Does filtering and smoothing of average evoked potentials really pay? A statistical comparison.

Averaging of sweeps to obtain evoked potentials provides an unsatisfactory reduction of the background activity for a small number of stimuli. A posteriori Wiener filtering, time varying filtering, and smoothing of the average EP have been proposed to meet this problem. As to a posteriori Wiener filtering, a controversy regarding its merits has been going on for several years. The present paper gives a statistical comparison of the above methods, based on real data of two groups of subjects (flash evoked potentials in 41 subjects, pattern reversal evoked potentials in 9 subjects). It is shown that most of the improvement of the filtering approaches was due to an attenuation effect, without any improvement in smoothness of the potentials. The strength of the attenuation introduced by the filtering approaches depended on the specific underlying signal-to-noise ratio. This effect led to an artificially enhanced interindividual variability and could intraindividually lead to biased topographical distribution, when several electrodes are considered. The smoothing method did not show this undesired feature, but, when applying strong smoothing, this method also rendered sizable distortions of the potentials.

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.

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.

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.

Modelling compound action potentials of peripheral nerves in situ. I. Model description: evidence for a non-linear relation between fibre diameter and velocity.

Compound sensory nerve action potentials of sural nerves of healthy volunteers have been measured at 7 different distances between the site of stimulation and the site of recording. The results are used to adjust a model description for the generation of compound nerve action potentials. A fairly good agreement between experiment and model is obtained when a weak non-linear, S-shaped relation between external fibre diameter and propagation velocity of the single fibre action potential is assumed. This result corresponds surprisingly well with findings in "classical' reconstruction attempts reported in the literature.

Relation between EEG and disability scores in multiple sclerosis.

A disability-scoring and an EEG-spectral analysis was performed in patients suffering from clinically and biochemically (oligoclonal aspect in cerebro-spinal fluid) proven multiple sclerosis at two stages of their illness, that are before and after a short intensive immuno suppressive therapy. In both stages a relationship is found between the degree of disability and the amount of beta activity in the fronto-central area and the amount of the theta activity in the temporal area. Significant improvement in the clinical state of the patient and a marked increase of the mean alpha frequency in the parietooccipital region could be demonstrated after short intensive immunosuppressive therapy.

Spectro-temporal representations and time-varying spectra of evoked potentials. A methodological investigation.

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, a spectro-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 of short-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.

A posteriori time-varying filtering of averaged evoked potentials. I. Introduction and conceptual basis.

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 he 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 time-invariant "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.

A posteriori time-varying filtering of averaged evoked potentials. II. Mathematical and computational aspects.

The problem of estimating an unknown transient signal, given an ensemble of waveforms, in which this signal appears as a nonrandom component in the presence of additive noise is considered. This problem is solved by generalizing the method of "a posteriori 'Wiener' filtering". In the new method, the ensemble average is filtered by a time-varying system which is based on estimated time-varying power spectra of signal and noise. The nature of this system, and the computational procedures involved, are discussed in detail. A software package for time-varying filtering is briefly described. Application of the method is illustrated by a simulation example, which also provides a comparison to time-invariant a posteriori "Wiener" filtering.

The discriminating role of EEG power spectra in dyslexic children.

The 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.

EEG spectra in children aged 8, 9 and 10 years. Reference values.

In a group of normal children aged 8, 9 and 10 years power density spectra of a 100-second EEG period were made over four different brain regions. All children had to undergo medical and psychological examinations prior to this EEG and the criteria for normality were based on these examinations. Reference values of the power density distribution for children between 8 and 10.5 years are given.

[The effects of ordinary provocation on the power density spectrum. A clinical electroencephalographic study (author's transl)]. / Die normalen Provokationen im Leistungsspektrum Eine klinisch-elektroenzephalographische Untersuchung.

In the search for methods which might increase the discrimination of the EEG, the power density spectra of a random sample of patients during different conditions were analysed. Power density spectra were made during eyes closed, eyes open, after hyperventilation, during photic stimulation, and during auditory stimulation. It proved that the spectra may change under different provocation tests, sometimes in a rather peculiar way. The conclusion was, that these analyses improve explicit the EEG diagnosis, a problem however is the time required for spectra interpretation.

A volume conductor study of compound action potentials of nerves in situ: the forward problem.

A computer model for the stimulation of compound nerve action potentials, based on superposition of volume conducted single nerve fibre potentials, is presented. The model assumes that the intracellular fibre potential, the fibre diameter distribution and the electrical conductivities of different tissues are known. Volume conductor fields are calculated in the spatial frequency domain. The influence of important parameters in the model is evaluated numerically. It is shown that it is necessary to give up the usual assumption of homogeneity and isotropy in the extracellular medium. In the present model parameters are introduced which allow an overall description of the complex morphological and physiological structure of the nerve trunk. Simulation results indicate that the model is a rather promising tool in studying the main properties of compound action potentials which up till now have not been sufficiently well understood.

Estimation of signal and noise spectra by special averaging techniques with application to a posteriori "Wiener" filtering.

This paper deals with the problem of separating the spectra of signal and noise in ensembles where the signal can be considered as an invariant component and the noise as a stationary additive background. Several methods are discussed and compared on the basis of a statistical analysis of the first two moments of the estimators for signal and noise spectra. As a consequence a procedure is proposed which provides a flexible compromise between estimation accuracy and computational effort. The application of this procedure to a posteriori "Wiener" filtering is compared with a more common, but time consuming, technique.

Reliability of Doppler sonography in extracranial cerebro-vascular stenosis.

Of our patients who underwent quantitative Doppler sonography (Haemato-Tacho-Graphy), 382 underwent carotid angiography and 61 vertebral artery angiography. The reliability of the method proved to be between 90 and 95 percent for carotid as well as for vertebral arteries. The HTG proved to be a useful addition to angiography in patients with extracranial cerebro-vascular stenosis.