Biomedical subjects

Klaus Mathiak

Publications and source records attributed to Klaus Mathiak.

30 records · Page 2Linked to original sources

PubMed2004

Temporal organization of "internal speech" as a basis for cerebellar modulation of cognitive functions.

The sequencing of smooth and rhythmically "sculptured" words and phrases at a speaker's habitual speech rate (4 Hz to 6 Hz) critically depends on the cerebellum. Besides overt performance, the cerebellum also seems to organize the syllabic structure of "auditory verbal imagery" or "internal speech"--that is, a prearticulatory but otherwise fully elaborated and temporally organized representation of verbal utterances. As a consequence, cerebellar disorders may compromise cognitive operations that involve a speech code, such as verbal working memory, or disrupt cognitive processes that encompass linguistic mediation. Besides the temporal organization of syllable strings at a prearticulatory level, cerebellar patients are impaired in speech perception tasks requiring the encoding of durational parameters of the acoustic signal. The hemodynamic responses associated with these two aspects of verbal-acoustic communication--internal speech and speech perception--were found to be organized along the rostro-caudal direction within paravermal aspects of the superior right cerebellar hemisphere. Those areas of the right cerebellar hemisphere thus might provide a common platform for the computation of temporal aspects of verbal utterances in the domains of both speech production and perception.

Brain↗

PubMed2003

Cortisol has different effects on human memory for emotional and neutral stimuli.

Adrenal stress hormones are considered to play a role in memory enhancement of emotionally arousing events. To investigate the effects of cortisol on human emotional memory, subjects were administered hydrocortisone (25 mg) or placebo and presented with either an emotionally arousing or a neutral story. Memory for the story was tested 1 week later. In all memory tests, subjects who viewed the emotional story scored better for the emotionally arousing story parts, indicating that arousal enhances memory. In memory of details, cortisol showed an interaction with story valence but no main effect: cortisol enhanced memory for details of the neutral story version, but impaired memory for details of the emotionally arousing version. We thus confirm a non-linear interaction between cortisol and arousal on memory formation.

Adult↗

PubMed2003

The right supratemporal plane hears the distance of objects: neuromagnetic correlates of virtual reality.

The neural mechanisms of auditory distance perception, a function of great biological importance, are poorly understood. Where not overruled by conflicting factors such as echoes or visual input, sound intensity is perceived as conveying distance information. We recorded neuromagnetic responses to amplitude variations over both supratemporal planes, with and without auditory spatial simulations. In the absence of other cues for distance, including those provided by auditory virtual reality, amplitude changes elicited enhanced preattentive responses over the right temporal lobe, indicating hemispheric lateralization of the 'where' pathway in the human. Lesion studies in monkeys and humans have shown that the rostral part of the right superior temporal cortex contributes to spatial awareness in the visual domain. Our data indicate that the distance to a sound source is processed within the adjacent right auditory cortex, thus extending the recent model of a right-hemisphere temporal multisensory matrix that subserves the integration of space-related data across visual and auditory modalities.

Adult↗

PubMed2003

Reconstruction of extended cortical sources for EEG and MEG based on a Monte-Carlo-Markov-chain estimator.

A new procedure to model extended cortical sources from EEG and MEG recordings based on a probabilistic approach is presented. The method (SPMECS) was implemented within the framework of maximum likelihood estimators. Neuronal activity generating EEG or MEG signals was characterized by the number of sources and their location and extension. Based on the noise distribution of the measured data, source configurations were associated with the according value of the likelihood function. To find the most likely source, i.e., the maximum likelihood estimator, and its level of confidence, a stochastic solver (Metropolis algorithm) was applied. The method presented supports the incorporation of virtually any constraint, e.g., based on physiological and anatomical a priori knowledge. Thus, ambiguity of the ill-posed inverse problem was reduced considerably by confining sources to the cortical surface extracted from individual MR images. The influence of different levels and types of noise on the outcome was investigated by means of simulations. Somatosensory evoked magnetic fields analyzed by the method presented suggest that larger extended cortical areas are involved in the processing of combined finger stimulation as compared to single finger stimulation.

Algorithms↗

PubMed2003

Processing of dynamic aspects of speech and non-speech stimuli: a whole-head magnetoencephalography study.

Clinical and experimental data indicate higher proficiency of the left hemisphere in encoding dynamic acoustic events such as rapid formant transitions (30-40 ms) that distinguish consonant-vowel syllables such as /ba/ or /da/. In order to further elucidate the underlying neurophysiological mechanisms, discrimination of /bi/-like formant transitions of variable duration (18, 36, 54, or 72 ms) from a steady-state /i/-like vowel was investigated by means of whole-head magnetoencephalography (MEG) both during visual distraction and selective attention. Voiced speech-like as well as unvoiced non-speech stimuli, matched for spectral envelope, served as test materials. Based on an oddball design, magnetic mismatch fields (MMF) were determined during an early (170-210 ms) and a late (230-290 ms) time window. Selective attention toward the deviant events resulted in enhanced MMFs particularly within the left hemisphere, indicating attention-dependent left-lateralized processing of dynamic auditory events across both the speech and non-speech domains. Perceptual discrimination improved along with transient lengthening. Accordingly, early MMF was, as a rule, enlarged in case of longer as compared to shorter transients. The 36-ms transitions yielded attention- and voicing-dependent deviations from the linear regression of MMF strength on transition duration. Considering the predominance of 30- to 40-ms formant transients across the world's languages, these findings indicate an adaptation or predisposition of the human perceptual system to the spectral/temporal characteristics of prototypical speech sounds. Signal voicing had no significant main effect on MMF strength despite superior perceptual performance in case of voiced as compared to voiceless target stimuli.

Acoustic Stimulation↗

PubMed2003

Parametric analysis of rate-dependent hemodynamic response functions of cortical and subcortical brain structures during auditorily cued finger tapping: a fMRI study.

A multitude of functional imaging studies revealed a mass activation effect at the level of the sensorimotor cortex during repetitive finger-tapping or finger-to-thumb opposition tasks in terms of either a stepwise or a monotonic relationship between movement rate and hemodynamic response. With respect to subcortical structures of the centralmotor system, there is, by contrast, some preliminary evidence for nonlinear rate/response functions within basal ganglia and cerebellum. To further specify these hemodynamic mechanisms, functional magnetic resonance imaging (fMRI) was performed during a finger-tapping task in response to acoustic stimuli (six different frequencies: 2.0, 2.5, 3.0, 4.0, 5.0 and 6.0 Hz; applied via headphones). Passive listening to the same auditory stimuli served as a control condition. Statistical evaluation of the obtained data considered two approaches: categorical and parametric analysis. As expected, the magnitude of the elicited hemodynamic response within left sensorimotor cortex (plateau phase at frequencies above 4 Hz) and mesiofrontal cortex paralleled movement rate. The observed bipartite mesial response pattern, most presumably, reflects functional compartmentalization of supplementary motor area (SMA) in a rostral component (pre-SMA) and in a caudal (SMA proper) component. At the level of the cerebellum, two significant hemodynamic responses within the hemisphere ipsilateral to the hand engaged into finger tapping (anterior/posterior quadrangular lobule and posterior quadrangular lobule) could be observed. Both activation foci exhibited a stepwise rate/response function. In accordance with clinical data, these data indicate different cerebellar contributions to motor control at frequencies below or above about 3 Hz, respectively. Caudate nucleus, putamen, and external pallidum of the left hemisphere displayed, by contrast, a negative linear rate/response relationship. The physiological significance of these latter findings remains to be clarified.

Adult↗

PubMed2003

Physiological self-regulation of regional brain activity using real-time functional magnetic resonance imaging (fMRI): methodology and exemplary data.

A brain-computer interface (BCI) based on real-time functional magnetic resonance imaging (fMRI) is presented which allows human subjects to observe and control changes of their own blood oxygen level-dependent (BOLD) response. This BCI performs data preprocessing (including linear trend removal, 3D motion correction) and statistical analysis on-line. Local BOLD signals are continuously fed back to the subject in the magnetic resonance scanner with a delay of less than 2 s from image acquisition. The mean signal of a region of interest is plotted as a time-series superimposed on color-coded stripes which indicate the task, i.e., to increase or decrease the BOLD signal. We exemplify the presented BCI with one volunteer intending to control the signal of the rostral-ventral and dorsal part of the anterior cingulate cortex (ACC). The subject achieved significant changes of local BOLD responses as revealed by region of interest analysis and statistical parametric maps. The percent signal change increased across fMRI-feedback sessions suggesting a learning effect with training. This methodology of fMRI-feedback can assess voluntary control of circumscribed brain areas. As a further extension, behavioral effects of local self-regulation become accessible as a new field of research.

Adult↗

PubMed2002

The influence of critical bands on neuromagnetic fields evoked by speech stimuli in humans.

The various classes of speech sounds differ in their configuration of acoustic features. Vowels are characterized by specific local maxima of spectral energy distribution (formants). Using whole-head magnetoencephalography, the impact of variation of the first (F1) and second formant (F2) on the evoked N1m component (100 ms latency) was studied based on an oddball design. F1 changes yielded N1m enhancements in parallel to the spectral distance between standard and deviant stimuli. By contrast, F2 shifts gave rise to a non-linear relationship: the N1m effect flattened out above a range of two Barks. This frequency domain accords to critical band characteristics of the peripheral and central auditory system. The differences of early neuronal encoding of both formants relate to the predominant role of F2 for the encoding of stop consonants.

Adult↗

PubMed2002

Cerebellum and speech perception: a functional magnetic resonance imaging study.

A variety of data indicate that the cerebellum participates in perceptual tasks requiring the precise representation of temporal information. Access to the word form of a lexical item requires, among other functions, the processing of durational parameters of verbal utterances. Therefore, cerebellar dysfunctions must be expected to impair word recognition. In order to specify the topography of the assumed cerebellar speech perception mechanism, a functional magnetic resonance imaging study was performed using the German lexical items "Boden" ([bodn], Engl. "floor") and "Boten" ([botn], "messengers") as test materials. The contrast in sound structure of these two lexical items can be signaled either by the length of the wordmedial pause (closure time, CLT; an exclusively temporal measure) or by the aspiration noise of wordmedial "d" or "t" (voice onset time, VOT; an intrasegmental cue). A previous study found bilateral cerebellar disorders to compromise word recognition based on CLT whereas the encoding of VOT remained unimpaired. In the present study, two series of "Boden - Boten" utterances were resynthesized, systematically varying either in CLT or VOT. Subjects had to identify both words "Boden" and "Boten" by analysis of either the durational parameter CLT or the VOT aspiration segment. In a subtraction design, CLT categorization as compared to VOT identification (CLT - VOT) yielded a significant hemodynamic response of the right cerebellar hemisphere (neocerebellum Crus I) and the frontal lobe (anterior to Broca's area). The reversed contrast ( VOT - CLT) resulted in a single activation cluster located at the level of the supratemporal plane of the dominant hemisphere. These findings provide first evidence for a distinct contribution of the right cerebellar hemisphere to speech perception in terms of encoding of durational parameters of verbal utterances. Verbal working memory tasks, lexical response selection, and auditory imagery of word strings have been reported to elicit activation clusters of a similar location. Conceivably, representation of the temporal structure of speech sound sequences represents the common denominator of cerebellar participation in cognitive tasks acting on a phonetic code.

Acoustic Stimulation↗

PubMed2002

Mismatch responses to randomized gradient switching noise as reflected by fMRI and whole-head magnetoencephalography.

The central auditory system of the human brain uses a variety of mechanisms to analyze auditory scenes, among others, preattentive detection of sudden changes in the sound environment. Electroencephalography (EEG) and magnetoencephalography (MEG) provide a measure to monitor neuronal cortical currents. The mismatch negativity (MMN) or field (MMNm) reflect preattentive activation in response to deviants within a sequence of homogenous auditory stimuli. Functional magnetic resonance imaging (fMRI) allows for a higher spatial resolution as compared to the extracranial electrophysiological techniques. The image encoding gradients of echo planar imaging (EPI) sequences, however, elicit an interfering background noise. To circumvent this shortcoming, the present study applied multi-echo EPI mimicking an auditory oddball design. The gradient trains (SOA = 800 msec, 94.5 dB SPL, stimulus duration = 152 msec) comprised amplitude (-9 dB) and duration (76 msec) deviants in a randomized sequence. Moreover, the scanner noise was recorded and applied in a whole-head MEG device to validate the properties of this specific material. Robust fMRI activation patterns emerged in response to the deviant gradient switching. Changes in amplitude activated the entire auditory cortex, whereas the duration deviants elicited right-lateralized signal increase in secondary areas. The recorded scanner noise evoked reliably right-lateralized mismatch MEG responses. Source localization was in accordance with activation of secondary auditory cortex. The presented paradigm provides a robust and feasible tool to study the functional anatomy of early cognitive auditory processing in clinical populations such as schizophrenia.

Acoustic Stimulation↗

PubMed2002

Functional cerebral asymmetries of pitch processing during dichotic stimulus application: a whole-head magnetoencephalography study.

Dichotic listening (DL) studies indicate higher proficiency of the right cerebral hemisphere in processing the pitch of auditory events. Especially, acoustic stimuli of a rich harmonic structure such as square waves (complex tones) elicit a left ear advantage (LEA) under dichotic stimulus application. In order to investigate the timing of early sensory encoding at the level of the supratemporal plane, whole-head magnetoencephalography (MEG; 151 channels) recordings were performed in 20 right-handed subjects using an oddball paradigm based on dichotically applied complex tones. In contrast to electroencephalography (EEG) and event related potentials (ERP), this technique separately measures neuronal activity of left and right auditory cortex. Neuromagnetic responses were obtained both during preattentive stimulus processing, as well as during a pitch detection task. Rare stimuli presented to the left ear elicited a stronger magnetic analogue of mismatch negativity (MMNm) over both hemispheres and gave rise to shorter latencies of the contralateral mismatch fields than right ear deviants. In conclusion, the present data provide first evidence for functional laterality effects even at the level of preattentive pitch processing within the auditory cortex.

Acoustic Stimulation↗

PubMed2002

Hemispheric lateralization of the processing of consonant-vowel syllables (formant transitions): effects of stimulus characteristics and attentional demands on evoked magnetic fields.

It is still unsettled in how far temporal resolution of dynamic acoustic events (formant transitions) or phonetic/linguistic processes contribute to predominant left-hemisphere encoding of consonant-vowel syllables. To further elucidate the underlying mechanisms, evoked magnetic fields in response to consonant-vowel events (synthetic versus spoken) were recorded (oddball design: standards=binaural/ba/, deviants=dichotic/ba/-/da/; 20 right-handed subjects) under different attentional conditions (visual distraction versus stimulus identification). Spoken events yielded a left-lateralized peak phase of the mismatch field (MMF; 150-200ms post-stimulus onset) in response to right-ear deviants during distraction. By contrast, pre-attentive processing of synthetic items gave rise to a left-enhanced MMF onset (100ms), but failed to elicit later lateralization effects. In case of directed attention, synthetic deviants elicited a left-pronounced MMF peak resembling the pre-attentive response to natural syllables. These interactions of MMF asymmetry with signal structure and attentional load indicate two distinct successive left-lateralization effects: signal-related operations and representation of 'phonetic traces'. Furthermore, a right-lateralized early MMF component (100ms) emerged in response to natural syllables during pre-attentive processing and to synthetic stimuli in case of directed attention. Conceivably, these effects indicate right hemisphere operations prior to phonetic evaluation such as periodicity representation. Two distinct time windows showed correlations between dichotic listening performance and ear effects on magnetic responses reflecting early gain factors (ca. 75ms post-stimulus onset) and binaural fusion strategies (ca. 200ms), respectively. Finally, gender interacted with MMF lateralization, indicating different processing strategies in case of artificial speech signals.

Adult↗