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Eric Polizzi - One of the best experts on this subject based on the ideXlab platform.
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domain decomposition approaches for accelerating Contour Integration eigenvalue solvers for symmetric eigenvalue problems
Numerical Linear Algebra With Applications, 2018Co-Authors: Vassilis Kalantzis, James Kestyn, Eric Polizzi, Yousef SaadAbstract:Summary This paper discusses techniques for computing a few selected eigenvalue–eigenvector pairs of large and sparse symmetric matrices. A recently developed class of techniques to solve this type of problems is based on integrating the matrix resolvent operator along a complex Contour that encloses the interval containing the eigenvalues of interest. This paper considers such Contour Integration techniques from a domain decomposition viewpoint and proposes two schemes. The first scheme can be seen as an extension of domain decomposition linear system solvers in the framework of Contour Integration methods for eigenvalue problems, such as FEAST. The second scheme focuses on integrating the resolvent operator primarily along the interface region defined by adjacent subdomains. A parallel implementation of the proposed schemes is described, and results on distributed computing environments are reported. These results show that domain decomposition approaches can lead to reduced run times and improved scalability.
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density matrix based algorithm for solving eigenvalue problems
Physical Review B, 2009Co-Authors: Eric PolizziAbstract:A new numerical algorithm for solving the symmetric eigenvalue problem is presented. The technique deviates fundamentally from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms) or other Davidson-Jacobi techniques, and takes its inspiration from the Contour Integration and density matrix representation in quantum mechanics. It will be shown that this new algorithm - named FEAST - exhibits high efficiency, robustness, accuracy and scalability on parallel architectures. Examples from electronic structure calculations of Carbon nanotubes (CNT) are presented, and numerical performances and capabilities are discussed.
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density matrix based algorithm for solving eigenvalue problems
Physical Review B, 2009Co-Authors: Eric PolizziAbstract:A new numerical algorithm for solving the symmetric eigenvalue problem is presented. The technique deviates fundamentally from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms) or other Davidson-Jacobi techniques, and takes its inspiration from the Contour Integration and density matrix representation in quantum mechanics. It will be shown that this new algorithm - named FEAST - exhibits high efficiency, robustness, accuracy and scalability on parallel architectures. Examples from electronic structure calculations of Carbon nanotubes (CNT) are presented, and numerical performances and capabilities are discussed.
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density matrix based algorithm for solving eigenvalue problems
Physical Review B, 2009Co-Authors: Eric PolizziAbstract:A fast and stable numerical algorithm for solving the symmetric eigenvalue problem is presented. The technique deviates fundamentally from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms) or other Davidson-Jacobi techniques and takes its inspiration from the Contour Integration and density-matrix representation in quantum mechanics. It will be shown that this algorithm---named FEAST---exhibits high efficiency, robustness, accuracy, and scalability on parallel architectures. Examples from electronic structure calculations of carbon nanotubes are presented, and numerical performances and capabilities are discussed.
Luigi Acerbi - One of the best experts on this subject based on the ideXlab platform.
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the role of sensory uncertainty in simple Contour Integration
PLOS Computational Biology, 2020Co-Authors: Yanli Zhou, Luigi AcerbiAbstract:Perceptual organization is the process of grouping scene elements into whole entities. A classic example is Contour Integration, in which separate line segments are perceived as continuous Contours. Uncertainty in such grouping arises from scene ambiguity and sensory noise. Some classic Gestalt principles of Contour Integration, and more broadly, of perceptual organization, have been re-framed in terms of Bayesian inference, whereby the observer computes the probability that the whole entity is present. Previous studies that proposed a Bayesian interpretation of perceptual organization, however, have ignored sensory uncertainty, despite the fact that accounting for the current level of perceptual uncertainty is one of the main signatures of Bayesian decision making. Crucially, trial-by-trial manipulation of sensory uncertainty is a key test to whether humans perform near-optimal Bayesian inference in Contour Integration, as opposed to using some manifestly non-Bayesian heuristic. We distinguish between these hypotheses in a simplified form of Contour Integration, namely judging whether two line segments separated by an occluder are collinear. We manipulate sensory uncertainty by varying retinal eccentricity. A Bayes-optimal observer would take the level of sensory uncertainty into account-in a very specific way-in deciding whether a measured offset between the line segments is due to non-collinearity or to sensory noise. We find that people deviate slightly but systematically from Bayesian optimality, while still performing "probabilistic computation" in the sense that they take into account sensory uncertainty via a heuristic rule. Our work contributes to an understanding of the role of sensory uncertainty in higher-order perception.
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the role of sensory uncertainty in simple Contour Integration
bioRxiv, 2019Co-Authors: Yanli Zhou, Luigi AcerbiAbstract:Abstract Perceptual organization is the process of grouping scene elements into whole entities. A classic example is Contour Integration, in which separate line segments are perceived as continuous Contours. Uncertainty in such grouping arises from scene ambiguity and sensory noise. Some classic Gestalt principles of Contour Integration, and more broadly, of perceptual organization, have been re-framed in terms of Bayesian inference, whereby the observer computes the probability that the whole entity is present. Previous studies that proposed a Bayesian interpretation of perceptual organization, however, have ignored sensory uncertainty, despite the fact that accounting for the current level of perceptual uncertainty is one the main signatures of Bayesian decision making. Crucially, trial-by-trial manipulation of sensory uncertainty is a key test to whether humans perform near-optimal Bayesian inference in Contour Integration, as opposed to using some manifestly non-Bayesian heuristic. We distinguish between these hypotheses in a simplified form of Contour Integration, namely judging whether two line segments separated by an occluder are collinear. We manipulate sensory uncertainty by varying retinal eccentricity. A Bayes-optimal observer would take the level of sensory uncertainty into account – in a very specific way – in deciding whether a measured offset between the line segments is due to non-collinearity or to sensory noise. We find that people deviate slightly but systematically from Bayesian optimality, while still performing “probabilistic computation” in the sense that they take into account sensory uncertainty via a heuristic rule. Our work contributes to an understanding of the role of sensory uncertainty in higher-order perception. Author summary Our percept of the world is governed not only by the sensory information we have access to, but also by the way we interpret this information. When presented with a visual scene, our visual system undergoes a process of grouping visual elements together to form coherent entities so that we can interpret the scene more readily and meaningfully. For example, when looking at a pile of autumn leaves, one can still perceive and identify a whole leaf even when it is partially covered by another leaf. While Gestalt psychologists have long described perceptual organization with a set of qualitative laws, recent studies offered a statistically-optimal – Bayesian, in statistical jargon – interpretation of this process, whereby the observer chooses the scene configuration with the highest probability given the available sensory inputs. However, these studies drew their conclusions without considering a key actor in this kind of statistically-optimal computations, that is the role of sensory uncertainty. One can easily imagine that our decision on whether two Contours belong to the same leaf or different leaves is likely going to change when we move from viewing the pile of leaves at a great distance (high sensory uncertainty), to viewing very closely (low sensory uncertainty). Our study examines whether and how people incorporate uncertainty into Contour Integration, an elementary form of perceptual organization, by varying sensory uncertainty from trial to trial in a simple Contour Integration task. We found that people indeed take into account sensory uncertainty, however in a way that subtly deviates from optimal behavior.
Charles D Gilbert - One of the best experts on this subject based on the ideXlab platform.
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Contour saliency in primary visual cortex
Neuron, 2006Co-Authors: Valentin Piech, Charles D GilbertAbstract:Contour Integration is an important intermediate stage of object recognition, in which line segments belonging to an object boundary are perceptually linked and segmented from complex backgrounds. Contextual influences observed in primary visual cortex (V1) suggest the involvement of V1 in Contour Integration. Here, we provide direct evidence that, in monkeys performing a Contour detection task, there was a close correlation between the responses of V1 neurons and the perceptual saliency of Contours. Receiver operating characteristic analysis showed that single neuronal responses encode the presence or absence of a Contour as reliably as the animal's behavioral responses. We also show that the same visual Contours elicited significantly weaker neuronal responses when they were not detected in the detection task, or when they were unattended. Our results demonstrate that contextual interactions in V1 play a pivotal role in Contour Integration and saliency.
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lateral connectivity and contextual interactions in macaque primary visual cortex
Neuron, 2002Co-Authors: Dan D Stettler, Aniruddha Das, Jean Bennett, Charles D GilbertAbstract:Two components of cortical circuits could mediate Contour Integration in primary visual cortex (V1): intrinsic horizontal connections and feedback from higher cortical areas. To distinguish between these, we combined functional mapping with a new technique for labeling axons, a recombinant adenovirus bearing the gene for green fluorescent protein (GFP), to determine the extent, density, and orientation specificity of V1 intrinsic connections and V2 to V1 feedback. Both connections cover portions of V1 representing regions of visual space up to eight times larger than receptive fields as classically defined, though the intrinsic connections are an order of magnitude denser than the feedback. Whereas the intrinsic connections link similarly oriented domains in V1, V2 to V1 feedback displays no such specificity. These findings suggest that V1 intrinsic horizontal connections provide a more likely substrate for Contour Integration.
Peter J Uhlhaas - One of the best experts on this subject based on the ideXlab platform.
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perceptual organization in ketamine users preliminary evidence of deficits on night of drug use but not 3 days later
Journal of Psychopharmacology, 2007Co-Authors: Peter J Uhlhaas, Isobel Millard, Leslie Muetzelfeldt, Valerie H Curran, Celia J A MorganAbstract:N-methyl-D-aspartate (NMDA)-receptor antagonists such as ketamine can induce transient schizophrenia-like symptoms and cognitive dysfunctions in healthy voLunteers similar to those observed in patients with schizophrenia. Perceptual organization deficits have been documented in schizophrenia and are thought to be related to some symptoms associated with the illness. The current study was designed to determine whether people who repeatedly self-administer ketamine would also show deficits in perceptual organization. Using a psychophysically well-controlled measure of Contour Integration, we compared a group of recreational users (n = 16) to a group of poly-drug using controls (n = 16). Contour Integration performance was measured on the night of drug use and 3 days later when drug free. The results showed that on the night of drug use, ketamine produced a dysfunction in Contour Integration however, this was not present 3 days Later when drug free. Levels of dissociation were also higher in ketamine users o...
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evidence for impaired visual context processing in schizotypy with thought disorder
Schizophrenia Research, 2004Co-Authors: Steven M Silverstein, Peter J Uhlhaas, William A. Phillips, P G LovellAbstract:Visual context processing was examined in relation to schizotypy in a large nonclinical university population. Schizotypal status was assessed with the Schizotypal Personality Questionnaire (SPQ) [Schizophr. Bull. 17 (1991) 555]. Schizotypal (n=32) and non-schizotypal (n=37) subjects were tested on a Contour Integration task (where context processing is necessary for good performance) and a visual size perception task (where context processing impairs accurate performance). In addition, a short form of the Thought Disorder Index (TDI) [Psychol. Assess. 5 (1993) 75] was administered to 28 schizotypal subjects. Thought disordered schizotypal subjects showed significantly impaired performance on the Contour Integration task but more accurate performance on the visual size perception task. These results support the hypothesis that deficits in visual context processing are the manifestation of a larger disturbance of cognitive coordination in schizotypy and schizophrenia.
Steven M Silverstein - One of the best experts on this subject based on the ideXlab platform.
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the spatial range of Contour Integration deficits in schizophrenia
Experimental Brain Research, 2012Co-Authors: Ilona Kovacs, Steven M Silverstein, Brian P Keane, M Deanna, Cameron S Carter, James M Gold, Angus W Macdonald, Daniel J RaglandAbstract:Contour Integration (CI) refers to the process that represents spatially separated elements as a unified edge or closed shape. Schizophrenia is a psychiatric disorder characterized by symptoms such as hallucinations, delusions, disorganized thinking, inappropriate affect, and social withdrawal. Persons with schizophrenia are impaired at CI, but the specific mechanisms underlying the deficit are still not clear. Here, we explored the hypothesis that poor patient performance owes to reduced feedback or impaired longer-range lateral connectivity within early visual cortex—functionally similar to that found in 5- to 6-year old children. This hypothesis predicts that as target element spacing increases from .7 to 1.4° of visual angle, patient impairments will become more pronounced. As a test of the prediction, 25 healthy controls and 36 clinically stable, asymptomatic persons with schizophrenia completed a CI task that involved determining whether a subset of Gabor elements formed a leftward or rightward pointing shape. Adjacent shape elements were spaced at either .7 or 1.4° of visual angle. Difficulty in each spacing condition depended on the number of noise elements present. Patients performed worse than controls overall, both groups performed worse with the larger spacing, and the magnitude of the between-group difference was not amplified at the larger spacing. These results show that CI deficits in schizophrenia cannot be explained in terms of a reduced spatial range of Integration, at least not when the shape elements are spaced within 1.5°. Later-developing, low-level integrative mechanisms of lateral connectivity and feedback appear not to be differentially impaired in the illness.
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evidence for impaired visual context processing in schizotypy with thought disorder
Schizophrenia Research, 2004Co-Authors: Steven M Silverstein, Peter J Uhlhaas, William A. Phillips, P G LovellAbstract:Visual context processing was examined in relation to schizotypy in a large nonclinical university population. Schizotypal status was assessed with the Schizotypal Personality Questionnaire (SPQ) [Schizophr. Bull. 17 (1991) 555]. Schizotypal (n=32) and non-schizotypal (n=37) subjects were tested on a Contour Integration task (where context processing is necessary for good performance) and a visual size perception task (where context processing impairs accurate performance). In addition, a short form of the Thought Disorder Index (TDI) [Psychol. Assess. 5 (1993) 75] was administered to 28 schizotypal subjects. Thought disordered schizotypal subjects showed significantly impaired performance on the Contour Integration task but more accurate performance on the visual size perception task. These results support the hypothesis that deficits in visual context processing are the manifestation of a larger disturbance of cognitive coordination in schizotypy and schizophrenia.
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perceptual organization the disorganization syndrome and context processing in chronic schizophrenia
Schizophrenia Research, 2000Co-Authors: Steven M Silverstein, Ilona Kovacs, Rodney Corry, Carolyn ValoneAbstract:Schizophrenia patients' perceptual organization abilities were assessed with a psychophysically well-controlled measure of Contour Integration. Compared with psychiatric and staff controls, schizophrenia patients were less able to detect Contours comprising Gabor elements as the detection of these Contours relied increasingly on long-range spatial interactions. Impaired task performance was also found to correlate significantly with higher levels of disorganized symptomatology. These data provide further evidence for impaired perceptual grouping in schizophrenia. In addition, the findings support the hypothesis that a common cortical processing algorithm involving contextual coordination is impaired in schizophrenia, leading to reduced binding of object features in vision, and reduced contextual disambiguation of linguistic information during thought and speech.