3,827 research outputs found
Neurally Implementable Semantic Networks
We propose general principles for semantic networks allowing them to be
implemented as dynamical neural networks. Major features of our scheme include:
(a) the interpretation that each node in a network stands for a bound
integration of the meanings of all nodes and external events the node links
with; (b) the systematic use of nodes that stand for categories or types, with
separate nodes for instances of these types; (c) an implementation of
relationships that does not use intrinsically typed links between nodes.Comment: 32 pages, 12 figure
Episodic Memory, Theta-Activity and Schizophrenia
People with schizophrenia are known to have difficulties with episodic
memory (EM). The purpose of this investigation was to examine the relationship
between theta-power and: i) behavioural measures of EM performance, ii) event-
related potential (ERP) indices of recollection and, iii) measures of schizophrenia
symptomatology. In doing so, the aim was to gain a better understanding of the
basic neural mechanisms that contribute to successful EM performance, and how
these may differ for people with schizophrenia. The present investigation adopted
an endophenotypic approach and collected measures of schizotypy from student
participants to minimise patient factors that can confound interpretations. Fifty-
four participants were asked to complete a reality-monitoring exclusion EM
paradigm whilst electroencephalogram (EEG) data were collected. Measures of
theta-power and ERPs were time-locked to words presented during the retrieval
phase. There was a significant positive correlation between theta-power over Fz
between 600-1000ms post-stimulus presentation and estimates of recollection in
the imagine condition as well as a significant negative correlation between these
measures of theta-power for perceive items and ERP indices of recollection for
imagine items. There was also a significant positive correlation between
measures of frontal theta-power in the imagine condition and negative
schizotypy. The epoch employed means it is likely these measures of theta-
power reflect processes contributing to the content-specific retrieval of imagined
items, and post-retrieval processes acting in service of differentiating imagined
items in EM. Results are discussed in terms of suggestions for interventions and
directions for future research
Task-related edge density (TED) - a new method for revealing large-scale network formation in fMRI data of the human brain
The formation of transient networks in response to external stimuli or as a
reflection of internal cognitive processes is a hallmark of human brain
function. However, its identification in fMRI data of the human brain is
notoriously difficult. Here we propose a new method of fMRI data analysis that
tackles this problem by considering large-scale, task-related synchronisation
networks. Networks consist of nodes and edges connecting them, where nodes
correspond to voxels in fMRI data, and the weight of an edge is determined via
task-related changes in dynamic synchronisation between their respective times
series. Based on these definitions, we developed a new data analysis algorithm
that identifies edges in a brain network that differentially respond in unison
to a task onset and that occur in dense packs with similar characteristics.
Hence, we call this approach "Task-related Edge Density" (TED). TED proved to
be a very strong marker for dynamic network formation that easily lends itself
to statistical analysis using large scale statistical inference. A major
advantage of TED compared to other methods is that it does not depend on any
specific hemodynamic response model, and it also does not require a
presegmentation of the data for dimensionality reduction as it can handle large
networks consisting of tens of thousands of voxels. We applied TED to fMRI data
of a fingertapping task provided by the Human Connectome Project. TED revealed
network-based involvement of a large number of brain areas that evaded
detection using traditional GLM-based analysis. We show that our proposed
method provides an entirely new window into the immense complexity of human
brain function.Comment: 21 pages, 11 figure
Characterization And Perturbation Of Functional Networks That Support Human Memory
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
Oscillatory, Computational, and Behavioral Evidence for Impaired GABAergic Inhibition in Schizophrenia
The dysconnection hypothesis of schizophrenia (SZ) proposes that psychosis is best understood in terms of aberrant connectivity. Specifically, it suggests that dysconnectivity arises through aberrant synaptic modulation associated with deficits in GABAergic inhibition, excitation-inhibition balance and disturbances of high-frequency oscillations. Using a computational model combined with a graded-difficulty visual orientation discrimination paradigm, we demonstrate that, in SZ, perceptual performance is determined by the balance of excitation-inhibition in superficial cortical layers. Twenty-eight individuals with a DSM-IV diagnosis of SZ, and 30 age- and gender-matched healthy controls participated in a psychophysics orientation discrimination task, a visual grating magnetoencephalography (MEG) recording, and a magnetic resonance spectroscopy (MRS) scan for GABA. Using a neurophysiologically informed model, we quantified group differences in GABA, gamma measures, and the predictive validity of model parameters for orientation discrimination in the SZ group. MEG visual gamma frequency was reduced in SZ, with lower peak frequency associated with more severe negative symptoms. Orientation discrimination performance was impaired in SZ. Dynamic causal modeling of the MEG data showed that local synaptic connections were reduced in SZ and local inhibition correlated negatively with the severity of negative symptoms. The effective connectivity between inhibitory interneurons and superficial pyramidal cells predicted orientation discrimination performance within the SZ group; consistent with graded, behaviorally relevant, disease-related changes in local GABAergic connections. Occipital GABA levels were significantly reduced in SZ but did not predict behavioral performance or oscillatory measures. These findings endorse the importance, and behavioral relevance, of GABAergic synaptic disconnection in schizophrenia that underwrites excitation-inhibition balance
The relationship between multisensory associative learning and multisensory integration
Integrating sensory information from multiple modalities leads to more precise and efficient perception and behaviour. The process of determining which sensory information should be perceptually bound is reliant on both low-level stimulus features, as well as multisensory associations learned throughout development based on the statistics of our environment. Here, we explored the relationship between multisensory associative learning and multisensory integration using encephalography (EEG) and behavioural measures. Sixty-one participants completed a three-phase study. First, participants were exposed to novel audiovisual shape-tone pairings with frequent and infrequent stimulus pairings and completed a target detection task. EEG recordings of the mismatch negativity (MMN) and P3 were calculated as neural indices of multisensory associative learning. Next, the same learned stimulus pairs were presented in audiovisual as well as unisensory auditory and visual modalities while both early (\u3c100 ms) and late neural indices of multisensory integration were recorded. Finally, participants completed an analogous behavioural speeded-response task, with behavioural indices of multisensory gain calculated using the Race Model. Significant relationships were found in fronto-central and occipital areas between neural measures of associative learning and both early and late indices of multisensory integration in frontal and centro-parietal areas, respectively. Participants who showed stronger indices of associative learning also exhibited stronger indices of multisensory integration of the stimuli they learned to associate. Furthermore, a significant relationship was found between neural index of early multisensory integration and behavioural indices of multisensory gain. These results provide insight into the neural underpinnings of how higher-order processes such as associative learning guide multisensory integration
Exogenous spatial precuing reliably modulates object processing but not object substitution masking
Object substitution masking (OSM) is used in behavioral and imaging studies to investigate processes associated with the formation of a conscious percept. Reportedly, OSM occurs only when visual attention is diffusely spread over a search display or focused away from the target location. Indeed, the presumed role of spatial attention is central to theoretical accounts of OSM and of visual processing more generally (Di Lollo, Enns, & Rensink, Journal of Experimental Psychology: General 129:481–507, 2000). We report a series of five experiments in which valid spatial precuing is shown to enhance the ability of participants to accurately report a target but, in most cases, without affecting OSM. In only one experiment (Experiment 5) was a significant effect of precuing observed on masking. This is in contrast to the reliable effect shown across all five experiments in which precuing improved overall performance. The results are convergent with recent findings from Argyropoulos, Gellatly, and Pilling (Journal of Experimental Psychology: Human Perception and Performance 39:646–661, 2013), which show that OSM is independent of the number of distractor items in a display. Our results demonstrate that OSM can operate independently of focal attention. Previous claims of the strong interrelationship between OSM and spatial attention are likely to have arisen from ceiling or floor artifacts that restricted measurable performance
The Youth Mental Health Risk and Resilience Study (YouR-Study)
Abstract
Background
The transition from adolescence to adulthood is associated with the emergence of psychosis and other mental health problems, highlighting the importance of this developmental period for the understanding of developing psychopathology and individual differences in risk and resilience. The Youth Mental Health Risk and Resilience Study (YouR-Study) aims to identify neurobiological mechanisms and predictors of psychosis-risk with a state-of-the-art neuroimaging approach (Magnetoencephalography, Magnetic Resonance Spectroscopy, Magnetic Resonance Imaging) in combination with core psychological processes, such as affect regulation and attachment, that have been implicated in the development and maintenance of severe mental health problems.
Methods/Design
One hundred participants meeting clinical high-risk criteria (CHR) for psychosis through the Comprehensive Assessment of At-Risk Mental State and Schizophrenia Proneness Instrument, Adult Version, in the age range from 16 to 35 years of age will be recruited. Mental-state monitoring up to a total of 2\ua0years will be implemented to detect transition to psychosis. In addition, a sample of n \u2009=\u200940 help-seeking participants will be recruited who do not meet CHR-criteria, a group of n \u2009=\u200950 healthy control participants and a sample of n \u2009=\u200925 patients with first-episode psychosis. MEG-activity will be obtained during auditory and visual tasks to examine neural oscillations and event-related fields. In addition, we will obtain estimates of GABA and Glutamate levels through Magnetic Resonance Spectroscopy (MRS) to examine relationships between neural synchrony and excitatory-inhibition (E/I) balance parameters. Neuroimaging will be complemented by detailed neuropsychological assessments as well as psychological measures investigating the impact of childhood abuse, attachment experiences and affect regulation.
Discussion
The YouR-study could potentially provide important insights into the neurobiological mechanisms that confer risk for psychosis as well as biomarkers for early diagnosis of severe mental health problems. Moreover, we expect novel data related to the contribution of affect regulation and attachment-processes in the development of mental health problems, leading to an integrative model of early stage psychosis and the factors underlying risk and resilience of emerging psychopathology
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