J. P. C. de Weerd

A posteriori time-varying filtering of averaged evoked potentials

This paper forms a preface and introduction to a new method for the estimation of evoked potentials: a posteriori time-varying filtering. A simple evoked potential model, consisting of a transient signal and additive noise, is discussed and the underlying assumptions explicitly formulated. Assuming this model, the problem of estimating the signal from an ensemble is considered from the statistical and communication engineering point of view, along with a brief survey of the pertinent literature. It is explained why ensemble averaging, in general, does not provide the best estimate in the mean-square error sense. After a summary of the controversial aspects of timeinvariant “a posteriori ‘Wiener’ filtering”, it is indicated how that method can be generalized to a time-varying counterpart, which is able to handle the essentially transient character of evoked potential waveforms. Finally, the new method is presented on a conceptual level and its application illustrated by examples.

Solution Methods of Electrical Field Problems in Physiology

The forward problem in electrophysiology¿the computation of the potential distribution due to a known electrical source in a known volume conductor¿is discussed. Three methods of solution are considered: 1) the finite difference method 2) a discretized integral equation method 3) the analytic method.

Theory and practice of a posteriori “Wiener” filtering of average evoked potentials

In this study a theoretical and practical analysis of the technique of a posteriori “Wiener” filtering of averaged evoked potentials is presented. It is shown that spectral smoothing plays a central role in obtaining a reliable estimate. Attention is paid to some practical problems that arise when the method is applied in discrete time. In an illustrative case study results are compared to theoretical Wiener filtering, while the influence of slow amplitude modulation has also been investigated. Some of the conflicting results regarding the method that have been reported in several recent papers are discussed. It is concluded, that when the method is applied with prudence, it may lead to an improved estimation of those evoked potentials that can be assumed to be approximately stationary. If the latter assumption is not valid, application of a time-varying filter technique is recommended instead.

Spectro-Temporal Representations and Time-Varying Spectra of Evoked Potentials

Evoked potential waveforms are generally of a dynamic, transient character. Consequently, their spectral energy distribution cannot be adequately described by time-invariant representations, such as the power density spectrum. Obviously, aspectro-temporal description is needed. Appropriate means for obtaining such descriptions are discussed, on the basis of theoretical considerations concerning simultaneous time-frequency representations and methods ofshort-time spectral analysis. With reference to the “uncertainty principle”, particular attention is paid to time-bandwidth products of various filter types, used in relation with the latter technique. It is concluded that the method of bandpass filtering with proportional bandwidth filters, having cosine transfer functions, arises as a suitable solution in evoked potential analysis. The results of applying this method to somatosensory, visual, and brainstem auditory evoked potentials are presented.

Accuracy of QRS detection in relation to the analysis of high-frequency components in the electrocardiogram

The detection of high-frequency components in the QRS complex by means of coherent signal averaging is affected by inaccuracy in the time reference. Jitter of the time reference or trigger, which is derived from the low-frequency QRS complex, will be caused by the noise in the QRS complex. A theory is developed by which the trigger jitter can be predicted from the properties of signal and noise for a system consisting of a filter and a single-level, dual-level or peak detector. The theory is applicable when the noise is additive and, under certain conditions, also when the noise is multiplicative. Using this theory the trigger jitter of a given filter-level detection system is compared with that of an optimal detection system consisting of a matched filter and peak detector. The theoretical trigger jitter of the above-mentioned detectors has been computer for e.c.g. recordings of 23 individuals, as a function of different filter settings and and with the assumption that the noise was additive. This resulted in an average trigger jitter of 0·2±0·1 ms for the optimal system, while for the peak detector and the dual level detector the jitter was slightly higher provided that the QRS complexes were symmetrical after filtering. With the effects of ventilation taken into account (multiplicative noise) it is shown that dual-level detection is considerably more accurate than single-level detection. A description of the dual-level detector is also presented.

Modelling compound action potentials of peripheral nerves in situ. II. A study of the influence of temperature

Compound nerve action potentials of sural nerves of healthy volunteers have been measured at different temperatures. In the range from 21 to 39 degrees C a linear temperature dependence of the propagation velocity, related to the fastest fibres, is found (slope 1.9 m/sec/degree C). The causes of amplitude and shape alterations in the compounds action potentials (CAPs) with changing temperature are analysed with the help of a model. It appears that the changes in the measured CAPs are well explained by known temperature effects on the single fibre level. Both the considerable increase in CAP duration after cooling and the relative constancy of the CAP amplitude were reproduced by the model. Close fits to the actually measured CAPs can be obtained when some crucial parameter values are optimized. The results of this procedure are presented.

The Discriminating Role of EEG Power Spectra in Dyslexic Children

SummaryThe aim of this investigation is to compare EEG power-density spectra between dyslexic and normal children. The children have been selected on the basis of specific reading and writing tasks; those with severe organic diseases and mental disorders were excluded.Groups aged 8, 9 and 10 years have been compared. In the 8-year-old group the power in the alpha band is higher in normals, in the 9-year-old group there was a higher power of the mu rhythm in normals and in the 10-year-old group the power in the temporal theta band is lower in normals. A possible relation between these findings and psychomotor development is discussed.ZusammenfassungDas Ziel der vorliegenden Untersuchung ist ein Vergleich der EEG-Breitbandfrequenzanalyse beim dyslexischen und beim normalen Kind. Die dyslexischen Kinder wurden aufgrund spezifischer Lese- und Schreibstörungen ausgesucht, wobei solche mit schweren organischen Hirnfunktionsstörungen oder anderen psychischen Anomalien ausgeschlossen wurden. Der Vergleich geschah zwischen zwei Gruppen von Kindern im Alter von 8, 9 und 10 Jahren.Bei den 8jährigen war die Spannung im Alphaband bei den Normalen höher, bei den 9jährigen erschien bei den Normalen die Spannung des mu-Rhythmus höher, und bei den 10jährigen war die Spannung im temporalen Thetaband bei den Normalen niedriger. Es wurde auf die möglichen Beziehungen zwischen diesen Feststellungen und der psychomotorischen Entwicklung in der Diskussion eingegangen.

On the optimal choice of a recording electrode in electroneurography

The influence of the type and size of recording electrodes on compound nerve action potentials was studied in order to establish optimal prerequisites for the application of quantitative analysis methods. A realistic volume conductor model was used to assess the dependence of the recorded signal on the position of the recording electrode with respect to the nerve. The analysis of both compound action potentials (CAPs) and single fibre action potentials (SFAPs) leads to the conclusion that near-nerve recording gives substantially more information. This applies in particular to the fine details in the CAP, such as notches in the main complex and the so-called late components. Subsequently, the influence of needle electrode bare tip length on the recorded signal was investigated by spatially averaging the potentials over the lead-off area. These results were combined with experimentally obtained noise levels for different stainless steel needle electrodes in order to determine the signal-to-noise ratio as a function of bare tip length. It was concluded that for near-nerve recording a needle tip of 1-2 mm gives optimal results in terms of wave shape preservation and signal-to-noise ratio.

Somatosensory evoked response in controlled A-alpha sensory fiber disease

SummaryIn contrast to the distal sensory response, the somatosensory evoked response (SSER) is detectable in patients with A-alpha sensory fiber diseases such as Friedreichs ataxia.In four patients with this disease a combined histological and electrophysiological investigation of sensory propagation was performed. The sural nerve was analyzed in terms of the various fiber diameters and the distal sensory and somatosensory evoked cortical responses recorded after sural and median nerve (finger) stimulation. A distinct evoked response could be detected with rather low amplitudes and retarded latency times, while in general no distal sensory response was found. The anatomical and physiological consequences are discussed.ZusammenfassungIm Gegensatz zu den sensiblen Nervenaktionspotentialen ist das Somato-sensible Evoked-Potential bei Patienten mit einer Friedreichschen Ataxie (eine A-alpha-sensible Nervenfasernerkrankung) gut zu erregen.Bei vier Patienten mit dieser Erkrankung haben wir eine histologische und elektrophysiologische Untersuchung vorgenommen. Die N.-suralis- und N.-ulnaris-sensiblen Nervenaktionspotentiale und Evoked Response sind analysiert worden. Kombiniert hiermit wurde eine Analyse der Faserdurchschnitte einer N.-suralis-Biopsie durchgeführt. Die anatomischen und physiologischen Konsequenzen sind diskutiert worden.

Modelling compound action potentials of peripheral nerves in situ. III. Nerve propagation in the refractory period.

Compound nerve action potentials (CAPs) of sural nerves of healthy persons were measured in the relative refractory period of the fibres. Double stimuli were used with interstimulus intervals ranging from 0.7 to 4.0 msec. The CAP latency increase, its amplitude decrease and its wave shape alterations with decreasing interstimulus interval are discussed, using a model description. Recovery functions for the single fibre action potential in the relative refractory period are defined. These recovery functions, however, cannot sufficiently explain the CAP amplitude diminutions actually measured. Apparently in the relative refractory period an increasing number of fibres ceases to propagate an action potential when the stimulus interval is shortened. This effect, which is caused by subthreshold stimulation, predominantly involves the slower fibres. The usual derivation of refractory period parameters in clinical practice is reviewed.