Wienbruch, Christian
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Distinct cognitive mechanisms in a gambling task share neural mechanisms
Distinct psychological processes have been proposed to unfold in decision-making. The time course of neural mechanisms supporting these processes has not been fully identified. The present MEG study examined spatio-temporal activity related to components of decision-making proposed to support reward valuation, reward prediction, and outcome evaluation. Each trial presented information on reward value (10 or 50 cents) and reward probability (10%, 50%, or 90%). Brain activity related to those inputs and to outcome feedback was evaluated via electromagnetic responses in source space. Distributed dipole activity reflected reward value and reward probability 150–350 ms after information arrival. Neural responses to reward-value information peaked earlier than those to reward-probability information. Results suggest that valuation, prediction, and outcome evaluation share neural structures and mechanisms even on a relatively fine time scale.
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
Abnormal slow wave mapping (ASWAM) : a tool for the investigation of abnormal slow wave activity in the human brain
Slow waves in the delta and theta frequency range, normal signs of deactivated networks in sleep stages, are considered 'abnormal' when prominent in the waking state and when generated in circumscribed brain areas. Structural cortical lesions, e.g. related to stroke, tumors, or scars, generate focal electric and magnetic slow wave activity in the penumbra. Focal concentrations of slow wave activity exceeding those of healthy subjects have also been found in individuals suffering from psychiatric disorders without obvious structural brain damage. Hence, identification and mapping of abnormal slow wave activity might contribute to the investigation of cortical indications of psychopathology. Here I propose a method for abnormal slow wave mapping (ASWAM), based on a 5 min resting magnetoencephalogramm (MEG) and equivalent current dipole fitting to sources in the 1-4 Hz frequency band (delta) in anatomically defined cortical regions. The method was tested in a sample of 116 healthy subjects (59 males), with the aim to provide a basis for later comparison with patient samples. As to be expected, delta dipole density was low in healthy subjects. However, its distribution differed between genders with fronto-central>posterior dipole density in male and posterior dominance in female participants, which was not significantly related to either age or head size. Results suggest that this method allows the identification of ASWA, so that comparison against Z-scores from a larger normal control group might assist diagnostic purposes in patient groups. As specific distributions seem to reflect differences between genders, this should be considered also in the analysis of patient samples.
Mapping the brain's orchestration during speech comprehension : task-specific facilitation of regional synchrony in neural networks
Background: How does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension.
Results: Amplitude-modulated (sinusoidal 41.5 Hz) auditory verbal and nonverbal stimuli served to drive steady-state oscillations in neural networks involved in speech comprehension. Stimuli were presented to 12 subjects in the following conditions (a) an incomprehensible string of words, (b) the same string of words after being introduced as a comprehensible sentence by proper articulation, and (c) nonverbal stimulations that induced a 600-Hz tone, a scale, and a melody. Coherence, defined as correlated activation of magnetic steady state fields across brain areas and measured as simultaneous activation of current dipoles in source space (Minimum-Norm-Estimates), increased within left-temporal-posterior areas when the sound string was perceived as a comprehensible sentence. Intra-hemispheric coherence was larger within the left than the right within the right than the left hemisphere for the sentence (condition (b) relative to all other conditions), and tended to be larger within the right than the left hemisphere for nonverbal stimuli (condition (c), tone and melody relative to the other conditions), leading to a more pronounced hemispheric asymmetry for nonverbal than verbal material.
Conclusions: We conclude that coherent neuronal network activity may index encoding of verbal information on the sentence level and can be used as a tool to investigate auditory speech comprehension.
Local Heschl's Gyrus-based coordinate system for intersubject comparison of M50 auditory response modeled by single equivalent current dipole
Allocating electromagnetic auditory responses to active regions in the human auditory cortex can be difficult because of high interindividual variability of the relevant structures. Location and orientation of the primary auditory cortex (Heschl's Gyrus) and the temporal plane vary with individual features such as age, gender, handedness, or between healthy subjects and patients with a psychiatric disorder (e.g., schizophrenia). Here, we propose a reference coordinate system that considers the individual MRI-based position, orientation and length of the primary auditory cortex to account for interindividual variability. Transformation of the M50 dipole localizations in this new HG-(Heschl's-Gyrus)-coordinate system, accomplished for 10 healthy subjects and 10 schizophrenia patients, confirmed group difference more precisely than other registration methods. We suggest to use the HG-coordinate system for localization of functional measures and evaluation of brain activity differences between groups or measurement conditions.
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.
Phonological but not auditory discrimination is impaired in dyslexia
Deficient phonological skills are considered to be a core problem in developmental dyslexia. Children with dyslexia often demonstrate poorer performance than non-impaired readers when categorizing speech-sounds. Using the automatic mismatch response, we show that in contrast to this deficit at the behavioural level, neurophysiological responding in dyslexic children indicates their ability to automatically discriminate syllables. Therefore, the phonological deficit is unlikely to be caused by a temporal deficit or by a noisy functional organization in the respective representational cortex. We obtained measures of reading, spelling and categorical speechperception from 58 dyslexic children and 21 control children. The children also participated in magnetoencephalographic measurements while being stimulated acoustically with the syllables /ba/ and /da/ in an oddball paradigm. Mismatch field (MMF) amplitudes between standard and deviant stimuli were obtained. Dyslexic children performed more poorly than control children on all test measures. However, the groups did not differ in MMF amplitude or latency. No correlations were found between MMF amplitudes and behavioural performance.These results were obtained with a large sample size and thus speak robustly against a general deficit in auditory discrimination in dyslexia. These results are compatible with the idea that decoding difficulties occur later in the processing stream where access to the phonological lexicon is attempted.
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.
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.
Decoupling neural networks from reality : dissociative Experiences in Torture Victims Are Reflected in Abnormal Brain Waves in Left Frontal Cortex
From a neuroscience perspective, little is known about the long-term effect of torture. Dissociative experiences and posttraumatic stress disorder are often the results of this experience. We examined psychological dissociation within a group of 23 torture victims and report its manifestations within neural networks in the human brain. In particular, we observed that disccoiative experiences are associated with slow abnormal brain waves generated in left ventrolateral frontal cortex. Given that focal slow waves often result from depriving neural networks of major input, the present results may indicate decoupling of frontal affective processors from left cortical language areas. This interpretation is consistent with the fact that disturbed access to structured verbal memory concerining traumatic events is a core feature of the dissociative experience.