Power spectrum and intracranial EEG patterns at seizure onset in partial epilepsy

Intracranial electroencephalographic patterns of seizure onset during preoperative assessment with intracerebral and subdural electrodes have been correlated with surgical outcome in 15 patients with partial epilepsy assessed for surgery. The presence and topography of electrodecremental events, high frequency activity, irregular sharp waves intermixed with slow activity, spike-wave activity and rhythmic ictal transformation at seizure onset were studied in a total of 78 complex partial seizures. Raw traces from intracerebral and subdural recordings were assessed visually in conjunction with changes in the following spectral variables (calculated for consecutive 1.28 sec epochs): amplitude (sum of amplitude of all components within a frequency band), activity, mobility, and complexity. The time course of these variables during preictal and ictal periods was displayed and assessed. This technique proved effective for detecting low-amplitude high-frequency activity and subtle electrodecremental events. It was concluded that: (a) most patients (12/15) showed early electrodecremental events, generalised or local, mainly involving frequencies below 40 Hz; (b) generalised electrodecremental events at onset did not imply poor outcome; (c) localised high-frequency activity, between 20 and 80 Hz, was associated with a good outcome.

Fractal analysis of electroencephalographic signals intracerebrally recorded during 35 epileptic seizures: evaluation of a new method for synoptic visualisation of ictal events

Traditional electroencephalography (EEG) produces a large volume display of brain electrical activity, which creates problems particularly in assessment of long periods of intracranial, stereoelectroencephalographic (SEEG) recording. A method for fractal analysis that describes 100 SEEG data points in terms of a single estimate of fractal dimension (1 < FD < 2) is reported; the central processing unit time costs amount to approximately 2 min/Mbyte of input signal (using a Sun SPARCstation LX). The diagnostic sensitivity of this method, applied to quantification and synoptic visualisation of SEEG signals recorded during 35 epileptic seizures in 7 patients, is evaluated. It is found that the method consistently defines ictal onset in terms of rapid relative increase in FD across several channels. Clinically severe seizures are characterised by more intense and generalised ictal changes in FD than clinically less severe events. For all 7 patients, and for 75% of individual seizures, "fractal diagnoses" of anatomically defined ictal onset zone coincided closely with ictal onset zone independently determined by inspection of traditional EEG displays of the same data. We conclude that the method is a computationally feasible way to achieve substantial reduction in the volume of SEEG data without undue loss of diagnostically important information in the primary signal.

Intracerebral propagation of interictal activity in partial epilepsy: implications for source localisation

The hypothesis that focal scalp EEG and MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The time course and spatial distribution of interictal activity recorded simultaneously by surface electrodes and by electrodes next to mesial temporal structures in 12 patients being assessed for epilepsy surgery have been studied to estimate the degree of confinement of neural activity present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of interictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlations between the amplitudes of deep and surface recordings, together with previous reports on the amplitude of scalp signals produced by artificially implanted dipoles suggest that the ratio of deep to surface activity recorded during interictal epileptiform activity on the scalp is around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but is generated in the underlying neocortex. Also, time delays of up to 220 ms recorded between interictal paroxysms at different recording sites show that interictal epileptiform activity can propagate neuronally within several milliseconds to relatively remote cortex. Large areas of archicortex and neocortex can then be simultaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres that trigger local phenomena which in turn give rise to sharp/slow waves or spikes, and (3) propagation along the neocortex. The low ratio of deep-to-surface signal on the scalp and the simultaneous activation of large neocortical areas can yield spurious equivalent dipoles localised in deeper structures. Frequent interictal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surface is independent of their capacity to trigger seizures. It is concluded that: (1) the deep-to-surface ratios of electromagnetic fields from deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate for surgical assessment; (3) the correct localisation of the onset of interictal activity does not necessarily imply the onset of seizures in the region or in the same hemisphere. It is suggested that, until volume conduction and neurophysiological propagation can be distinguished, semiempirical correlations between symptomatology, surgical outcome, and detailed presurgical modeling of the neocortical projection patterns by combined MEG, EEG, and MRI could be more fruitful than source localization with unrealistic source models.

MEG and EEG in epilepsy: is there a difference?

Interictal epileptiform activity recorded by scalp EEG, foramen ovale electrodes and MEG is discussed. Gross differences in waveform between the electric and magnetic records are discussed in the light of intracranial depth recordings.

A new technique for fractal analysis applied to human, intracerebrally recorded, ictal electroencephalographic signals

Application of a new method of fractal analysis to human, intracerebrally recorded, ictal electroencephalographic (EEG) signals is reported. 'Frameshift-Richardson' (FR) analysis involves estimation of fractal dimension (1 < FD < 2) of consecutive, overlapping 10-s epochs of digitised EEG data; it is suggested that this technique offers significant operational advantages over use of algorithms for FD estimation requiring preliminary reconstruction of EEG data in phase space. FR analysis was found to reduce substantially the volume of EEG data, without loss of diagnostically important information concerning onset, propagation and evolution of ictal EEG discharges. Arrhythmic EEG events were correlated with relatively increased FD; rhythmic EEG events with relatively decreased FD. It is proposed that development of this method may lead to: (i) enhanced definition and localisation of initial ictal changes in the EEG presumed due to multi-unit activity; and (ii) synoptic visualisation of long periods of EEG data.

An ASSEMBLER routine for on-line graphic display and averaging of data acquired on a personal microcomputer

An ASSEMBLER routine is described for data acquisition and "on-line" averaging, artefact rejection and graphic display of data on a personal microcomputer (IBM compatible). The user determines the number of input channels, sampling frequency, number of samples, input range, stimulation frequency (epoch frequency) and the number of epochs to be acquired and averaged. Data from each epoch are scanned in search of saturating artefacts and will be added to previous epochs if none is found. Data are then graphically displayed as voltage versus time before acquiring next epoch. Display options can be defined by the user at run time by means of the keyboard and include: display of last epoch, display of the average, storage screen and refreshing screen after every epoch. High data transfer rates and program speed allows for high stimulation rates in the presence of on line graphic display. The computer then behaves as a multichannel digital oscilloscope with access to large memory buffers, disk storage, high averaging capabilities, artefact rejection and wide potential for data analysis. Its applications to the recording of magnetic and electric evoked responses are illustrated. The program is available from the authors.

The effects of electrical stimulation of A and C visceral afferent fibres on the excitability of viscerosomatic neurones in the thoracic spinal cord of the cat

Single unit electrical activity has been recorded from viscerosomatic neurons in the lower thoracic spinal cord of decerebrate spinalized cats. The responses of the cells to electrical stimulation of afferent fibres in the splanchnic (SPLN) nerve and the effects of repetitive stimulation of somatic and visceral afferent C-fibres have been studied. Four groups of viscerosomatic neurones could be distinguished according to the type of visceral afferent input of the cells: (1) A-only cells (32.9%), driven only by stimulation of A delta afferent fibres in the SPLN nerve; (2) C-only cells (3%), driven only by stimulation of C afferent fibres in the SPLN nerve; (3) A + C cells (45.7%), driven by both A delta and C afferent fibres in the SPLN nerve; and (4) A + C? cells (18.6%), driven by A delta visceral afferents and showing signs of responsiveness to C-fibres though lacking a distinct response volley to visceral C-fibre activation. Two cells of the A + C group and located in lamina I of the dorsal horn responded to SPLN nerve stimulation in a manner consistent with the afferent fibre composition of the nerve, that is, showed evidence of strong monosynaptic links with SPLN afferent C-fibres and weaker responses to SPLN A delta afferents. Excitability changes of viscerosomatic neurones ('wind up', 'wind down' and changes in background activity) were also observed in the majority of neurones following electrical stimulation of somatic and of visceral afferent C-fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

A fast data acquisition system for neurophysiological signals based on a personal microcomputer

A general purpose analog-to-digital conversion system and its interface for a low-cost personal computer (Commodore 64) are described. Special emphasis has been put on achieving a high sampling frequency rate (up to 20,000 samples/s) and a high discrimination (12 bits). It has likewise been attempted to increase the number of input channels (up to 8, by means of a multiplexer) and averaging capability since such features may be useful in a great number of neurobiological studies. Commercially available hardware elements have been employed and software has been developed in BASIC and 6510 Machine Code.