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Non-invasive single-trial monitoring of human movement-related brain activation based on DC-magnetoencephalography.
Neuroimaging techniques, such as fMRI, PET and near-infrared spectroscopy, monitor task-related neuronal activations in the brain indirectly through the associated neurovascular/metabolic responses. To assess the primary neuronal activations directly, magnetoencephalography was combined here with a mechanical modulation of the head-to-sensor position and signal separation via independent component analysis. In all of five subjects this approach allowed to monitor the time evolution of DC fields (<0.1 Hz) over the left hemisphere related to complex finger movements of the right hand alternating with rest periods (30 s each). Throughout the recording period of 30 min, stable task-related DC fields were recordable in a single-trial mode, i.e. without any averaging. DC-MEG opens up the possibility of analysing non-invasively cortical DC-activity also in stroke, migraine or epilepsy patients.
Adult↗
Event-related fMRI of the somatosensory system using electrical finger stimulation.
Cortical signal intensity changes due to brief (1 s) innocuous electrical stimuli applied to the second and fifth finger of the right hand were measured by means of fMRI at 1.5 T. The activation pattern in this event-related fMRI approach closely resembled that obtained in recent block-design studies. Activations were found in contralateral primary (SI) and bilaterally in secondary (SII) somato-sensory cortex as well as in posterior parietal cortex, insula, and supplementary motor area (SMA). In SI, the somatotopic organization of the hand area is demonstrated, more clearly to be seen in area 3b than in area 1 and 2. In conclusion, the feasibility to employ event-related somatosensory stimulation paradigms in fMRI studies is demonstrated.
Adult↗
The eloquence of silent cortex: analysis of afferent input to deafferented cortex in arm amputees.
Cortical reorganisation after limb amputation includes topographic displacements of body representation areas and changes of areal extent. Remarkably, truncated nerves, which had innervated amputated limb parts and remained in the residual limbs, can retain access to the deafferented somatosensory cortex. Using somatosensory evoked potentials (SEP) we characterized afferences from electrically stimulated truncated nerves to the brachial plexus and cortex in 12 arm amputees. While peripheral responses were highly variable, thalamocortical input to S-1, as reflected by the primary cortical SEP component, was present in 11 of 12 patients. Despite long-term deafferentation, macroscopic phenomena of inhibition/refractoriness, as assessed by stimulus rate variations, appeared to be changed only marginally. Thus, deafferented cortex remains responsive when given artificial phantom input and could provide a neuronal substrate for spontaneous phantom limb sensations, including phantom pain.
Adult↗
Neurovascular coupling analyzed non-invasively in the human brain.
Most functional brain imaging methods detect neuronal activations indirectly through the accompanying neurovascular response. Here, we demonstrate that a novel methodological approach, the combination of DC-magnetoencephalography (DC-MEG) and near-infrared spectroscopy (NIRS), allows non-invasive assessment of the dynamics of neurovascular coupling in the human brain: detecting directly slow neuronal processes (with time constants of 30s), DC-MEG revealed, even in unaveraged recordings, sustained neuronal activations at pericentral hand cortices contralateral to repetitive finger movements; these were accompanied by the ensuing local vascular response showing similar dynamical features as quantified by simultaneously recorded NIRS. This non-invasive approach opens a new avenue for the understanding of neurovascular coupling during physiological tasks as well as in diseases involving slow neuronal depolarization shifts and alterations of blood flow, such as stroke or migraine.
Adult↗
Does familiarity facilitate the cortical processing of music sounds?
Automatic cortical sound discrimination, as indexed by the mismatch negativity (MMN) component of the auditory evoked potential, is facilitated for familiar speech sounds (phonemes). In musicians as compared to non-musicians, an enhanced MMN has been observed for complex non-speech sounds. Here, musically trained subjects were presented with sequences of either familiar (tonal) or structurally matched unfamiliar (atonal) triad chords, both with either fixed or randomly transposed chord root pitch. The MMN elicited by deviant chords did not differ for familiar and unfamiliar triad sequences, and was undiminished even to unfamiliar deviant sounds which were consciously undetectable. Only subsequent attention-related components indicated facilitated cognitive processing of familiar sounds, corresponding to higher behavioral detection scores.
Acoustic Stimulation↗
Ain't no rhythm fast enough: EEG bands beyond beta.
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Beta Rhythm↗
Linking 600-Hz "spikelike" EEG/MEG wavelets ("sigma-bursts") to cellular substrates: concepts and caveats.
Somatosensory evoked human EEG and magnetoencephalographic (MEG) responses comprise a brief burst of low-amplitude, high-frequency (approximately 600 Hz) spikelike wavelets ("sigma-bursts") superimposed on the primary cortical response (e.g., the N20 to electrical median nerve stimulation). The recent surge of interest in these macroscopic sigma-burst responses is energized by the prospect of monitoring noninvasively, highly synchronized and rapidly repeating population spikes generated in the human thalamic and cortical somatosensory system. Thus, analyses of spike-related sigma-bursts could uniquely complement conventional low-frequency EEG/MEG, reflecting mass excitatory and inhibitory postsynaptic potentials that potentially also incorporate subthreshold activities of undetermined functional relevance. Recent studies using spatiotemporal source analysis of multichannel recordings identified regional burst sources subcortically (near-thalamic) as well as cortically. At the primary somatosensory cortex, sigma-burst generators showed the well-established homuncular somatotopic ordering. Functionally, the 600-Hz burst appears to comprise multiple subcomponents with differential sensitivity to stimulus rate, intensity, sleep-wake cycle, tactile interference, subject age, and certain movement disorders. A plenitude of cellular candidates contributing to burst generation at different levels can already now be envisaged, including cuneothalamic and thalamocortical relay cells, as well as cortical bursting pyramidal cells and fast-spiking inhibitory interneurons. Although cellular burst coding might serve to relay information with high efficiency, concepts to link macroscopic sigma-bursts and cellular substrates call for additional study.
Brain Stem↗
Patterns of disturbed impulse propagation in multiple sclerosis identified by low and high frequency somatosensory evoked potential components.
In human median nerve somatosensory evoked potentials (SSEPs), high frequency (600 Hz) oscillations (HFOs) are superimposed onto the low frequency SSEP component N20. High frequency oscillations are generated both in deep axon segments of thalamo-cortical projection neurons and at the primary somatosensory cortex. The present study aimed to test the hypothesis that HFOs might be more sensitive to temporal dispersion caused by demyelinating lesions in multiple sclerosis (MS) than the N20. The authors recorded HFOs in median nerve SSEPs in 50 patients with definite MS and in 30 healthy controls. Three patterns of SSEP alterations were found: (1) abolished HFOs with either normal (11% of stimulated limbs), or delayed N20 (16% of stimulated limbs); (2) an attenuation of N20 amplitude with preserved HFOs (13%); and (3) a mixture of both patterns (21%). The first pattern--normal N20 with abolished HFOs--indicates that the HFOs are a sensitive marker of slight demyelination. The second pattern is suggestive of a mainly axonal lesion type, while the third pattern points to a combined axonal/demyelinating process or a conduction block. Analysis of HFOs allows identification of slight demyelinating processes in MS patients in whom the N20 SSEP component remains unaffected. The HFOs provide a tool to distinguish different patterns of disturbed impulse propagation.
Adult↗
Epidural abscess of the cervical spine with osteomyelitis of the odontoid process.
STUDY DESIGN: A case report. OBJECTIVES: To document the rare condition of staphylococcal osteomyelitis of the odontoid process and to increase knowledge about the clinical characteristics and favorable outcome if patients are managed appropriately. SUMMARY OF BACKGROUND DATA: Osteomyelitis of the odontoid process caused by Staphylococcus aureus is a rare disease. A handful of cases have been reported within the last 30 years. Destructive odontoid peg involvement is most commonly associated with rheumatoid disease, which has a distinct clinical course compared with that of bacterial infection. METHODS: Two patients with bacterial osteomyelitis of the odontoid peg underwent medical and surgical treatment. They were observed for 3 years. All authors were involved in the care of these patients. RESULTS: Close monitoring of the patients' neurologic status and the use of noninvasive imaging techniques to evaluate the cervical spine led to an individualized treatment plan including antibiotic medication and transoral surgery with good outcomes in both cases. CONCLUSIONS: Awareness of the occurrence of bacterial osteomyelitis of the odontoid process, with or without neurologic symptoms, in patients with neck pain and fever may lead to earlier detection of this potentially critical condition, which has an excellent prognosis when treated early and appropriately.
Abscess↗
The effect of artifact rejection by signal-space projection on source localization accuracy in MEG measurements.
The consequences of artifact suppression by means of signal-space projection on dipole localization accuracy for magnetoencephalography measurements are studied. Approximate analytical formulas, equivalent to the Cramer-Rao bound, are presented and verified by Monte Carlo simulations which relate the increase of localization error for individual coordinates to the similarity of the artifact field and respective (contravariant) quadrupole fields obtained by differentiating the dipole field with respect to its origin. The expressions simplify significantly for dipoles placed below the center of the measuring system giving rise to highly symmetric field patterns. Formulas are presented both for single- and for multiple-artifact rejection. As illustrative examples artifact fields are constructed which a) lead to highly decreasing signal-to-noise ratio and goodness-of-fit (GOF), while the localization error is unaffected for all coordinates and b) lead to an increase of localization error while the SNR and the GOF stays constant. Finally, the rich structure of localization error increase is demonstrated for a class of artifact fields originating from artifact current dipoles.
Artifacts↗
Artifact reduction in magnetoneurography based on time-delayed second-order correlations.
Artifacts in magnetoneurography data due to endogenous biological noise sources, like the cardiac signal, can be four orders of magnitude higher than the signal of interest. Therefore, it is important to establish effective artifact reduction methods. We propose a blind source separation algorithm using only second-order temporal correlations for cleaning biomagnetic measurements of evoked responses in the peripheral nervous system. The algorithm showed its efficiency by eliminating disturbances originating from biological and technical noise sources and successfully extracting the signal of interest. This yields a significant improvement of the neuro-magnetic source analysis.
Algorithms↗
Independent component analysis of noninvasively recorded cortical magnetic DC-fields in humans.
We apply a recently developed multivariate statistical data analysis technique--so called blind source separation (BSS) by independent component analysis--to process magnetoencephalogram recordings of near-dc fields. The extraction of near-dc fields from MEG recordings has great relevance for medical applications since slowly varying dc-phenomena have been found, e.g., in cerebral anoxia and spreading depression in animals. Comparing several BSS approaches, it turns out that an algorithm based on temporal decorrelation successfully extracted a dc-component which was induced in the auditory cortex by presentation of music. The task is challenging because of the limited amount of available data and the corruption by outliers, which makes it an interesting real-world testbed for studying the robustness of ICA methods.
Acoustic Stimulation↗
Current multipole expansion to estimate lateral extent of neuronal activity: a theoretical analysis.
High-resolution magnetoencephalography (MEG) allows for a detailed description of focal neuronal current sources going far beyond the dipole approximation which merely indicates the center and magnitude of neuronal activity. Higher order multipole coefficients can be related to other bulk properties, like spatial extent or curvature. The possibility and limitations of measuring spatial extent by interpreting reconstructed multipole coefficients was tested under realistic noise conditions and for model misspecifications; for this analysis the primary cortical response ("N20") to electric median nerve stimulation was modeled by a one dimensional source distribution. The forward calculation was done analytically up to octapolar order for a spherical volume conductor. The multipole expansion is shown to estimate the lateral source extent with negligible bias; this estimate is to first-order stable against additional source features, like gyral curvature or spatial extent in a second direction (gyral depth, neuronal length). For a dipole moment of 20 nAm a lateral extent of 2 cm can be detected for a realistic noise level with large but experimentally still reasonable effort. Approximating a realistic head model by a sphere results in errors larger than the extent to be estimated; accordingly, studies on human cortical evoked responses will require multipole fitting in realistic head models.
Humans↗
Cardiac artifact subspace identification and elimination in cognitive MEG data using time-delayed decorrelation.
To reduce physiological artifacts in magnetoencephalographic (MEG) and electroencephalographic recordings, a number of methods have been applied in the past such as principal component analysis, signal-space projection, regression using secondary information, and independent component analysis. This method has become popular as it does not have constraints such as orthogonality between artifact and signal or the need for a priori information. Applying the time-delayed decorrelation algorithm to raw data from a visual stimulation MEG experiment, we show that several of the independent components can be attributed to the cardiac artifact. Calculating an average cardiac activity shows that physiologically different excitation states of the heart produce similar field distributions in the MEG sensor system. This is equivalent to differing spectral properties of cardiac field distributions in the raw data. As a consequence, the algorithm combines, e.g., the R peak and the T wave of the cardiac cycle into a single component and the one-to-one assignment of each independent component with a physiological source is not justified in this case. To improve the signal quality of visually evoked fields, the multidimensional cardiac artifact subspace is suppressed from the data. To assess the preservation of the evoked signal after artifact suppression, a geometrical and a temporal measure are introduced. The suppression of cardiac and alpha wave artifacts allows, in our experimental setting, the reduction of the number of epochs to one half while preserving the visually evoked signal.
Algorithms↗
The BCI Competition 2003: progress and perspectives in detection and discrimination of EEG single trials.
Interest in developing a new method of man-to-machine communication--a brain-computer interface (BCI)--has grown steadily over the past few decades. BCIs create a new communication channel between the brain and an output device by bypassing conventional motor output pathways of nerves and muscles. These systems use signals recorded from the scalp, the surface of the cortex, or from inside the brain to enable users to control a variety of applications including simple word-processing software and orthotics. BCI technology could therefore provide a new communication and control option for individuals who cannot otherwise express their wishes to the outside world. Signal processing and classification methods are essential tools in the development of improved BCI technology. We organized the BCI Competition 2003 to evaluate the current state of the art of these tools. Four laboratories well versed in EEG-based BCI research provided six data sets in a documented format. We made these data sets (i.e., labeled training sets and unlabeled test sets) and their descriptions available on the Internet. The goal in the competition was to maximize the performance measure for the test labels. Researchers worldwide tested their algorithms and competed for the best classification results. This paper describes the six data sets and the results and function of the most successful algorithms.
Adult↗
Brain-computer communication and slow cortical potentials.
A thought translation device (TTD) has been designed to enable direct brain-computer communication using self-regulation of slow cortical potentials (SCPs). However, accuracy of SCP control reveals high intersubject variability. To guarantee the highest possible communication speed, some important aspects of training SCPs are discussed. A baseline correction of SCPs can increase performance. Multichannel recordings show that SCPs are of highest amplitude around the vertex electrode used for feedback, but in some subjects more global distributions were observed. A new method for control of eye movement is presented. Sequential effects of trial-to-trial interaction may also cause difficulties for the user. Finally, psychophysiological factors determining SCP communication are discussed.
Adult↗
BCI Competition 2003--Data set III: probabilistic modeling of sensorimotor mu rhythms for classification of imaginary hand movements.
Brain-computer interfaces require effective online processing of electroencephalogram (EEG) measurements, e.g., as a part of feedback systems. We present an algorithm for single-trial online classification of imaginary left and right hand movements, based on time-frequency information derived from filtering EEG wideband raw data with causal Morlet wavelets, which are adapted to individual EEG spectra. Since imaginary hand movements lead to perturbations of the ongoing pericentral mu rhythm, we estimate probabilistic models for amplitude modulation in lower (10 Hz) and upper (20 Hz) frequency bands over the sensorimotor hand cortices both contra- and ipsilaterally to the imagined movements (i.e., at EEG channels C3 and C4). We use an integrative approach to accumulate over time evidence for the subject's unknown motor intention. Disclosure of test data labels after the competition showed this approach to succeed with an error rate as low as 10.7%.
Algorithms↗