Josef Halámek

Variability of Phase Shift Between Blood Pressure and Heart Rate Fluctuations A Marker of Short-Term Circulation Control

Background We postulated that the variability of the phase shift between blood pressure and heart rate fluctuation near the frequency of 0.10 Hz might be useful in assessing autonomic circulatory control. Methods and Results We tested this hypothesis in 4 groups of subjects: 28 young, healthy individuals; 13 elderly healthy individuals; 25 patients with coronary heart disease; and 19 patients with a planned or implanted cardioverterdefibrillator (ICD recipients). Data from 5 minutes of free breathing and at 2 different, controlled breathing frequencies (0.10 and 0.33 Hz) were used. Clear differences (P<0.001) in variability of phase were evident between the ICD recipients and all other groups. Furthermore, at a breathing frequency of 0.10 Hz, differences in baroreflex sensitivity (P<0.01) also became evident, even though these differences were not apparent at the 0.33‐Hz breathing frequency. Conclusions—The frequency of 0.10 Hz represents a useful and potentially important one for controlled breathing, at which differences in blood pressure‐RR interactions become evident. These interactions, whether computed as a variability of phase to define stability of the blood pressure‐heart rate interaction or defined as the baroreflex sensitivity to define the gain in heart rate response to blood pressure changes, are significantly different in patients at risk for sudden arrhythmic death. In young versus older healthy individuals, only baroreflex gain is different, with the variability of phase being similar in both groups. These measurements of short‐term circulatory control might help in risk stratification for sudden cardiac death. (Circulation. 2003;108:292‐297.)

Interictal high-frequency oscillations indicate seizure onset zone in patients with focal cortical dysplasia.

Interictal high-frequency oscillations (HFO) were recently identified in recordings from depth macroelectrodes in epileptic patients. StereoEEG (SEEG) recordings were analyzed in four patients with medically intractable partial seizures due to focal cortical dysplasia type IIA. Characteristics of HFO within seizure onset zone (SOZ), irritative zone, and remote brain areas were investigated. Whilst the rate of occurrence for ripples (80-200 Hz) was significantly higher in recordings from within than outside the SOZ, the rate of fast ripples (200-450 Hz) was less reliable index of SOZ. Interestingly, the mean powers across subjects were significantly higher within than outside the SOZ in both ripple and fast ripple frequency ranges. Our study demonstrates a capacity of interictal HFO to detect the SOZ in focal cortical dysplasias.

Executive functions processed in the frontal and lateral temporal cortices: intracerebral study.

OBJECTIVE The study was designed to investigate the neurocognitive network in the frontal and lateral temporal cortices that is activated by the complex cognitive visuomotor tasks of letter writing. METHODS Eight epilepsy surgery candidates with implanted intracerebral depth electrodes performed two tasks involving the writing of single letters. The first task consisted of copying letters. In the second task, the patients were requested to write any other letter. The cognitive load of the second task was increased mainly by larger involvement of the executive functions. The task-related ERD/ERS of the alpha, beta and gamma rhythms was studied. RESULTS The alpha and beta ERD as the activational correlate of writing of single letters was found in the sensorimotor cortex, anterior cingulate, premotor, parietal cortices, SMA and the temporal pole. The alpha and beta ERD linked to the increased cognitive load was present moreover in the dorsolateral and ventrolateral prefrontal cortex, orbitofrontal cortex and surprisingly also the temporal neocortex. Gamma ERS was detected mostly in the left motor cortex. CONCLUSIONS Particularly the temporal neocortex was activated by the increased cognitive load. SIGNIFICANCE The lateral temporal cortex together with frontal areas forms a cognitive network processing executive functions.

Ictal and peri-ictal oscillations in the human basal ganglia in temporal lobe epilepsy

Preictal, ictal, and postictal oscillations in the basal ganglia were analyzed. Five persons with temporal lobe epilepsy who were candidates for surgery had diagonal depth electrodes implanted in the basal ganglia: four of them in the putamen, and one in the pallidum and caudate. Time-frequency and power spectral analyses were used to analyze the EEG. Significant frequency components of 2-10 Hz were consistently observed in the basal ganglia. The frequency of this component slowed during seizures. There was a significant ictal increase in power spectral density in all frequency ranges. The changes in the basal ganglia were consistent while seizure activity spread over the cortex, and partially persisted after the clinical seizure ended. They were inconsistent in the period after seizure onset. Seizures originating in the mesiotemporal structures can affect physiological rhythms in the basal ganglia. The basal ganglia did not generate epileptiform EEG activity. An inhibitory role for the basal ganglia during temporal lobe seizures is suggested.

High‐resolution digital quadrature detection

We illustrate the principles of digital quadrature detection and call attention to its various benefits (ghost‐free spectra and high immunity to low‐frequency interference) and its intrinsic capability of generating data sets with different aliasing behaviors. A function describing the filtering efficiency is introduced, and the digital filters of our detector are compared with their analog counterparts of conventional nuclear magnetic resonance spectrometers. With an appropriate analog‐to‐digital converter (ADC), our digital detector has a dynamic range which is essentially limited by the analog noise, and increases when the spectral bandwidth is reduced. These nearly ideal performances are achieved through dithering, which randomizes the quantization error and oversampling, which reduces the quantization noise in the band of interest. We introduce a ‘‘figure of merit’’ for AD converters which estimates the noise performances of ADCs, and allows to compare products which achieve different compromises between speed and accuracy. The distortions due to the nonlinearities of the ADCs are analyzed through simulations. We find that the majority of the spurious signals (i.e., the errors other than noise) occur outside the band of interest, and are disposed through digital filtering. An unexpected result of the simulation is that, in some circumstances (e.g., large‐scale narrowband dithering), an increase in the number of bits of the ADC may actually reduce the distortion‐free dynamic range. In Sec. VIII we analyze practical problems like the role of the aperture jitter and the selection of the sampling frequency.

Neural correlates of affective picture processing — A depth ERP study

Using functional neuroimaging techniques (PET and fMRI), various cortical, limbic, and paralimbic structures have been identified in the last decade as neural substrates of human emotion. In this study we used a novel approach (intracerebral recordings of event-related potentials) to add to our knowledge of specific brain regions involved in affective picture processing. Ten intractable epileptic patients undergoing pre-surgical depth electrode recording viewed pleasant, neutral, and unpleasant pictures and intracerebral event-related potentials (ERPs) were recorded. A total of 752 cortical and subcortical sites were investigated. Significant differences in ERPs to unpleasant as compared to neutral or pleasant pictures were frequently and consistently observed in recordings from various brain areas--the mesial temporal cortex (the amygdala, the hippocampus, the temporal pole), the lateral temporal cortex, the mesial prefrontal cortex (ACC and the medial frontal gyrus), and the lateral prefrontal cortex. Interestingly, the mean latencies of responses to emotional stimuli were somewhat shorter in the frontal lobe structures (with evidently earlier activation within lateral prefrontal areas when compared to mesial prefrontal cortex) and longer in the temporal lobe regions. These differences, however, were not significant. Additional clearly positive findings were observed in some rarely investigated regions--in the posterior parietal cortex, the precuneus, and the insula. An approximately equivalent number of positive findings was revealed in the left and right hemisphere structures. These results are in agreement with a multisystem model of human emotion, distributed far beyond the typical limbic system and substantially comprising lateral aspects of both frontal lobes as well.

SIGNAL PROCESSING IN TRANSCEIVERS FOR NUCLEAR MAGNETIC RESONANCE AND IMAGING

The relationships between signal processing schemes and expected performances of several types of NMR transceiver are discussed to show how emerging digital technologies may be used in future systems. We identify a transceiver architecture where accuracy of the analog signal processing functions is not critical, and top performances are obtained through real‐time digital signal processing. Our receiver, comprising an oversampling analog‐to‐digital converter scheme, achieves a dynamic range of 12 bits at bandwidths above 1 MHz, and a dynamic range in excess of 16 bits at bandwidths <15 kHz. We give an outline of specifications and hardware design issues of the main parts of this transceiver and demonstrate experimentally the superior performances of our receiver.

Dynamic coupling between heart rate and ventricular repolarisation

Abstract A novel model for the coupling between ventricular repolarisation and heart rate (QT/RR) is presented. It is based upon a transfer function (TRF) formalism that describes the static and dynamic properties of this coupling, i.e., the behaviour after a sudden change in heart rate. Different TRF models were analysed by comparing their capability to describe experimental data collected from 19 healthy volunteers using several RR stimulation protocols: (i) rest with deep breathing at 0.1 Hz; (ii) tilt with controlled breathing at 0.1 and 0.33 Hz; and (iii) cycling. A search for the best TRF led to unambiguous identification of a three-parameter model as the most suitable descriptor of QT/RR coupling. Compared with established static models (linear or power-law), our model predictions are substantially closer to the experimental results, with errors ∼50% smaller. The shape of the frequency and step responses of the TRF presented is essentially the same for all subjects and protocols. Moreover, each TRF may be uniquely identified by three parameters obtained from the step response, which are believed to be of physiological relevance: (i) gain for slow RR variability; (ii) gain for fast RR variability; and (iii) time during which QT attains 90% of its steady-state value. The TRF successfully describes the behaviour of the RR control following an abrupt change in RR interval, and its parameters may offer a tool for detecting pharmacologically induced changes, particularly those leading to increased arrhythmogenic risk.

SignalPlant: an open signal processing software platform.

The growing technical standard of acquisition systems allows the acquisition of large records, often reaching gigabytes or more in size as is the case with whole-day electroencephalograph (EEG) recordings, for example. Although current 64-bit software for signal processing is able to process (e.g. filter, analyze, etc) such data, visual inspection and labeling will probably suffer from rather long latency during the rendering of large portions of recorded signals. For this reason, we have developed SignalPlant-a stand-alone application for signal inspection, labeling and processing. The main motivation was to supply investigators with a tool allowing fast and interactive work with large multichannel records produced by EEG, electrocardiograph and similar devices. The rendering latency was compared with EEGLAB and proves significantly faster when displaying an image from a large number of samples (e.g. 163-times faster for 75  ×  10(6) samples). The presented SignalPlant software is available free and does not depend on any other computation software. Furthermore, it can be extended with plugins by third parties ensuring its adaptability to future research tasks and new data formats.

On the Time Course of Synchronization Patterns of Neuronal Discharges in the Human Brain during Cognitive Tasks

Using intracerebral EEG recordings in a large cohort of human subjects, we investigate the time course of neural cross-talk during a simple cognitive task. Our results show that human brain dynamics undergo a characteristic sequence of synchronization patterns across different frequency bands following a visual oddball stimulus. In particular, an initial global reorganization in the delta and theta bands (2–8 Hz) is followed by gamma (20–95 Hz) and then beta band (12–20 Hz) synchrony.