Human cortical neurons in epileptogenic foci: comparison of inter-ictal firing patterns to those of "epileptic" neurons in animals.

Abstract 1. 1. Extracellular recordings of the spontaneous firing patterns of cortical neurons were obtained in patients undergoing craniotomy for the surgical excision of an epileptogenic focus. 2. 2. Because many kinds of experimental “epileptic” foci in animals exhibit cells with high frequency (200–500/sec) bursts of action potentials, lasting 5–50 msec and recurring many times per second, we explored the electrocorticographically defined epileptogenic focus in humans in search of bursting firing patterns. 3. 3. In addition to normal firing patterns, many cells near the focus exhibited “epileptic” bursting firing patterns. Sometimes, normal and bursting cells could be recorded simultaneously, indicating that cells in proximity to one another are not uniform in such firing properties. 4. 4. Such high frequency bursts are not typically the result of artifact such as micro-electrode pressure upon a cell, or heartbeat or respiratory movements of the cortex. Most bursting cells were obtained under operating conditions involving local anesthesia, but similar results were seen under general anesthesia. 5. 5. Bursts were not necessarily synchronized with the EEG sharp waves, nor with bursts from other simultaneously recorded neurons. 6. 6. Attempts were made to modify the burst patterns by arousing a sleeping patient and by electrical stimulation of the adjacent cortical surface. While some modifications in the rate of recurrence of the bursts could be obtained, the timing patterns of the first few spikes within a burst did not change readily. 7. 7. High frequency tonic firing was also seen. Some such activity could be observed to undergo spontaneous changes from silence to high frequency tonic firing and then to bursts. 8. 8. Within a burst, the timing of spikes may be very repeatable (stereotyped bursts) in some cases. In a few cases, the structured bursts reported in chronic monkey foci have been observed, where there seems to be a characteristic pause in the firing after the first one or two spikes and then a resumption of high frequency firing in a manner identical to the stereotyped bursts. These structured timing patterns have been considered a clue towards the identification of primarily dysfunctional epileptic neurons (in contrast to normal cells recruited into bursting firing patterns by an abnormally large synaptic input). 9. 9. We would conclude that there is a good correspondence between the chronic alumina “epileptic” foci in animals and the human disease, insofar as the inter-ictal firing patterns of neurons near the focus is concerned.