein ◽  
S. Das ◽  
R. Gorniak ◽  
M. R. Sperling ◽  
...  
Keyword(s):  
Functional Connectivity ◽  
Episodic Memory ◽  
Temporal Lobe ◽  
Medial Temporal Lobe ◽  
Memory Task ◽  
Memory Processing ◽  
High Frequencies ◽  
Functional Connections ◽  
Brain Functions ◽  
Encoding And Retrieval

AbstractThe medial temporal lobe (MTL) is a locus of episodic memory in the human brain. It is comprised of cytologically distinct subregions that, in concert, give rise to successful encoding and retrieval of context-dependent memories. However, the functional connections between these subregions are poorly understood. To determine functional connectivity among MTL subregions, we had 126 subjects fitted with indwelling electrodes perform a verbal memory task, and asked how encoding or retrieval correlated with interregional synchronization. Using phase-based measures of connectivity, we found that synchronous theta (4-8 Hz) activity underlies successful episodic memory, whereas high-frequencies exhibit desynchronization. Moreover, theta functional connectivity during encoding aligned with key anatomic connections, including critical links between the entorhinal cortex, dentate gyrus, and CA1 of the hippocampus. Retrieval-associated networks demonstrated enhanced involvement of the subiculum, reflecting a substantial reorganization of the encoding-associated network. We posit that coherent theta activity within the MTL marks periods of successful memory, but distinct patterns of connectivity dissociate key stages of memory processing.Significance StatementThe brain functions through the interaction of its distinct parts, but little is known about how such connectivity dynamics relate to learning and memory. We used a large dataset of 126 human subjects with intracranial electrodes to assess patterns of electrical connectivity within the medial temporal lobe – a key region for memory processing – as they performed a memory task. We discovered that unique networks of time-varying, low-frequency interactions correlate with memory encoding and retrieval, specifically in the theta band. Simultaneously, we observed elevated spectral power at high frequencies in these same regions. The result is a complete map of physiological dynamics within the MTL, highlighting how a reorganization of theta networks support distinct memory operations.

scholarly journals Effects of amnesia on processing in the hippocampus and default mode network during a naturalistic memory task: a case study

2018 ◽  
Author(s):  
Christiane Oedekoven ◽  
James L. Keidel ◽  
Stuart Anderson ◽  
Angus Nisbet ◽  
Chris Bird
Keyword(s):  
Functional Connectivity ◽  
Episodic Memory ◽  
Memory Performance ◽  
Memory Task ◽  
Structural Mri ◽  
Brain Regions ◽  
Left Hippocampus ◽  
Cortical Regions ◽  
The Right ◽  
Encoding And Retrieval

Despite their severely impaired episodic memory, individuals with amnesia are able to comprehend ongoing events. Online representations of a current event are thought to be supported by a network of regions centred on the posterior midline cortex (PMC). By contrast, episodic memory is widely believed to be supported by interactions between the hippocampus and these cortical regions. In this MRI study, we investigated the encoding and retrieval of lifelike events (video clips) in a patient with severe amnesia likely resulting from a stroke to the right thalamus, and a group of 20 age-matched controls. Structural MRI revealed grey matter reductions in left hippocampus and left thalamus in comparison to controls. We first characterised the regions activated in the controls while they watched and retrieved the videos. There were no differences in activation between the patient and controls in any of the regions. We then identified a widespread network of brain regions, including the hippocampus, that were functionally connected with the PMC in controls. However, in the patient there was a specific reduction in functional connectivity between the PMC and a region of left hippocampus when both watching and attempting to retrieve the videos. A follow up analysis revealed that in controls the functional connectivity between these regions when watching the videos was correlated with memory performance. Taken together, these findings support the view that the interactions between the PMC and the hippocampus enable the encoding and retrieval of multimodal representations of the contents of an event.

scholarly journals Increased task-dependent functional connectivity by stimulation of the hippocampal network predicts memory enhancement

2019 ◽  
Author(s):  
Kristen N. Warren ◽  
Molly S. Hermiller ◽  
Aneesha S. Nilakantan ◽  
Joel L. Voss
Keyword(s):  
Functional Connectivity ◽  
Episodic Memory ◽  
Memory Retrieval ◽  
Medial Temporal Lobe ◽  
Network Connectivity ◽  
Network Control ◽  
Retrieval Task ◽  
Memory Processing ◽  
Hippocampal Network ◽  
Hippocampal Networks

AbstractSuccessful episodic memory involves dynamic increases in the coordination of activity across distributed hippocampal networks, including the posterior-medial network (PMN) and the anterior-temporal network (ATN). We tested whether this up-regulation of functional connectivity during memory processing can be enhanced within hippocampal networks by noninvasive stimulation, and whether such task-dependent connectivity enhancement predicts episodic memory improvement. Participants received stimulation targeting either the PMN or an out-of-network control location. We compared the effects of stimulation on fMRI connectivity measured during an autobiographical memory retrieval task versus during rest within the PMN and the ATN. PMN-targeted stimulation significantly increased connectivity during memory retrieval versus rest within the PMN. This effect was not observed in the ATN, or in either network due to control out-of-network stimulation. Task-dependent increases in connectivity due to PMN-targeted stimulation within the medial temporal lobe predicted improved performance of a separate episodic memory test. It is therefore possible to enhance the task-dependent regulation of hippocampal network connectivity that supports memory processing using noninvasive stimulation.

A Neuroanatomical Construct for the Amnesic Effects of Propofol

2002 ◽  
Vol 97 (2) ◽  
pp. 329-337 ◽  
Author(s):  
Robert A. Veselis ◽  
Ruth A. Reinsel ◽  
Vladimir A. Feshchenko ◽  
Ann M. Dnistrian
Keyword(s):  
Blood Flow ◽  
Cerebral Blood Flow ◽  
Episodic Memory ◽  
Temporal Lobe ◽  
Medial Temporal Lobe ◽  
Regional Cerebral Blood Flow ◽  
Memory Task ◽  
Statistical Parametric Mapping ◽  
Regional Cerebral Blood ◽  
Word Memory

Background This study was designed to identify neuroanatomical locations of propofol's effects on episodic memory by producing minimal and maximal memory impairment during conscious sedation. Drug-related changes in regional cerebral blood flow (rCBF) were located in comparison with rCBF increases during a simple word memory task. Methods Regional cerebral blood flow changes were assessed in 11 healthy volunteers using H215O positron emission tomography (PET) and statistical parametric mapping (SPM99) at 600 and 1,000 ng/ml propofol target concentrations. Study groups were based on final recognition scores of auditory words memorized during PET scanning. rCBF changes during propofol administration were compared with those during the word memory task at baseline. Results Nonoverlapping memory effects were evident: low (n = 4; propofol concentration 523 +/- 138 ng/ml; 44 +/- 13% decrement from baseline memory) and high (n = 7; 829 +/- 246 ng/ml; 87 +/- 6% decrement from baseline) groups differed in rCBF reductions primarily in right-sided prefrontal and parietal regions, close to areas activated in the baseline memory task, particularly R dorsolateral prefrontal cortex (Brodmann area 46; x, y, z = 51, 38, 22). The medial temporal lobe region exhibited relative rCBF increases. Conclusions As amnesia becomes maximal, rCBF reductions induced by propofol occur in brain regions identified with working memory processes. In contrast, medial temporal lobe structures were resistant to the global CBF decrease associated with propofol sedation. The authors postulate that the episodic memory effect of propofol is produced by interference with distributed cortical processes necessary for normal memory function rather than specific effects on medial temporal lobe structures.

scholarly journals Electrical stimulation in hippocampus and entorhinal cortex impairs spatial and temporal memory

2017 ◽  
Author(s):  
Abhinav Goyal ◽  
Jonathan Miller ◽  
Andrew J. Watrous ◽  
Sang Ah Lee ◽  
Tom Coffey ◽  
...  
Keyword(s):  
Episodic Memory ◽  
Entorhinal Cortex ◽  
Temporal Lobe ◽  
Cognitive Processes ◽  
Brain Stimulation ◽  
Medial Temporal Lobe ◽  
Memory Task ◽  
Temporal Organization ◽  
Electrical Brain Stimulation ◽  
Neurosurgical Patients

AbstractThe medial temporal lobe (MTL) is widely implicated in supporting episodic memory and navigation, but its precise functional role in organizing memory across time and space remains elusive. Here we examine the specific cognitive processes implemented by MTL structures (hippocampus and entorhinal cortex) to organize memory by using electrical brain stimulation, leveraging its ability to establish causal links between brain regions and features of behavior. We studied neurosurgical patients of both sexes who performed spatial-navigation and verbal-episodic memory tasks while brain stimulation was applied in various regions during learning. During the verbal memory task, stimulation in the MTL disrupted the temporal organization of encoded memories such that items learned with stimulation tended to be recalled in a more randomized order. During the spatial task, MTL stimulation impaired subjects’ abilities to remember items located far away from boundaries. These stimulation effects were specific to the MTL. Our findings thus provide the first causal demonstration in humans of the specific memory processes that are performed by the MTL to encode when and where events occurred.Significance StatementNumerous studies have implicated the medial temporal lobe (MTL) in encoding spatial and temporal memories, but they have not been able to causally demonstrate the nature of the cognitive processes by which this occurs in real-time. Electrical brain stimulation is able to demonstrate causal links between a brain region and a given function with high temporal precision. By examining behavior in a memory task as subjects received MTL stimulation, we provide the first causal evidence demonstrating the role of the MTL in organizing the spatial and temporal aspects of episodic memory.

Medial temporal lobe dysfunction during encoding and retrieval of episodic memory in non-demented APOE ε4 carriers

Neuroscience ◽  
2010 ◽  
Vol 168 (2) ◽  
pp. 487-497 ◽  
Author(s):  
J. Kukolja ◽  
C.M. Thiel ◽  
T. Eggermann ◽  
K. Zerres ◽  
G.R. Fink
Keyword(s):  
Episodic Memory ◽  
Temporal Lobe ◽  
Medial Temporal Lobe ◽  
Apoe Ε4 ◽  
Encoding And Retrieval

Children show adult-like hippocampal pattern separation for familiar but not novel events

2020 ◽  
Author(s):  
Susan L. Benear ◽  
Elizabeth A. Horwath ◽  
Emily Cowan ◽  
M. Catalina Camacho ◽  
Chi Ngo ◽  
...  
Keyword(s):  
Episodic Memory ◽  
Temporal Lobe ◽  
Medial Temporal Lobe ◽  
Developmental Change ◽  
Pattern Separation ◽  
Developmental Changes ◽  
Similarity Analysis ◽  
Separation Processes ◽  
Parahippocampal Cortex ◽  
Pattern Similarity

The medial temporal lobe (MTL) undergoes critical developmental change throughout childhood, which aligns with developmental changes in episodic memory. We used representational similarity analysis to compare neural pattern similarity for children and adults in hippocampus and parahippocampal cortex during naturalistic viewing of clips from the same movie or different movies. Some movies were more familiar to participants than others. Neural pattern similarity was generally lower for clips from the same movie, indicating that related content taxes pattern separation-like processes. However, children showed this effect only for movies with which they were familiar, whereas adults showed the effect consistently. These data suggest that children need more exposures to stimuli in order to show mature pattern separation processes.

scholarly journals Early stages of tau pathology and its associations with functional connectivity, atrophy and memory

Brain ◽  
2021 ◽  
Author(s):  
David Berron ◽  
Jacob W Vogel ◽  
Philip S Insel ◽  
Joana B Pereira ◽  
Long Xie ◽  
...  
Keyword(s):  
Functional Connectivity ◽  
Entorhinal Cortex ◽  
Temporal Lobe ◽  
Medial Temporal Lobe ◽  
Memory Impairment ◽  
Memory Performance ◽  
Hippocampal Atrophy ◽  
Tau Pathology ◽  
Β Amyloid ◽  
With Memory

Abstract In Alzheimer’s disease, postmortem studies have shown that the first cortical site where neurofibrillary tangles appear is the transentorhinal region, a subregion within the medial temporal lobe that largely overlaps with area 35, and the entorhinal cortex. Here we used tau-PET imaging to investigate the sequence of tau pathology progression within the human medial temporal lobe and across regions in the posterior-medial system. Our objective was to study how medial temporal tau is related to functional connectivity, regional atrophy, and memory performance. We included 215 β-amyloid negative cognitively unimpaired, 81 β-amyloid positive cognitively unimpaired and 87 β-amyloid positive individuals with mild cognitive impairment, who each underwent [18]F-RO948 tau and [18]F-flutemetamol amyloid PET imaging, structural T1-MRI and memory assessments as part of the Swedish BioFINDER-2 study. First, event-based modelling revealed that the entorhinal cortex and area 35 show the earliest signs of tau accumulation followed by the anterior and posterior hippocampus, area 36 and the parahippocampal cortex. In later stages, tau accumulation became abnormal in neocortical temporal and finally parietal brain regions. Second, in cognitively unimpaired individuals, increased tau load was related to local atrophy in the entorhinal cortex, area 35 and the anterior hippocampus and tau load in several anterior medial temporal lobe subregions was associated with distant atrophy of the posterior hippocampus. Tau load, but not atrophy, in these regions was associated with lower memory performance. Further, tau-related reductions in functional connectivity in critical networks between the medial temporal lobe and regions in the posterior-medial system were associated with this early memory impairment. Finally, in patients with mild cognitive impairment, the association of tau load in the hippocampus with memory performance was partially mediated by posterior hippocampal atrophy. In summary, our findings highlight the progression of tau pathology across medial temporal lobe subregions and its disease-stage specific association with memory performance. While tau pathology might affect memory performance in cognitively unimpaired individuals via reduced functional connectivity in critical medial temporal lobe-cortical networks, memory impairment in mild cognitively impaired patients is associated with posterior hippocampal atrophy.

The Effect of Divided Attention on Encoding and Retrieval in Episodic Memory Revealed by Positron Emission Tomography

2000 ◽  
Vol 12 (2) ◽  
pp. 267-280 ◽  
Author(s):  
Tetsuya Iidaka ◽  
Nicole D. Anderson ◽  
Shitij Kapur ◽  
Roberto Cabez ◽  
Fergus I. M. Craik
Keyword(s):  
Positron Emission Tomography ◽  
Prefrontal Cortex ◽  
Episodic Memory ◽  
Divided Attention ◽  
Memory Performance ◽  
Memory Task ◽  
Anterior Cingulate ◽  
Emission Tomography ◽  
Positron Emission ◽  
Encoding And Retrieval

The effects of divided attention (DA) on episodic memory encoding and retrieval were investigated in 12 normal young subjects by positron emission tomography (PET). Cerebral blood flow was measured while subjects were concurrently performing a memory task (encoding and retrieval of visually presented word pairs) and an auditory tone-discrimination task. The PET data were analyzed using multivariate Partial Least Squares (PLS), and the results revealed three sets of neural correlates related to specific task contrasts. Brain activity, relatively greater under conditions of full attention (FA) than DA, was identified in the occipital-temporal, medial, and ventral-frontal areas, whereas areas showing relatively more activity under DA than FA were found in the cerebellum, temporo-parietal, left anterior-cingulate gyrus, and bilateral dorsolateral-prefrontal areas. Regions more active during encoding than during retrieval were located in the hippocampus, temporal and the prefrontal cortex of the left hemisphere, and regions more active during retrieval than during encoding included areas in the medial and right-prefrontal cortex, basal ganglia, thalamus, and cuneus. DA at encoding was associated with specific decreases in rCBF in the left-prefrontal areas, whereas DA at retrieval was associated with decreased rCBF in a relatively small region in the right-prefrontal cortex. These different patterns of activity are related to the behavioral results, which showed a substantial decrease in memory performance when the DA task was performed at encoding, but no change in memory levels when the DA task was performed at retrieval.

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