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Stefan Knecht

Publications and source records attributed to Stefan Knecht.

At least 37 records · Page 2Linked to original sources
PubMed2004

Responsiveness to repeated speech stimuli persists in left but not right auditory cortex.

Activation of the auditory cortex habituates with repeated stimulation. While behaviorally adaptive in most circumstances, decreasing auditory responsiveness could interfere with speech perception. We therefore tested whether auditory habituation differs for speech and non-speech stimuli and for left and right auditory cortex. We examined seven right-handed subjects in whom we had determined left-hemispheric language dominance by event-related blood flow assessment. We recorded magnetoencephalographic-evoked responses to trains of four sine tones or vowels and measured the decrement from the first to the last stimulus of the response component about 100 ms after stimulus onset (N1). For the sine tones there was a decrement in both hemispheres. Conversely, for vowels there was significant attenuation of the auditory decrement in the left compared with the right hemisphere (p=0.017). This left-hemisphere persistence in auditory responsiveness to vowels demonstrates that the human brain processes speech stimuli differently than non-speech stimuli and that the left-hemisphere plays a dominant role in this speech-specific auditory processing.

Acoustic Stimulation↗
PubMed2004

Levodopa: faster and better word learning in normal humans.

Dopamine is a potent modulator of learning and has been implicated in the encoding of stimulus salience. Repetition, however, as required for the acquisition and reacquisition of sensorimotor or cognitive skills (e.g., in aphasia therapy), decreases salience. We here tested whether increasing brain levels of dopamine during repetitive training improves learning success. Forty healthy humans took 100mg of the dopamine precursor levodopa or placebo daily for 5 days in a randomized double-blind and parallel-group design. Ninety minutes later on each day, subjects were trained on an artificial vocabulary using a high-frequency repetitive approach. Levodopa significantly enhanced the speed, overall success, and long-term retention of novel word learning in a dose-dependent manner. These findings indicate new ways to potentiate learning in a variety of domains if conventional training alone fails.

Adult↗
PubMed2004

Influence of somatosensory input on motor function in patients with chronic stroke.

In healthy volunteers, reduction of somatosensory input from one hand leads to rapid performance improvements in the other hand. Thus, it is possible that reduction of somatosensory input from the healthy hand can influence motor function in the paretic hand of chronic stroke patients with unilateral hand weakness. To test this hypothesis, we had 13 chronic stroke patients perform motor tasks with the paretic hand and arm during cutaneous anesthesia of the healthy hand and healthy foot in separate sessions. Performance of a finger tapping task, but not a wrist flexion task, improved significantly with anesthesia of the hand, but not the foot. This effect progressed with the duration of anesthesia and correlated with baseline motor function. We conclude that cutaneous anesthesia of the healthy hand elicits transient site-specific improvements in motor performance of the moderately paretic hand in patients with chronic stroke, consistent with interhemispheric competition models of sensorimotor processing.

Adult↗
PubMed2004

Determining the hemispheric dominance of spatial attention: a comparison between fTCD and fMRI.

Human brain mapping allows the systematic assessment of interindividual differences in functional brain anatomy. Functional transcranial Doppler sonography (fTCD) is an imaging tool that allows for fast and mobile assessment of hemispheric lateralization of task-related brain activation. It is ideal to screen large cohorts of subjects. The goal of the present study was to investigate whether fTCD and functional magnetic resonance imaging (fMRI) determine hemispheric lateralization of brain activation related to visuospatial attention concordantly. Used together, fMRI and fTCD may then open up a wide range of potential applications in neuroscience. Fifteen subjects were examined both with fTCD and fMRI while they judged accuracy of line bisections (Landmark task). For fTCD, the maximal mean difference in stimulus-related relative cerebral blood flow velocity changes in the left and right middle cerebral arteries was assessed as the lateralization index LI(fTCD). For fMRI, two approaches were used to determine hemispheric dominance. First, we measured brain activity as the extent of the activated region, i.e., the number of activated voxels above a statistical threshold. Second, we calculated the magnitude of the fMRI signal change between the activation and the control task within a region of interest. Results of fTCD and fMRI were concordant in every single case. Therefore, scanning large cohorts with fTCD for hemispheric dominance during Landmark task will provide results consistent with fMRI. FMRI can then be used for in depth assessment of the specific patterns of activation.

Adult↗
PubMed2004

Fracture of the odontoid process complicating tardive dystonia.

We report on a 57-year-old woman with frequent and powerful retrocollis. The involuntary movements combined with rheumatoid arthritis and osteoporosis led to a fracture of the axial odontoid process. Neurologists with a focus on movement disorders should screen for diseases putting the patient at risk for spinal lesions and consider early treatment.

Anti-Dyskinesia Agents↗
PubMed2004

The investigation of functional brain lateralization by transcranial Doppler sonography.

Functional transcranial Doppler sonography (fTCD) adds to the techniques of functional imaging. fTCD measures cerebral perfusion changes related to neural activation in a way comparable to functional magnetic resonance tomography. fTCD contends itself with comparison of averaged, event-related blood flow velocity changes within the territories of two cerebral arteries, for example the left versus the right middle cerebral artery. It can thus serve to evaluate the functional lateralization of higher cognitive functions like hemispheric language dominance (HLD). We present typical applications of fTCD by summarizing studies employing the technique. Then, the physical and physiological underpinnings of fTCD are reviewed. After a brief description of a prototype paradigm for assessing HLD, a detailed outline of the fTCD data analysis is presented. Caveats for fTCD, like other functional imaging techniques, are that the validity of results depends on adequate control of the task parameters, particularly cooperation and reference conditions. We complete the review with examinations of the reliability and validity of the fTCD technique. We conclude that fTCD can be employed to substitute the invasive amobarbital procedure to determine language lateralization in individual patients before undergoing brain surgery. Because of its easy applicability, robustness and mobility, fTCD can also be used to examine many subjects (including children) to obtain representative data on the variability of lateralization of higher cognitive functions, or to scan for atypical patterns of lateralization.

Algorithms↗
PubMed2004

D-amphetamine boosts language learning independent of its cardiovascular and motor arousing effects.

D-Amphetamine (AMPH) was effective in a number of studies on motor and language recovery after stroke, but given safety concerns, its general use after stroke is still debated. Most stroke patients are excluded from treatment because of a significant risk of cardiovascular dysregulation. AMPH acts on multiple transmitter systems, and mainly the noradrenergic actions are related to the cardiovascular effects. If AMPH's cardiovascular and arousal effects were correlated with its plasticity-enhancing effects in humans, this would imply that desired and undesired effects are inevitably tied. If not, improved cerebral reorganization may not be mediated by AMPH's arousing effects and could be achieved with substances lacking the undesired cardiovascular effects. As a model for language recovery after stroke, we used a prospective, randomized, double-blind, placebo-controlled design and taught 40 healthy male subjects an artificial vocabulary of 50 concrete nouns over the course of five consecutive training days (high-frequency training). The associative learning principle involved higher co-occurrences of 'correct' picture-pseudoword pairings as compared to 'incorrect' pairings. Subjects received either AMPH (0.25 mg/kg) or placebo 90 min prior to training on each day. Novel word learning was significantly faster and better in the AMPH as compared to the placebo group. Increased learning success was maintained 1 month post-training. No correlation was found between training success and drug-induced increases in blood pressure, heart rate, or a facilitation of simple motor reaction time. Our data show that AMPH's plasticity-enhancing effect in humans is not related to its cardiovascular arousal. This suggests that the beneficial effects in stroke patients could also be obtained by less cardiovascular active drugs.

Adult↗
PubMed2004

Prefrontal cortex asymmetry for memory encoding of words and abstract shapes.

Previous work suggested a differential contribution of prefrontal cortex (PFC) to successful encoding depending on the stimulus material. Here, we tested the hypothesis that encoding of words preferentially involves the left PFC, while encoding of nonverbal items (abstract shapes) relies on the right PFC. We used an experimental design that evaluated encoding of both words and abstract shapes in the same healthy volunteers. A transient virtual lesion of the left or the right PFC was elicited with transcranial magnetic stimulation (TMS) while subjects memorized verbal and nonverbal items. We found that encoding of verbal material was disrupted by left PFC stimulation, whereas encoding of nonverbal material was disrupted by right PFC stimulation. These results demonstrate a functionally relevant lateralization of prefrontal contribution for verbal and nonverbal memory encoding.

Adult↗
PubMed2004

Task repetition can affect functional magnetic resonance imaging-based measures of language lateralization and lead to pseudoincreases in bilaterality.

Repeated functional magnetic resonance imaging (fMRI) during learning and recovery can inform us about functional reorganization in the brain. We examined how, in the absence of reorganization, simple task repetition affects measures of fMRI activation. We studied fMRI activation over 10 consecutive sessions of silent word generation in a healthy subject. Additionally, we performed functional transcranial Doppler sonography (fTCD) to learn about the temporal pattern of corresponding changes in cerebral blood flow velocity (CBFV) and pulsatility. With repetition, word generation-associated increases in heart rate diminished steadily. Task repetition also led to a net increase in CBFV bilaterally and to a bilateral increase in the number of activated voxels on fMRI. As a result, whereas the absolute interhemispheric difference of activated voxels remained constant, there was a decrease in the standard fMRI index for language lateralization [LIfMRI = 100 (activated voxels in left hemisphere - voxels in right)/(voxels in left + voxels in right)]. Thus task repetition can lead to changes in task-related autonomic drive and an augmentation of bihemispheric blood flow. This can mimic increasing bilaterality of brain activation.

Adult↗
PubMed2004

Transcranial direct current stimulation disrupts tactile perception.

The excitability of the cerebral cortex can be modulated by various transcranial stimulation techniques. Transcranial direct current stimulation (tDCS) offers the advantage of portable equipment and could, therefore, be used for ambulatory modulation of brain excitability. However, modulation of cortical excitability by tDCS has so far mostly been shown by indirect measures. Therefore, we examined whether tDCS has a direct behavioral/perceptional effect. We compared tactile discrimination of vibratory stimuli to the left ring finger prior to, during and after tDCS applied for 7 min at 1-mA current intensity in 13 subjects. Stimulation was pseudorandomized into cathodal, anodal and sham conditions in a within-subject design. The active electrode was placed over the corresponding somatosensory cortex at C4 according to the 10-20 EEG system and the reference electrode at the forehead above the contralateral orbita. Cathodal stimulation compared with sham induced a prolonged decrease of tactile discrimination, while anodal and sham stimulation did not. Thus, cortical processing can be modulated in a behaviorally/perceptually meaningful way by weak transcranial current stimulation applied through portable technology. This finding offers a new perspective for the treatment of conditions characterized by alterations of cortical excitability.

Adult↗
PubMed2004

Specific and nonspecific effects of transcranial magnetic stimulation on picture-word verification.

Repetitive transcranial magnetic stimulation (rTMS) can temporarily impair or improve performance, including language processing. It remains unclear, however, (i) which scalp sites are most appropriate to achieve the desired effects and (ii) which experimental setups produce facilitation or inhibition of language functions. We assessed the effects of TMS at different stimulation sites on picture-word verification in healthy volunteers. Twenty healthy volunteers with left language lateralization, as determined by functional transcranial Dopplersonography, performed picture-word verification prior to and after rTMS (1 Hz for 600 s at 110% of subjects' resting motor thresholds). Stimulation sites were the classical language areas (Broca's and Wernicke's), their homolog brain regions of the right hemisphere, and the occipital cortex. Additionally, sham stimulation over Broca's area was applied in a subsample of 11 subjects. As a control task, 10 volunteers performed a colour-tone matching task under the same experimental conditions. There was a general nonspecific arousal effect for both verum and sham TMS for both the picture-word verification and for the control task. However, superimposed there were opposite effects on picture-word verification for stimulation of Wernicke's area and Broca's area, namely a relative inhibition in the case of Wernicke's area and a relative facilitation in the case of Broca's area. These results demonstrate that low frequency rTMS has both general arousing effects and domain-specific effects.

Adult↗
PubMed2004

Word learning can be achieved without feedback: implications for aphasia therapy.

PURPOSE: Children acquire new words through exposure, without the necessity for explicit feedback by caregivers. In aphasia therapy, feedback to the patient is considered an important asset even though the empirical base demonstrating superior learning with online feedback is lacking. The present study examined if healthy adults and patients with chronic aphasia can acquire a new lexicon by intense frequency of exposure alone. METHODS: We compared learning rates with "frequency of exposure alone" (no-feedback condition: n=19 healthy adults; two patients with chronic Broca's and Wernicke's aphasia, respectively) with a condition where subjects additionally received online feedback (feedback condition; n=19). The learning principle was higher statistical co-occurrences of "correct" picture-pseudoword pairings as compared to "incorrect" pairings. In the feedback condition, immediate online feedback on the correctness of respective choices was additionally provided. RESULTS: Both healthy groups successfully acquired the vocabulary. Feedback led to a slight initial acceleration of learning but did not improve latency to peak performance or long-term retention of lexical knowledge. These findings show that high frequency interactive exposure is a potent word learning mechanism in adults and that feedback is not crucial. This is further corroborated by our successful training of two patients with chronic aphasia without online feedback. CONCLUSIONS: Our findings demonstrate that word re-learning in aphasia could benefit from maximizing on the frequency of exposure and exploiting the therapeutic principle of "massed practice", which has been successful in physical rehabilitation after stroke. Additionally, economizing on feedback may prevent patients becoming discouraged by continuous confrontation with their deficits.

Adult↗
PubMed2003

Changing cortical excitability with low-frequency transcranial magnetic stimulation can induce sustained disruption of tactile perception.

Transcranial magnetic stimulation (TMS) is promising as a therapeutic tool, and TMS of the motor system has served as a model for regionally specific modulations of cortical excitability. It is unclear, however, to what extent response characteristics of the motor cortex are representative of other brain systems. We wanted to determine whether TMS could induce a sustained disruption of somatosensory processing beyond the stimulation duration, similar to observations in the motor system. We applied 1-Hz TMS at 110% of subjects' motor thresholds for a variable duration over the right and left somatosensory cortex before subjects performed a tactile frequency discrimination task with the left hand. Tactile discrimination was impaired only after TMS over the right somatosensory cortex (analysis of variance: p <.01). The duration of this impairment correlated with the duration of the preceding TMS; the effect lasted approximately 2 min after 5 min of TMS, 4 min after 10 min of TMS, and 8 min after 20 min of TMS. Two conclusions arise: 1) low-frequency TMS can interfere with tactile perception in a robust and sustained way, and 2) TMS dosing parameters effective in the motor system are also effective in the somatosensory system and may reflect a modality-independent response characteristic of the cerebral cortex.

Adult↗
PubMed2003

Functional magnetic resonance imaging mirrors recovery of visual perception after repetitive tachistoscopic stimulation in patients with partial cortical blindness.

We investigated three patients with partial cortical blindness after brain injury by means of functional magnetic resonance imaging (fMRI) before and after the application of a daily visual stimulation-therapy over a period of 6 months. Before therapy, fMRI data showed a severely reduced blood-oxygen-level-dependent (BOLD) signal in primary visual cortex when compared to healthy volunteers. Following several months of rehabilitative therapy a neuropsychological improvement of visual functions was accompanied by an increase in BOLD signal of residual perilesional regions whereas fMRI data of the control group remained unchanged. A high capacity of functional recovery and synaptic plasticity of surviving perilesional neuronal structures of primary visual cortex followed by an increased input into post-connected visual areas can be discussed as a basis for the reoccurrence of visual functions.

Adult↗
PubMed2003

Learning of tactile frequency discrimination in humans.

Learning is based on the remodeling of neural connections in the brain. The purpose of the present study was to examine the extent to which training-induced improvements in tactile frequency discrimination in humans are correlated with an increase of cortical representations in the primary somatosensory cortex. Healthy male subjects (n = 16) were trained in a tactile frequency discrimination task of the left ring finger. During the first 15 days of training, there was a steep improvement in frequency discrimination, which generalized from the trained finger to its homologue on the opposite hand, and to a lesser extent, to the other fingers on both hands. During the following 15 days of training, there was only a minor improvement in tactile frequency discrimination. Retention of improved performance in frequency discrimination 30 days after training was demonstrated for all digits. Cortical finger representation in the primary somatosensory cortex, as measured by magnetic source imaging, did not change during training. Because of the generalized training effect and the lack of detectable increase in the cortical field evoked from the trained finger, we assume that skill improvement was mediated predominantly by regions outside the primary somatosensory cortex.

Adult↗
PubMed2003

Language perception activates the hand motor cortex: implications for motor theories of speech perception.

The precise mechanisms of how speech may have developed are still unknown to a large extent. Gestures have proven a powerful concept for explaining how planning and analysing of motor acts could have evolved into verbal communication. According to this concept, development of an action-perception network allowed for coding and decoding of communicative gestures. These were manual or manual/articulatory in the beginning and then became increasingly elaborate in the articulatory mode. The theory predicts that listening to the 'gestures' that compose spoken language should activate an extended articulatory and manual action-perception network. To examine this hypothesis, we assessed the effects of language on cortical excitability of the hand muscle representation by transcranial magnetic stimulation. We found the hand motor system to be activated by linguistic tasks, most notably pure linguistic perception, but not by auditory or visuospatial processing. The amount of motor system activation was comparable in both hemispheres. Our data support the theory that language may have evolved within a general and bilateral action-perception network.

Adult↗