Wienbruch, Christian
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Dipole parameter estimation of M50 auditory evoked fields applied to the study of training-induced neuroplasticity in schizophrenia
In a first step three different methods for dipole localization of M50 auditory sources evoked by the double click paradigm were compared in order to differentiate between 12 schizophrenic patients (1 female) and 10 controls (1 female). Dependent variables were sensory gating ratios (dipolemoment of themagnetic counterpart of click two divided by click one) and hemispheric asymmetry (left vs. right hemispheric dipole location). MEG was measured using a 148 channels magnetometer system. Dipoleswere fitted using (1) a spherical head model with 34 MEG channels over the left and right temporal lobe, respectively, (2) a spherical head model with 68MEGchannels over both temporal lobes, (3) a boundary element model (BEM) based on an averaged magnetic resonance imaging (MRI) dataset (using 68 MEG channels). Differentiation between patients and controls for both gating ratios and anterior-posterior asymmetry was most precise when dipole moments were calculated with the BEM model. Thus, in a second step, the BEM method was used to investigate changes across 4 weeks, during which patients completed a cognitive training program. In patients, gating ratios were decreased after training but no changes in the asymmetry were noticed. Stability of measures was confirmed in controls.
The spatio-temporal pattern of reward processing : magnetoencephalographic responses to value appraisal and reward prediction in a gambling task
Reward processing is frequently examined in decision-making designs, as they involve essential components like evaluation of value and reward prediction. The present study examined magnetoencephalographic (MEG) correlates of such components in 20 volunteers: subjects had to decide, whether or not to gamble for 10 or 50 c (Eurocent), which they could win with 10%, 50% or 90% chance. MEG responses to the visually presented value (10 or 50 c) and chance (10, 50 or 90%) stimuli, analyzed using Minimum Norm Estimates (MNE), distinguished value evaluation and reward prediction in time windows between 150 and 350 ms after stimulus onset in different brain areas: righthemispheric temporo-parietal dipole activity 150 230 ms distinguished value evaluation (po .01), whereas the chance prediction varied with right-hemispheric temporo-parietal dipole activity at 215 255 ms (p o .05), bilateral fronto-temporal dipole activity at 235 275 ms (p o .01) and frontal dipole activity at 250 350 ms (p o .05). Frontal activity was larger and decision time was longer on risky trials (decision to gamble at 50% chance). Activation of the same region by both cues (value and chance) suggests that reward processing comprises the interaction of preferred value and expectancy of outcome, while the course of activity suggests a consecutively activated neuronal network of reward processing, including posterior temporal to prefrontal regions.
Altered oscillatory brain dynamics after repeated traumatic stress
Background:
Repeated traumatic experiences, e.g. torture and war, lead to functional and structural cerebral changes, which should be detectable in cortical dynamics. Abnormal slow waves produced within circumscribed brain regions during a resting state have been associated with lesioned neural circuitry in neurological disorders and more recently also in mental illness.
Methods
Using magnetoencephalographic (MEG-based) source imaging, we mapped abnormal distributions of generators of slow waves in 97 survivors of torture and war with posttraumatic stress disorder (PTSD) in comparison to 97 controls.
Results:
PTSD patients showed elevated production of focally generated slow waves (1 4 Hz), particularly in left temporal brain regions, with peak activities in the region of the insula. Furthermore, differential slow wave activity in right frontal areas was found in PTSD patients compared to controls.
Conclusion:
The insula, as a site of multimodal convergence, could play a key role in understanding the pathophysiology of PTSD, possibly accounting for what has been called posttraumatic alexithymia, i.e., reduced ability to identify, express and regulate emotional responses to reminders of traumatic events. Differences in activity in right frontal areas may indicate a dysfunctional PFC, which may lead to diminished extinction of conditioned fear and reduced inhibition of the amygdala.
Abnormal oscillatory brain dynamics in schizophrenia : a sign of deviant communication in neural network?
Background: Slow waves in the delta (0.5 - 4 Hz) frequency range are indications of normal activity in sleep. In neurological disorders, focal electric and magnetic slow wave activity is generated in the vicinity of structural brain lesions. Initial studies, including our own, suggest that the distribution of the focal concentration of generators of slow waves (dipole density in the delta frequency band) also distinguishes patients with psychiatric disorders such as schizophrenia, affective disorders, and posttraumatic stress disorder.
Methods: The present study examined the distribution of focal slow wave activity (ASWA: abnormal slow wave activity) in 116 healthy subjects, 76 inpatients with schizophrenic or schizoaffective diagnoses and 42 inpatients with affective (ICD-10: F3) or neurotic/reactive (F4) diagnoses using a newly refined measure of dipole density. Based on 5-min resting magnetoencephalogram (MEG), sources of activity in the 1-4 Hz frequency band were determined by equivalent dipole fitting in anatomically defined cortical regions.
Results: Compared to healthy subjects the schizophrenia sample was characterized by significantly more intense slow wave activity, with maxima in frontal and central areas. In contrast, affective disorder patients exhibited less slow wave generators mainly in frontal and central regions when compared to healthy subjects and schizophrenia patients. In both samples, frontal ASWA were related to affective symptoms.
Conclusion: In schizophrenic patients, the regions of ASWA correspond to those identified for gray matter loss. This suggests that ASWA might be evaluted as a measure of altered neuronal network architecture and communication, which may mediate psychophathological signs.
Error related fields: localizing the magnetic equivalent of the ERN
Introduction:
Research in the field of error processing and error related brain activity has a long history in the field of neuropsychology. It has been found in numerous EEG studies, that a typical brain potential arises following an erroneous response. This error related negativity (ERN) and the accompanying positivity (Pe) are thought to be related to error monitoring and feedback. This is crucial for the detection of errors and correction of actions in the framework of organization of complex behaviors and high-level goals. Yet so far, only a single published study investigated the magnetoencephalographic equivalent to the ERN (mERN) by means of single dipole modeling. This study as well as previous EEG works suggests the major source of the ERN to be located in the anterior cingulate cortex (ACC).
Here, we implemented a computerized version of the paper-and-pencil d2-test, to measure the error related field in the MEG and to localize the generator of the ERN using a distributed source model.
Methods:
13 (5 male/ 8 female) student volunteers participated in this study. We measured the MEG during a 15-minute stimulation period with a 148-channel magnetometer system (MAGNES 2500 WH, 4D Neuroimaging, San Diego, USA). The stimulation consisted of the presentation of the d2-test stimuli and the participants had to indicate via button press whether the stimulus was a d accompanied by two marks or not. Correct and incorrect button presses were recorded. 4 Second snippets around the triggers were extracted and analyzed. Raw data was 1 Hz high pass filtered and trials containing artifacts were excluded from analysis. Trial number in the two conditions was equalized, in order to assure equal signal-to-noise ratio.
After averaging the single trials, a nonparametric permutation test was used to identify clusters of activation in time and sensor space. Subsequently, time-windows of significant effects found in the ERF analysis were modeled in source space using a linearly constrained minimum variance (lcmv) beamformer. Statistical differences on the source level were confirmed using a dependent samples t-test. All analysis steps were performed using Fieldtrip (http://www.ru.nl/neuroimaging/fieldtrip/).
Results:
Cluster analysis revealed a parietal cluster of activation between 80-160 ms after motor response, where the incorrect responses elicited significantly larger field amplitude than the correct responses (p
Functional re-recruitment of dysfunctional brain areas predicts language recovery in chronic aphasia
Functional recovery in response to a brain lesion, such as a stroke, can even occur years after the incident and may be accelerated by effective rehabilitation strategies. In eleven chronic aphasia patients, we administered a short-term intensive language training to improve language functions and to induce cortical reorganization under rigorously controlled conditions. Overt naming performance was assessed during functional magnetic resonance imaging (fMRI) prior to and immediately after the language training. Regions of interest (ROIs) for statistical analyses were constituted by areas with individually determined abnormally high densities of slow wave generators (identified by magnetoencephalography prior to the language intervention) that clustered mainly in left perilesional areas. Three additional individually defined regions served to control for the specificity of the results for the selected respective target region: the homologue area of the individual patient's lesion, the mirror image of the delta ROI in the right hemisphere and left hemispheric regions that did not produce a significant amount of slow wave activity. Treatment-induced changes of fMRI brain activation were highly correlated with improved naming of the trained pictures, but selectively within the pre-training dysfunctional perilesional brain areas. Our results suggest that remodeling of cortical functions is possible even years after a stroke. The behavioral gain seems to be mediated by brain regions that had been partially deprived from input after the initial stroke. We therefore provide first time direct evidence for the importance of treatment-induced functional reintegration of perilesional areas in a heterogeneous sample of chronic aphasia patients.
If-then planning modulates the P300 in children with attention deficit hyperactivity disorder
Children with attention deficit disorder have difficulties with tasks that require response inhibition. We measured EEG data of nonmedicated childen with ADHS and control children in two conditions, a neutral condition and a condition that involved making if-then plans. If-then plans improved response inhibition and increased the P300 inchildren with ADHS compared with the neutral condition. The present results encourage the application of self-regulation using if-then plans in addition or as an alternative to common medical therapy.
Electromagnetic brain activity in higher frequency bands during automatic word processing indicates recovery of function in aphasia
AIM: Little is known about how treatment affects the neural substrate of language function in stroke sufferers. In the present study authors investigated neuronal correlates of treatment induced recovery of language functions in patients with chronic aphasia. METHODS: In 10 chronic aphasia patients and 10 age- and gender-matched control participants, evoked high-frequency activity (HFA, >20 Hz) was determined from the magnetoencephalogram in an automatic word recognition task, in which content, function, and pseudowords were visually presented at fast rate (350-ms). Recording was repeated after 2 weeks, in aphasics after intensive language training to evaluate training effects, in controls to establish HFA stability. RESULTS: In the first recording, bilateral HFA distribution in controls contrasted right-hemispheric predominance in the patients. After training, this right>left asymmetry in aphasics was reduced to a bilateral pattern similar to controls. While word class did not substantially affect HFA patterns in the two groups, enhanced right-hemispheric HFA in the patients varied with better language function (test performance) prior to training, while after training, left-temporal function- and pseudoword evoked HFA varied with performance in tests of written language. CONCLUSIONS: Results suggest that HFA might serve as a measure in the evaluation of rehabilitation efforts in chronic aphasia: enhanced right-hemispheric HFA might indicate compensatory activation of contralateral language areas, which tends towards patterns comparable to normal subjects after effective language training.