Source Localization Analysis Research Articles

Performance Analysis of Matched-Field Source Localization Under Spatially Correlated Noise Field

Matched-field processing (MFP) concerns estimation of source locations by exploiting full wave modeling of acoustic waveguide propagation. Typical MFP performance demonstrates a threshold behavior, that is, below some signal-to-noise ratio (SNR), the mean-square error (MSE) increases dramatically. This threshold phenomenon has been intensively investigated previously under spatially white noise field. In this paper, we develop approaches to study MFP performance in the presence of spatially correlated noises. Both the discrete interference and surface-generated noise are considered, and a so-called method of intervals errors (MIE) is derived in the context of the maximum likelihood estimate (MLE). Performance analysis of both narrowband and broadband MFP is implemented for source localization in a realistic shallow water environment. Simulation results suggest that 1) adding either spatially correlated noise increases the MLE threshold SNR compared to the case with white noise only, but effects at high SNR are different, MSE decreasing at the same rate with a point interference while decaying in a slower pace with surface-generated noises; 2) with a strong discrete interference, interference position, rather than strength, matters more in determining the MLE performance, and broadband processing effectively reduces the threshold SNR; 3) effect of the surface-generated noises is highly frequency dependent, and processing using a higher frequency would be preferred; 4) overall the surface-generated noise is more difficult to attack by an adaptive method such as the MLE.

The relationship between the visual evoked potential and the gamma band investigated by blind and semi-blind methods

Gamma Band Activity (GBA) is increasingly studied for its relation with attention, change detection, maintenance of working memory and the processing of sensory stimuli. Activity around the gamma range has also been linked with early visual processing, although the relationship between this activity and the low frequency visual evoked potential (VEP) remains unclear. This study examined the ability of blind and semi-blind source separation techniques to extract sources specifically related to the VEP and GBA in order to shed light on the relationship between them. Blind (Independent Component Analysis—ICA) and semi-Blind (Functional Source Separation—FSS) methods were applied to dense array EEG data recorded during checkerboard stimulation. FSS was performed with both temporal and spectral constraints to identify specifically the generators of the main peak of the VEP (P100) and of the GBA. Source localisation and time-frequency analyses were then used to investigate the properties and co-dependencies between VEP/P100 and GBA. Analysis of the VEP extracted using the different methods demonstrated very similar morphology and localisation of the generators. Single trial time frequency analysis showed higher GBA when a larger amplitude VEP/P100 occurred. Further examination indicated that the evoked (phase-locked) component of the GBA was more related to the P100, whilst the induced component correlated with the VEP as a whole. The results suggest that the VEP and GBA may be generated by the same neuronal populations, and implicate this relationship as a potential mediator of the correlation between the VEP and the Blood Oxygenation Level Dependent (BOLD) effect measured with fMRI.

Antero-Posterior EEG Spectral Power Gradient as a Correlate of Extraversion and Behavioral Inhibition

Several studies have shown that individual-specific patterns of cortical spectral power distribution are relatively stable across time and experimental conditions. The antero-posterior EEG spectral power gradient (APSPG) emerged as the most prominent feature associated with important personality characteristics. In this study this phenomenon is further investigated in relation to its stability and association with different personality traits. It has been shown that APSPG is generally more pronounced during resting baseline than during inter-trial interval and post-stimulus period. Its association with personality variables is similar for different frequency bands and is mostly preserved during different experimental conditions. Relatively higher oscillatory activity in frontal than in posterior cortical sites is more frequently observed in subjects with higher behavioral inhibition (BIS) and lower Sociability. Source localization analysis showed that both high BIS/low Sociability and high APSPG are associated with higher oscillatory activity in medial cortices associated with emotion processing (mostly the cingulate gyrus). This association could be tentatively explained by higher vigilance and emotional tension in introverted and behaviorally inhibited subjects.

Source Localization of Event-Related Potential Effects Differentiates between Vascular Dementia and Alzheimer's Disease

Background: VaD and AD have different underlying pathogenic mechanisms, and may produce distinct signatures in brain activity that could aid in differential diagnosis. We investigate possible differential markers based on source localization of electrophysiological correlates of memory processing in VaD and AD. Methods: Advances in high resolution whole head EEG recordings, together with accurate conductivity models of head tissues, mean that reasonably accurate distributed source models can now be established. We used 128-channel EEG to measure amplitudes, latencies and topographies of the N2, P4, and P7 in AD and VaD patients and controls during an episodic memory task (all groups N = 8). We then used source localization methods (GeoSource; LAURA) to determine the cortical generators for the best performing ERP diagnostic markers. Results: The posterior N2 is prominent in controls and AD, but attenuated in VaD. This N2 effect localizes to the central occipital cortex in AD and controls, but not in VaD. The prefrontal P4 is prominent in controls, and evident in VaD, but attenuated in AD. The P4 localizes in all groups to the medial temporal region, but with varying intensity and timing. The right parietal P7 effect is prominent in controls, but attenuated in AD and VaD. The P7 localizes to prefrontal cortex in controls, but involves only right posterior cortex in VaD, and central posterior cortex in AD. Conclusions: Source localization analysis shows that scalp ERP differences between AD and VaD can be unambiguously attributed to different underlying generators within the brain. We hypothesize that these effects reflect disease-specific neuropathology, and offer new functional biomarkers to aid in differential diagnosis of AD and VaD.

Investigation of Periventricular white Matter Hyperintensities Prone to Progression from Amnestic MCI to Alzheimer's Disease

region, but with varying intensity and timing. The right parietal P7 effect is prominent in controls, but attenuated in AD and VaD. The P7 localizes to prefrontal cortex in controls, but involves only right posterior cortex in VaD, and central posterior cortex in AD. Conclusions: Source localization analysis shows that scalp ERP differences between AD and VaD can be unambiguously attributed to different underlying generators within the brain. We hypothesize that these effects reflect disease-specific neuropathology, and offer new functional biomarkers to aid in differential diagnosis of AD and VaD.

On the Origin of the N400 Effects: An ERP Waveform and Source Localization Analysis in Three Matching Tasks

The question of the cognitive nature and the cerebral origins of the event-related potential (ERP) N400 component has frequently been debated. Here, the N400 effects were analyzed in three tasks. In the semantic task, subjects decided whether sequentially presented word pairs were semantically related or unrelated. In the phonologic (rhyme detection) task, they decided if words were phonologically related or not. In the image categorization task, they decided whether images were categorically related or not. Difference waves between ERPs to unrelated and related conditions (defined here as the N400 effect) demonstrated a greater amplitude and an earlier peak latency effect in the image than in semantic and phonologic tasks. In contrast, spatial correlation analysis revealed that the maps computed during the peak of the N400 effects were highly correlated. Source localization computed from these maps showed the involvement in all tasks of the middle/superior temporal gyrus. Our results suggest that these qualitatively similar N400 effects index the same cognitive content despite differences in the representational formats (words vs. images) and the types of mismatch (semantic vs. phonological) across tasks.

Robustness analysis and new hybrid algorithm of wideband source localization for acoustic sensor networks

Wideband source localization using acoustic sensor networks has been drawing a lot of research interest recently in wireless communication applications, such as cellular handset localization, global positioning systems (GPS), and land navigation technologies, etc. The maximum-likelihood is the predominant objective which leads to a variety of source localization approaches. However, the appropriate optimization (search) algorithms are still being pursuit by researchers since different aspects about the effectiveness of such algorithms have to be addressed on different circumstances. In this paper, we focus on the two popular source localization methods for wideband acoustic signals, namely the alternating projection (AP) algorithm and the expectation maximization (EM) algorithm. We explore the respective limitations of these two methods and design a new hybrid approach thereupon. Through Monte Carlo simulations, we demonstrate that the trade-off can be achieved between the computational complexity and the localization accuracy using our newly proposed scheme. Moreover, we present the new robustness analysis for the source localization algorithms. We derive the Cramer-Rao lower bound (CRLB) involving the source spectral estimation error and thus prove that the new hybrid algorithm is more efficient than the EM algorithm. By employing the Gaussianity test, we also quantify the statistical mismatch between the actual statistics of the sensor signals and the underlying Gaussian model. We show that the Gaussianity measure can be a reliable robustness figure for source localization.

Reduced NoGo-anteriorisation during continuous performance test in deletion syndrome 22q11.2

Deletion syndrome 22q11.2 (DS22q11.2) is a high-risk factor for psychiatric disorders. Alterations in brain morphology and function including the anterior cingulate cortex (ACC) are suggested to underlie the increased psychiatric disposition. We assessed response-inhibition in patients with DS22q11.2 ( n = 13) and healthy controls ( n = 13) matched for age, sex, and handedness by means of a Go–NoGo-Task during recording of a multi-channel electroencephalography (EEG). Analysis of event-related potentials (P300) resulted in an aberrant topographical pattern and NoGo-anteriorisation (NGA) as a parameter of medial prefrontal function was significantly reduced in patients with DS22q11.2 compared to controls. Differences in IQ between groups did not account for the findings. Source localization analysis (LORETA) revealed diminished left temporal brain activation during the Go-condition, but no altered ACC activation in DS22q11 during the NoGo-condition. Despite recent reports of structural alterations of the ACC in DS22q11.2 our findings suggest that response-inhibition mediated by the ACC is not impaired in DS22q11.2.

Poor reading skills may involve a failure to focus attention

A source localization analysis was carried out to provide brain functional and structural assessments of individuals with poor reading skills. Standardized low-resolution brain electromagnetic tomography was used to locate sources of P2 and P3 event-related potential components in normal readers and in poor reader children performing a cued continuous performance task. Cue-elicited P2 sources in the right superior parietal gyrus were smaller in 37 poor readers than in 40 normal readers. Poor readers showed a higher P3 activation in response to a false target in frontal and frontorbital regions than normal readers. These results suggest that reading disabilities may be attributed to failures in attentional focalization for incoming stimuli.

Food-Evoked Changes in Humans

Two experiments investigate effects related to food intake in humans. In Experiment 1, we measured startle response modulation while study participants ate ice cream, yoghurt, and chocolate. Statistical analysis revealed that ice cream intake resulted in the most robust startle inhibition compared to no food. Contrasting females and males, we found significant differences related to the conditions yoghurt and chocolate. In females, chocolate elicited the lowest response amplitude followed by yoghurt and ice cream. In males, chocolate produced the highest startle response amplitude even higher than eating nothing, whereas ice cream produced the lowest. Assuming that high response amplitudes reflect aversive motivation while low response amplitudes reflect appetitive motivational states, it is interpreted that eating ice cream is associated with the most appetitive state given the alternatives of chocolate and yoghurt across gender. However, in females alone eating chocolate, and in males alone eating ice cream, led to the most appetitive state. Experiment 2 was conducted to describe food intake-related brain activity by means of source localization analysis applied to electroencephalography data (EEG). Ice cream, yoghurt, a soft drink, and water were compared. Brain activity in rostral portions of the superior frontal gyrus was found in all conditions. No localization differences between conditions occurred. While EEG was found to be insensitive, startle response modulation seems to be a reliable method to objectively quantify motivational states related to the intake of different foods.

Event-related delta and theta synchronization during explicit and implicit emotion processing

Emotion information processing may occur in two modes which are differently represented in conscious awareness. Fast online processing involves coarse-grained analysis of salient features, and is not represented in conscious awareness; offline processing takes hundreds of milliseconds to generate fine-grained analysis, and is represented in conscious awareness. These processing modes may be studied using event-related electroencephalogram theta and delta synchronization as a marker of emotion processing. Two experiments were conducted, which differed on the mode of emotional information presentation. In the explicit mode subjects were explicitly instructed to evaluate the emotional content of presented stimuli; in the implicit mode they performed a gender discrimination task. Firstly, we show that in both experiments theta and delta synchronization is stronger upon presentation of “emotional” than “neutral” stimuli, and in subjects who are more sensitive, or experience higher emotional involvement than in less sensitive or detached subjects. Secondly, we show that in the implicit mode theta and delta synchronization is more pronounced in an early (before 250 ms post-stimulus) processing stage, whereas in the explicit mode it is more pronounced in a later processing stage. Source localization analysis showed that implicit processing of angry and happy (relative to neutral) faces is associated with higher early (before 250 ms) theta synchronization in the right parietal cortex and the right insula, respectively. Explicit processing of angry and happy faces is associated with higher late (after 250 ms) theta synchronization in the left temporal lobe and bilateral prefrontal cortex, respectively.

Beamforming array techniques for acoustic emission monitoring of large concrete structures

This paper introduces a novel method of acoustic emission (AE) analysis which is particularly suited for field applications on large plate-like reinforced concrete structures, such as walls and bridge decks. Similar to phased-array signal processing techniques developed for other non-destructive evaluation methods, this technique adapts beamforming tools developed for passive sonar and seismological applications for use in AE source localization and signal discrimination analyses. Instead of relying on the relatively weak P-wave, this method uses the energy-rich Rayleigh wave and requires only a small array of 4–8 sensors. Tests on an in-service reinforced concrete structure demonstrate that the azimuth of an artificial AE source can be determined via this method for sources located up to 3.8 m from the sensor array, even when the P-wave is undetectable. The beamforming array geometry also allows additional signal processing tools to be implemented, such as the VESPA process (VElocity SPectral Analysis), whereby the arrivals of different wave phases are identified by their apparent velocity of propagation. Beamforming AE can reduce sampling rate and time synchronization requirements between spatially distant sensors which in turn facilitates the use of wireless sensor networks for this application.

Brain responses to repeated visual experience among low and high sensation seekers: Role of boredom susceptibility

To better understand individual differences in sensation seeking and its components, including boredom susceptibility and experience seeking, we examined brain responses of high and low sensation seekers during repeated visual experience. Individuals scoring in the top and bottom quartiles from a college-aged population on the Brief Sensation-Seeking Scale (BSSS) participated in an event-related potentials (ERPs) experiment. Line drawings of common objects were randomly intermixed and presented 1–3 times. Sixty-four channel ERP responses were recorded while participants classified items as “man-made” or “not man-made” in a repetition priming task. The two groups showed different ERP responses at frontal electrode sites after seeing a visual stimulus for 400–800 ms. The frontal late positive components (LPC) showed different habituation of ERP responses to new and studied repeated objects between high and low sensation seekers. Source localization analysis (LORETA) indicated that during visual stimulus adaptation the ventral pre-frontal cortex showed lack of frontal involvement among high sensation seekers. Furthermore, frontal LPC latencies during repeated visual exposure correlated with boredom susceptibility and experience seeking subscales. The distinct profiles of brain responses to repeated visual experience in high and low sensation seekers provide evidence that individual differences in neural adaptation can be linked to personality dimensions.

The Importance of Electroencephalogram Assessment for Autistic Disorders

Abstract Autistic disorders are a set of complex syndromes that lead to challenges impacting communication, behavior repertoire, and social skills. The etiology of autism is unknown but is likely epigenetic in nature. It is likely associated with an inflammatory process leading to neuroinflammation in early childhood. Autistic disorders include seizures in approximately one-third of the cases and there are often regions of brain dysfunction associated with neural connectivity anomalies. The electroencephalogram (EEG) is presented as a premiere tool to assess these difficulties due to its' non-invasive nature, availability and utility in detailing these difficulties. Techniques for seizure detection, monitoring, and tracing their propagation are shown. Similar approaches can then be utilized for assessing EEG oscillations, which are at the heart of these neuronal regulation dysfunctions. Autistic disorders are clearly associated with regions of dysfunction and quantitative electroencephalogram strategies for assessing these impairments are shown. These include techniques for increasing the specificity and spatial resolution of the EEG such as source localization and independent components analysis. Lastly, advanced methods for assessing the neural connectivity problems that underlie the difficulties of these children are presented. EEG assessment, when processed and analyzed with the most advanced techniques, can be invaluable in the evaluation of autistic disorders.

Magnetoencephalographic gamma power reduction in patients with schizophrenia during resting condition.

The "default network" represents a baseline condition of brain function and is of interest in schizophrenia research because its component brain regions are believed to be aberrant in the disorder. We hypothesized that magnetoencephalographic (MEG) source localization analysis would reveal abnormal resting activity within particular frequency bands in schizophrenia. Eyes-closed resting state MEG signals were collected for two comparison groups. Patients with schizophrenia (N = 38) were age-gender matched with healthy control subjects (N = 38), and with a group of unmedicated unaffected siblings of patients with schizophrenia (N = 38). To localize 3D-brain regional differences, synthetic aperture magnetometry was calculated across established frequency bands as follows: delta (0.9-4 Hz), theta (4-8 Hz), alpha (8-14 Hz), beta (14-30 Hz), gamma (30-80 Hz), and super-gamma (80-150 Hz). PRINCIPLE OBSERVATIONS: Patients with schizophrenia showed significantly reduced activation in the gamma frequency band in the posterior region of the medial parietal cortex. As a group, unaffected siblings of schizophrenia patients also showed significantly reduced activation in the gamma bandwidth across similar brain regions. Moreover, using the significant region for the patients and examining the gamma band power gave an odds ratio of 6:1 for reductions of two standard deviations from the mean. This suggests that the measure might be the basis of an intermediate phenotype. MEG resting state analysis adds to the evidence that schizophrenic patients experience this condition very differently than healthy controls. Whether this baseline difference relates to network abnormalities remains to be seen.

ERP correlates of superordinate category activation

The human semantic network is hierarchically organized, containing superordinate, basic and subordinate levels. Various impairments are thought to be connected with abnormal access to superordinate concepts. We devised an ERP paradigm to examine the activation of superordinate versus otherwise related concepts in 20 healthy participants. Following the presentation of a typical category member an arrow indicated whether the appropriate superordinate category had to be generated (categorization task) or an otherwise related word (relation task). To control task execution, a second word was presented for which a match-mismatch-judgment was required. Reaction times, accuracy rates and ERPs after the second word showed that participants successfully accessed the superordinate category name and that verification in the categorization task was faster and easier than in the relation task. Comparison of ERPs after the arrow revealed topographical, Global Field Power (GFP), and onset latency differences between the two tasks and thus indicated the involvement of at least partially different neural generators. Source localization analysis confirmed that brain regions were activated that were also identified in previous experiments with semantic task. The paradigm seems to be suitable for further examination of superordinate activation processes and evaluation of impairments such as thought disorders in schizophrenic patients.

An electrophysiological approach to investigations of sensory dysfunction in schizophrenia

Sensory dysfunction has been shown to be a part of the pathophysiology of schizophrenia. Nowadays we have an objective, non-invasive tool with which to measure neural manifestations of sensory dysfunction. Defined as time-locked changes to external stimuli in the EEG, event-related potentials (ERPs) provide an objective index of information processing in the human brain. Importantly, ERPs may be analyzed through a variety of approaches such as conventional ERP analysis, analysis in the time-frequency domain, microstate segmentation and topographical analysis, as well as source localisation analysis. Each of the methods gives distinct information; they also supplement each other. Here, an attempt is made to verify the validity of combining different approaches to study sensory dysfunction in neuropsychiatric disorders. For example, the data from a schizophrenic patient and an age- and sex-matched healthy subject generate a picture of the events which emerges after visual, proprioceptive and simultaneous presentation of stimuli in both modalities. This approach, though time-consuming, allows the visualisation of changes appearing in the malfunctioning brain as compared to the healthy brain. These methods could ultimately lead to a better establishment of one or more endophenotypes for the schizophrenic disorders. They might also serve as a way to track changes in response to various medications and therapies.

Evoked potentials to nociceptive stimuli delivered by CO 2 or Nd:YAP lasers

Objective This study compares the amplitude, latency, morphology, scalp topography and intracranial generators of laser-evoked potentials (LEPs) to CO 2 and Nd:YAP laser stimuli. Methods LEPs were assessed in 11 healthy subjects (6 men, mean age 39 ± 10 years) using a 32-channel acquisition system. Laser stimuli were delivered on the dorsum of both hands (intensity slightly above pain threshold), and permitted to obtain lateralised (N1) and vertex components (N2-P2) with similar scalp distribution for both types of lasers. Results The N1-YAP had similar latencies but significantly higher amplitudes relative to N1-CO 2. The N2-P2 complex showed earlier latencies, higher amplitudes (N2) and more synchronised responses when using Nd:YAP stimulation. The distribution of intracranial generators assessed with source localization analyses (sLORETA) was similar for Nd:YAP and CO 2 lasers. The insular, opercular, and primary sensorimotor cortices were active during the N1 time-window, whereas the anterior midcingulate, supplementary motor areas and mid-anterior insulae were active concomitant to the N2-P2 complex. Conclusions Earlier latencies and larger amplitudes recorded when using Nd:YAP pulses suggest a more synchronized nociceptive afferent volley with this type of laser. Significance This, together with its handy utilization due to optic fibre transmission, may favour the use of Nd:YAP lasers in clinical settings.

Aesthetic value of paintings affects pain thresholds

Pain is modulated by cognitive factors, including attention and emotions. In this study we evaluated the distractive effect of aesthetic appreciation on subjectively rated pain (visual analogue scale;VAS) and multi-channel evoked potentials induced by CO 2 laser stimulation of the left hand in twelve healthy volunteers. Subjects were stimulated by laser in the absence of other external stimulation (baseline condition) and while looking at different paintings they had previously rated as beautiful, neutral or ugly. The view of paintings previously appreciated as beautiful produced lower pain scores and a clear inhibition of the P2 wave amplitude, localized in the anterior cingulate cortex; the inhibition of P2 wave amplitude was lesser or not significant during the presentation of the ugly or neutral paintings, respectively. Dipole source localization analysis of the LEP peaks showed significant changes during different conditions, with a shift from the posterior to the anterior right cingulated cortex while looking at paintings previously rated as beautiful. Our results provide evidence that pain may be modulated at cortical level by the aesthetic content of the distracting stimuli.

Attentional load modifies early activity in human primary visual cortex

Recent theories of selective attention assume that the more attention is required by a task, the earlier are irrelevant stimuli filtered during perceptual processing. Previous functional MRI studies have demonstrated that primary visual cortex (V1) activation by peripheral distractors is reduced by higher task difficulty at fixation, but it remains unknown whether such changes affect initial processing in V1 or subsequent feedback. Here we manipulated attentional load at fixation while recording peripheral visual responses with high-density EEG in 28 healthy volunteers, which allowed us to track the exact time course of attention-related effects on V1. Our results show a modulation of the earliest component of the visual evoked potential (C1) as a function of attentional load. Additional topographic and source localization analyses corroborated this finding, with significant load-related differences observed throughout the first 100 ms post-stimulus. However, this effect was observed only when stimuli were presented in the upper visual field (VF), but not for symmetrical positions in the lower VF. Our findings demonstrate early filtering of irrelevant information under increased attentional demands, thus supporting models that assume a flexible mechanism of attentional selection, but reveal important functional asymmetries across the VF.