Jürgen Dammers

Neurophysiological correlates of the recognition of facial expressions of emotion as revealed by magnetoencephalography

MEG correlates of the recognition of facial expressions of emotion were studied in four healthy volunteers. Subjects performed a facial emotion recognition task and a control task involving recognition of complex objects including faces. Facial emotion recognition activated inferior frontal cortex, amygdala and different parts of temporal cortex in a relatively consistent time sequence. The characteristics of these activations were clearly different from those recorded during the control task. Most interesting was the fact that faces evoked different MEG responses as a function of task demands, i.e., the activations recorded during facial emotion recognition were different from those recorded during simple face recognition in the control task. These findings support the assumption that MEG is able to specifically identify the activation pattern of the brain when recognition of the emotional expression of a face is performed.

A novel approach to the human connectome: Ultra-high resolution mapping of fiber tracts in the brain

Signal transmission between different brain regions requires connecting fiber tracts, the structural basis of the human connectome. In contrast to animal brains, where a multitude of tract tracing methods can be used, magnetic resonance (MR)-based diffusion imaging is presently the only promising approach to study fiber tracts between specific human brain regions. However, this procedure has various inherent restrictions caused by its relatively low spatial resolution. Here, we introduce 3D-polarized light imaging (3D-PLI) to map the three-dimensional course of fiber tracts in the human brain with a resolution at a submillimeter scale based on a voxel size of 100 μm isotropic or less. 3D-PLI demonstrates nerve fibers by utilizing their intrinsic birefringence of myelin sheaths surrounding axons. This optical method enables the demonstration of 3D fiber orientations in serial microtome sections of entire human brains. Examples for the feasibility of this novel approach are given here. 3D-PLI enables the study of brain regions of intense fiber crossing in unprecedented detail, and provides an independent evaluation of fiber tracts derived from diffusion imaging data.

Time course of regional brain activations during facial emotion recognition in humans

Recognition of facial expressions of emotions is very important for communication and social cognition. Neuroimaging studies showed that numerous brain regions participate in this complex function. To study spatiotemporal aspects of the neural representation of facial emotion recognition we recorded neuromagnetic activity in 12 healthy individuals by means of a whole head magnetoencephalography system. Source reconstructions revealed that several cortical and subcortical brain regions produced strong neural activity in response to emotional faces at latencies between 100 and 360 ms that were much stronger than those to neutral as well as to blurred faces. Orbitofrontal cortex and amygdala showed affect-related activity at short latencies already within 180 ms after stimulus onset. Some of the emotion-responsive regions were repeatedly activated during the stimulus presentation period pointing to the assumption that these reactivations represent indicators of a distributed interacting circuitry.

Real Time Processing of Affective and Cognitive Stimuli in the Human Brain Extracted from MEG Signals

The magnetoencephalography (MEG) signal was recorded while subjects watched a video containing separate blocks of affective and cognitive advertisements and recalled slides extracted from the video a day later. An earlier behavioural study using the same video material showed that the affective advertisements were better recalled and that administration of propranolol (a beta-adrenergic blocker) abolished this effect. Magnetic field tomography (MFT) was used to extract tomographic estimates of activity millisecond by millisecond from the continuous MEG signal. Statistically significant differences between affective and cognitive blocks were identified in posterior and prefrontal areas. Cognitive blocks produced stronger activity in posterior parietal areas and superior prefrontal cortex in all three subjects. Affective blocks modulated activity in orbitofrontal and retrosplenial cortex, amygdala and brainstem. Individual contributions to the statistical maps were traced in real time from milliseconds to many seconds. Time-locked responses from the recall session were used to compare average and single trial MFT solutions and to combine activations from all subjects into a common anatomical space. The last step produced statistically significant increases in occipital and inferior ventral cortex between 100 and 200 ms compared to a prestimulus baseline.

High-Resolution Fiber Tract Reconstruction in the Human Brain by Means of Three-Dimensional Polarized Light Imaging

Functional interactions between different brain regions require connecting fiber tracts, the structural basis of the human connectome. To assemble a comprehensive structural understanding of neural network elements from the microscopic to the macroscopic dimensions, a multimodal and multiscale approach has to be envisaged. However, the integration of results from complementary neuroimaging techniques poses a particular challenge. In this paper, we describe a steadily evolving neuroimaging technique referred to as three-dimensional polarized light imaging (3D-PLI). It is based on the birefringence of the myelin sheaths surrounding axons, and enables the high-resolution analysis of myelinated axons constituting the fiber tracts. 3D-PLI provides the mapping of spatial fiber architecture in the postmortem human brain at a sub-millimeter resolution, i.e., at the mesoscale. The fundamental data structure gained by 3D-PLI is a comprehensive 3D vector field description of fibers and fiber tract orientations – the basis for subsequent tractography. To demonstrate how 3D-PLI can contribute to unravel and assemble the human connectome, a multiscale approach with the same technology was pursued. Two complementary state-of-the-art polarimeters providing different sampling grids (pixel sizes of 100 and 1.6 μm) were used. To exemplarily highlight the potential of this approach, fiber orientation maps and 3D fiber models were reconstructed in selected regions of the brain (e.g., Corpus callosum, Internal capsule, Pons). The results demonstrate that 3D-PLI is an ideal tool to serve as an interface between the microscopic and macroscopic levels of organization of the human connectome.

Real-time neural activity and connectivity in healthy individuals and schizophrenia patients.

Processing of facial information is distributed across several brain regions, as has been shown recently in many neuroimaging studies. Disturbances in accurate face processing have been repeatedly demonstrated in different stages of schizophrenia. Recently, electroencephalography (EEG) and tomographic analysis of average magnetoencephalographic (MEG) data were used to define the latencies of significant regional brain activations in healthy and schizophrenic subjects elicited during the recognition of facial expression of emotions. The current study re-examines these results using tomographic analysis of single trial MEG data. In addition to the areas identified by the analysis of the average MEG data, statistically significant activity is identified in several other areas, including a sustained increase in the right amygdala activity in response to emotional faces in schizophrenic subjects. The single trial analysis demonstrated that the reduced activations identified from the average MEG signal of schizophrenic subjects is due to high variability across single trials rather than reduced activity in each single trial. In control subjects, direct measures of linkage demonstrate distinct stages of processing of emotional faces within well-defined network of brain regions. Activity in each node of the network, confined to 30 to 40 ms latency windows, is linked to earlier and later activations of the other nodes of the network. In schizophrenic subjects, no such well-defined stages of processing were observed. Instead, the activations, although strong were poorly linked to each other, managing only isolated links between pairs of areas.

Pattern reversal visual evoked responses of V1/V2 and V5/MT as revealed by MEG combined with probabilistic cytoarchitectonic maps.

Pattern reversal stimulation provides an established tool for assessing the integrity of the visual pathway and for studying early visual processing. Numerous magnetoencephalographic (MEG) and electroencephalographic (EEG) studies have revealed a three-phasic waveform of the averaged pattern reversal visual evoked potential/magnetic field, with components N75(m), P100(m), and N145(m). However, the anatomical assignment of these components to distinct cortical generators is still a matter of debate, which has inter alia connected with considerable interindividual variations of the human striate and extrastriate cortex. The anatomical variability can be compensated for by means of probabilistic cytoarchitectonic maps, which are three-dimensional maps obtained by an observer-independent statistical mapping in a sample of ten postmortem brains. Transformed onto a subjects brain under consideration, these maps provide the probability with which a given voxel of the subjects brain belongs to a particular cytoarchitectonic area. We optimize the spatial selectivity of the probability maps for V1 and V2 with a probability threshold which optimizes the self- vs. cross-overlap in the population of postmortem brains used for deriving the probabilistic cytoarchitectonic maps. For the first time, we use probabilistic cytoarchitectonic maps of visual cortical areas in order to anatomically identify active cortical generators underlying pattern reversal visual evoked magnetic fields as revealed by MEG. The generators are determined with magnetic field tomography (MFT), which reconstructs the current source density in each voxel. In all seven subjects, our approach reveals generators in V1/V2 (with a greater overlap with V1) and in V5 unilaterally (right V5 in three subjects, left V5 in four subjects) and consistent time courses of their stimulus-locked activations, with three peak activations in V1/V2 (contributing to C1m/N75m, P100m, and N145m) and two peak activations in V5 (contributing to P100m and N145m). The reverberating V1/V2 and V5 activations demonstrate the effect of recurrent activation mechanisms including V1 and extrastriate areas and/or corticofugal feedback loops. Our results demonstrate that the combined investigation of MEG signals with MFT and probabilistic cytoarchitectonic maps significantly improves the anatomical identification of active brain areas.

Integration of Amplitude and Phase Statistics for Complete Artifact Removal in Independent Components of Neuromagnetic Recordings

In magnetoencephalography (MEG) and electroencephalography (EEG), independent component analysis is widely applied to separate brain signals from artifact components. A number of different methods have been proposed for the automatic or semiautomatic identification of artifact components. Most of the proposed methods are based on amplitude statistics of the decomposed MEG/EEG signal. We present a fully automated approach based on amplitude and phase statistics of decomposed MEG signals for the isolation of biological artifacts such as ocular, muscle, and cardiac artifacts (CAs). The performance of different artifact identification measures was investigated. In particular, we show that phase statistics is a robust and highly sensitive measure to identify strong and weak components that can be attributed to cardiac activity, whereas a combination of different measures is needed for the identification of artifacts caused by ocular and muscle activity. With the introduction of a rejection performance parameter, we are able to quantify the rejection quality for eye blinks and CAs. We demonstrate in a set of MEG data the good performance of the fully automated procedure for the removal of cardiac, ocular, and muscle artifacts. The new approach allows routine application to clinical measurements with small effect on the brain signal.

Early sensory encoding of affective prosody: Neuromagnetic tomography of emotional category changes

In verbal communication, prosodic codes may be phylogenetically older than lexical ones. Little is known, however, about early, automatic encoding of emotional prosody. This study investigated the neuromagnetic analogue of mismatch negativity (MMN) as an index of early stimulus processing of emotional prosody using whole-head magnetoencephalography (MEG). We applied two different paradigms to study MMN; in addition to the traditional oddball paradigm, the so-called optimum design was adapted to emotion detection. In a sequence of randomly changing disyllabic pseudo-words produced by one male speaker in neutral intonation, a traditional oddball design with emotional deviants (10% happy and angry each) and an optimum design with emotional (17% happy and sad each) and nonemotional gender deviants (17% female) elicited the mismatch responses. The emotional category changes demonstrated early responses (<200 ms) at both auditory cortices with larger amplitudes at the right hemisphere. Responses to the nonemotional change from male to female voices emerged later ( approximately 300 ms). Source analysis pointed at bilateral auditory cortex sources without robust contribution from other such as frontal sources. Conceivably, both auditory cortices encode categorical representations of emotional prosodic. Processing of cognitive feature extraction and automatic emotion appraisal may overlap at this level enabling rapid attentional shifts to important social cues.

The temporal dynamics of insula activity to disgust and happy facial expressions: A magnetoencephalography study

The insula has consistently been shown to be involved in processing stimuli that evoke the emotional response of disgust. Recently, its specificity for processing disgust has been challenged and a broader role of the insula in the representation of interoceptive information has been suggested. Studying the temporal dynamics of insula activation during emotional processing can contribute valuable information pertaining to this issue. Few studies have addressed the insulas putative specificity to disgust and the dynamics of its underlying neural processes. In the present study, neuromagnetic responses of 13 subjects performing an emotional continuous performance task (CPT) to faces with disgust, happy, and neutral expressions were obtained. Magnetic field tomography extracted the time course of bilateral insula activities. Right insula activation was stronger to disgust and happy than neutral facial expressions at about 200 ms after stimulus onset. Later only at about 350 ms after stimulus onset the right insula was activated stronger to disgust than happy facial expressions. Thus, the early right insula response reflects activation to emotionally arousing stimuli regardless of valence, and the later right insula response differentiates disgust from happy facial expressions. Behavioral performance but not the insula activity differed between 100 ms and 1000 ms presentation conditions. Present findings support the notion that the insula is involved in the representation of interoceptive information.