378 BRAIN MECHANISMS AND LEARNING 



Once the independent stage of secondary discharge has been reached, 

 local changes in cellular excitability may be demonstrated by alterations in 

 evoked potentials. For example, one measure of excitability change is the 

 amplitude of the direct cortical response (DCR) to the same electrical 

 stimulus presented before, and 7 days after, development of the focus. 

 Electrodes were implanted in various cortical regions of the hemisphere 

 contralateral to the ethyl chloride lesion (Fig. 4) so that a bipolar stimu- 

 lating electrode consisting of two 75 micron silver wires cemented to- 



pic. 4 



Direct cortical response tested before, and 7 days after, production of an etiiyl cliloridc lesion. 

 The derivations are from implanted electrodes as indicated in the diagram. The area of the 

 ethyl chloride lesion is cross hatched. Variations in duration and configuration of the evoked 

 potentials are due to differences in inter-electrode distance and in orientation of the recording 

 electrode with respect to the stimulating electrodes. Photographs consist of ten to forty 

 superimposed sweeps. Calibration: 15 msec. 



gether and exposed only at their tips rested approximately 2 mm. away 

 from a monopolar steel needle used as a recording electrode. The reference 

 electrode was a steel screw in the calvarium. Direct cortical responses were 

 tested repeatedly at each of these sites before producing the ethyl chloride 

 lesions. Responses obtained in each area are demonstrated in the first 

 column of oscillograph tracings. These tracings were made by super- 

 imposition often to forty responses and thus serve to indicate the stability 

 and, therefore, reliability of amplitude measurements under these circum- 

 stances. An ethyl chloride lesion was then produced in the left visual 



