218 Information Storage and Neural Control 



contralateral to the center of the isolated slab. Once the epileptic 

 lesion had begun to discharge actively it too was isolated in the 

 same way. It was thus possible to compare the properties of neurally 

 isolated non-epileptic tissue in one hemisphere with similarly 

 isolated but epileptic tissue in a comparable region of the opposite 

 hemisphere in the same animal. 



Although many different test situations were investigated, only 

 one will be discu.ssed at this time. Approximately three months 

 after the cortical isolations were made the animals were prepared 

 for an acute experiment. Wide exposure of both cerebral hemi- 

 spheres and a tracheotomy were performed under ether anesthesia 

 after which the ether was allowed to dissipate and the animals 

 were maintained under Flaxedil and artificial respiration. The 

 pial surface was covered with warm mineral oil or saline. Epilep- 

 tiform after-discharges were induced in the intact normal cortex 

 outside the isolated zones either by direct electrical stimulation 

 or by placement of small pledgets of filter paper soaked in Metrazol. 

 Propagation of these after-discharges was monitored by means of 

 recording electrodes distributed throughout the intact cortex and 

 within the isolated area. The extent to which high voltage dis- 

 charge originating externally spreads across the solution of neural 

 continuity to excite cells within the isolated zone is considered to 

 be a measure of the excitability of those cells. 



In our experience it was rare indeed for paroxysmal discharge 

 to cross the neural gap and excite non-epileptic isolated cortex 

 (Fig. 22). As may be seen in Figure 22 this was true even when 

 the epileptiform activity was of extremely high voltage, long 

 duration, and spread quite readily to the opposite hemisphere. 

 On the other hand the isolated epileptic tissue of the mirror focus 

 was quite easily invaded by epileptiform activity arising ex- 

 ternally (Fig. 23). 



In the experiment illustrated in Figure 23, tungsten micro- 

 electrodes having tip diameters of 1-5 micra were inserted to 

 a depth of 500-1000 micra into the isolated slab. Since a search 

 for spontaneously firing units was rarely successful it was necessary 

 to rely upon multiple placements at a depth where unit discharge 

 might reasonably be expected in connection with surface electro- 

 graphic paroxysms. Microelectrode recording was employed in 



