Correlations between EEG and multiple unit activity of cortifugal axons at the onset of drug induced seizures in cats.

The onset of sustained multiple unit activity (MUA) in corticofugal axons was found to precede the onset of tonic EEG activity in 91 of 92 seizures produced by administering pentylenetetrazol, strychnine, or dieldrin to cats. Latency differences from 0-8 sec were observed with a mean of 3 sec. A trend was noted for shorter latencies with subsequent seizures in the same subject. No differences in latency were seen for spontaneous as compared to evoked seizures. Much variation was encountered in the pattern of MUA and EEG development at seizure onset. In general seizures with well-defined EEG components of desynchronization, tonic and clonic activity had the greatest latency differences and the least correspondence between MUA and EEG. Seizures with the abrupt emergence of tonic or clonic activity had the smallest latency differences and the best correspondence between MUA and EEG. Large changes in the activity along corticofugal axons were observed without EEG correlates. The evidence supports the concept that the degree of synchrony within the cortical (pyramidal) cell population is critical to the emergence and development of EEG during the tonic and clonic phases of a seizure. The data suggest that, in some cases, important changes in corticofugal output may contribute to and reinforce the activity in subcortical loci implicated in generalized seizure onset.