Brain Electric Field Research Articles

Concurrency in electrical neuroinformatics: parallel computation for studying the volume conduction of brain electrical fields in human head tissues

SummaryAdvances in human brain neuroimaging for high‐temporal and high‐spatial resolutions will depend on localization of electroencephalography (EEG) signals to their cortex sources. The source localization inverse problem is inherently ill‐posed and depends critically on the modeling of human head electromagnetics. We present a systematic methodology to analyze the main factors and parameters that affect the EEG source‐mapping accuracy. These factors are not independent, and their effect must be evaluated in a unified way. To do so requires significant computational capabilities to explore the problem landscape, quantify uncertainty effects, and evaluate alternative algorithms. Bringing high‐performance computing to this domain is necessary to open new avenues for neuroinformatics research. The head electromagnetics forward problem is the heart of the source localization inverse. We present two parallel algorithms to address tissue inhomogeneity and impedance anisotropy. Highly accurate head modeling environments will enable new research and clinical neuroimaging applications. Cortex‐localized dense‐array EEG analysis is the next‐step in neuroimaging domains such as early childhood reading, understanding of resting‐state brain networks, and models of full brain function. Therapeutic treatments based on neurostimulation will also depend significantly on high‐performance computing integration. Copyright © 2015 John Wiley & Sons, Ltd.

Developmental changes in spontaneous electrocortical activity and network organization from early to late childhood

We investigated the development of spontaneous (resting state) cerebral electric fields and their network organization from early to late childhood in a large community sample of children. Critically, we examined electrocortical maturation across one-year windows rather than creating aggregate averages that can miss subtle maturational trends. We implemented several novel methodological approaches including a more fine grained examination of spectral features across multiple electrodes, the use of phase-lagged functional connectivity to control for the confounding effects of volume conduction and applying topological network analyses to weighted cortical adjacency matrices. Overall, there were major decreases in absolute EEG spectral density (particularly in the slow wave range) across cortical lobes as a function of age. Moreover, the peak of the alpha frequency increased with chronological age and there was a redistribution of relative spectral density toward the higher frequency ranges, consistent with much of the previous literature. There were age differences in long range functional brain connectivity, particularly in the alpha frequency band, culminating in the most dense and spatially variable networks in the oldest children. We discovered age-related reductions in characteristic path lengths, modularity and homogeneity of alpha-band cortical networks from early to late childhood. In summary, there is evidence of large scale reorganization in endogenous brain electric fields from early to late childhood, suggesting reduced signal amplitudes in the presence of more functionally integrated and band limited coordination of neuronal activity across the cerebral cortex.

Inter-subject Variability in Electric Fields of Motor Cortical tDCS.

The sources of inter-subject variability in the efficacy of transcranial direct current stimulation (tDCS) remain unknown. One potential source of variations is the brain's electric field, which varies according to each individual's anatomical features. We employed an approach that combines imaging and computational modeling to quantitatively study the extent and primary causes of inter-subject variation in tDCS electric fields. Anatomically-accurate models of the head and brain of 24 males (age: 38.63±11.24 years) were constructed from structural MRI. Finite-element method was used to computationally estimate the electric fields for tDCS of the motor cortex. Surface-based inter-subject registration of the electric field and functional MRI data was used for group level statistical analysis. We observed large differences in each individual's electric field patterns. However, group level analysis revealed that the average electric fields concentrated in the vicinity of the primary motor cortex. The variations in the electric fields in the hand motor area could be characterized by a normal distribution with a standard deviation of approximately 20% of the mean. The cerebrospinal fluid (CSF) thickness was the primary factor influencing an individual's electric field, thereby explaining 50% of the inter-individual variability, a thicker layer of CSF decreasing the electric field strength. The variability in the electric fields is related to each individual's anatomical features and can only be controlled using detailed image processing. Age was found to have a slight negative effect on the electric field, which might have implications on tDCS studies on aging brains.

Pediatric stroke and transcranial direct current stimulation: methods for rational individualized dose optimization.

Background: Transcranial direct current stimulation (tDCS) has been investigated mainly in adults and doses may not be appropriate in pediatric applications. In perinatal stroke where potential applications are promising, rational adaptation of dosage for children remains under investigation.Objective: Construct child-specific tDCS dosing parameters through case study within a perinatal stroke tDCS safety and feasibility trial.Methods: 10-year-old subject with a diagnosis of presumed perinatal ischemic stroke and hemiparesis was identified. T1 magnetic resonance imaging (MRI) scans used to derive computerized model for current flow and electrode positions. Workflow using modeling results and consideration of dosage in previous clinical trials was incorporated. Prior ad hoc adult montages vs. de novo optimized montages provided distinct risk benefit analysis. Approximating adult dose required consideration of changes in both peak brain current flow and distribution which further tradeoff between maximizing efficacy and adding safety factors. Electrode size, position, current intensity, compliance voltage, and duration were controlled independently in this process.Results: Brain electric fields modeled and compared to values previously predicted models (Datta et al., 2011; Minhas et al., 2012). Approximating conservative brain current flow patterns and intensities used in previous adult trials for comparable indications, the optimal current intensity established was 0.7 mA for 10 min with a tDCS C3/C4 montage. Specifically 0.7 mA produced comparable peak brain current intensity of an average adult receiving 1.0 mA. Electrode size of 5 × 7 cm2 with 1.0 mA and low-voltage tDCS was employed to maximize tolerability. Safety and feasibility confirmed with subject tolerating the session well and no serious adverse events.Conclusion: Rational approaches to dose customization, with steps informed by computational modeling, may improve guidance for pediatric stroke tDCS trials.

Dosage Considerations for Transcranial Direct Current Stimulation in Children: A Computational Modeling Study

Transcranial direct current stimulation (tDCS) is being widely investigated in adults as a therapeutic modality for brain disorders involving abnormal cortical excitability or disordered network activity. Interest is also growing in studying tDCS in children. Limited empirical studies in children suggest that tDCS is well tolerated and may have a similar safety profile as in adults. However, in electrotherapy as in pharmacotherapy, dose selection in children requires special attention, and simple extrapolation from adult studies may be inadequate. Critical aspects of dose adjustment include 1) differences in neurophysiology and disease, and 2) variation in brain electric fields for a specified dose due to gross anatomical differences between children and adults. In this study, we used high-resolution MRI derived finite element modeling simulations of two healthy children, ages 8 years and 12 years, and three healthy adults with varying head size to compare differences in electric field intensity and distribution. Multiple conventional and high-definition tDCS montages were tested. Our results suggest that on average, children will be exposed to higher peak electrical fields for a given applied current intensity than adults, but there is likely to be overlap between adults with smaller head size and children. In addition, exposure is montage specific. Variations in peak electrical fields were seen between the two pediatric models, despite comparable head size, suggesting that the relationship between neuroanatomic factors and bioavailable current dose is not trivial. In conclusion, caution is advised in using higher tDCS doses in children until 1) further modeling studies in a larger group shed light on the range of exposure possible by applied dose and age and 2) further studies correlate bioavailable dose estimates from modeling studies with empirically tested physiologic effects, such as modulation of motor evoked potentials after stimulation.

Transcranial Current Brain Stimulation (tCS): Models and Technologies

In this paper, we provide a broad overview of models and technologies pertaining to transcranial current brain stimulation (tCS), a family of related noninvasive techniques including direct current (tDCS), alternating current (tACS), and random noise current stimulation (tRNS). These techniques are based on the delivery of weak currents through the scalp (with electrode current intensity to area ratios of about 0.3-5 A/m2) at low frequencies (typically < 1 kHz) resulting in weak electric fields in the brain (with amplitudes of about 0.2-2 V/m). Here we review the biophysics and simulation of noninvasive, current-controlled generation of electric fields in the human brain and the models for the interaction of these electric fields with neurons, including a survey of in vitro and in vivo related studies. Finally, we outline directions for future fundamental and technological research.

Instrumental methods in the diagnostics of locked-in syndrome

The locked-in syndrome (LiS) is typically characterized by a paralysis of almost all body muscles combined with intact cognitive functions. In practice, there are often additional brain damages besides the one directly causing LiS. These damages can lead to cognitive impairment, which substantially complicates the diagnosis of LiS. At the level of behavior, therefore, the disease can be confused with akinetic mutism, vegetative state (syn. unresponsive wakefulness state) and some other conditions. Using instrumental methods in addition to behavioral diagnostics helps to avoid diagnostic errors and to improve prognosis of rehabilitation of such patients. These methods, which include measurements of brain electric or magnetic fields, electrical potential of muscles, blood flow and oxygen consumption in the brain, are reviewed in this paper.

Scale-Free Properties of the Functional Magnetic Resonance Imaging Signal during Rest and Task

It has been shown recently that a significant portion of brain electrical field potentials consists of scale-free dynamics. These scale-free brain dynamics contain complex spatiotemporal structures and are modulated by task performance. Here we show that the fMRI signal recorded from the human brain is also scale free; its power-law exponent differentiates between brain networks and correlates with fMRI signal variance and brain glucose metabolism. Importantly, in parallel to brain electrical field potentials, the variance and power-law exponent of the fMRI signal decrease during task activation, suggesting that the signal contains more long-range memory during rest and conversely is more efficient at online information processing during task. Remarkably, similar changes also occurred in task-deactivated brain regions, revealing the presence of an optimal dynamic range in the fMRI signal. The scale-free properties of the fMRI signal and brain electrical field potentials bespeak their respective stationarity and nonstationarity. This suggests that neurovascular coupling mechanism is likely to contain a transformation from nonstationarity to stationarity. In summary, our results demonstrate the functional relevance of scale-free properties of the fMRI signal and impose constraints on future models of neurovascular coupling.

Differences in quantitative EEG between frontotemporal dementia and Alzheimer’s disease as revealed by LORETA

Objective To determine the electrophysiological characteristics of frontotemporal dementia (FTD) and the distinction with Alzheimer’s disease (AD). Methods We performed analyses of global field power (GFP) which is a measure of whole brain electric field strength, and EEG neuroimaging analyses with sLORETA (standardized low resolution electromagnetic tomography), in the mild stages of FTD ( n = 19; mean age = 68.11 ± 7.77) and AD ( n = 19; mean age = 69.42 ± 9.57) patients, and normal control (NC) subjects ( n = 22; mean age = 66.13 ± 6.02). Results In the GFP analysis, significant group effects were observed in the delta (1.5–6.0 Hz), alpha1 (8.5–10.0 Hz), and beta1 (12.5–18.0 Hz) bands. In sLORETA analysis, differences in activity were observed in the alpha1 band (NC > FTD) in the orbital frontal and temporal lobe, in the delta band (AD > NC) in widespread areas including the frontal lobe, and in the beta1 band (FTD > AD) in the parietal lobe and sensorimotor area. Conclusions Differential patterns of brain regions and EEG frequency bands were observed between the FTD and AD groups in terms of pathological activity. Significance FTD and AD patients in the early stages displayed different patterns in the cortical localization of oscillatory activity across different frequency bands.

Spatiotemporal Analysis of Multichannel EEG: CARTOOL

This paper describes methods to analyze the brain's electric fields recorded with multichannel Electroencephalogram (EEG) and demonstrates their implementation in the software CARTOOL. It focuses on the analysis of the spatial properties of these fields and on quantitative assessment of changes of field topographies across time, experimental conditions, or populations. Topographic analyses are advantageous because they are reference independents and thus render statistically unambiguous results. Neurophysiologically, differences in topography directly indicate changes in the configuration of the active neuronal sources in the brain. We describe global measures of field strength and field similarities, temporal segmentation based on topographic variations, topographic analysis in the frequency domain, topographic statistical analysis, and source imaging based on distributed inverse solutions. All analysis methods are implemented in a freely available academic software package called CARTOOL. Besides providing these analysis tools, CARTOOL is particularly designed to visualize the data and the analysis results using 3-dimensional display routines that allow rapid manipulation and animation of 3D images. CARTOOL therefore is a helpful tool for researchers as well as for clinicians to interpret multichannel EEG and evoked potentials in a global, comprehensive, and unambiguous way.

The electric field is a sufficient physical determinant of the human magnetic sense

The onset and offset of weak low-frequency magnetic fields triggered evoked potentials in human subjects that could be detected using nonlinear analysis, but not by means of time averaging. Because the magnetic fields and their induced electric fields were both present in the brain, their respective role in producing the effect on brain activity could not be ascertained. We inquired whether a biophysical coupling mechanism involving only the electric field could explain the occurrence of the brain potentials. An external electric field capable of producing a brain electric field comparable to that induced by the magnetic stimuli was identified by finite-element analysis. The electroencephalogram from 23 subjects was measured from six scalp derivations in the presence and absence of the external electric field, and the presence of evoked potentials was assessed using nonlinear and linear analyses. Evoked potentials were observed in all but one subject (p < 0.05 in each subject); the potentials had the same latency, duration, and distribution of magnitudes as seen in the earlier studies, and were detectable only by means of nonlinear analysis. Using a realistic physical model of an ion channel, we showed that transduction of an electric field could be explained by assuming that the field exerted a force on glycocalyx molecules attached to a channel gate. The evoked potentials described here, as well as those observed previously in response to magnetic stimuli, were probably triggered by the induced electric field.

Signs of Mental Activity: An Essential Guidebook

Event Related Potentials: A Methods Handbook. Todd C. Handy (Ed.). 2005. Cambridge, MA: Bradford/MIT Press, 404 pp., $60.00.The dramatic increase in interest in functional magnetic resonance imaging in recent years has been accompanied by a wealth of literature on the scientific foundations of the procedure and the highly complex methodological issues that must be considered during experimental design and data analysis. To some extent, progress in this exciting area has eclipsed significant developments in other areas of neuroimaging, particularly the “neuromagnetic” approaches that study time-locked brain electrical potentials or magnetic field changes produced in the course of processing perceptual, cognitive, and motor events. However, there has been increasing recognition that the study of event-related electrical potentials and neuromagnetic responses remains a critical complement to “hemodynamic” approaches because of their exquisite temporal resolution and because they provide a more direct index of neural activity related to stimulus events than procedures dependent on the coupling of brain activity with cerebral blood flow. In this context, Event Related Potentials: A Methods Handbook, edited by Todd C. Handy, represents an important and long overdue handbook. It provides practical and concise information on ERP methods that is comparable to literature available for fMRI techniques.

Evidence for unaltered brain electrical topography during prefrontal response control in cycloid psychoses

Objective Prefrontal structures such as the anterior cingulate cortex (ACC) play a decisive role in processes of action monitoring and response control, functions often impaired in schizophrenia. Patients with cycloid psychoses exhibit some characteristic neurophysiological features not indicative of the cerebral hypofrontality observed in schizophrenia. This study aimed at examining if cycloid psychoses—unlike schizophrenias—involve a normal brain-electrical topography during a task demanding prefrontal response control. Methods Thirty-seven patients with cycloid psychoses and 37 healthy controls were investigated electrophysiologically while performing a Continuous Performance Test (CPT). Topographical analyses were conducted to individually quantify the Nogo-anteriorisation (NGA) as a neurophysiological index of prefrontal response control. Results The patients exhibited an unaltered topography with a mean NGA not significantly different from the controls. They did, however, differ from the control group regarding their Global Field Power (GFP), with a significantly reduced GFP ( p<0.001) and decreased latencies ( p<0.01) during Nogo trials. On a behavioral level, patients exhibited prolonged reaction times and an increased rate of omission errors. Conclusions The investigated patients showed an activation of specific (presumably frontal) brain areas during Nogo trials, resulting in a frontalisation of the brain-electrical field comparable to the control group. However, the strength of this activation was apparently reduced. The patients' unaltered topographical pattern contrasts with previous findings in schizophrenic patients and supports the hypothesis that cycloid psychoses entail less severe prefrontal deficits than schizophrenias, which might be an indication of different biological backgrounds for both groups of endogenous psychoses.

Global Approach to Multichannel Electroencephalogram Analysis for Diagnosis and Clinical Evaluation in Mild Alzheimer’s Disease

To establish an early diagnosis of Alzheimer’s disease (AD), we evaluated brain spatial dynamics and cognitive function in mild AD. Seventeen patients with the diagnosis of mild AD and 17 age-matched controls were examined for Ω (global complexity), Σ (total power) and Φ (generalized frequency) by 19-channel electroencephalography (EEG). As a result, the mild AD group showed significantly higher Ω values than the control group. The Φ values were highly correlated with the Mini-Mental State Examination scores and the full IQ and performance IQ scores of the Japanese Wechsler Adult Intelligence Scale Revised. These results indicate an increase in spatial complexity of the brain electric field in mild AD, as well as a close relationship between slowing of the global frequency of field changes and the cognitive decline in mild AD.

Altered response control and anterior cingulate function in attention-deficit/hyperactivity disorder boys.

To investigate mechanisms and structures underlying prefrontal response control and inhibition in boys suffering from attention-deficit/hyperactivity disorder (ADHD). Sixteen boys with ADHD and 19 healthy controls were investigated electrophysiologically during performance of a visual Go-Nogo task (Continuous Performance Test, CPT). An electrophysiological source localization method was employed to further analyze the data. The ADHD boys showed a significantly diminished central Nogo-P3, due to a lack of Nogo-related frontalization of the positive brain electrical field in this group. This two-dimensional effect was associated with a significantly reduced activation of the anterior cingulate cortex (ACC) in the ADHD boys in the Nogo condition of the CPT. Both groups did not significantly differ regarding the amplitude of the Nogo-N2. The results indicate deficits in prefrontal response control in unmedicated ADHD boys that do not seem to be specifically inhibitory in nature. A supposed dysfunction of the ACC in ADHD was confirmed.

Electrical neuroimaging based on biophysical constraints

This paper proposes and implements biophysical constraints to select a unique solution to the bioelectromagnetic inverse problem. It first shows that the brain's electric fields and potentials are predominantly due to ohmic currents. This serves to reformulate the inverse problem in terms of a restricted source model permitting noninvasive estimations of Local Field Potentials (LFPs) in depth from scalp-recorded data. Uniqueness in the solution is achieved by a physically derived regularization strategy that imposes a spatial structure on the solution based upon the physical laws that describe electromagnetic fields in biological media. The regularization strategy and the source model emulate the properties of brain activity's actual generators. This added information is independent of both the recorded data and head model and suffices for obtaining a unique solution compatible with and aimed at analyzing experimental data. The inverse solution's features are evaluated with event-related potentials (ERPs) from a healthy subject performing a visuo-motor task. Two aspects are addressed: the concordance between available neurophysiological evidence and inverse solution results, and the functional localization provided by fMRI data from the same subject under identical experimental conditions. The localization results are spatially and temporally concordant with experimental evidence, and the areas detected as functionally activated in both imaging modalities are similar, providing indices of localization accuracy. We conclude that biophysically driven inverse solutions offer a novel and reliable possibility for studying brain function with the temporal resolution required to advance our understanding of the brain's functional networks.

Reduced response-inhibition in obsessive–compulsive disorder measured with topographic evoked potential mapping

Recent neuroimaging studies have suggested that a hyperactivity of the frontal-striate neuronal circuits, including the orbitofrontal cortex and the basal ganglia, mediates the symptomatology of obsessive–compulsive disorder (OCD). However, there is also some evidence that the superior frontal cortex is less activated in OCD, and this local hypoactivity has been shown to be negatively associated with the symptomatology. As the superior frontal cortex is believed to be involved in inhibitory control, this study investigated the brain electrical activity during response inhibition in OCD. Twelve patients with OCD and 12 healthy controls performed a cued Go–NoGo task (continuous performance test), while event-related potentials were registered with 21 electrodes. Patients reacted significantly faster than controls, but did not differ from controls regarding the error rate. As a main result, we found a reduced frontal activity during the NoGo condition in OCD, which was condensed in a reduced anteriorisation of the brain electrical field. We suggest that this inhibitory deficit in OCD has a major contribution to the pathophysiology of OCD, which is underscored by the fact that the anteriorisation during the NoGo condition (NGA) was negatively correlated with the symptomatology as measured by the Yale-Brown Obsessive-Compulsive Scale.

Chronic schizophrenics with positive symptomatology have shortened EEG microstate durations.

In young, first-episode, never-treated schizophrenics compared with controls, (a) generally shorter durations of EEG microstates were reported (Koukkou et al., Brain Topogr 6 (1994) 251; Kinoshita et al., Psychiatry Res Neuroimaging 83 (1998) 58), and (b) specifically, shorter duration of a particular class of microstates (Koenig et al., Eur Arch Psychiatry Clin Neurosci 249 (1999) 205). We now examined whether older, chronic schizophrenic patients with positive symptomatology also show these characteristics. Multichannel resting EEG (62.2 s/subject) from two subject groups, 14 patients (36.1+/-10.2 years old) and 13 controls (35.1+/-8.2 years old), all males, was analyzed into microstates using a global approach for microstate analysis that clustered the microstates into 4 classes (Koenig et al., 1999). (a) Hypothesis testing of general microstate shortening supported a trend (P=0.064). (b) Two-way repeated measure ANOVA (two subject groupsx4 microstate classes) showed a significant group effect for microstate duration. Posthoc tests revealed that a microstate class with brain electric field orientation from left central to right central-posterior had significantly shorter microstates in patients than controls (68.5 vs. 76.1 ms, P=0.034). The results were in line with the results from young, never-treated, productive patients, thus suggesting that in schizophrenic information processing, one class of mental operations might intermittently cause deviant mental constructs because of premature termination of processing.

Dynamics of brain electric field during recall of Salpuri dance performance.

The brain wave activity of a professional Salpuri dancer was observed while the subject recalled her performance of the Salpuri dance when sitting in a chair with closed eyes. As she recalled the feeling of the ecstatic trance state induced by the dance, an increase in alpha brain activity was observed together with marked frontal midline theta activity. Compared to a resting state, the dynamics of the electrical activity in the brain showed an increase in the global field power integral and a decrease in generalized frequency and spatial complexity.

Topographic ERP changes due to displacements of visuo-spatial attention

In order to explore the cerebral processes of visuo-spatial attention shifts, the spatio-temporal characteristics of ERPs to targets following a spatial cue were examined topographically. Subjects were asked to respond selectively to a target presented 200 ms after offset of a spatial cue. Valid targets (75%) appeared at cue position; invalid targets (25%) were presented at a different position than the cue. In valid trials, an anterior negative deflection of Nd and a single-peaked P3 were observed with shorter reaction time. In invalid trials, a posterior N2–P3 complex was observed with delayed reaction time. Spatio-temporal patterns of brain electric fields (BEF) to valid and invalid trials differed between 240 and 280 ms after target presentation. Regardless of target locations, the BEF in valid trials was characterized by anterior negative/posterior positive distribution. On invalid trials, where subjects were forced to shift their visuo-spatial attention from the cue to the target, the BEF was characterized by an anterior positive/posterior negative distribution for all cue-target pairs except those presented in the upper visual field. Therefore, the BEF within 240–280 ms after the target presentation might be concerned with cerebral processes of visuo-spatial attention shift, i.e., with recapturing/evaluating target.