The Experts below are selected from a list of 1242102 Experts worldwide ranked by ideXlab platform
John J Foxe - One of the best experts on this subject based on the ideXlab platform.
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the neurophysiology of human biological motion processing a high density electrical mapping study
NeuroImage, 2011Co-Authors: Aaron Krakowski, John J Foxe, Lars A Ross, Adam C Snyder, Pejman Sehatpour, Simon P KellyAbstract:The neural processing of biological motion (BM) is of profound experimental interest since it is often through the movement of another that we interpret their immediate intentions. Neuroimaging points to a specialized cortical network for processing biological motion. Here, high-density electrical mapping and Source-Analysis techniques were employed to interrogate the timing of information processing across this network. Participants viewed point-light-displays depicting standard body movements (e.g. jumping), while event-related potentials (ERPs) were recorded and compared to ERPs to scrambled motion control stimuli. In a pair of experiments, three major phases of BM-specific processing were identified: 1) The earliest phase of BM-sensitive modulation was characterized by a positive shift of the ERP between 100 and 200 ms after stimulus onset. This modulation was observed exclusively over the right hemisphere and Source-Analysis suggested a likely generator in close proximity to regions associated with general motion processing (KO/hMT). 2) The second phase of BM-sensitivity occurred from 200 to 350 ms, characterized by a robust negative-going ERP modulation over posterior middle temporal regions bilaterally. Source-Analysis pointed to bilateral generators at or near the posterior superior temporal sulcus (STS). 3) A third phase of processing was evident only in our second experiment, where participants actively attended the BM aspect of the stimuli, and was manifest as a centro-parietal positive ERP deflection, likely related to later cognitive processes. These results point to very early sensory registration of biological motion, and highlight the interactive role of the posterior STS in analyzing the movements of other living organisms.
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right hemisphere control of visuospatial attention line bisection judgments evaluated with high density electrical mapping and Source Analysis
NeuroImage, 2003Co-Authors: John J Foxe, Daniel C Javitt, Mark E MccourtAbstract:Abstract The “line-bisection” task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170–400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (∼170–190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (∼190–240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (∼240–400 ms), the topography becomes dominated by this right central parietal negativity. Inverse Source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.
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right hemisphere control of visuospatial attention line bisection judgments evaluated with high density electrical mapping and Source Analysis
NeuroImage, 2003Co-Authors: John J Foxe, Daniel C Javitt, Mark E MccourtAbstract:The "line-bisection" task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170-400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (approximately 170-190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (approximately 190-240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (approximately 240-400 ms), the topography becomes dominated by this right central parietal negativity. Inverse Source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.
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the spatiotemporal dynamics of illusory contour processing combined high density electrical mapping Source Analysis and functional magnetic resonance imaging
The Journal of Neuroscience, 2002Co-Authors: Micah M Murray, Glenn R Wylie, Beth A Higgins, Daniel C Javitt, Charles E Schroeder, John J FoxeAbstract:Because environmental information is often suboptimal, visual perception must frequently rely on the brain's reconstruction of contours absent from retinal images. Illusory contour (IC) stimuli have been used to investigate these "filling-in" processes. Intracranial recordings and neuroimaging studies show IC sensitivity in lower-tier area V2, and to a lesser extent V1. Some interpret these data as evidence for feedforward processing of IC stimuli, beginning at lower-tier visual areas. On the basis of lesion, visual evoked potentials (VEP), and neuroimaging evidence, others contend that IC sensitivity is a later, higher-order process. Whether IC sensitivity seen in lower-tier areas indexes feedforward or feedback processing remains unresolved. In a series of experiments, we addressed the spatiotemporal dynamics of IC processing. Centrally presented IC stimuli resulted in early VEP modulation (88-100 msec) over lateral-occipital (LOC) scalp--the IC effect. The IC effect followed visual response onset by 40 msec. Scalp current density topographic mapping, Source Analysis, and functional magnetic resonance imaging results all localized the IC effect to bilateral LOC areas. We propose that IC sensitivity described in V2 and V1 may reflect predominantly feedback modulation from higher-tier LOC areas, where IC sensitivity first occurs. Two additional observations further support this proposal. The latency of the IC effect shifted dramatically later (approximately 120 msec) when stimuli were laterally presented, indicating that retinotopic position alters IC processing. Immediately preceding the IC effect, the VEP modulated with inducer eccentricity--the configuration effect. We interpret this to represent contributions from global stimulus parameters to scene Analysis. In contrast to the IC effect, the topography of the configuration effect was restricted to central parieto-occipital scalp.
Daniel C Javitt - One of the best experts on this subject based on the ideXlab platform.
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right hemisphere control of visuospatial attention line bisection judgments evaluated with high density electrical mapping and Source Analysis
NeuroImage, 2003Co-Authors: John J Foxe, Daniel C Javitt, Mark E MccourtAbstract:The "line-bisection" task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170-400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (approximately 170-190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (approximately 190-240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (approximately 240-400 ms), the topography becomes dominated by this right central parietal negativity. Inverse Source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.
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right hemisphere control of visuospatial attention line bisection judgments evaluated with high density electrical mapping and Source Analysis
NeuroImage, 2003Co-Authors: John J Foxe, Daniel C Javitt, Mark E MccourtAbstract:Abstract The “line-bisection” task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170–400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (∼170–190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (∼190–240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (∼240–400 ms), the topography becomes dominated by this right central parietal negativity. Inverse Source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.
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the spatiotemporal dynamics of illusory contour processing combined high density electrical mapping Source Analysis and functional magnetic resonance imaging
The Journal of Neuroscience, 2002Co-Authors: Micah M Murray, Glenn R Wylie, Beth A Higgins, Daniel C Javitt, Charles E Schroeder, John J FoxeAbstract:Because environmental information is often suboptimal, visual perception must frequently rely on the brain's reconstruction of contours absent from retinal images. Illusory contour (IC) stimuli have been used to investigate these "filling-in" processes. Intracranial recordings and neuroimaging studies show IC sensitivity in lower-tier area V2, and to a lesser extent V1. Some interpret these data as evidence for feedforward processing of IC stimuli, beginning at lower-tier visual areas. On the basis of lesion, visual evoked potentials (VEP), and neuroimaging evidence, others contend that IC sensitivity is a later, higher-order process. Whether IC sensitivity seen in lower-tier areas indexes feedforward or feedback processing remains unresolved. In a series of experiments, we addressed the spatiotemporal dynamics of IC processing. Centrally presented IC stimuli resulted in early VEP modulation (88-100 msec) over lateral-occipital (LOC) scalp--the IC effect. The IC effect followed visual response onset by 40 msec. Scalp current density topographic mapping, Source Analysis, and functional magnetic resonance imaging results all localized the IC effect to bilateral LOC areas. We propose that IC sensitivity described in V2 and V1 may reflect predominantly feedback modulation from higher-tier LOC areas, where IC sensitivity first occurs. Two additional observations further support this proposal. The latency of the IC effect shifted dramatically later (approximately 120 msec) when stimuli were laterally presented, indicating that retinotopic position alters IC processing. Immediately preceding the IC effect, the VEP modulated with inducer eccentricity--the configuration effect. We interpret this to represent contributions from global stimulus parameters to scene Analysis. In contrast to the IC effect, the topography of the configuration effect was restricted to central parieto-occipital scalp.
Bin He - One of the best experts on this subject based on the ideXlab platform.
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3 dynamic effective connectivity of epileptic networks determined with high density eeg Source Analysis
Clinical Neurophysiology, 2012Co-Authors: Gijs Plomp, Bin He, Remi Tyrand, Laura Astolfi, M Seeck, Christoph M Michel, S VulliemozAbstract:Introduction Analyzing the dynamic behaviour of epileptic networks could help to better understand the way pathologic neural activity propagates, and leads to spikes, seizures, and their electro-clinical and cognitive manifestations, with implications for epilepsy surgery candidates. Methods In 6 patients with temporal lobe epilepsy we studied effective connectivity of large-scale cortical networks at high temporal resolution around interictal spikes, recorded with high density (256 channels) EEG. The cortical electric Source activity was obtained for 90 cortical regions of interest (ROI) using a distributed inverse solution. Multivariate, time-varying (millisecond resolution), and frequency-resolved (1–50 Hz) Granger causality Analysis (Partial Directed Coherence) was applied to the Source signal for all ROIs. In all patients subsequent intracranial recording or surgical resection was used for validation. Results Information flow occurred predominantly in the theta and beta bands. The key driving structures where located in the anterior and medial temporal regions, with peak information transfer before the spike maximum. We found fast-varying connectivity patterns between the antero-medial and lateral temporal lobe and basal frontal lobe, but also transient transfer towards the contralateral temporal lobe. In two patients with a multifocal irritative zone, we found evidence of connectivity from the main anterior temporal driver towards the secondary spike focus remote from the epileptogenic zone. Conclusion EEG-based time-varying effective connectivity of epileptic spikes provides a clear characterization of the epileptic networks that is concordant with invasive electro-clinical findings. This could have major clinical implications for tailoring resective, disconnective, and functional surgery.
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ictal Source Analysis localization and imaging of causal interactions in humans
NeuroImage, 2007Co-Authors: Lei Ding, Gregory A Worrell, Terrence D Lagerlund, Bin HeAbstract:Abstract We propose a new integrative approach to characterize the structure of seizures in the space, time, and frequency domains. Such characterization leads to a new technical development of ictal Source Analysis for the presurgical evaluation of epilepsy patients. The present new ictal Source Analysis method consists of three parts. First, a three-dimensional Source scanning procedure is performed by a spatio-temporal FINE Source localization method to locate the multiple Sources responsible for the time evolving ictal rhythms at their onsets. Next, the dynamic behavior of the Sources is modeled by a multivariate autoregressive process (MVAR). Lastly, the causal interaction patterns among the Sources as a function of frequency are estimated from the MVAR modeling of the Source temporal dynamics. The causal interaction patterns indicate the dynamic communications between Sources, which are useful in distinguishing the primary Sources responsible for the ictal onset from the secondary Sources caused by the ictal propagation. The present ictal Analysis strategy has been applied to a number of seizures from five epilepsy patients, and their results are consistent with observations from either MRI lesions or SPECT scans, which indicate its effectiveness. Each step of the ictal Source Analysis is statistically evaluated in order to guarantee the confidence in the results.
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classification of motor imagery tasks for brain computer interface applications by means of two equivalent dipoles Analysis
IEEE Transactions on Neural Systems and Rehabilitation Engineering, 2005Co-Authors: Baharan Kamousi, Bin HeAbstract:We have developed a novel approach using Source Analysis for classifying motor imagery tasks. Two-equivalent-dipoles Analysis was proposed to aid classification of motor imagery tasks for brain-computer interface (BCI) applications. By solving the electroencephalography (EEG) inverse problem of single trial data, it is found that the Source Analysis approach can aid classification of motor imagination of left- or right-hand movement without training. In four human subjects, an averaged accuracy of classification of 80% was achieved. The present study suggests the merits and feasibility of applying EEG inverse solutions to BCI applications from noninvasive EEG recordings.
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motor imagery classification by means of Source Analysis for brain computer interface applications
Journal of Neural Engineering, 2004Co-Authors: Lei Ding, Bin HeAbstract:We report a pilot study of performing classification of motor imagery for brain–computer interface applications, by means of Source Analysis of scalp-recorded EEGs. Independent component Analysis (ICA) was used as a spatio-temporal filter extracting signal components relevant to left or right motor imagery (MI) tasks. Source Analysis methods including equivalent dipole Analysis and cortical current density imaging were applied to reconstruct equivalent neural Sources corresponding to MI, and classification was performed based on the inverse solutions. The classification was considered correct if the equivalent Source was found over the motor cortex in the corresponding hemisphere. A classification rate of about 80% was achieved in the human subject studied using both the equivalent dipole Analysis and the cortical current density imaging Analysis. The present promising results suggest that the Source Analysis approach could manifest a clearer picture on the cortical activity, and thus facilitate the classification of MI tasks from scalp EEGs.
Michael Scherg - One of the best experts on this subject based on the ideXlab platform.
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Source Analysis of interictal spikes in polymicrogyria loss of relevant cortical fissures requires simultaneous eeg to avoid meg misinterpretation
NeuroImage, 2005Co-Authors: Thomas Bast, Georgia Ramantani, Tobias Boppel, Tanja Metzke, Ozdin Ozkan, Christoph Stippich, Angelika Seitz, Andre Rupp, D Rating, Michael SchergAbstract:Abstract Purpose: Multiple Source Analysis of interictal EEG and MEG spikes was used to identify irritative zones in polymicrogyria (PMG). Spike onset times and Source localization were compared between both modalities. PMG is characterized by a marked loss of deep cortical fissures. Hence, differences between EEG and MEG were expected since MEG signals are predominantly generated from tangentially orientated neurons in fissures. Patients: We studied 7 children and young adults (age 7.5 to 19 years) with localization-related epilepsy and unilateral polymicrogyria (PMG) as defined from anatomical MRI. Methods: 122-channel whole-head MEG and 32-channel EEG were recorded simultaneously for 25 to 40 min. Using the BESA program, interictal spikes were identified visually and used as templates to search for similar spatio-temporal spike patterns throughout the recording. Detected similar spikes ( r > 0.85) were averaged, high-pass filtered (5 Hz) to enhance spike onset, and subjected to multiple spatio-temporal Source Analysis. Source localization was visualized by superposition on T1-weighted MRI and compared to the lesion. Results: Nine spike types were identified in seven patients (2 types in 2 patients). Eight out of nine EEG Sources and seven MEG Sources modeling spike onset were localized within the visible lesion. EEG spike onset preceded MEG significantly in two spike types by 19 and 25 ms. This was related to radial onset activity in EEG while MEG localized propagated activity. In one case, the earliest MEG spike activity was localized to the normal hemisphere while the preceding radial EEG onset activity was localized within the lesion. Distances between EEG and MEG onset Sources varied markedly between 9 and 51 mm in the eight spike types with concordant lateralization. Conclusion: Interictal irritative zones were localized within the lesion in PMG comparable to other malformations, e.g., FCD. Discrepancies in MEG and EEG were related to the lack of deep fissures in PMG. In two cases, MEG was blind to the onset of radial interictal spike activity and localized propagated spike activity. In two other cases, MEG localized to the more peripheral parts of the irritative zone. Simultaneous EEG recordings with MEG and multiple Source Analysis are required to avoid problems of MEG interpretation.
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eeg Source Analysis and fmri reveal two electrical Sources in the fronto parietal operculum during subepidermal finger stimulation
NeuroImage, 2005Co-Authors: Andrej Stancak, Hubert Polacek, Jiři Vrana, Rosa Rachmanova, Karsten Hoechstetter, Jaroslav Tintra, Michael SchergAbstract:Using functional magnetic resonance imaging (fMRI) and electroencephalographic (EEG) Source dipole Analysis in 10 normal subjects, two electrical Source dipoles in the contralateral fronto-parietal operculum were identified during repetitive painful subepidermal stimulation of the right index finger. The anterior Source dipole peaking at 79 ± 8 ms (mean ± SD) was located in the frontal operculum, and oriented tangentially toward the cortical surface. The posterior Source dipole peaking at 118 ± 12 ms was located in the upper bank of the Sylvian fissure corresponding to the second somatosensory cortex (S2). The orientations of the posterior Source dipoles displayed large variability, but differed significantly (P < 0.05) from the orientations of the anterior Source dipoles. Electrical Sources and fMRI clusters were also observed in ipsilateral fronto-parietal operculum. However, due to low signal-to-noise ratio of ipsilateral EEG Sources in individual recordings, separation of Sources into anterior and posterior clusters was not performed. Combined fMRI and Source dipole EEG Analysis of individual data suggests the presence of two distinct electrical Sources in the fronto-parietal operculum participating in processing of somatosensory stimuli. The anterior region of the fronto-parietal operculum shows earlier peak activation than the posterior region.
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eeg and meg Source Analysis of single and averaged interictal spikes reveals intrinsic epileptogenicity in focal cortical dysplasia
Epilepsia, 2004Co-Authors: Thomas Bast, Christoph Stippich, Angelika Seitz, Andre Rupp, D Rating, Oezdin Oezkan, Sabine Rona, Susanne Fauser, Josef Zentner, Michael SchergAbstract:Summary: Purpose: Simultaneous interictal EEG and magnetoencephalography (MEG) recordings were used for noninvasive Analysis of epileptogenicity in focal cortical dysplasia (FCD). The results of two different approach methods (multiple Source Analysis of averaged spikes and single dipole peak localization of single spikes) were compared with pre- and postoperative anatomic magnetic resonance imaging (MRI). Patients: We studied nine children and adolescents (age, 3.5–15.9 years) with localization-related epilepsy and FCD diagnosis based on MRI. Five patients underwent epilepsy surgery, two of them after long-term recording with subdural grid electrodes, and one after intraoperative electrocorticography. Methods: The 122-channel whole-head MEGs and 33-channel EEGs were recorded simultaneously for 25 to 40 min. Interictal spikes were identified visually and used as templates to search for similar spatiotemporal spike patterns throughout the recording. With the BESA program, similar spikes (r > 0.85) were detected, averaged, high-pass filtered (5 Hz) to enhance spike onset, and subjected to multiple spatiotemporal Source Analysis with a multishell spherical head model. Peak activity from single spikes was modeled by single dipoles for the same subset of spikes. Source localization was visualized by superposition on T1-weighted MRI and compared with the lesion identified in T1- and T2-weighted MRI. In the five cases undergoing epilepsy surgery, the results were correlated with invasive recordings, postoperative MRI, and outcome. Results: In all cases, the Analysis of averaged spikes showed a localization of onset- and peak-related Sources within the visible lesion for both EEG and MEG. Of the single spikes, 128 (45%; total 284) were localizable at the peak in MEG, and 170 (60%) in EEG. Of these, 91% localized within the lesion with MEG, and 93.5% with EEG. In three of five patients operated on, the resected area included the onset zones of averaged EEG and MEG spike activity. These patients had excellent postoperative outcome, whereas the others did not become seizure free. Conclusions: Consistent MEG and EEG spike localization in the lesional zone confirmed the hypothesis of intrinsic epileptogenicity in FCD.
Lourens J. Waldorp - One of the best experts on this subject based on the ideXlab platform.
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the wald test and crame spl acute r rao bound for misspecified models in electromagnetic Source Analysis
IEEE Transactions on Signal Processing, 2005Co-Authors: Lourens J. Waldorp, Hilde M. Huizenga, Raoul P. P. P. GrasmanAbstract:By using signal processing techniques, an estimate of activity in the brain from the electro- or magneto-encephalogram (EEG or MEG) can be obtained. For a proper Analysis, a test is required to indicate whether the model for brain activity fits. A problem in using such tests is that often, not all assumptions are satisfied, like the assumption of the number of shells in an EEG. In such a case, a test on the number of Sources (model order) might still be of interest. A detailed Analysis is presented of the Wald test for these cases. One of the advantages of the Wald test is that it can be used when not all assumptions are satisfied. Two different, previously suggested, Wald tests in electromagnetic Source Analysis (EMSA) are examined: a test on Source amplitudes and a test on the closeness of Source pairs. The Wald test is analytically studied in terms of alternative hypotheses that are close to the null hypothesis (local alternatives). It is shown that the Wald test is asymptotically unbiased, that it has the correct level and power, which makes it appropriate to use in EMSA. An accurate estimate of the Crame/spl acute/r-Rao bound (CRB) is required for the use of the Wald test when not all assumptions are satisfied. The sandwich CRB is used for this purpose. It is defined for nonseparable least squares with constraints required for the Wald test on amplitudes. Simulations with EEG show that when the sensor positions are incorrect, or the number of shells is incorrect, or the conductivity parameter is incorrect, then the CRB and Wald test are still good, with a moderate number of trials. Additionally, the CRB and Wald test appear robust against an incorrect assumption on the noise covariance. A combination of incorrect sensor positions and noise covariance affects the possibility of detecting a Source with small amplitude.
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spatiotemporal eeg meg Source Analysis based on a parametric noise covariance model
IEEE Transactions on Biomedical Engineering, 2002Co-Authors: Hilde M. Huizenga, Lourens J. Waldorp, J C De Munck, Raoul P. P. P. GrasmanAbstract:A method is described to incorporate the spatiotemporal noise covariance matrix into a spatiotemporal Source Analysis. The essential feature is that the estimation problem is split into two parts. First, a model is fitted to the observed noise covariance matrix. This model is a Kronecker product of a spatial and a temporal matrix. The spatial matrix models the spatial covariances by a function dependent on sensor distance. The temporal matrix models the temporal covariances as lag dependent. In the second part, Sources are estimated given this noise model, which can be done very efficiently due to the Kronecker formulation. An application to real electroencephalogram (EEG) data shows that the noise model fits the data very well. Simulation results show that the resulting Source estimates are more precise than those obtained from a standard Analysis neglecting the noise covariance. In addition, the estimated standard errors of the Source parameter estimates are far more precise than those obtained from a standard Analysis. Finally, the Source parameter standard errors are used to investigate the effects of temporal sampling. It is shown that increasing the sampling by a factor x, decreases the standard errors of all Source parameters with the square root of x.
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estimated generalized least squares electromagnetic Source Analysis based on a parametric noise covariance model eeg meg
IEEE Transactions on Biomedical Engineering, 2001Co-Authors: Lourens J. Waldorp, Hilde M. Huizenga, Conor V Dolan, Peter C M MolenaarAbstract:Estimated generalized least squares (EGLS) electromagnetic Source Analysis is used to downweight noisy and correlated data. Standard EGLS requires many trials to accurately estimate the noise covariances and, thus, the Source parameters. Alternatively, the noise covariances can be modeled parametrically. Only the parameters of the model describing the noise covariances need to be estimated and, therefore, less trials are required. This method is referred to as parametric EGLS (PEGLS). In this paper, PEGLS is developed and its performance is tested in a simulation study and in a pseudoempirical study.
