[Intraoperative neurophysiological monitoring improves outcome in neurosurgery]. / Intraoperatives neurophysiologisches Monitoring verbessert das Outcome in der Neurochirurgie.

Intraoperative Neurophysiological Mo-nitoring (IONM) identifies eloquent areas or nerves fibers during neurosurgical interventions and monitors their function. For several interventions IONM has become mandatory in neurosurgery. IONM increases patient safety during surgery as the risk of neurological deficits is reduced. Safer surgery reduces the time needed for the intervention and thereby reduces risk. IONM contributes to complete resection of tumors, which in turn prolongs patients' survival. Complicated surgical interventions associated with an elevated risk of neurological deficits have only become possible due to IONM. IONM comprises a variety of procedures that are selected for a particular intervention. With appropriate selection of the procedures IONM has been shown to improve neurological and functional outcome after neurosurgical interventions.

Thalamocortical theta coherence in neurological patients at rest and during a working memory task.

We simultaneously recorded the local field potential (LFP) in the thalamus and the electroencephalogram (EEG) on the scalp of 5 patients suffering from neurogenic pain, epilepsy and movement disorders. In an earlier study [], we have investigated the slowing of EEG and the high thalamocortical coherence in the framework of thalamocortical dysrhythmia, the common underlying pathophysiology. The current study focuses on the effects of different cognitive conditions. When patients rested with eyes closed, a theta peak dominated the EEG spectra. The peak height was reduced upon opening the eyes, reminiscent of the classical alpha blocking. This peak reduction also appeared in the thalamic LFP recording. When patients activated their working memory by counting backwards, the theta peak increased in scalp EEG or in the LFP recorded in thalamic nuclei VA/VL. The coherence estimates between EEG and LFP ranged between 21% and 76% for different patients and cognitive conditions (mean: 50%). The involvement of both cortex and thalamus in working memory and the high thalamocortical coherence underline, in addition to cortico-cortical interactions, the importance of thalamocortical modules in the generation of higher cognitive functions.

Different frequencies for different scales of cortical integration: from local gamma to long range alpha/theta synchronization.

Cortical activity and perception are not driven by the external stimulus alone; rather sensory information has to be integrated with various other internal constraints such as expectations, recent memories, planned actions, etc. The question is how large scale integration over many remote and size-varying processes might be performed by the brain. We have conducted a series of EEG recordings during processes thought to involve neuronal assemblies of varying complexity. While local synchronization during visual processing evolved in the gamma frequency range, synchronization between neighboring temporal and parietal cortex during multimodal semantic processing evolved in a lower, the beta1 (12-18 Hz) frequency range, and long range fronto-parietal interactions during working memory retention and mental imagery evolved in the theta and alpha (4-8 Hz, 8-12 Hz) frequency range. Thus, a relationship seems to exist between the extent of functional integration and the synchronization-frequency. In particular, long-range interactions in the alpha and theta ranges seem specifically involved in processing of internal mental context, i.e. for top-down processing. We propose that large scale integration is performed by synchronization among neurons and neuronal assemblies evolving in different frequency ranges.

Synchronization between temporal and parietal cortex during multimodal object processing in man.

A series of recordings in cat visual cortex suggest that synchronous activity in neuronal cell ensembles serves to bind the different perceptual qualities belonging to one object. We provide evidence that similar mechanisms seem also to be observable in human subjects for the representation of supramodal entities. Electroencephalogram (EEG) was recorded from 19 scalp electrodes (10/20 system) in 19 human subjects and EEG amplitude and coherence were determined during presentation of objects such as house, tree, ball. Objects were presented in three different ways: in a pictorial presentation, as spoken words and as written words. In order to find correlates of modality-independent processing, we searched for patterns of activation common to all three modalities of presentation. The common pattern turned out to be an increase of coherence between temporal and parietal electrodes in the 13-18 Hz beta1 frequency range. This is evidence that population activity of temporal cortex and parietal cortex shows enhanced coherence during presentation of semantic entities. Coherent activity in this low-frequency range might play a role for binding of multimodal ensembles.

Synchronization between prefrontal and posterior association cortex during human working memory.

We measured coherence between the electroencephalogram at different scalp sites while human subjects performed delayed response tasks. The tasks required the retention of either verbalizable strings of characters or abstract line drawings. In both types of tasks, a significant enhancement in coherence in the theta range (4-7 Hz) was found between prefrontal and posterior electrodes during 4-s retention intervals. During 6-s perception intervals, far fewer increases in theta coherence were found. Also in other frequency bands, coherence increased; however, the patterns of enhancement made a relevance for working memory processes seem unlikely. Our results suggest that working memory involves synchronization between prefrontal and posterior association cortex by phase-locked, low frequency (4-7 Hz) brain activity.

Nonlinear analysis of epileptic activity in rabbit neocortex.

We report on the nonlinear analysis of electroencephalogram (EEG) recordings in the rabbit visual cortex. Epileptic seizures were induced by local penicillin application and triggered by visual stimulation. The analysis procedures for nonlinear signals have been developed over the past few years and applied primarily to physical systems. This is an early application to biological systems and the first to EEG data. We find that during epileptic activity, both global and local embedding dimensions are reduced with respect to nonepileptic activity. Interestingly, these values are very low (dE approximately equal to 3) and do not change between preictal and tonic stages of epileptic activity, also the Lyapunov dimension remains constant. However, between these two stages the manifestations of the local dynamics change quite drastically, as can be seen, e.g., from the shape of the attractors. Furthermore, the largest Lyapunov exponent is reduced by a factor of about two in the second stage and characterizes the difference in dynamics. Thus, the occurrence of clinical symptoms associated with the tonic seizure activity seems to be mainly related to the local dynamics of the nonlinear system. These results thus seem to give a strong indication that the dynamics remains much the same in these stages of behavior, and changes are due to alterations in model parameters and consequent bifurcations of the observed orbits.

Persistent patterns of brain activity: an EEG coherence study of the positive effect of music on spatial-temporal reasoning.

Motivated by predictions from the structured trion model of the cortex, behavioral experiments have demonstrated a causal short-term enhancement of spatial-temporal reasoning in college students following exposure to a Mozart sonata, but not in control conditions. The coherence analysis of electroencephalogram (EEG) recordings is well suited to the neurophysiological investigation of this behavioral enhancement. Here we report the presence of right frontal and left temporo-parietal coherent activity induced by listening to Mozart which carried over into the spatial-temporal tasks in three of our seven subjects. This carry-over effect was compared to EEG coherence analysis of spatial-temporal-tasks after listening to text. We suggest that these EEG coherence results provide the beginnings of understanding of the neurophysiological basis of the causal enhancement of spatial-temporal reasoning by listening to specific music. The observed long-lasting coherent EEG pattern might be evidence for structured sequences in cortical dynamics which extend over minutes.

Origin of the High-Frequency Doublet in the Vibrational Spectrum of Vitreous SiO2

The vibrational properties of amorphous SiO2 were studied within first-principles density functional theory. The calculated spectrum is in good agreement with neutron data, showing, in particular, a double peak in the high-frequency region. This doublet results from different local modes of the tetrahedral subunits and cannot be ascribed to a longitudinal-optic-transverse-optic (LO-TO) effect. This solves a long-standing controversy about the origin of the doublet in neutron spectra. A LO-TO splitting is recovered only when the long-wavelength limit is probed, as in optical experiments. These findings should be a general feature of tetrahedral AX2 amorphous networks.