INTRACEREBRAL PROPAGATION OF INTERICTAL ACTIVITY IN PARTIAL EPILEPSY - IMPLICATIONS FOR SOURCE LOCALIZATION

The hypothesis MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The ti me course and spatial distribution of interictal activity recorded sim ultaneously by surface electrodes and by electrodes next to mesial tem poral structures in 12 patients being assessed for epilepsy surgery ha ve been studied to estimate the degree of confinement of neural activi ty present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of inter ictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlatio ns between the amplitudes of deep and surface recordings, together wit h previous reports on the amplitude of scalp signals produced by;artif icially implanted dipoles suggest that the ratio of deep to surface ac tivity recorded during interictal epileptiform activity on the scalp i s around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but i s generated in the underlying neocortex. Also, time delays of up to 22 0 ms recorded between interictal paroxysms at different recording site s show that interictal epileptiform activity can propagate neuronally within cortex. Large areas of archicortex and neocortex can then be si multaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres t hat trigger rise to sharp/slow along the neocortex. The low ratio of t he simultaneous activation of large neocortical areas can yield spurio us equivalent dipoles localised in deeper structures. Frequent interic tal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surf ace is independent of their capacity to trigger seizures. It is conclu ded that: (1) the deep-to-surface ratios of electromagnetic fields fro m deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate models for interictal acti vity for surgical assessment; (3) the correct localisation of the onse t of interictal activity does not necessarily imply the onset of seizu res in the region or in the same hemisphere. It is suggested that, unt il volume conduction and neurophysiological propagation can be disting uished, semiempirical correlations between symptomatology, surgical ou tcome, and detailed presurgical modelling of the neocortical projectio n patterns by combined MEG, EEG, and MRI could be more fruitful than s ource localisation with unrealistic source models.