RELATIONSHIP OF EPILEPTOGENICITY TO CORTICAL ORGANIZATION

Epileptogenic foci were produced in the visual cortex of 34 cats by the use of isotonic sodium penicillin in a specially designed applicator permitting replacement of the penicillin with Ringer solution (or vice versa) without affecting microelectric stability. Micropipettes (30-50 MΩ) were advanced obliquely through the cortex. Serially isolated cells were classified according to their response characteristics during the epileptiform surface wave and physiological characteristics, which included receptive-field location, preferred axis of movement (preferred orientation), and ocular dominance. Results were as follows: The preferred axis of movement was calculated repeatedly on single cells, allowing determination of the difference between each single estimate and the mean value. The preferred axis of movement determinations from data obtained from the same unit before and after application of penicillin did not vary beyond this limit. Some cells that were strongly directionally asymmetrical became less so after penicillin application. Ocular dominance was unaffected by the application of penicillin. Cell manifesting typical bursts of spikes during the epileptiform surface wave were considered to be within the epileptogenic focus. Criteria for identifying cells within the epileptogenic focus were specified through analysis of interspike intervals as a function of elapsed time. Approximately 50% of cells affected by penicillin became unresponsive to physiological light stimulation. Bursts of spikes occurring in responsive cells during the epileptiform wave could not be differentiated from bursts occurring in nonresponsive cells. All cells with typical bursts were considered to be within the focus. The data support the view that the pathophysiological property of epileptogenicity is manifest as a population response in a mammalian cortex. The minimal epileptogenic aggregate is greater than a single neuron, at least at the margins of the focus. This minimal epileptogenic aggregate is probably no smaller than the number of cells with the same preferred axis of movement and ocular dominance for a single retinal field site. An integral multiple of such minimal aggregates would therefore constitute the epileptogenic focus.