376 



BRAIN MECHANISMS AND LEARNING 



following ablation or neuronal isolation of the primary lesion. Electro- 

 graphically, this dependent stage of homotopic discharge may be character- 

 ized by the fact that secondary spikes appear only at the time that spike 

 discharges are evident in the primary region (and often only in response to 

 the larger of the primary spikes) and have an appreciable latency following 

 the primary discharge when measured with cathode ray oscillography 

 (Fig. 2)- 



1-0 A R 459 



5-7 





Fig. I 



Electroencephalogram of an unanacsthetized rabbit 24 hours (A) and 3 days (B) after produc- 

 tion of an ethyl chloride lesion. The site and extent of the lesion is indicated by the cross 

 hatched area on the diagram. Derivations are bipolar from implanted electrodes over the 

 indicated regions. Calibration is 50 microvolts and i second. 



After a given period of time (24 hours to 7 days in the rabbit, 4 weeks to 

 3 months in the cat; about 8 weeks in the monkey) the secondary contra- 

 lateral focus becomes independent in that it does not disappear upon 

 ablation or coagulation of the prmiary region of discharge. Electro- 

 graphically, the independent stage of homotopic discharge is characterized 

 by random spikes and paroxysmal bursts which bear no relation in time 

 to the activity present in the primary lesion (Fig. 3). 



