Interhemispheric interactions in seizures of focal onset: data from human intracranial recordings

Interactions between the two hemispheres were studied during seizures recorded in 8 epileptic patients having chronic intracranial electrodes. Seven had temporal lobe foci and one a fronto-central focus. Strength of interaction was measured by the coherence between the EEGs from symmetrical contralateral locations. Time delays of a few milliseconds between discharges were computed by the coherence and phase method. The evolution of interactions was followed from the time a seizure of focal onset had become bilateral to its end. It was found that interhemispheric coherence was generally low throughout seizures, highest values being reached early in the seizure at the time of spread, or at the very end. Time delays most often indicated a lead from the side of onset, whether they were measured early or late in the seizure. Exceptions to these results were found in 2 of 3 patients with bilateral independent onsets: interhemispheric coherence was higher and time leads were always from the same side, independently of the side of onset. If it is assumed that high interhemispheric coherence is mediated by direct connections such as corpus callosum and anterior commissure, then these results can be interpreted as follows: the major commissures do not play an important role in contralateral spread of temporal lobe seizures although they are sometimes active, particularly at initial spread and at seizure end. Time leads from the side of onset indicate that the focus retains an influence over the contralateral discharge throughout the seizure. A different situation may exist with independent bitemporal foci.