Interhemispheric relations during bilateral spike-and-wave activity.

Temporal relationships between homologous EEG channels of the two hemispheres were studied in patients whose EEGs showed bilateral synchronous spike-and-wave activity. Group A (seven patients) had generalized corticoreticular epilepsy and no sign of a localized predominant epileptogenic area. In group B (12 patients), the bilateral spike-and-wave activity was present in conjunction with a localized area of predominant epileptogenicity demonstrated by EEG, radiological, or clinical examinations. The measurement of small time differences between two homologous channels was performed by transforming the slope of the phase characteristic of the cross-spectrum into time, when the interchannel coherence was sufficiently high. Although measurements were not possible in every case (because of a lack of coherence or nonlinearity of the phase), results clearly indicated that the spike-and-waves in group A did not present significant interhemispheric time differences, whereas those of group B frequently presented a lead time (average 15 msec) from the side with the localized epileptogenic area. The method can be clinically useful to differentiate primary from secondary bilateral synchrony. The concept of secondary bilateral synchrony and the possible pathophysiological mechanisms explaining the time differences are discussed.