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Alfons Schnitzler

Publications and source records attributed to Alfons Schnitzler.

45 records · Page 3Linked to original sources
PubMed2003

The cerebral oscillatory network of voluntary tremor.

It has recently been shown that resting tremor in Parkinson's disease is associated with oscillatory neural coupling in an extensive cerebral network comprising a cerebello-diencephalic-cortical loop and cortical motor, somatosensory and posterior parietal areas contralateral to the tremor hand. The aim of the present study was to investigate whether this oscillatory brain network exclusively reflects a pathophysiological state in parkinsonian resting tremor or whether it constitutes a fundamental feature of physiological motor control. We investigated cerebro-muscular and cerebro-cerebral coupling in 11 healthy subjects imitating typical antagonistic parkinsonian tremor. We recorded brain activity with a 122-channel whole-head neuromagnetometer and surface EMGs of the forearm extensor. Analysis of cerebro-muscular and cerebro-cerebral coherence revealed oscillatory coupling in the same brain structures that comprise the oscillatory network of parkinsonian resting tremor. Interestingly, similar to parkinsonian resting tremor, cerebro-cerebral coherences often showed a significant peak at twice the simulated tremor frequency. The most striking differences between parkinsonian patients, as investigated in a previous study and healthy subjects imitating the antagonistic resting tremor were a reduction of the coupling between primary sensorimotor cortex and a diencephalic structure--most likely the thalamus--and an enhancement of the coupling between premotor and primary sensorimotor cortex. Our results indicate that the coupling of oscillatory activity within a cerebello-diencephalic-cortical loop constitutes a basic feature of physiological motor control. Thus, our data are consistent with the hypothesis that parkinsonian resting tremor involves oscillatory cerebro-cerebral coupling in a physiologically pre-existing network.

Adult↗
PubMed2003

Cortical activations associated with auditorily paced finger tapping.

We investigated neuromagnetic responses during an auditorily paced synchronization task using a 122-channel whole-head neuromagnetometer. Eight healthy right handed subjects were asked to synchronize left and right unilateral finger taps to a regular binaural pacing signal. Synchronization of the right hand with an auditory pacing signal is known to be associated with three tap-related neuromagnetic sources localized in the contralateral primary sensorimotor cortex. While the first source represents the neuromagnetic correlate of the motor command the second one reflects somatosensory feedback due to the finger movement. The functional meaning of the third source, which is also localized in the primary somatosensory cortex is still unclear. On the one hand this source represents a neuromagnetic correlate of somatosensory feedback due to the finger tap. On the other hand it has been suggested that the function of this source could additionally represent a cognitive process, which enables the subject to monitor the time distance between taps and clicks. The aim of the present study was to elucidate the function of this source, which would fundamentally reform the meaning of the primary somatosensory cortex in the timing of movements with respect to external events. The data of the present study demonstrate that the three sources in the contralateral sensorimotor cortex are stronger related to the tap than to the click. This result contradicts the assumption of a cognitive process localized in the primary somatosensory cortex. Thus, activation in the primary somatosensory cortex most likely represents exclusively somatosensory feedback and no further cognitive processes.

Acoustic Stimulation↗
PubMed2003

The cerebral oscillatory network of parkinsonian resting tremor.

Data from experiments in MPTP monkeys as well as from invasive and non-invasive recordings in patients with Parkinson's disease suggest an abnormal synchronization of neuronal activity in the generation of resting tremor in Parkinson's disease. In six patients with tremor-dominant idiopathic Parkinson's disease, we recorded simultaneously surface electromyograms (EMGs) of hand muscles, and brain activity with a whole-head magnetoencephalography (MEG) system. Using a recently developed analysis tool (Dynamic Imaging of Coherent Sources; DICS), we determined cerebro-muscular and cerebro-cerebral coherence as well as the partial coherence between cerebral areas and muscle, and localized coherent sources within the individual MRI scans. The phase lag between the EMG and cerebral activity was determined by means of a Hilbert transform of both signals. After overnight withdrawal from medication, patients showed typical Parkinson's disease resting tremor (4-6 Hz). This tremor was associated with strong coherence between the EMG of forearm muscles and activity in the contralateral primary motor cortex (M1) at tremor frequency but also at double tremor frequency. Phase lags between M1 activity and EMG were between 15 and 25 ms (M1 activity leading) at single, but also at double tremor frequency, corresponding well to the corticomuscular conduction time. Furthermore, significant coherence was observed between M1 and medial wall areas (cingulate/supplementary motor area; CMA/SMA), lateral premotor cortex (PM), diencephalon, secondary somatosensory cortex (SII), posterior parietal cortex (PPC) and the contralateral cerebellum at single tremor and, even stronger at double tremor frequency. Spectra of coherence between thalamic activity and cerebellum as well as several brain areas revealed additional broad peaks around 20 Hz. Power spectral analysis of activity in all central areas indicated the strongest frequency components at double tremor frequency. Partial coherence analysis and the calculation of phase shifts revealed a strong bidirectional coupling between the EMG and diencephalic activity and a direct afferent coupling between the EMG and SII and the PPC. In contrast, the cerebellum, SMA/CMA and PM show little evidence for direct coupling with the peripheral EMG but seem to be connected with the periphery via other cerebral areas (e.g. M1). In summary, our results demonstrate tremor-related oscillatory activity within a cerebral network, with abnormal coupling in a cerebello-diencephalic-cortical loop and cortical motor (M1, SMA/CMA, PM) and sensory (SII, PPC) areas contralateral to the tremor hand. The main frequency of cerebro-cerebral coupling corresponds to double the tremor frequency.

Adult↗
PubMed2002

Reduced inhibition within primary motor cortex in patients with poststroke focal motor seizures.

BACKGROUND: Following an ischemic brain lesion, the affected cortex undergoes structural and functional changes that may lead to increased cortical excitability or decreased inhibitory neuronal activity, resulting in the occurrence of poststroke epileptic seizures in 6 to 10% of patients with stroke. METHODS: To assess motor cortical excitability, transcranial magnetic stimulation (TMS) was used to determine the silent period (SP) duration in 84 consecutive patients with ischemic stroke. RESULTS: In a subpopulation of six patients (38 to 72 years old) a significant decrease of the SP duration (mean 116 +/- 14 msec) was detected in either the arm or the leg on the affected side as compared to the corresponding unaffected limb (mean 231 +/- 32 msec). This electrophysiologic abnormality was clinically associated with focal motor seizures in five of the six patients, whereas none of the other 76 patients with normal or prolonged SP durations developed seizures or epilepsy. CONCLUSIONS: Silent period shortening in this group reflects decreased inhibitory activity that may partly be related to functional or structural impairment of GABAergic interneurons. TMS may be of value for determining patients with stroke at risk for developing poststroke seizures.

Adult↗
PubMed2002

Cortical representation of first and second pain sensation in humans.

Single painful stimuli evoke two successive and qualitatively distinct sensations referred to as first and second pain sensation. Peripherally, the neural basis of this phenomenon is a dual pathway for pain with Adelta and C fibers mediating first and second pain, respectively. Yet, the differential cortical correlates of both sensations are largely unknown. We therefore used magnetoencephalography to record and directly compare first and second pain-related cortical responses to cutaneous laser stimuli in humans. Our results show that brief painful stimuli evoke sustained cortical activity corresponding to sustained pain perception comprising early first pain-related and late second pain-related components. Cortical activity was located in primary (S1) and secondary (S2) somatosensory cortices and anterior cingulate cortex. Time courses of activations disclosed that first pain was particularly related to activation of S1 whereas second pain was closely related to anterior cingulate cortex activation. Both sensations were associated with S2 activation. These results correspond to the different perceptual characteristics of both sensations and probably reflect different biological functions of first and second pain. First pain signals threat and provides precise sensory information for an immediate withdrawal, whereas second pain attracts longer-lasting attention and motivates behavioral responses to limit further injury and optimize recovery.

Adult↗
PubMed2002

Rapid mapping of finger representations in human primary somatosensory cortex applying neuromagnetic steady-state responses.

We analyzed somatosensory evoked steady-state fields in order to localize finger representations in the hand area of the primary somatosensory cortex (S1). Using a 122-channel whole-head neuromagnetometer we recorded in six healthy subjects neuromagnetic responses to high frequency electrical stimuli delivered simultaneously to digit I, II, III and V at 22, 24, 27 and 30 Hz, respectively, and to transient stimulation of each single digit with a frequency of 3 Hz. Responses were averaged separately for each digit and were modeled by single equivalent current dipoles. Both conditions yielded the typical somatotopic finger representations within S1 hand area. Dipole locations did not differ significantly between the transient and the steady-state stimulation. Therefore, simultaneous high-frequency stimulation of the digits seems to be a reliable method for rapid and detailed mapping of the S1 hand area. This procedure has potential advantages over recording of transient responses. With simultaneous steady-state stimulation the measurement times are reduced to 2 min for mapping the whole hand area. Because of this our method probably increases spatial accuracy and permits repeated short interval recordings, e.g. in experiments studying short term plasticity.

Adult↗
PubMed2002

Critical flicker frequency for quantification of low-grade hepatic encephalopathy.

Subclinical hepatic encephalopathy (SHE) is currently diagnosed by psychometric tests or neurophysiologic techniques. In view of its sociomedical relevance, simple and reproducible tests for routine diagnosis are required. This study evaluates critical flicker-frequency thresholds for quantification of low-grade hepatic encephalopathy. A total of 115 patients (92 with cirrhosis, 23 controls) were analyzed for HE severity (mental state, computerized psychometric tests), and the threshold frequencies at which light pulses are perceived as fused (fusion frequency) or flickering light (critical flicker frequency [CFF]). CFF was a highly reproducible parameter with little age, day-time, and training dependency. CFFs in cirrhotic patients without HE (HE 0) were not different from those found in noncirrhotic controls. Significantly lower CFFs were found in cirrhotic patients with subclinical or manifest HE, and the various HE groups separated from each other at a high level of significance (P <.01). By using a CFF cut-off value of 39 Hz, a 100% separation of patients with manifest HE from noncirrhotic controls and HE 0 cirrhotic patients was obtained. SHE patients separated from HE 0 cirrhotic patients with high sensitivity (55%) and specificity (100%). The HE severity-dependent differences were found in both, alcoholic and posthepatitic cirrhosis. Statistically significant correlations (P <.01) were found between CFFs and individual psychometric tests. Aggravation of preexisting HE after transjugular intrahepatic portosystemic stent shunt (TIPS) implantation was accompanied by a corresponding decrease of CFF, whereas improvement of HE increased CFF. In conclusion, CFF is a sensitive, simple, and reliable parameter for quantification of low-grade HE severity in cirrhotic patients and may be useful for the detection and monitoring of SHE.

Aged↗
PubMed2002

Cortical representation of venous nociception in humans.

Painful sensations can be evoked by application of thermal, mechanical, and chemical stimuli to the blood vessels. The cortical substrates of these sensations are unknown. We therefore used whole-head magnetoencephalography to record cortical responses to painful laser stimuli applied cutaneously and intravenously to the dorsum of the hand in healthy human subjects. Similar to the cutaneous stimuli, venous stimulation nearly simultaneously activated the contralateral primary and the bilateral secondary somatosensory cortices. In the venous stimulation condition, all activation peaks were about 50 ms earlier than in the cutaneous stimulation condition. Locations of responses to both stimuli did not differ. These results show that the afferent volley from the veins reaches the cerebral cortex significantly earlier than that from the skin. This might be due to differences in peripheral conduction velocity. Apart from this, these findings demonstrate that venous nociception shares the cortical representation of cutaneous nociception in human somatosensory cortices. Thus the cortical representation of nociceptive processing from tissues of mesodermal and ectodermal origin appears to be similar.

Adult↗