Formation of visual memories controlled by gamma power phase-locked to alpha oscillations

Neuronal oscillations provide a window for understanding the brain dynamics that organize the flow of information from sensory to memory areas. While it has been suggested that gamma power reflects feedforward processing and alpha oscillations feedback control, it remains unknown how these oscillations dynamically interact. Magnetoencephalography (MEG) data was acquired from healthy subjects who were cued to either remember or not remember presented pictures. Our analysis revealed that in anticipation of a picture to be remembered, alpha power decreased while the cross-frequency coupling between gamma power and alpha phase increased. A measure of directionality between alpha phase and gamma power predicted individual ability to encode memory: stronger control of alpha phase over gamma power was associated with better memory. These findings demonstrate that encoding of visual information is reflected by a state determined by the interaction between alpha and gamma activity

Cross-Frequency Power Correlations Reveal the Right Superior Temporal Gyrus as a Hub Region During Working Memory Maintenance

In the present study, we characterized within- and cross-frequency power correlations from magnetoencephalography (MEG) data in order to understand how different brain regions cooperate as a network to maintain working memory representations with several features. The working memory items were composed of spatially arranged dots supposedly requiring both the dorsal and the ventral stream to be engaged during maintenance. Using a beamforming technique, we localized memory-dependent sources in the alpha, beta, and gamma bands. After the single-trial power values were extracted from these frequency bands with respect to each source, we calculated the correlations within- and cross-frequency bands. The following general picture emerged: gamma power in right superior temporal gyrus (STG) during working memory maintenance was correlated with numerous other sources in the alpha band in prefrontal, parietal, and posterior regions. In addition, the power correlations within the alpha band showed correlations across posterior-parietal-frontal regions. From these findings, we suggest that the STG dominated by gamma activity serves as a hub region for the network nodes responsible for the retention of the stimulus used in this study, which is likely to depend on both the “where-” and the “what-” visual system simultaneously. The present study demonstrates how oscillatory dynamics reflecting the interaction between cortical areas can be investigated by means of cross-frequency power correlations in source space. This methodological framework could be of general utility when studying functional network properties of the working brain

Cross-Frequency Power Correlations Reveal the Right Superior Temporal Gyrus as a Hub Region During Working Memory Maintenance

In the present study, we characterized within- and cross-frequency power correlations from magnetoencephalography (MEG) data in order to understand how different brain regions cooperate as a network to maintain working memory representations with several features. The working memory items were composed of spatially arranged dots supposedly requiring both the dorsal and the ventral stream to be engaged during maintenance. Using a beamforming technique, we localized memory-dependent sources in the alpha, beta, and gamma bands. After the single-trial power values were extracted from these frequency bands with respect to each source, we calculated the correlations within- and cross-frequency bands. The following general picture emerged: gamma power in right superior temporal gyrus (STG) during working memory maintenance was correlated with numerous other sources in the alpha band in prefrontal, parietal, and posterior regions. In addition, the power correlations within the alpha band showed correlations across posterior-parietal-frontal regions. From these findings, we suggest that the STG dominated by gamma activity serves as a hub region for the network nodes responsible for the retention of the stimulus used in this study, which is likely to depend on both the “where-” and the “what-” visual system simultaneously. The present study demonstrates how oscillatory dynamics reflecting the interaction between cortical areas can be investigated by means of cross-frequency power correlations in source space. This methodological framework could be of general utility when studying functional network properties of the working brain

Whole-brain structural and functional neuroimaging of individuals who attempted suicide and people who did not: A systematic review and exploratory coordinate-based meta-analysis

Suicide is the cause of death of approximately 800,000 people a year. Despite the relevance of this behaviour, risk assessment tools rely on clinician experience and subjective ratings. Given that previous suicide attempts are the single strongest predictors of future attempts, we designed a systematic review and coordinate-based meta-analysis to demonstrate whether neuroimaging features can help distinguish individuals who attempted suicide from subjects who did not. Out of 5,659 publications from PubMed, Scopus, and Web of Science, we summarised 102 experiments and meta-analysed 23 of them. A cluster in the right superior temporal gyrus, a region implicated in emotional processing, might be functionally hyperactive in individuals who attempted suicide. No statistically significant differences in brain morphometry were evidenced. Furthermore, we used JuSpace to show that this cluster is enriched in 5-HT1A heteroreceptors in the general population. This exploratory meta-analysis provides a putative neural substrate linked to previous suicide attempts. Heterogeneity in the analytical techniques and weak or absent power analysis of the studies included in this review currently limit the applicability of the findings, the replication of which should be prioritised

부호화에 대한 주의 조절의 뇌 기능적 신경망: 양분 그래프 필트레이션 기반의 오실레이션 관계 연구

학위논문 (박사)-- 서울대학교 대학원 : 협동과정 인지과학전공, 2015. 2. 이동수.To encode relevant information and also suppress irrelevant one is suggested to be important mechanism for efficient memory functioning in the brain. The neural correlates of such memory regulation has been suggested as intra-regional interactions between alpha and gamma cross-frequency power. The current study expanded the cross-frequency interactions into a large-scale regional network for memory regulation. I analyzed the data wherein twenty three healthy subjects were instructed to remember (Remember) or not to remember (No-Remember) the following picture item by a cue during magnetoencephalography (MEG) recording. In this setting, network was modeled by correlation between regions for alpha power during cue presentation and regions for gamma power during item presentation, yielding a bipartite graph. Then, graph filtration method was applied to bipartite graph, wherein the changes of connected components between two sets of nodes (i.e., region) were quantified into two invariant measures: a barcode and single linkage distance. This procedure was called bipartite graph filtration in this study. As a result, as for the global connectivity reflected by the barcode, task condition was significantly different for gamma power during item presentation. In addition, subjects with steeper barcode in Remember than in No-Remember condition showed higher compliance to task instruction. Also, for Remember condition, subjects with fast merging pattern in alpha power during cue presentation than the pattern in gamma power during item presentation showed better memory performance. These findings suggested that encoding regulation is achieved by large-scale regional interactions, and the cognitive function is captured by distinctive pattern of network. As for the local connectivity reflected by single linkage distance, the connectivity between the left dorsolateral superior frontal gyrus for alpha power and the left insula for gamma power was significantly closer in Remember than in No-Remember condition. The superior frontal gyrus was included in dorsal attention network, suggesting dorsal attentional regulation of later long-term memory in the insular cortex. In conclusion, encoding regulation was investigated by large-scale brain interactions using bipartite graph filtration, showing the method allows to examine the time-lagged interactions between different frequency powers in an MEG network study for the first time.Abstract i Contents iv List of Figures vii List of Tables viii 1. Introduction 1 1.1. The control mechanism for efficient memory function 1 1.2. Brain oscillation in encoding regulation 2 1.2.1. The previous study of encoding regulation in terms of long-term memory 3 1.2.2. The proposal from the previous study 9 1.3. The distributed network organization for cognitive function 13 1.4. Large-scale brain network based on bipartite graph filtration 14 1.5. The aim and hypothesis the present study 17 2. Materials and Methods 19 2.1. Participants 19 2.2. Experimental paradigm and procedure 20 2.3. Behavioral measures 21 2.4. MEG measurement 22 2.5. Structural MR image acquisition 23 2.6. Data analysis 23 2.7. Spectral analysis 24 2.8. Source analysis 25 2.9. Network analysis 27 2.9.1. Network construction 27 2.9.2. Graph filtration 33 2.10. Statistical analysis 36 3. Results 40 3.1. Behavioral performance 40 3.2. Global network property measured by barcode 40 3.3. Local network property measured by single linkage distance 50 4. Discussion 56 4.1. Multi-scale connectivity pattern for encoding regulation is distinctive of functional state and task performance 57 4.2. Top-down controls for encoding in insular cortex is predictive of better performance 59 4.3. The estimation of interaction between alpha and gamma band power 62 4.4. Graph filtration method for investigating brain network structure 64 4.5. Modeling a large-scale network in cerebral cortex using electrophysiological data 65 5. Conclusion 69 References 76 국문 초록 86Docto

Corrélats neuronaux de la mémoire de travail en magnétoencéphalographie à l’état de repos

Purpose: There are few studies demonstrating the link between neural oscillations in magnetoencephalography (MEG) at rest and cognitive performance. Working memory is one of the most studied cognitive processes and is the ability to manipulate information on items kept in short-term memory. Heister & al. (2013) showed correlation patterns between brain oscillations at rest in MEG and performance in a working memory task (n-back). These authors showed that delta/theta activity in fronto-parietal areas is related to working memory performance. In this study, we use resting state MEG oscillations to validate these correlations with both of verbal (VWM) and spatial (SWM) working memory, and test their specificity in comparison with other cognitive abilities. Methods: We recorded resting state MEG and used clinical neuropsychological tests to assess working memory performance in 18 volunteers (6 males and 12 females). The other neuropsychological tests of the WAIS-IV were used as control tests to assess the specificity of the correlation patterns with working memory. We calculated means of Power Spectrum Density for different frequency bands (delta, 1-4Hz; theta, 4-8Hz; alpha, 8-13Hz; beta, 13-30Hz; gamma1, 30-59Hz; gamma2, 61-90Hz; gamma3, 90-120Hz; large gamma, 30-120Hz) and correlated MEG power normalised for the maximum in each frequency band at the sensor level with working memory performance. We then grouped the sensors showing a significant correlation by using a cluster algorithm. Results: We found positive correlations between both types of working memory performance and clusters in the bilateral posterior and right fronto-temporal regions for the delta band (r2 =0.73), in the fronto-middle line and right temporal regions for the theta band (r2 =0.63) as well as in the parietal regions for the alpha band (r2 =0.78). Verbal working memory and spatial working memory share a common fronto-parietal cluster of sensors but also show specific clusters. These clusters are specific to working memory, as compared to those obtained for other cognitive abilities and right posterior parietal areas, specially in slow frequencies, appear to be specific to working memory process. Conclusions: Slow frequencies (1-13Hz) but more precisely in delta/theta bands (1-8Hz), recorded at rest with magnetoencephalography, predict working memory performance and support the role of a fronto-parietal network in working memory.L’étude et la caractérisation des oscillations cérébrales au repos en magnétoencéphalographie (MEG) en relation avec les performances cognitives ont été peu étudiées. La mémoire de travail permet la manipulation de l’information sur des éléments qui y sont temporairement stockés. Heister et al. (2013) ont étudié l’association entre la mémoire de travail et les oscillations cérébrales au repos. Leurs résultats mettent en évidence un lien entre la performance dans une tâche de mémoire de travail (n-back) et une activité delta/thêta fronto-pariétale droite. Notre projet utilise la MEG au repos pour valider ces corrélations avec des tests standardisés de mémoire de travail verbale et spatiale, et tester leur spécificité en comparaison avec d’autres capacités cognitives. Méthodologie: Nous avons enregistré 18 participants volontaires (6 hommes et 12 femmes) en magnétoencéphalographie de repos. Nous avons évalué les capacités de mémoire de travail verbale et spatiale au moyen de l’Indice de Mémoire de Travail de l’Échelle d'Intelligence de Wechsler pour Adultes - quatrième édition (WAIS-IV), et le sous test d’Addition Spatiale de l’Échelle Clinique de Mémoire de Wechsler - quatrième édition. Nous avons aussi calculé les corrélations avec les autres Indices du WAIS-IV pour évaluer la spécificité des patrons de corrélation observés avec la mémoire de travail. Nous avons moyenné la puissance de la décomposition spectrale pour chaque bande de fréquence (delta, 1-4Hz; thêta, 4-8Hz; alpha, 8-13Hz; beta, 13- 30Hz; gamma1, 30-59Hz; gamma2, 61-90Hz; gamma3, 90-120Hz; large gamma, 30-120Hz) puis nous avons corrélé cette puissance normalisée par le maximum de chaque bande au niveau des capteurs avec la performance dans un test de mémoire de travail. Nous avons ensuite regroupé les capteurs significatifs en cluster d'intérêt. Résultats: Nous avons mis en évidence une corrélation positive entre la performance en mémoire de travail et les régions fronto-pariétale droite pour la bande delta (r2=0,73), fronto-temporale médiale droite pour la bande thêta (r2=0,63), et pariéto-centrale pour la bande alpha (r2=0,78). Les résultats suggèrent que les mémoires de travail verbale et spatiale partagent un même réseau fronto-pariétal. Chaque type de mémoire de travail a aussi des corrélations spécifiques dans des régions différentes pour certaines banques de fréquence. Comparée à d'autres habiletés cognitives, la mémoire de travail est associée à des patrons de corrélations spécifiques avec le MEG au repos et une région pariétale droite qui semble spécialisée dans les basses fréquences à la mémoire de travail. Conclusions: Les basses fréquences (1-13Hz) et plus précisément delta/thêta (1-8Hz), enregistrées au repos en MEG dans les régions frontales et pariétales, permettent de prédire la performance de mémoire de travail ce qui supporte le rôle d’un réseau fronto-pariétal

Oscillations cérébrales et performances cognitives : études à l'état de repos en MEG chez des sujets contrôles et des survivants de cancer pédiatrique

Cette étude s’intéresse au lien entre les dynamiques cérébrales et les capacités cognitives, cette problématique a déjà été explorée auparavant en imagerie cérébrale, notamment à l’aide de tâches effectuées pendant l’imagerie. Cependant la caractérisation de l’activité spontanée a principalement été faite soit avec une faible précision spatiale (capteur EEG/MEG), soit en IRMf qui a une faible résolution temporelle. L’objectif de cette thèse est de caractériser l’activité spontanée au repos au niveau cortical associée à différents processus cognitifs et leur performance. Le second chapitre cherche à établir les corrélats neuronaux de la performance de la mémoire au repos à l’aide des puissances spectrales localisées au niveau des sources corticales. Le troisième chapitre cherche à répliquer les méthodes utilisées dans l’article 1 avec les mêmes participants, mais dans un autre domaine cognitif afin d’établir les corrélats neuronaux de la fluence verbale ainsi que de discriminer une composante verbale et exécutive. Ces deux composantes ont été mises en évidence en utilisant une factorisation avec un test purement exécutif (Trail making test- condition 4) et un autre purement verbal (richesse du vocabulaire). Dans le quatrième chapitre, nous répliquons encore la méthode de l’article 1 avec les mêmes sujets, mais sur un test d’apprentissage verbal. Lors de l’apprentissage verbal, deux stratégies d’apprentissage (sériel et sémantique) possibles sont utilisées de manière concurrente, nous avons cherché à établir si des différences comportementales se traduisaient par des patrons d’activation différents. Dans le cinquième chapitre, nous avons cherché à établir des différences fonctionnelles entre les survivants de la leucémie et des sujets contrôles, puis à établir un lien entre la neurotoxicité et le déficit cognitif rencontré chez cette population, finalement nous avons établi un modèle intégrant neurotoxicité, performance cognitive et marqueur neurophysiologique fonctionnel cérébral. Cette recherche aura approfondi les connaissances sur l’état de repos et principalement fourni les premiers travaux qui mettent en lien l’activité cérébrale spontanée au repos au niveau des sources corticales avec plusieurs tests neuropsychologiques comportementaux. Les résultats ont amené des patrons d’activation spatio-fréquentielle différents, démontrant des spécificités reliées à certains tests comportementaux ou des traitements de l’information (sériel ou sémantique). Finalement les travaux sur les survivants de la leucémie ont montré que l’état de repos pouvait caractériser le fonctionnement des déficits cognitifs à long terme et être un marqueur de remédiation pour de futurs traitements.This study is interested in the link between brain dynamics and cognitive abilities. This problem has already been explored before in brain imaging, notably with the help of task performed during imaging. However, the characterization of spontaneous activity has mainly been done either with weak spatial resolution (EEG/MEG sensor) or in fMRI which has a low temporal resolution. The objective of this thesis is to characterize the spontaneous activity at rest at the cortical level associated with different cognitive processes and their performance. The second chapter seeks to establish the neural correlates of resting memory performance using spectral powers localized at cortical sources. The third chapter seeks to replicate the methods used in article 1 with the same participants but in another cognitive domain in order to establish the neural correlates of verbal fluency as well as to discriminate a verbal and an executive component. These two components were highlighted using a factorization with a purely executive test (Trail making test-condition 4) and another purely verbal one (vocabulary richness). In the fourth chapter, we replicate the method of article 1 with the same subjects, but on a verbal learning test. During verbal learning, two possible learning strategies (serial and semantic) are used concurrently, we sought to establish whether behavioural differences translate into different activation patterns. In the fifth chapter, we sought to establish functional differences between leukemia survivors and control subjects, then to search for a link between neurotoxicity and the cognitive deficit encountered in this population; finally we established a model integrating neurotoxicity, cognitive performance and functional neurophysiological brain markers. This research will have deepened the knowledge on the resting state and mainly provided the first works that link the spontaneous brain activity at rest at the level of cortical sources with several behavioural neuropsychological tests. The results led to different spatio-frequential activation patterns, showing specificities related to certain behavioural tests or information processing (serial or semantic). Finally, work on leukemia survivors has shown that resting states could characterize the functioning of long-term cognitive deficits and be a remediation marker for future treatments

Examining the Transient Neural Dynamics Underlying Working Memory Maintenance for Complex Visual Stimuli

Working memory (WM) is the temporary storage of information to accomplish a future goal. The WM delay period is the time after encoding but before retrieval when information is being maintained, typically in the absence of relevant stimuli. Understanding how the brain supports maintenance during the delay period, and how neural activity and connectivity are related to memory is critical for advancing both basic knowledge as well as informing declines in memory and cognition related to neurodegenerative diseases and healthy aging. An open question in the field of WM research is how information is stored during this delay period. One theory suggests persistent neural activity supports the storage of information while another theory suggests rapid synaptic weight changes (i.e., an activity-silent mechanism). While support exists for both theories, numerous confounds complicate the experiments designed to distinguish between these two theories. Most notably, studies typically use simple stimuli with short, predictable, unfilled delay periods, with few studies examining this open question using complex visual stimuli. For this dissertation four EEG experiments were conducted to answer three questions: 1) Does the type of maintenance technique used during the delay period modulate the neural activity for complex visual stimuli? 2) What are the load-dependent delay activity and connectivity patterns associated with maintenance of complex visual stimuli? And 3) How do patterns of delay activity and connectivity change when attention is sustained during unpredictable delay period durations? In the first set of experiments, we examined the role of rehearsal in maintaining complex visual stimuli. We looked at the impact of rehearsal versus suppression of rehearsal using novel naturalistic scenes that contained semantic content and phase-scrambled scenes that lacked semantic content. The benefit of rehearsal was associated with increases in theta and alpha band amplitude, but only when the stimuli lacked semantic content. The overall pattern of change in amplitude was similar for rehearsal and suppression of rehearsal, regardless of the type of stimulus. In the second set of experiments, we examined the role of attention in maintaining complex visual stimuli. We used a similar set of stimuli to the first experiments, employing a load manipulation (low load-2 images, high load-5 images) while perceptual interference was present. While increasing the WM load, particularly with complex stimuli, places a greater demand on attentional resources, interfering stimuli may compete for the available resources. This was confirmed in the examination of theta and alpha amplitude, as amplitude was reduced for the high WM load as compared with the low WM load. We also analyzed functional connectivity to identify the underlying brain networks that facilitated performance for the low load condition and identified three supporting networks, including a frontal- posterior temporal network that is responsible for filtering the interfering stimuli. Additionally, in a separate experiment using similar stimuli, we randomly varied the delay interval (short-2 sec, medium-5 sec, and long-9 sec) from trial to trial, which ensured sustained attention throughout the delay period because participants did not know when the delay period would end and the probe would appear. The delay activity associated with complex visual stimuli suggests a pattern of transient delay activity for medium and long delay periods, regardless of load, with an early increase in event-related synchronous activity (ERS) in alpha and lower beta activity (until 2-3 secs) followed by an extended period of event-related desynchronous (ERD) in alpha and beta band activity, in parietal and parieto-occipital regions. Sustained delay activity (i.e., ERS in alpha activity followed by a return to baseline) was only observed for the short delay interval (~2 sec). Our results suggest that the pattern of ERS reflects an early period of goal setting, in which attention is focused inward to prevent interference. As the delay interval increases, the pattern of ERD reflects ongoing maintenance and associations with stored semantic knowledge. Finally, we compared the underlying brain networks that supported maintenance across all experiments using connectivity measures. This comparison identified a different frontoparietal network that is implicated in cognitive control and was found to be involved in both effortful maintenance (i.e., for stimuli lacking semantic content) as well as maintenance that places an increased demand on attentional resources (e.g., interference present or increased intervals). These results provide some support for the persistent delay activity hypothesis, as there were changes in delay activity from baseline throughout the delay period in all experiments, regardless of maintenance technique, WM load or delay period interval. Furthermore, the delay period connectivity analyses for all experiments implicate fronto-parietal and temporal networks in supporting maintenance and suggest a flexible role of attention (e.g., filtering of interfering stimuli, control over attention) that varies based on task demands. Together, these delay activity and connectivity findings inform the ongoing debate about the neural dynamics that support visual WM