Klaus Mathiak

Principles of a brain-computer interface (BCI) based on real-time functional magnetic resonance imaging (fMRI)

A brain-computer interface (BCI) based on functional magnetic resonance imaging (fMRI) records noninvasively activity of the entire brain with a high spatial resolution. We present a fMRI-based BCI which performs data processing and feedback of the hemodynamic brain activity within 1.3 s. Using this technique, differential feedback and self-regulation is feasible as exemplified by the supplementary motor area (SMA) and parahippocampal place area (PPA). Technical and experimental aspects are discussed with respect to neurofeedback. The methodology now allows for studying behavioral effects and strategies of local self-regulation in healthy and diseased subjects.

Enhancement of BOLD-contrast sensitivity by single-shot multi-echo functional MR imaging.

Improved data acquisition and processing strategies for blood oxygenation level‐dependent (BOLD)‐contrast functional magnetic resonance imaging (fMRI), which enhance the functional contrast‐to‐noise ratio (CNR) by sampling multiple echo times in a single shot, are described. The dependence of the CNR on T2*, the image encoding time, and the number of sampled echo times are investigated for exponential fitting, echo summation, weighted echo summation, and averaging of correlation maps obtained at different echo times. The method is validated in vivo using visual stimulation and turbo proton echoplanar spectroscopic imaging (turbo‐PEPSI), a new single‐shot multi‐slice MR spectroscopic imaging technique, which acquires up to 12 consecutive echoplanar images with echo times ranging from 12 to 213 msec. Quantitative T2*‐mapping significantly increases the measured extent of activation and the mean correlation coefficient compared with conventional echoplanar imaging. The sensitivity gain with echo summation, which is computationally efficient provides similar sensitivity as fitting. For all data processing methods sensitivity is optimum when echo times up to 3.2 T2* are sampled. This methodology has implications for comparing functional sensitivity at different magnetic field strengths and between brain regions with different magnetic field inhomogeneities. Magn Reson Med 42:87–97, 1999.

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.

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 BodenBoten 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 Brocas area). The reversed contrast (VOT CLT) resulted in a single activation cluster located at the level of the supra-temporal 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.

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 speakers 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.

Discrimination of temporal information at the cerebellum: functional magnetic resonance imaging of nonverbal auditory memory

Until recently, the cerebellum was held to play its chief role in motor control. By contrast, Keele and Ivry (1990) proposed that it may subserve time estimation within the perceptual domain as well. In accordance with this suggestion, speech perception requiring minute differentiation of time intervals was found compromised by cerebellar pathology a subsequent functional magnetic resonance imaging (fMRI) study found hemodynamic activation of the right neocerebellum under these conditions. In the current fMRI investigation a non-speech task involving duration storage and comparison yielded significant hemodynamic responses within the lateral Crus I area of the right cerebellar hemisphere. Concomitantly, a left prefrontal cluster was observed. The present fMRI study employed single-shot double-echo echo-planar imaging (EPI) to reduce image distortion and acquisition time with whole-brain coverage (TE = 28 and 66 ms, TR = 5 s, 28 slices, TA = 2.8 s). Twelve healthy subjects performed two tasks: identifying pauses between tones as short or long (30-130 ms) and deciding which of two successive pauses was longer. The activation pattern in the discrimination task was analogous to that seen during speech perception and verbal working memory (WM) tasks. We suggest that the storage of precise temporal structures relies on a cerebellar-prefrontal loop. This network allows for temporal organization of verbal sequences and phoneme encoding based on durational operations in a linguistic context.

Evaluation of motion and realignment for functional magnetic resonance imaging in real time.

Functional magnetic resonance imaging in real time is an emerging tool for the assessment of dynamic changes in brain activation. The short response latency (tens of seconds) renders the technique more sensitive to motion artifacts. Motion correction in real time requires computationally efficient algorithms which can be executed on a complete 3D data set within a single time of repetition cycle. In this study, a method to evaluate motion and realign functional images in real time implemented on standard imaging hardware is introduced. The detection of activity in correlation maps is improved, and artifactual edge enhancements are reduced. As the estimation of large movements is stable, this algorithm is attractive for clinical studies with uncooperative patients. Magn Reson Med 45:167–171, 2001.

Encoding of temporal speech features (formant transients) during binaural and dichotic stimulus application: a whole-head magnetencephalography study.

Spoken-word recognition depends upon the encoding of relevant information bearing elements of the acoustic speech signal. For example, relatively rapid shifts of spectral energy distribution (formant transients) cue the perception of stop consonant-vowel (CV) syllables such as /ba/, /ga/, and /da/. A variety of data indicate left-hemisphere superiority with respect to the processing of formant transients. To further delineate the underlying neurophysiological mechanisms, evoked cortical fields in response to CV syllables (oddball design; frequent stimulus=binaural /ga/; four deviant constellations: Binaural /ba/, binaural /da/, left /da/ (left ear deviant)-right /ga/, right /da/ (right ear deviant)-left /ga/) were recorded by means of whole-head magnetencephalography (MEG; 151 channels) under two different conditions of attentional demands (visual distraction versus reaction to prespecified stimuli). (a) During binaural stimulus presentation attention toward target events resulted in a significantly enhanced mismatch field (MMNm, magnetic analogue to the mismatch negativity) over the left as compared to the right hemisphere. In contrast, preattentive processing of the CV syllables failed MMNm lateralization effects. (b) Dichotic application of /da/ elicited a larger contralateral MMNm amplitude in subjects with right ear advantage (REA) at behavioral testing. In addition, right ear deviants yielded a stronger ipsilateral response than the left ear cognates. Taken together, these data indicate bilateral preattentive processing and subsequent attention-related predominant left-hemisphere encoding of formant transients at the level of the supratemporal plane. Furthermore, REA during dichotic application of CV syllables seems to be linked to functional dissociation of the two hemispheres during auditory processing.

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.

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.