SPIKE-WAVE COMPLEXES AND FAST COMPONENTS OF CORTICALLY GENERATED SEIZURES - III - SYNCHRONIZING MECHANISMS

The intracortical and thalamocortical synchronization of spontaneously occurring or bicuculline-induced seizures, consisting of spike-wave ( SW) or polyspike-wave (PSW) complexes at 2-3 Hz and fast runs at 10-15 Hz, was investigated in cats under ketamine-xylazine anesthesia. We u sed single and dual simultaneous intracellular recordings from cortica l areas 5 and 7, and extracellular recordings of unit firing and field potentials from neocortical areas 5, 7, 17, 18, as well as related th alamic nuclei. The evolution of time delays between paroxysmal depolar izing events in single neurons or neuronal pools recorded from adjacen t and distant sites was analyzed by using I) sequential cross-correlat ions between field potentials, 2) averaged activities triggered by the spiky component of cortical SW/PSW complexes, and 3) time histograms between neuronal discharges. In all instances, the paroxysmal activiti es recorded from the dorsal thalamus lagged the onset of seizures in n eocortex. The time lags between simultaneously impaled cortical neuron s were significantly smaller during SW complexes than during the prior epochs of slow oscillation. During seizures, as during the slow oscil lation, the intracortical synchrony was reduced with increased distanc e between different cortical sites. Dual intracellular recordings show ed that, during the same seizure, time lags were not constant and, ins tead, reflected alternating precession of the recorded foci. After tra nsection between areas 5 and 7, the intracortical synchrony was lost, but corticothalamocortical volleys could partially restore seizure syn chrony. These data show that the neocortex leads the thalamus during S W/PSW seizures, that time lags between cortical foci are not static, a nd that thalamus may assist synchronization of SW/PSW seizures after d isconnection of intracortical synaptic linkages.