PubMed Health⌕ Search

Biomedical subjects

T Elbert

Publications and source records attributed to T Elbert.

At least 19 recordsLinked to original sources
PubMed2004

One set of sounds, two tonotopic maps: exploring auditory cortex with amplitude-modulated tones.

OBJECTIVE: The possibility of simultaneously observing activation of primary and secondary auditory cortices has been demonstrated by Engelien et al. [Hear Res 2000;148:153-60]. METHODS: Such a dual monitoring by means of neuromagnetic recordings can be achieved when a subject is stimulated by brief pulses of 40Hz-modulated tones. Depending on the frequency filter applied, either the steady-state field (SSF) or the N1m can be extracted from the evoked magnetic field complex. RESULTS: Using this "combined" (two-maps) paradigm with 4 carrier frequencies, we show that it is possible to synchronously screen two tonotopic maps--one map each reflected either by the SSF or the N1m. Indicators are the systematic variation in the location (higher frequencies are more posterior) and orientation (higher frequencies oriented differently in the sagittal plane) of the equivalent current dipole (ECD). These parameters were compared with those obtained from "classic" (one map) paradigms in which either a pure tone elicits an N1m or a 40 Hz continuous (3 s) stimulation produces an SSF. Overall the results were similar, however, systematic differences between the paradigms were found for ECD localization, dipole strength, amplitude, and phase. CONCLUSIONS AND SIGNIFICANCE: One possible interpretation of these results is that different tonotopically arranged cortical fields were involved in the generation of the components.

Acoustic Stimulation↗
PubMed2002

Comparison of data transformation procedures to enhance topographical accuracy in time-series analysis of the human EEG.

We describe a methodology to apply current source density (CSD) and minimum norm (MN) estimation as pre-processing tools for time-series analysis of single trial EEG data. The performance of these methods is compared for the case of wavelet time-frequency analysis of simulated gamma-band activity. A reasonable comparison of CSD and MN on the single trial level requires regularization such that the corresponding transformed data sets have similar signal-to-noise ratios (SNRs). For region-of-interest approaches, it should be possible to optimize the SNR for single estimates rather than for the whole distributed solution. An effective implementation of the MN method is described. Simulated data sets were created by modulating the strengths of a radial and a tangential test dipole with wavelets in the frequency range of the gamma band, superimposed with simulated spatially uncorrelated noise. The MN and CSD transformed data sets as well as the average reference (AR) representation were subjected to wavelet frequency-domain analysis, and power spectra were mapped for relevant frequency bands. For both CSD and MN, the influence of noise can be sufficiently suppressed by regularization to yield meaningful information, but only MN represents both radial and tangential dipole sources appropriately as single peaks. Therefore, when relating wavelet power spectrum topographies to their neuronal generators, MN should be preferred.

Artifacts↗
PubMed2001

Gender differences in functional hemispheric asymmetry during processing of vowels as reflected by the human brain magnetic response.

A number of findings indicate gender differences in language-related functional hemispheric brain asymmetry. To test if such gender-specific laterality is already present at the level of vowel-processing, the auditory evoked magnetic field was recorded in healthy right-handed male and female participants in response to the German synthetic vowels [a], [e] and [i]. Female participants exhibited stronger N100m responses than male participants over the left hemisphere. This observation was highly reliable across repeated experimental sessions. The present lateralization shows that previous findings suggesting a stronger left-hemispheric dominance for verbal material in males than in females can not be generalized to basic speech elements. Furthermore, the present results support the importance of controlling for gender ratio in studies of phonetic processing.

Acoustic Stimulation↗
PubMed2001

Source distribution of neuromagnetic slow waves and MEG-delta activity in schizophrenic patients.

BACKGROUND: Schizophrenic patients exhibit more activity in the electroencephalographic delta and theta frequency range than do control subjects. Using magnetic source imaging (MSI) our study aimed to explore this phenomenon in the magnetoencephalogram (MEG), the distribution of its sources, and associations between symptom profiles and sources of low-frequency activity in the brain. METHODS: Whole-head MEG recordings were obtained from 28 schizophrenic patients and 20 healthy control subjects during a resting condition. The generators of the focal magnetic slow waves were located employing a single moving dipole model. Distributed or multiple delta and theta sources were captured by the minimum norm estimate. RESULTS: Both localization procedures showed slow wave activity to be enhanced in schizophrenic patients compared with control subjects. Focal slow wave activity differed most between groups in frontotemporal and in posterior regions. Slow wave activity was associated with symptom characteristics in that positive symptoms varied with frontal delta and theta activity. CONCLUSIONS: Results indicate that activity in low-frequency bands in schizophrenic patients exceeds the activity of control subjects in distinct areas, and that this focal clustering of neuromagnetic slow waves may be related to psychopathologic characteristics.

Adult↗
PubMed2001

Altered hemispheric asymmetry of auditory magnetic fields to tones and syllables in schizophrenia.

BACKGROUND: A growing body of literature suggests that schizophrenic patients often do not show the normal brain hemispheric asymmetry. We have found this for simple tones presented to the right ear in a previous study. In this study we extended this investigation to left ear stimulation and verbal stimuli. METHODS: With a whole-head neuromagnetometer, contra- and ipsilateral auditory-evoked magnetic fields in response to tones (1000 Hz) and to the syllables ("ba") delivered to the left and right ears in separate runs were compared between schizophrenic patients (n = 17) and healthy control subjects (n = 15). RESULTS: In response to tones, all control subjects showed the expected asymmetry (contralateral predominance) of the auditory-evoked magnetic N100m (dipole moment). In the patient sample asymmetry was reversed following tones presented to the left ear in 47% and following tones to the right ear in 24%. In response to syllables, the asymmetry was similar between groups. In patients compared with control subjects the N100m was located more anterior without asymmetry between hemispheres. CONCLUSIONS: Results suggest that deviation from the normal functional lateralization in schizophrenia appears in a proportion of patients at a basic stage of auditory processing, but may be compensated for at higher levels such as the processing of syllables.

Acoustic Stimulation↗
PubMed2001

Auditory temporal processing deficit in dyslexia is associated with enhanced sensitivity in the visual modality.

Developmental dyslexia has been associated with a deficit in temporal processing, but it is controversial whether the postulated deficit is pansensory or limited to the auditory modality. We present psychophysical assessment data of auditory and visual temporal processing abilities in children with dyslexia. While none of the dyslexic children displayed temporal processing abnormalities in the visual sensory modality, dyslexics with poor auditory temporal scores reached high-level visual performance. Our results do not confirm the hypothesis of a general temporal processing deficit for dyslexia but suggest that limitations in auditory temporal processing might be compensated for by a well-functioning visual sensory modality.

Adaptation, Physiological↗
PubMed2001

Human large-scale oscillatory brain activity during an operant shaping procedure.

The present study aimed at examining the oscillatory brain-electric correlates of human operant learning using high-density electroencephalography (EEG). Induced gamma-band activity (GBA) was studied using a fixed-interval reinforcement schedule with a variable limited hold period, which was decreased depending on response accuracy. Thus, participants' behavior was shaped during the course of the learning session. After each response, numbers indicating the money value of that response served as reinforcing stimuli. Random reinforcement and self-paced button pressing without reinforcement were added as control conditions. GBA around 40 Hz was enhanced at posterior electrodes in response to visual feedback stimuli during shaping and random reward compared to the self-paced pressing condition where no visual feedback was provided. Furthermore, shaping was associated with a pronounced left frontal lower gamma (20-30 Hz) increase in response to feedback stimuli, whereas this pattern was not observed in the random reinforcement and self-paced pressing conditions. The present findings are in line with the notion that macroscopic high-frequency dynamics of neuronal cell assemblies may be regarded as a mechanism involved in learning and memory formation.

Adult↗
PubMed2001

Effects of emotional arousal in the cerebral hemispheres: a study of oscillatory brain activity and event-related potentials.

OBJECTIVE: The present study aimed at examining the time course and topography of oscillatory brain activity and event-related potentials (ERPs) in response to laterally presented affective pictures. METHODS: Electroencephalography was recorded from 129 electrodes in 10 healthy university students during presentation of pictures from the international affective picture system. Frequency measures and ERPs were obtained for pleasant, neutral, and unpleasant pictures. RESULTS: In accordance with previous reports, a modulation of the late positive ERP wave at parietal recording sites was found as a function of emotional arousal. Early mid gamma band activity (GBA; 30-45 Hz) at 80 ms post-stimulus was enhanced in response to aversive stimuli only, whereas the higher GBA (46-65 Hz) at 500 ms showed an enhancement of arousing, compared to neutral pictures. ERP and late gamma effects showed a pronounced right-hemisphere preponderance, but differed in terms of topographical distribution. CONCLUSIONS: Late gamma activity may represent a correlate of widespread cortical networks processing different aspects of emotionally arousing visual objects. In contrast, differences between affective categories in early gamma activity might reflect fast detection of aversive stimulus features.

Adult↗
PubMed2001

Neuromagnetic evidence for early semantic access in word recognition.

Magnetic brain responses recorded in the human magnetoencephalogram (MEG) distinguished between words with different semantics but carefully matched for frequency and length. Multiple recordings from a single subject showed that 100 ms following stimulus onset, significantly stronger neuromagnetic responses were elicited by words with strong multimodal semantic associations than by other word material. At this early processing step, there was a highly significant correlation (0.80) between the magnitude of brain responses to individual words recorded over parieto-occipital areas and their semantic association strengths. Subsequent to this early difference related to word meaning, additional differences in MEG responses emerged for words from different grammatical categories. Together, these results suggest that word meaning can be reflected by early neuromagnetic brain responses and before the grammatical information about the word is encoded.

Adult↗
PubMed2001

Constraint-induced therapy of chronic aphasia after stroke.

Patients with chronic aphasia were assigned randomly to a group to receive either conventional aphasia therapy or constraint-induced (CI) aphasia therapy, a new therapeutic technique requiring intense practice over a relatively short period of consecutive days. CI aphasia therapy is realized in a communicative therapeutic environment constraining patients to practice systematically speech acts with which they have difficulty. Patients in both groups received the same amount of treatment (30 to 35 hours) as 10 days of massed-practice language exercises for the CI aphasia therapy group (3 hours per day minimum; 10 patients) or over a longer period of approximately 4 weeks for the conventional therapy group (7 patients). CI aphasia therapy led to significant and pronounced improvements on several standard clinical tests, on self-ratings, and on blinded-observer ratings of the patients' communicative effectiveness in everyday life. Patients who received the control intervention failed to achieve comparable improvements. Data suggest that the language skills of patients with chronic aphasia can be improved in a short period by use of an appropriate massed-practice technique that focuses on the patients' communicative needs.

Adult↗
PubMed2001

Representational cortex in musicians. Plastic alterations in response to musical practice.

The lifelong ability to adapt to environmental needs is based on the capacity of the central nervous system for plastic alterations. In a series of neurophysiological experiments, we studied the impact of music and musical training in musicians on the specific functional organization in auditory and somatosensory representational cortex. In one such study, subjects listened to music from which one specific spectral frequency was removed. This led to rapid and reversible adaptation of neuronal responses in auditory cortex. Further experimental evidence demonstrated that long years of practice and training by professional musicians to enable them to reach their capacity is associated with enlarged cortical representations in the somatosensory and auditory domains. This tuning of neuronal representations was specifically observed for musical tones and was absent when pure sinusoidal tones were used as stimuli. In the somatosensory cortex, plastic changes proved to be specific for the fingers frequently used and stimulated. These changes were not detected in the fingers of the hand that were not involved in playing the particular instrument. Neuroplastic alterations also may be driven into a domain where they may become maladaptive. The clinical syndrome of focal hand dystonia that may occur in musicians who engage in forceful practice may be one such consequence. We will discuss the possibilities of reversing maladaptive responses leading to the successful treatment of focal hand dystonia, which relies on basic research about cortical reorganization. This example elucidates how neuroscientific progress can guide the development of practice guidelines and therapeutic measures for the benefit of professional musicians.

Auditory Cortex↗
PubMed2000

Differential activation in somatosensory cortex for different discrimination tasks.

Maps of the body surface in somatosensory cortex have been shown to be highly plastic, altering their configuration in response to changes in use of body parts. The current study investigated alterations in the functional organization of the human somatosensory cortex resulting from massed practice. Over a period of 4 weeks, subjects were given synchronous tactile stimulation of thumb (D1) and little finger (D5) for 1 hr/d. They had to identify the orientation of the stimuli. Neuroelectric source localization based on high-resolution EEG revealed that, when subjects received passive tactile stimulation of D1 or D5, the representations of the fingers in primary somatosensory cortex were closer together after training than before. There was also an apparently correlative tendency to anomalously mislocalize near-threshold tactile stimuli equally to the distant finger costimulated during training rather than preferentially to the finger nearest to the finger stimulated in a post-training test. However, when the stimulus discrimination had to be made, neuroelectric source imaging revealed that the digital representations of D1 and D5 were further apart after training than before. Thus, the same series of prolonged repetitive stimulations produced two different opposite effects on the spatial relationship of the cortical representations of the digits, suggesting that differential activation in the same region of somatosensory cortex is specific to different tasks.

Adult↗
PubMed2000

Atypical organisation of the auditory cortex in dyslexia as revealed by MEG.

Neuroanatomical and -radiological studies have converged to suggest an atypical organisation in the temporal bank of the left-hemispheric Sylvian fissure for dyslexia. Against the background of this finding, we applied high temporal resolution magnetoencephalography (MEG) to investigate functional aspects of the left-hemispheric auditory cortex in 11 right-handed dyslexic children (aged 8-13 years) and nine matched normal subjects (aged 8-14 years). Event-related field components during a passive oddball paradigm with pure tones and consonant-vowel syllables were evaluated. The first major peak of the auditory evoked response, the M80, showed identical topographical distributions in both groups. In contrast, the generating brain structures of the later M210 component were located more anterior to the earlier response in children with dyslexia only. Control children exhibited the expected activation of more posterior source locations of the component that appeared later in the processing stream. Since the group difference in the relative location of the M210 source seemed to be independent of stimulus category, it is concluded that dyslexics and normally literate children differ as to the organisation of their left-hemispheric auditory cortex.

Adolescent↗
PubMed2000

MEG gamma band activity in schizophrenia patients and healthy subjects in a mental arithmetic task and at rest.

OBJECTIVES: High frequency oscillations have been suggested as a correlate of cognitive processes and have recently also been implicated in aberrant forms of information processing. The present study investigated whether magnetoencephalographic (MEG) gamma band activity (20-71 Hz) can serve as an index of cognitive processes in the absence of external stimulation and to what extent gamma activity differs between healthy people and schizophrenia patients. METHODS: The amount and topography of MEG power in the gamma band range was examined in 15 schizophrenia patients and 15 healthy comparison subjects while performing a complex mental arithmetic task and at rest. RESULTS: In healthy subjects a left frontal and left fronto-temporal increase in gamma power was observed during mental arithmetic. Schizophrenia patients either failed to display such a task effect (30-45 Hz) or had reversed lateralization with enhanced activity over right frontal and right fronto-temporal regions under cognitive demands (45-71 Hz). In the frequency band from 60 to 71 Hz patients showed less gamma at fronto-temporal, posterio-temporal and occipital sites irrespective of the task. CONCLUSIONS: These results indicate, first, that gamma topography can index cognitive activation in a very complex and purely internal task. Second, groups differed in the pattern of activation during the task, a result which may be consistent with working memory dysfunction in schizophrenia. Third, the general topographic difference between healthy subjects and patients is in line with the notion of abnormalities in the thalamocortical circuit in schizophrenia.

Adult↗
PubMed2000

EEG brain mapping of phonological and semantic tasks in Italian and German languages.

OBJECTIVES: Event-related potential correlates of phonological encoding - as compared with lexical access and semantic categorization - were measured in two studies involving two groups of 14 German and 14 Italian subjects. METHODS: A two stimulus reaction time paradigm was used. Stimulus pairs presented one-by-one with 2 s inter-stimulus intervals (ISI) had to be matched with respect to lexical identity (word-picture) in a word comprehension task or with respect to the phonological representative of objects in a rhyming task. A semantic categorization task was added for the Italian sample. In both studies, the EEG was recorded from 26 scalp electrodes according to the 10-20 system. The slow negative potential during the ISI (CNV) was determined as the electrocortical correlate of preparation for and activation of the specific language-related task. RESULTS: In both samples, phonological encoding (rhyming) evoked a more pronounced CNV over the left- compared with the right-frontal area, while less lateralized central dominance of the CNV was found in the word comprehension task. Semantic categorization was accompanied by the least asymmetry of activity. CONCLUSIONS: Results indicate that the different degree of asymmetry induced by phonological and semantic processing may be determined from the scalp distribution of slow cortical potentials with cross-lingual reliability.

Acoustic Stimulation↗