Operantly conditioned firing patterns of epileptic neurons in the monkey motor cortex

In an awake rhesus monkey we operantly conditioned the activity of single precentral pyramidal tract cells near a chronic alumina-induced epileptic focus. Units chosen for conditioning fired predominantly in stereotyped high-frequency long-first-interval bursts. Most units also exhibited brief periods of tonic regular firing, typical of normal precentral cells. The proportion of spikes occurring as long-first-interval bursts was determined on the basis of interspike intervals and defined as the epileptic index. Operantly reinforcing transient increases in unit activity with applesauce produced increases in average rates in all nine cells, with no consistent change in the mean epileptic index. Reinforcing transient decreases in firing rate produced a clear decrease in average rate for two cells, no sustained rate changes in six, and an increase in one; the average epileptic index did not change consistently, although transient pauses in cell activity were invariably preceded and followed by long-first-interval bursts. Reinforcing decreases in the epileptic index produced a sustained drop in the number of long-first-interval bursts/min and a concomitant increase in both regular firing and total rate. Reinforcing an increase in epileptic index produced no consistent changes. These results suggest that firing patterns of epileptic cells may be synaptically modified in awake animals. Analysis of reinforced responses suggest that transient increases in synaptic drive generating higher rates may also decrease the proportion of long-first-interval bursts.