Common aspects of the development of a kindling epileptogenic focus in the prepyriform cortex of the dog and in the hippocampus of the rat: spontaneous interictal transients with changing polarities.

Long-term records of field potentials during kindling were analysed in 2 preparations: the prepyriform cortex (PPC) in dog and the CA1 subarea of the hippocampus in rat. Besides the occurrence of after-discharges directly following the kindling stimulus, short transients were found to occur spontaneously between seizures: spontaneous inter-ictal transients or SITs. In both the PPC and the hippocampus 2 different types of SITs were encountered. Both types corresponded to a dipole layer centred around the main local pyramidal neurones (PPC or hippocampus). One type occurred early in the course of kindling and had the same polarity as the local evoked potential (EP) produced by stimulation of the lateral olfactory tract in PPC or of the Schaffer collaterals in the hippocampus. The other type occurred at later stages and had a reversed polarity to the EP. These findings were interpreted as indicating that at the onset of kindling spontaneously occurring depolarizations corresponding to the early type of SITs are restricted to the level of the synapses to which the kindling stimulus is applied; at a later stage, when convulsions are likely to occur, large spontaneous depolarizations corresponding to the late type of SITs are generated at the level of the cell bodies.

Automatic detection and localization of epileptic foci.

A new method of automatic EEG analysis (ASD or automatic non-stationarity detection) derived from a parametric EEG model (autoregressive filter model) and based on inverse filtering, has been applied to scalp and subdural EEGs of epileptic patients. By this method it was possible to detect the occurrence of transient non-stationarities such as paroxysmal patterns of activity (spikes, spike-and-waves) characteristic of inter-ictal EEGs. The ASD has been implemented in a general purpose digital computer. The method allows: (a) statistical evaluation of paroxysmal patterns; (b) multi-channel analysis, where the interrelationships of different derivations are quantified and displayed by means of a clinically useful spatial map. The application of the ASD method to scalp and subdural EEGs, simultaneously recorded, has revealed that a number of transient non-stationarities detected in the scalp by the program yet not by visual inspection, coincide with clear paroxysmal patterns in subdural derivations. In this way conventional definitions of characteristic paroxysmal patterns at the scalp are put in question. Identical conclusions regarding the localization of an epileptogenic zone in 4 patients were obtained either using the conventional methods of recording electro-clinical seizures in EEGs of long duration or applying the ASD method to short EEG epochs (100 sec long, at most).