220 Information Storage and Neural Control 



order to avoid the ambiguity engendered when high amplitude 

 potentials arising externally are conducted in volume to the large 

 electrodes resting upon the surface of the isolated segment. Thus 

 in the first part of the tracings in the two experiments illustrated 

 in Figures 23A and B the large electrodes on the surface of isolated 

 cortex (Channel 3, Fig. 23A and Channel 2, Fig. 23B) record 

 potential variations precisely concordant in time with those in 

 the surrounding normal cortex where the seizure was initiated. 

 Not until seconds later did the microelectrode tracing reveal that 

 single elements within the slab had developed high frequency 

 self-sustained discharge. Since the high impedance of the micro- 

 electrode tip precludes recording at any distance, we cannot 

 escape the conclusion that nonsynaptic activation of ganglionic 

 elements within the isolated region had occurred. 



Although only negative evidence can be presented for the case 

 of non-epileptic isolated cortex, the contrast between that and 

 the ease with which invasion of epileptic zones can be demon- 

 strated has led to the conclusion that abnormal excitability per- 

 sists in the secondary epileptogenic focus for several months after 

 an isolation procedure which eliminated a self-reexcitation mech- 

 anism. Presumably, therefore, the persistence of abnormal behavior 

 in these cells depends upon structural or biochemical alterations 

 rather than upon continuing electrical input. 



On the basis of the reasoning discussed earlier the ribonucleic 

 acid distribution in the mirror focus was examined histochemically, 

 first with the methyl green pyronin method and subsequently 

 (with concordant results) with Azure B and Gallocyanin at 

 acid pH. After preliminary electrical studies had clearly indicated 

 the extent and distribution of both primary and secondary dis- 

 charging areas the animals were sacrificed and brains perfused 

 in situ. Serial sections were prepared and those from primary and 

 secondary foci were compared with those from electrically un- 

 involved areas of brain. Figure 24 demonstrates a small nest of 

 darkly stained cells in a section taken from the electrically defined 

 mirror focus. The border of the densely stained region is fairly 

 sharp, and to the left is the adjacent normal cortex, so that one 

 may compare the dye-binding property of normal cortical tissue 

 with that of the electrically abnormal zone on the right. A slightly 



