Models of source currents in the brain

Brain signatures predict communicative function of speech production in interaction

People normally know what they want to communicate before they start speaking. However, brain indicators of communication are typically observed only after speech act onset, and it is unclear when any anticipatory brain activity prior to speaking might first emerge, along with the communicative intentions it possibly reflects. Here, we investigated brain activity prior to the production of different speech act types, request and naming actions performed by uttering single words embedded into language games with a partner, similar to natural communication. Starting ca. 600 msec before speech onset, an event-related potential maximal at fronto-central electrodes, which resembled the Readiness Potential, was larger when preparing requests compared to naming actions. Analysis of the cortical sources of this anticipatory brain potential suggests a relatively stronger involvement of fronto-central motor regions for requests, which may reflect the speaker's expectation of the partner actions typically following requests, e.g., the handing over of a requested object. Our results indicate that different neuronal circuits underlying the processing of different speech act types activate already before speaking. Results are discussed in light of previous work addressing the neural basis of speech act understanding and predictive brain indexes of language comprehension.

Accuracy and spatial properties of distributed magnetic source imaging techniques in the investigation of focal epilepsy patients

Source localization of interictal epileptiform discharges (IEDs) is clinically useful in the presurgical workup of epilepsy patients. We aimed to compare the performance of four different distributed magnetic source imaging (dMSI) approaches: Minimum norm estimate (MNE), dynamic statistical parametric mapping (dSPM), standardized low-resolution electromagnetic tomography (sLORETA), and coherent maximum entropy on the mean (cMEM). We also evaluated whether a simple average of maps obtained from multiple inverse solutions (Ave) can improve localization accuracy. We analyzed dMSI of 206 IEDs derived from magnetoencephalography recordings in 28 focal epilepsy patients who had a well-defined focus determined through intracranial EEG (iEEG), epileptogenic MRI lesions or surgical resection. dMSI accuracy and spatial properties were quantitatively estimated as: (a) distance from the epilepsy focus, (b) reproducibility, (c) spatial dispersion (SD), (d) map extension, and (e) effect of thresholding on map properties. Clinical performance was excellent for all methods (median distance from the focus MNE = 2.4 mm; sLORETA = 3.5 mm; cMEM = 3.5 mm; dSPM = 6.8 mm, Ave = 0 mm). Ave showed the lowest distance between the map maximum and epilepsy focus (Dmin lower than cMEM, MNE, and dSPM, p = .021, p = .008, p < .001, respectively). cMEM showed the best spatial features, with lowest SD outside the focus (SD lower than all other methods, p < .001 consistently) and high contrast between the generator and surrounding regions. The average map Ave provided the best localization accuracy, whereas cMEM exhibited the lowest amount of spurious distant activity. dMSI techniques have the potential to significantly improve identification of iEEG targets and to guide surgical planning, especially when multiple methods are combined.

Empirical Comparison of Distributed Source Localization Methods for Single-Trial Detection of Movement Preparation

The development of technologies for the treatment of movement disorders, like stroke, is still of particular interest in brain-computer interface (BCI) research. In this context, source localization methods (SLMs), that reconstruct the cerebral origin of brain activity measured outside the head, e.g., via electroencephalography (EEG), can add a valuable insight into the current state and progress of the treatment. However, in BCIs SLMs were often solely considered as advanced signal processing methods that are compared against other methods based on the classification performance alone. Though, this approach does not guarantee physiological meaningful results. We present an empirical comparison of three established distributed SLMs with the aim to use one for single-trial movement prediction. The SLMs wMNE, sLORETA, and dSPM were applied on data acquired from eight subjects performing voluntary arm movements. Besides the classification performance as quality measure, a distance metric was used to asses the physiological plausibility of the methods. For the distance metric, which is usually measured to the source position of maximum activity, we further propose a variant based on clusters that is better suited for the single-trial case in which several sources are likely and the actual maximum is unknown. The two metrics showed different results. The classification performance revealed no significant differences across subjects, indicating that all three methods are equally well-suited for single-trial movement prediction. On the other hand, we obtained significant differences in the distance measure, favoring wMNE even after correcting the distance with the number of reconstructed clusters. Further, distance results were inconsistent with the traditional method using the maximum, indicating that for wMNE the point of maximum source activity often did not coincide with the nearest activation cluster. In summary, the presented comparison might help users to select an appropriate SLM and to understand the implications of the selection. The proposed methodology pays attention to the particular properties of distributed SLMs and can serve as a framework for further comparisons.

Near-instant automatic access to visually presented words in the human neocortex: neuromagnetic evidence

Rapid and efficient processing of external information by the brain is vital to survival in a highly dynamic environment. The key channel humans use to exchange information is language, but the neural underpinnings of its processing are still not fully understood. We investigated the spatio-temporal dynamics of neural access to word representations in the brain by scrutinising the brain's activity elicited in response to psycholinguistically, visually and phonologically matched groups of familiar words and meaningless pseudowords. Stimuli were briefly presented on the visual-field periphery to experimental participants whose attention was occupied with a non-linguistic visual feature-detection task. The neural activation elicited by these unattended orthographic stimuli was recorded using multi-channel whole-head magnetoencephalography, and the timecourse of lexically-specific neuromagnetic responses was assessed in sensor space as well as at the level of cortical sources, estimated using individual MR-based distributed source reconstruction. Our results demonstrate a neocortical signature of automatic near-instant access to word representations in the brain: activity in the perisylvian language network characterised by specific activation enhancement for familiar words, starting as early as ~70 ms after the onset of unattended word stimuli and underpinned by temporal and inferior-frontal cortices.

Probabilistic delay differential equation modeling of event-related potentials

"Dynamic causal models" (DCMs) are a promising approach in the analysis of functional neuroimaging data due to their biophysical interpretability and their consolidation of functional-segregative and functional-integrative propositions. In this theoretical note we are concerned with the DCM framework for electroencephalographically recorded event-related potentials (ERP-DCM). Intuitively, ERP-DCM combines deterministic dynamical neural mass models with dipole-based EEG forward models to describe the event-related scalp potential time-series over the entire electrode space. Since its inception, ERP-DCM has been successfully employed to capture the neural underpinnings of a wide range of neurocognitive phenomena. However, in spite of its empirical popularity, the technical literature on ERP-DCM remains somewhat patchy. A number of previous communications have detailed certain aspects of the approach, but no unified and coherent documentation exists. With this technical note, we aim to close this gap and to increase the technical accessibility of ERP-DCM. Specifically, this note makes the following novel contributions: firstly, we provide a unified and coherent review of the mathematical machinery of the latent and forward models constituting ERP-DCM by formulating the approach as a probabilistic latent delay differential equation model. Secondly, we emphasize the probabilistic nature of the model and its variational Bayesian inversion scheme by explicitly deriving the variational free energy function in terms of both the likelihood expectation and variance parameters. Thirdly, we detail and validate the estimation of the model with a special focus on the explicit form of the variational free energy function and introduce a conventional nonlinear optimization scheme for its maximization. Finally, we identify and discuss a number of computational issues which may be addressed in the future development of the approach.

Automatic ultrarapid activation and inhibition of cortical motor systems in spoken word comprehension

To address the hotly debated question of motor system involvement in language comprehension, we recorded neuromagnetic responses elicited in the human brain by unattended action-related spoken verbs and nouns and scrutinized their timecourse and neuroanatomical substrates. We found that already very early on, from ∼80 ms after disambiguation point when the words could be identified from the available acoustic information, both verbs and nouns produced characteristic somatotopic activations in the motor strip, with words related to different body parts activating the corresponding body representations. Strikingly, along with this category-specific activation, we observed suppression of motor-cortex activation by competitor words with incompatible semantics, documenting operation of the neurophysiological principles of lateral/surround inhibition in neural word processing. The extremely early onset of these activations and deactivations, their emergence in the absence of attention, and their similar presence for words of different lexical classes strongly suggest automatic involvement of motor-specific circuits in the perception of action-related language.

Neural dynamics of speech act comprehension: an MEG study of naming and requesting

The neurobiological basis and temporal dynamics of communicative language processing pose important yet unresolved questions. It has previously been suggested that comprehension of the communicative function of an utterance, i.e. the so-called speech act, is supported by an ensemble of neural networks, comprising lexico-semantic, action and mirror neuron as well as theory of mind circuits, all activated in concert. It has also been demonstrated that recognition of the speech act type occurs extremely rapidly. These findings however, were obtained in experiments with insufficient spatio-temporal resolution, thus possibly concealing important facets of the neural dynamics of the speech act comprehension process. Here, we used magnetoencephalography to investigate the comprehension of Naming and Request actions performed with utterances controlled for physical features, psycholinguistic properties and the probability of occurrence in variable contexts. The results show that different communicative actions are underpinned by a dynamic neural network, which differentiates between speech act types very early after the speech act onset. Within 50–90 ms, Requests engaged mirror-neuron action-comprehension systems in sensorimotor cortex, possibly for processing action knowledge and intentions. Still, within the first 200 ms of stimulus onset (100–150 ms), Naming activated brain areas involved in referential semantic retrieval. Subsequently (200–300 ms), theory of mind and mentalising circuits were activated in medial prefrontal and temporo-parietal areas, possibly indexing processing of intentions and assumptions of both communication partners. This cascade of stages of processing information about actions and intentions, referential semantics, and theory of mind may underlie dynamic and interactive speech act comprehension.

A passive exoskeleton can push your life up: application on multiple sclerosis patients

In the present study, we report the benefits of a passive and fully articulated exoskeleton on multiple sclerosis patients by means of behavioral and electrophysiological measures, paying particular attention to the prefrontal cortex activity. Multiple sclerosis is a neurological condition characterized by lesions of the myelin sheaths that encapsulate the neurons of the brain, spine and optic nerve, and it causes transient or progressive symptoms and impairments in gait and posture. Up to 50% of multiple sclerosis patients require walking aids and 10% are wheelchair-bound 15 years following the initial diagnosis. We tested the ability of a new orthosis, the “Human Body Posturizer”, designed to improve the structural and functional symmetry of the body through proprioception, in multiple sclerosis patients. We observed that a single Human Body Posturizer application improved mobility, ambulation and response accuracy, in all of the tested patients. Most importantly, we associated these clinical observations and behavioral effects to changes in brain activity, particularly in the prefrontal cortex.

Rapid extraction of lexical tone phonology in Chinese characters: a visual mismatch negativity study

Background

In alphabetic languages, emerging evidence from behavioral and neuroimaging studies shows the rapid and automatic activation of phonological information in visual word recognition. In the mapping from orthography to phonology, unlike most alphabetic languages in which there is a natural correspondence between the visual and phonological forms, in logographic Chinese, the mapping between visual and phonological forms is rather arbitrary and depends on learning and experience. The issue of whether the phonological information is rapidly and automatically extracted in Chinese characters by the brain has not yet been thoroughly addressed.

Methodology/Principal Findings

We continuously presented Chinese characters differing in orthography and meaning to adult native Mandarin Chinese speakers to construct a constant varying visual stream. In the stream, most stimuli were homophones of Chinese characters: The phonological features embedded in these visual characters were the same, including consonants, vowels and the lexical tone. Occasionally, the rule of phonology was randomly violated by characters whose phonological features differed in the lexical tone.

Conclusions/Significance

We showed that the violation of the lexical tone phonology evoked an early, robust visual response, as revealed by whole-head electrical recordings of the visual mismatch negativity (vMMN), indicating the rapid extraction of phonological information embedded in Chinese characters. Source analysis revealed that the vMMN was involved in neural activations of the visual cortex, suggesting that the visual sensory memory is sensitive to phonological information embedded in visual words at an early processing stage.

Attention to language: novel MEG paradigm for registering involuntary language processing in the brain

Previous research indicates that, under explicit instructions to listen to spoken stimuli or in speech-oriented behavioural tasks, the brain's responses to senseless pseudowords are larger than those to meaningful words; the reverse is true in non-attended conditions. These differential responses could be used as a tool to trace linguistic processes in the brain and their interaction with attention. However, as previous studies relied on explicit instructions to attend or ignore the stimuli, a technique for automatic attention modulation (i.e., not dependent on explicit instruction) would be more advantageous, especially when cooperation with instructions may not be guaranteed (e.g., neurological patients, children etc). Here we present a novel paradigm in which the stimulus context automatically draws attention to speech. In a non-attend passive auditory oddball sequence, rare words and pseudowords were presented among frequent non-speech tones of variable frequency and length. The low percentage of spoken stimuli guarantees an involuntary attention switch to them. The speech stimuli, in turn, could be disambiguated as words or pseudowords only in their end, at the last phoneme, after the attention switch would have already occurred. Our results confirmed that this paradigm can indeed be used to induce automatic shifts of attention to spoken input. At ~250ms after the stimulus onset, a P3a-like neuromagnetic deflection was registered to spoken (but not tone) stimuli indicating an involuntary attention shift. Later, after the word-pseudoword divergence point, we found a larger oddball response to pseudowords than words, best explained by neural processes of lexical search facilitated through increased attention. Furthermore, we demonstrate a breakdown of this orderly pattern of neurocognitive processes as a result of sleep deprivation. The new paradigm may thus be an efficient way to assess language comprehension processes and their dynamic interaction with those of attention allocation. It does it in an automatic and task-free fashion, indicating its potential benefit for assessing uncooperative clinical populations.

Complexity analysis of spontaneous brain activity: effects of depression and antidepressant treatment

Magnetoencephalography (MEG) allows the real-time recording of neural activity and oscillatory activity in distributed neural networks. We applied a non-linear complexity analysis to resting-state neural activity as measured using whole-head MEG. Recordings were obtained from 20 unmedicated patients with major depressive disorder and 19 matched healthy controls. Subsequently, after 6 months of pharmacological treatment with the antidepressant mirtazapine 30 mg/day, patients received a second MEG scan. A measure of the complexity of neural signals, the Lempel-Ziv Complexity (LZC), was derived from the MEG time series. We found that depressed patients showed higher pre-treatment complexity values compared with controls, and that complexity values decreased after 6 months of effective pharmacological treatment, although this effect was statistically significant only in younger patients. The main treatment effect was to recover the tendency observed in controls of a positive correlation between age and complexity values. Importantly, the reduction of complexity with treatment correlated with the degree of clinical symptom remission. We suggest that LZC, a formal measure of neural activity complexity, is sensitive to the dynamic physiological changes observed in depression and may potentially offer an objective marker of depression and its remission after treatment.

Ultra-rapid access to words in the brain

Rapid information processing in the human brain is vital to survival in a highly dynamic environment. The key tool humans use to exchange information is spoken language, but the exact speed of the neuronal mechanisms underpinning speech comprehension is still unknown. Here we investigate the time course of neuro-lexical processing by analyzing neuromagnetic brain activity elicited in response to psycholinguistically and acoustically matched groups of words and pseudowords. We show an ultra-early dissociation in cortical activation elicited by these stimulus types, emerging ∼50 ms after acoustic information required for word identification first becomes available. This dissociation is the earliest brain signature of lexical processing of words so far reported, and may help explain the evolutionary advantage of human spoken language.

Fast mapping of novel word forms traced neurophysiologically

Human capacity to quickly learn new words, critical for our ability to communicate using language, is well-known from behavioral studies and observations, but its neural underpinnings remain unclear. In this study, we have used event-related potentials to record brain activity to novel spoken word forms as they are being learnt by the human nervous system through passive auditory exposure. We found that the brain response dynamics change dramatically within the short (20 min) exposure session: as the subjects become familiarized with the novel word forms, the early (∼100 ms) fronto-central activity they elicit increases in magnitude and becomes similar to that of known real words. At the same time, acoustically similar real words used as control stimuli show a relatively stable response throughout the recording session; these differences between the stimulus groups are confirmed using both factorial and linear regression analyses. Furthermore, acoustically matched novel non-speech stimuli do not demonstrate similar response increase, suggesting neural specificity of this rapid learning phenomenon to linguistic stimuli. Left-lateralized perisylvian cortical networks appear to be underlying such fast mapping of novel word forms unto the brain’s mental lexicon.

Thalamocortical sensorimotor circuit in multiple sclerosis: an integrated structural and electrophysiological assessment

Demyelination and axonal damage are pathologic hallmarks of multiple sclerosis (MS), leading to loss of neuronal synchronization, functional disconnection amongst brain relays, and clinical sequelae. To investigate these properties, the primary component of the sensorimotor network was analyzed in mildly disabled Relapsing-Remitting MS patients without sensory symptoms at the time of the investigation. By magnetoencephalography (MEG), the recruitment pattern within the primary sensory (S1) and motor (M1) areas was estimated through the morphology of the early components of somatosensory evoked magnetic fields (SEFs), after evaluating the S1 responsiveness to sensory inputs from the contralateral arm. In each hemisphere, network recruitment properties were correlated with ispilateral thalamus volume, estimated by morphometric techniques upon high-resolution 3D structural magnetic resonance images (MRI). S1 activation was preserved, whereas SEF morphology was strikingly distorted in MS patients, marking a disruption of primary somatosensory network patterning. An unbalance of S1-M1 dynamic recruitment was documented and correlated with the thalamic volume reduction in the left hemisphere. These findings support the model of MS as a disconnection syndrome, with major susceptibility to damage experienced by nodes belonging to more frequently recruited and highly specialized networks.

Rapid cortical plasticity underlying novel word learning

Humans are unique in developing large lexicons as their communication tool. To achieve this, they are able to learn new words rapidly. However, neural bases of this rapid learning, which may be an expression of a more general cognitive mechanism, are not yet understood. To address this, we exposed our subjects to familiar words and novel spoken stimuli in a short passive perceptual learning session and compared automatic brain responses to these items throughout the learning exposure. Initially, we found enhanced activity for known words, indexing the ignition of their underlying memory traces. However, just after 14 min of learning exposure, the novel items exhibited a significant increase in response magnitude matching in size with that to real words. This activation increase, as we would like to propose, reflects rapid mapping of new word forms onto neural representations. Similar to familiar words, the neural activity subserving rapid learning of new word forms was generated in the left-perisylvian language cortex, especially anterior superior-temporal areas. This first report of a neural correlate of rapid learning suggests that our brain may effectively form new neuronal circuits online as it gets exposed to novel patterns in the sensory input. Understanding such fast learning is key to the neurobiological explanation of the human language faculty and learning mechanisms in general.

Interactions between language and attention systems: early automatic lexical processing?

An ongoing debate is whether and to what extent access to cortical representations is automatic or dependent on attentional processes. To address this, we modulated the level of attention on auditory input and recorded ERPs elicited by syllables completing acoustically matched words and pseudowords. Under nonattend conditions, the word-elicited response (peaking at approximately 120 msec) was larger than that to pseudowords, confirming early activation of lexical memory traces. However, when attention was directed toward the auditory input, such word-pseudoword difference disappeared. Whereas responses to words seemed unchanged by attentional variation, early pseudoword responses were modulated significantly by attention. Later on, attention modulated a positive deflection at approximately 230 msec and a second negativity at approximately 370 msec for all stimuli. The data indicate that the earliest stages of word processing are not affected by attentional demands and may thus possess certain automaticity, with attention effects on lexical processing accumulating after 150-200 msec. We explain this by robustness of preexisting memory networks for words whose strong internal connections guarantee rapid full-scale activation irrespective of the attentional resources available. Conversely, the processing of pseudowords, which do not have such stimulus-specific cortical representations, appears to be strongly modulated by the availability of attentional resources, even at its earliest stages. Topography analysis and source reconstruction indicated that left peri-sylvian cortices mediate attention effects on memory trace activation.

Effects of attention on what is known and what is not: MEG evidence for functionally discrete memory circuits

Recent results obtained with a neural-network model of the language cortex suggest that the memory circuits developing for words are both distributed and functionally discrete. This model makes testable predictions about brain responses to words and pseudowords under variable availability of attentional resources. In particular, due to their strong internal connections, the action-perception circuits for words that the network spontaneously developed exhibit functionally discrete activation dynamics, which are only marginally affected by attentional variations. At the same time, network responses to unfamiliar items - pseudowords - that have not been previously learned (and, therefore, lack corresponding memory representations) exhibit (and predict) strong attention dependence, explained by the different amounts of attentional resources available and, therefore, different degrees of competition between multiple memory circuits partially activated by items lacking lexical traces. We tested these predictions in a novel magnetoencephalography experiment and presented subjects with familiar words and matched unfamiliar pseudowords during attention demanding tasks and under distraction. The magnetic mismatch negativity (MMN) response to words showed relative immunity to attention variations, whereas the MMN to pseudowords exhibited profound variability: when subjects attended the stimuli, the brain response to pseudowords was larger than that to words (as typically observed in the N400); when attention was withdrawn, the opposite pattern emerged, with the response to pseudowords reduced below the response to words. Main cortical sources of these activations were localized to superior-temporal cortex. These results confirm the model's predictions and provide evidence in support of the hypothesis that words are represented in the brain as action-perception circuits that are both discrete and distributed.

Complexity analysis of spontaneous brain activity in attention-deficit/hyperactivity disorder: diagnostic implications

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is defined as the most common neurobehavioral disorder of childhood, but an objective diagnostic test is not available yet to date. Neurophychological, neuroimaging, and neurophysiological research offer ample evidence of brain and behavioral dysfunctions in ADHD, but these findings have not been useful as a diagnostic test.

METHODS

Whole-head magnetoencephalographic recordings were obtained from 14 diagnosed ADHD patients and 14 healthy children during resting conditions. Lempel-Ziv complexity (LZC) values were obtained for each channel and child and averaged in five sensor groups: anterior, central, left lateral, right lateral, and posterior.

RESULTS

Lempel-Ziv complexity scores were significantly higher in control subjects, with the maximum value in anterior region. Combining age and anterior complexity values allowed the correct classification of ADHD patients and control subjects with a 93% sensitivity and 79% specificity. Control subjects showed an age-related monotonic increase of LZC scores in all sensor groups, while children with ADHD exhibited a nonsignificant tendency toward decreased LZC scores. The age-related divergence resulted in a 100% specificity in children older than 9 years.

CONCLUSIONS

Results support the role of a frontal hypoactivity in the diagnosis of ADHD. Moreover, the age-related divergence of complexity scores between ADHD patients and control subjects might reflect distinctive developmental trajectories. This interpretation of our results is in agreement with recent investigations reporting a delay of cortical maturation in the prefrontal cortex.

Spatiotemporal signatures of large-scale synfire chains for speech processing as revealed by MEG

We report a new brain signature of memory trace activation in the human brain revealed by magnetoencephalography and distributed source localization. Spatiotemporal patterns of cortical activation can be picked up in the time course of source images underlying magnetic brain responses to speech and noise stimuli, especially the generators of the magnetic mismatch negativity. We found that acoustic signals perceived as speech elicited a well-defined spatiotemporal pattern of sequential activation of superior-temporal and inferior-frontal cortex, whereas the same identical stimuli, when perceived as noise, did not elicit temporally structured activation. Strength of local sources constituting large-scale spatiotemporal patterns reflected additional lexical and syntactic features of speech. Morphological processing of the critical sound as verb inflection led to particularly pronounced early left inferior-frontal activation, whereas the same sound functioning as inflectional affix of a noun activated superior-temporal cortex more strongly. We conclude that precisely timed spatiotemporal patterns involving specific cortical areas may represent a brain code of memory circuit activation. These spatiotemporal patterns are best explained in terms of synfire mechanisms linking neuronal populations in different cortical areas. The large-scale synfire chains appear to reflect the processing of stimuli together with the context-dependent perceptual and cognitive information bound to them.

Spatiotemporal patterns of brain activation during an action naming task using magnetoencephalography

Eight right-handed subjects were asked to silently generate a verb to a visual stimulus while the magnetic flux normal to the scalp surface was recorded with a whole-head neuromagnetometer. The spatiotemporal patterns of activation in lateral occipital, inferior parietal, superior temporal, basal temporal, and inferior frontal cortices were estimated using minimum estimation, a distributed source analysis methodology. Although there was significant variability among subjects, averaged data indicated that latencies of peak activation in these regions of interest progressed from posterior to anterior. Peak latencies were earliest in lateral occipital cortex and latest in pars opercularis and pars triangularis in the inferior frontal gyrus. Lateralization of activation was strongest in pars opercularis, which is part of classical Broca's area, with activation being stronger in this area within the left hemisphere in every subject. Results provide support for the use of magnetoencephalography in conjunction with MNE analysis for the purpose of lateralizing and localizing language-specific activation in frontal areas as well as the study of the spatiotemporal parameters of brain activation associated with cognitive function.

Early MEG activation dynamics in the left temporal and inferior frontal cortex reflect semantic context integration

Traditional views link semantic context integration to neurophysiological activity at 300-500 msec. To study possible early dynamics related to semantic context integration, we recorded, in passive oddball paradigm, magnetic evoked responses to spoken word pairs, the second word being either congruent or incongruent with the first one. The same experimental words were placed in orthogonally varied context, thus providing a strict control for any effects of acoustic, phonological, and psycholinguistic stimulus features. Responses to the same critical words were obtained also outside of semantic context. We found that regardless of their acoustic features, semantically incongruent stimuli elicited a brain response already at approximately 115 msec after the critical word onset. The same words did not produce such deflection in semantically legal context. The responses were maximal at left temporal and inferior frontal cortical sites, which was also confirmed by distributed current source analysis. The left temporal activation preceded the frontal one by approximately 16 msec. No late response dynamics (>350 msec) were found that would reflect the semantic modulation in this nonattend passive design, indicating the possible role of attention in generating the later responses. Our results suggest that the earliest brain processes of semantic context integration can occur at approximately 100 msec after the onset of spoken words in the left inferior frontal and superior temporal cortex.

Magnetoencephalography (MEG) determined temporal modulation of visual and auditory sensory processing in the context of classical conditioning to faces

Using magnetoencephalography (MEG), we determined the time course of sensory-evoked modulations during differential aversive conditioning to faces, with an aversive noise event (UCS). Conditioning was associated with the development of a differential event-related waveform peaking at approximately 150 ms. Source analysis indicated the localization of this modulation to ventral occipital regions. In the auditory domain, a modulation of auditory-evoked responses to a probe sound was evident in a late component emerging at approximately 180 ms over sensors in fronto-temporal regions. The findings indicate the time course in processing sensory stimuli can be altered on the basis of their acquired value.

Tracking speech comprehension in space and time

A fundamental challenge for the cognitive neuroscience of language is to capture the spatio-temporal patterns of brain activity that underlie critical functional components of the language comprehension process. We combine here psycholinguistic analysis, whole-head magnetoencephalography (MEG), the Mismatch Negativity (MMN) paradigm, and state-of-the-art source localization techniques (Equivalent Current Dipole and L1 Minimum-Norm Current Estimates) to locate the process of spoken word recognition at a specific moment in space and time. The magnetic MMN to words presented as rare "deviant stimuli" in an oddball paradigm among repetitive "standard" speech stimuli, peaked 100-150 ms after the information in the acoustic input, was sufficient for word recognition. The latency with which words were recognized corresponded to that of an MMN source in the left superior temporal cortex. There was a significant correlation (r = 0.7) of latency measures of word recognition in individual study participants with the latency of the activity peak of the superior temporal source. These results demonstrate a correspondence between the behaviorally determined recognition point for spoken words and the cortical activation in left posterior superior temporal areas. Both the MMN calculated in the classic manner, obtained by subtracting standard from deviant stimulus response recorded in the same experiment, and the identity MMN (iMMN), defined as the difference between the neuromagnetic responses to the same stimulus presented as standard and deviant stimulus, showed the same significant correlation with word recognition processes.

A method for measurement of joint kinematics in vivo by registration of 3-D geometric models with cine phase contrast magnetic resonance imaging data

A new method is presented for measuring joint kinematics by optimally matching modeled trajectories of geometric surface models of bones with cine phase contrast (cine-PC) magnetic resonance imaging data. The incorporation of the geometric bone models (GBMs) allows computation of kinematics based on coordinate systems placed relative to full 3-D anatomy, as well as quantification of changes in articular contact locations and relative velocities during dynamic motion. These capabilities are additional to those of cine-PC based techniques that have been used previously to measure joint kinematics during activity. Cine-PC magnitude and velocity data are collected on a fixed image plane prescribed through a repetitively moved skeletal joint. The intersection of each GBM with a simulated image plane is calculated as the model moves along a computed trajectory, and cine-PC velocity data are sampled from the regions of the velocity images within the area of this intersection. From the sampled velocity data, the instantaneous linear and angular velocities of a coordinate system fixed to the GBM are estimated, and integration of the linear and angular velocities is used to predict updated trajectories. A moving validation phantom that produces motions and velocity data similar to those observed in an experiment on human knee kinematics was designed. This phantom was used to assess cine-PC rigid body tracking performance by comparing the kinematics of the phantom measured by this method to similar measurements made using a magnetic tracking system. Average differences between the two methods were measured as 2.82 mm rms for anterior/posterior tibial position, and 2.63 deg rms for axial rotation. An intertrial repeatability study of human knee kinematics using the new method produced rms differences in anterior/posterior tibial position and axial rotation of 1.44 mm and 2.35 deg. The performance of the method is concluded to be sufficient for the effective study of kinematic changes caused to knees by soft tissue injuries.

Therapy-related reorganization of language in both hemispheres of patients with chronic aphasia

The brain processes of language recovery after stroke are poorly understood, partly because past research did not allow to differentiate the effects of spontaneous restitution processes from those of learning-related cortical reorganization. Here, we use a new approach offered by recently developed intense neuropsychological therapy methods, which allow for improving language functions within a short time period. Stroke patients with chronic aphasia received intense language therapy for 2 weeks and, over this period, improved their language performance as assessed using clinical tests. Neurophysiological activity elicited by words and pseudowords was measured before and after treatment. Over the therapy interval, early word evoked potentials (latency 250-300 ms) became significantly stronger whereas pseudoword responses did not change. Word-specific changes were documented by analyses of ERP amplitudes and root mean square values, which revealed interactions of the factors Assessment time (before vs. after therapy) and Wordness (word vs. pseudoword). Source localization using Minimum Norm Current Estimates showed that bilateral cortical sources activated by word stimuli contributed to the change, suggesting that neuronal networks distributed over both hemispheres are the substrate of cortical reorganization of language processing in intense aphasia therapy. Word-evoked differences in source strengths were significantly correlated with performance on a clinical language test, demonstrating a link between behavioral and neurophysiological changes. We suggest that the early word-evoked negativity might represent an index of reorganization of language after stroke and thus an aphasia recovery potential.

Brain signatures of meaning access in action word recognition

The brain basis of action words may be neuron ensembles binding language- and action-related information that are dispersed over both language- and action-related cortical areas. This predicts fast spreading of neuronal activity from language areas to specific sensorimotor areas when action words semantically related to different parts of the body are being perceived. To test this, fast neurophysiological imaging was applied to reveal spatiotemporal activity patterns elicited by words with different action-related meaning. Spoken words referring to actions involving the face or leg were presented while subjects engaged in a distraction task and their brain activity was recorded using high-density magnetoencephalography. Shortly after the words could be recognized as unique lexical items, objective source localization using minimum norm current estimates revealed activation in superior temporal (130 msec) and inferior frontocentral areas (142-146 msec). Face-word stimuli activated inferior frontocentral areas more strongly than leg words, whereas the reverse was found at superior central sites (170 msec), thus reflecting the cortical somatotopy of motor actions signified by the words. Significant correlations were found between local source strengths in the frontocentral cortex calculated for all participants and their semantic ratings of the stimulus words, thus further establishing a close relationship between word meaning access and neurophysiology. These results show that meaning access in action word recognition is an early automatic process ref lected by spatiotemporal signatures of word-evoked activity. Word-related distributed neuronal assemblies with specific cortical topographies can explain the observed spatiotemporal dynamics reflecting word meaning access.

Determinants of dominance: Is language laterality explained by physical or linguistic features of speech?

The nature of cerebral asymmetry of the language function is still not fully understood. Two main views are that laterality is best explained (1) by left cortical specialization for the processing of spectrally rich and rapidly changing sounds, and (2) by a predisposition of one hemisphere to develop a module for phonemes. We tested both of these views by investigating magnetic brain responses to the same brief acoustic stimulus, placed in contexts where it was perceived either as a noise burst with no resemblance of speech, or as a native language sound being part of a meaningless pseudoword. In further experiments, the same acoustic element was placed in the context of words. We found reliable left hemispheric dominance only when the sound was placed in word context. These results, obtained in a passive odd-ball paradigm, suggest that neither physical properties nor phoneme status of a sound are sufficient for laterality. In order to elicit left lateralized cortical activation in normal right-handed individuals, a rapidly changing spectrally rich sound with phoneme status needs to be placed in the context of frequently encountered larger language elements, such as words. This demonstrates that language laterality is bound to the processing of sounds as units of frequently occurring meaningful items and can thus be linked to the processes of learning and memory trace formation for such items rather than to their physical or phonological properties.

Evaluation of different cortical source localization methods using simulated and experimental EEG data

Different cortical source localization methods have been developed to directly link the scalp potentials with the cortical activities. Up to now, these methods are the only possible solution to noninvasively investigate cortical activities with both high spatial and time resolutions. However, the application of these methods is hindered by the fact that they have not been rigorously evaluated nor compared. In this paper, the performances of several source localization methods (moving dipoles, minimum Lp norm, and low resolution tomography (LRT) with Lp norm, p equal to 1, 1.5, and 2) were evaluated by using simulated scalp EEG data, scalp somatosensory evoked potentials (SEPs), and upper limb motor-related potentials (MRPs) obtained on human subjects (all with 163 scalp electrodes). By using simulated EEG data, we first evaluated the source localization ability of the above methods quantitatively. Subsequently, the performance of the various methods was evaluated qualitatively by using experimental SEPs and MRPs. Our results show that the overall LRT Lp norm method with p equal to 1 has a better source localization ability than any of the other investigated methods and provides physiologically meaningful reconstruction results. Our evaluation results provide useful information for choosing cortical source localization approaches for future EEG/MEG studies.

Activation in the anterior left auditory cortex associated with phonological analysis of speech input: localization of the phonological mismatch negativity response with MEG

The spatio-temporal dynamics of cortical activation underlying auditory word recognition, particularly its phonological stage, was studied with whole-head magnetoencephalography (MEG). Subjects performed a visuo-auditory priming task known to evoke the phonological mismatch negativity (PMN) response that is elicited by violations of phonological expectancies. Words and non-words were presented in separate conditions. In each of the 318 trials, the subjects first saw a word/non-word (e.g., 'cat') that was soon followed by a prime letter (e.g., 'h'). Their task was to replace mentally the sound of the first letter of the word/non-word with the prime letter, thus resulting in a new word/non-word (e.g., 'hat'). Finally, an auditory word/non-word either matching or mismatching with the anticipated item was presented. In most subjects, a PMNm followed by a later, N400m-like negativity was obtained in the left hemisphere to the mismatching auditory stimuli. A similar response pattern was obtained in the right hemisphere only in a few subjects. Source localization of the N1m, an index of acoustic analysis, and the PMNm and N400m-like responses was performed using L1 minimum-norm estimation. In the left hemisphere, the PMNm source for the words was significantly more anterior than the source of the N400m-like response; for the non-words, the PMNm source was significantly more anterior than the sources of the N1m and the N400m-like response. These results suggest that the left-hemisphere neuronal networks involved in sub-lexical phonological analysis are at least partly different from those responsible for the earlier (acoustic) and later (whole item) processing of speech input.

Keep it simple: a case for using classical minimum norm estimation in the analysis of EEG and MEG data

The present study aims at finding the optimal inverse solution for the bioelectromagnetic inverse problem in the absence of reliable a priori information about the generating sources. Three approaches to tackle this problem are compared theoretically: the maximum-likelihood approach, the minimum norm approach, and the resolution optimization approach. It is shown that in all three of these frameworks, it is possible to make use of the same kind of a priori information if available, and the same solutions are obtained if the same a priori information is implemented. In particular, they all yield the minimum norm pseudoinverse (MNP) in the complete absence of such information. This indicates that the properties of the MNP, and in particular, its limitations like the inability to localize sources in depth, are not specific to this method but are fundamental limitations of the recording modalities. The minimum norm solution provides the amount of information that is actually present in the data themselves, and is therefore optimally suited to investigate the general resolution and accuracy limits of EEG and MEG measurement configurations. Furthermore, this strongly suggests that the classical minimum norm solution is a valuable method whenever no reliable a priori information about source generators is available, that is, when complex cognitive tasks are employed or when very noisy data (e.g., single-trial data) are analyzed. For that purpose, an efficient and practical implementation of this method will be suggested and illustrated with simulations using a realistic head geometry.

Integrating sensory and motor mapping in a comprehensive MEG protocol: Clinical validity and replicability

Considerable evidence supports the idea of magnetoencephalography (MEG) being a valuable noninvasive tool for presurgical mapping of sensory and motor functions. In this study, we test the validity and replicability of a new experimental paradigm for simultaneous sensory and motor mapping using MEG recordings. This comprehensive sensorimotor protocol (CSSMP), where external mechanic stimulation serves as a cue for voluntary movements, allows the recording of sensory and motor cortical responses during a single activation task. The stability and replicability of MEG-derived recordings during this paradigm were tested in a group of eight neurologically normal volunteers and six patients with perirolandic lesions. We found that a common sensorimotor cortical network, engaging sensory (S1, S2) and motor (M1) areas, was reliably activated in all subjects and patients and that the results remained exceptionally stable over time. Additionally, the clinical validity of the MEG-derived maps of activation was tested through intraoperative electrocortical stimulation mapping in the group of patients. The MEG-derived anatomical maps for specific sensory (S1) and motor (M1) responses were verified, by direct cortical mapping, and confirmed with the patient's surgical outcome. The results of this validation study show that the so-called CSSMP is a reliable and reproducible method for assessing simultaneously sensory and motor areas. This method minimizes methodological problems and improves our knowledge of the spatiotemporal organization of the sensorimotor cortical network and helps to optimize the surgical management of patients with perirolandic lesions.

Distributed neuronal networks for encoding category-specific semantic information: the mismatch negativity to action words

Mismatch negativity (MMN), an index of experience-dependent memory traces, was used to investigate the processing of action-related words in the human brain. Responses to auditorily presented movement-related English words were recorded in a non-attend odd-ball protocol using a high-density electroencephalographic (EEG) set-up. MMN was calculated using responses to the same words presented as standard and deviant stimuli in different sessions to avoid contamination from phonetic-acoustic differences. The topography of the mismatch negativity to action words revealed an unusual centro-posterior distribution of the responses, suggesting that activity was at least in part generated posterior to usually observed frontal MMNs. Moreover, responses to hand-related word stimulus (pick) had a more widespread lateral distribution, whereas leg-related stimulus (kick) elicited a more focal dorsal negativity. These differences, remarkably reminiscent of sensorimotor cortex topography, were further assessed using distributed source analysis of the EEG signal (L2 minimum-norm current estimates). The source analysis also confirmed differentially distributed activation for the two stimuli. We suggest that these results indicate activation of distributed neuronal assemblies that function as category-specific memory traces for words and may involve sensorimotor cortical structures for encoding action words.

Word-specific cortical activity as revealed by the mismatch negativity

Neurophysiological brain activity evoked by individual spoken words and pseudowords was recorded and the mismatch negativity (MMN), an automatic index of experience-dependent auditory memory traces, was calculated. Consistent with earlier reported results, the MMN response to word-final syllables was enhanced compared with that elicited by the same syllables placed in a pseudoword context. Here we now demonstrate that the enhancement of the MMN elicited by two individual words showed different scalp topographies. The early word-specific brain activity is consistent with the assumption that the memory traces activated by individual words are carried by large neuronal ensembles that differ in their distributions over the cortex. Current source estimates localized the between-word differences in the right hemisphere and in parieto-occipital left-hemispheric areas. The differential brain responses to individual words appeared as early as approximately 100 ms after the recognition points of the words, suggesting that their specific memory traces become active almost immediately after the information in the acoustic input is sufficient for word identification.

Grammar Processing Outside the Focus of Attention: an MEG Study

To address the cerebral processing of grammar, we used whole-head high-density magnetoencephalography to record the brain's magnetic fields elicited by grammatically correct and incorrect auditory stimuli in the absence of directed attention to the stimulation. The stimuli were minimal short phrases of the Finnish language differing only in one single phoneme (word-final inflectional affix), which rendered them as either grammatical or ungrammatical. Acoustic and lexical differences were controlled for by using an orthogonal design in which the phoneme's effect on grammaticality was inverted. We found that occasional syntactically incorrect stimuli elicited larger mismatch negativity (MMN) responses than correct phrases. The MMN was earlier proposed as an index of preattentive automatic speech processing. Therefore, its modulation by grammaticality under nonattend conditions suggests that early syntax processing in the human brain may take place outside the focus of attention. Source analysis (single—dipole models and minimum-norm current estimates) indicated grammaticality dependent differential activation of the left superior temporal cortex suggesting that this brain structure may play an important role in such automatic grammar processing.

Spatiotemporal dynamics of neural language processing: an MEG study using minimum-norm current estimates

The inferior frontal and superior temporal areas in the left hemisphere are well-known to be crucial for language processing in most right-handed individuals. This has been established by classical neurological investigations and neuropsychological studies along with metabolic brain imaging have recently revealed converging evidence. Here, we use fast neurophysiological brain imaging, magnetoencephalography (MEG), and L1 Minimum-Norm Current Estimates to investigate the time course of cortical activation underlying the magnetic Mismatch Negativity elicited by a spoken word. Left superior-temporal areas became active 136 ms after the information in the acoustic input was sufficient for identifying the word, and activation of the left inferior-frontal cortex followed after an additional delay of 22 ms. By providing answers to the where- and when-questions of cortical activation, MEG recordings paired with current estimates of the underlying cortical sources may advance our understanding of the spatiotemporal dynamics of distributed neuronal networks involved in cognitive processing in the human brain.

Automatic processing of grammar in the human brain as revealed by the mismatch negativity

The Mismatch Negativity (MMN), a neurophysiological indicator of cognitive processing, was used to investigate grammatical processes in the absence of focused attention to language. Subjects instructed to watch a silent video film and to ignore speech stimuli heard grammatical and ungrammatical spoken word strings that were physically identical up to a divergence point where they differed between each other by a minimal acoustic event, the presence or the absence of a final -s sound. The sentence we come was presented as a rare deviant stimulus against the background of frequently occurring ungrammatical strings, and, in a different experiment, the ungrammatical string *we comes was the deviant in the reverse design. To control for effects related to differences between the critical words, come and comes, control conditions were used in which the same words were presented out of linguistic context. At 100-150 ms after the divergence point, the ungrammatical deviant stimulus elicited a larger MMN than the correct sentence at left-anterior recording sites. This difference was not seen under the out-of-context conditions. In the time range 100-400 ms after stimulus divergence, a spatiotemporal pattern of grammatically related effects was documented by statistically significant interactions of the word and context variables. Minimum-Norm Current Estimates of the cortical sources of the grammaticality effects revealed a main source in the left frontal cortex. We use a neurobiological model of serial order processing to provide a tentative explanation for the data.

Early influences of word length and frequency: a group study using MEG

In what way are linguistic word properties reflected in the neurophysiological brain response? During a memory task we presented written words orthogonally varied in length (long, short) and frequency (high, low). Brain responses of 15 subjects were recorded using a 148-channel magnetoencephalogram. Very early after stimulus onset (60 ms), long words led to significantly stronger activation than short words, as revealed by the global field power (GFP). Later on, low frequency words led to stronger brain responses than high frequency words. This effect depended on word length: it was seen 120-170 ms after stimulus onset, for short words only, but at 225-250 ms exclusively for long words. Source localisation revealed that effects due to word length were pronounced over occipital areas whereas frequency effected more widespread cortical areas with a strong focus over left occipitotemporal areas (visual word form areas).

Delayed striate cortical activation during spatial attention

Recordings of event-related potentials (ERPs) and event-related magnetic fields (ERMFs) were combined with functional magnetic resonance imaging (fMRI) to study visual cortical activity in humans during spatial attention. While subjects attended selectively to stimulus arrays in one visual field, fMRI revealed stimulus-related activations in the contralateral primary visual cortex and in multiple extrastriate areas. ERP and ERMF recordings showed that attention did not affect the initial evoked response at 60-90 ms poststimulus that was localized to primary cortex, but a similarly localized late response at 140-250 ms was enhanced to attended stimuli. These findings provide evidence that the primary visual cortex participates in the selective processing of attended stimuli by means of delayed feedback from higher visual-cortical areas.

Real time mapping of rat midbrain neural circuitry using auditory evoked potentials

Auditory evoked potentials were recorded in 360 homogeneously spaced sites, in a volume encapsulating the lateral lemniscus-inferior colliculus transition of anaesthetized rats, in order to calculate the electric field vector distribution with each moment in time referenced to the onset of sound presentation. Software, to conduct calculations and graphical representation, and hardware, to minimize neural damage upon recording, were developed in our laboratory. Our results indicate a smooth transition of both amplitude and direction of vectors, suggestive of sequentially activated sites with outward and inward ionic currents coherent with what is known of this part of the primary auditory pathway. That is, anatomical sites (neural generators) and latency for activation matches previous research of the auditory pathway, while adding a real time perspective to the anatomical substrates recruited during the auditory evoked response. An algorithm for calculating the divergent of the vector field, an estimate of the current source density inside the three-dimensional control volume, was used to infer the possible current sinks and sources generating the field potentials. This technique allowed a clear visualization of two distinct discharges arising from the lateral lemniscus towards the inferior colliculus, thus recording signal propagation, as a movie file, with 0.06 ms time resolution.

Cortical current density reconstruction of interictal epileptiform activity in temporal lobe epilepsy

OBJECTIVE

To investigate the value of cortical current density (CCD) reconstruction in localizing intracranial generators of interictal epileptiform activity in mesial and lateral temporal lobe epilepsy (TLE).

METHODS

Non-linear minimum L(1)-norm CCD reconstruction (with current sources restricted to the individual cortical surface and a realistic boundary element method (BEM) head model) was used to localize and to study the propagation of interictal epileptiform EEG activity in 13 pre-surgical patients with TLE.

RESULTS

In all but one patient with mesial temporal lesions, an initial activation maximum corresponding to the ascending part of averaged sharp waves was found in the ipsilateral anterior basolateral temporal lobe, mostly extending up to the affected mesial structures whose resection rendered the patients seizure-free. In all 3 patients with lateral temporal lesions, the activation was initially confined to temporal neocortex immediately adjacent to the epileptogenic lesion. Towards the peak of sharp waves, two patients showed a propagation of interictal activity to anterior and posterior and partly contralateral temporal regions. A conventional EEG analysis based on amplitude maxima or phase reversal would have missed the initial onset zone.

CONCLUSIONS

The findings demonstrate that CCD reconstruction can be a valuable additional non-invasive component in the multimodal pre-surgical evaluation of epilepsy patients.

Lateralization of activity associated with language function using magnetoencephalography: a reliability study

This study was conducted to investigate the reliability of magnetoencephalography in lateralizing and localizing brain activity associated with receptive language function. Sixteen right-handed adults with no history of neurologic disorder engaged in a continuous recognition memory task for visually presented words in two separate sessions. The magnetic flux normal to the scalp surface was measured with a whole-head neuromagnetometer during task performance. Using the total number of acceptable activity sources as an index, overall activation was greater in the left compared with the right hemisphere for all 16 subjects in both sessions. Sources of activity were consistently found in the temporoparietal areas of the left hemisphere in all subjects. Moreover, clusters of activity sources in this region either overlapped spatially or were found in close proximity across sessions. Medial and basal temporal lobe activity was also observed in most subjects during at least one session, and tended to be lateralized to left hemisphere. These results suggest that magnetoencephalography is a promising tool for determination of cerebral dominance for language and localization of temporal lobe language areas.