842 HANDBOOK OF PHVSlOI.Om' ^ NEl'ROPin'SIOLOm' II 



FIG. 3. Pyramidal complexes evoked by stimulation at various levels beneath cortical 

 surface in monkey (Dial anesthesia). Recording electrode in contralateral lateral column 

 between Ci and Cj. Stimulus through focal monopolar electrode on, within or beneath 

 contralateral motor arm area. Numbers under each trace indicate depth of stimulating 

 electrode in millimeters, measured from cortical surface. Cortex was 2.5 mm thick at point 

 of penetration. [From Fatten & Amassian (80). j 



FIG. 4. Effect of cortical stimulus site and stimulus strength on pyramidal discharge in 

 monkey (Dial anesthesia). Recording electrode below Ci in the lateral column contralateral 

 to stimulation. A. Responses elicited by stimulating point A Unset) with relative strengths 

 indicated by numbers under the traces. B. Responses evoked from point B; note high thresh- 

 old compared to .4. C Responses evoked by stimulating point B; upper trace, test response alone; 

 lower trace, test shock followed identical conditioning shock by 1 8 msec. Note in test response 

 there is obliteration of late (I) activity but persistence of the component with latency of 

 4 msec, (slowly conducted D wave). [From Patton & .\massian (80).] 



cortex has been removed (fig. 2). In the experiment 

 shown in figure 3, the cortex was left intact, but the 

 stimulating electrode was thrust for varying distances 

 into or through the cortex, which was 2.5 mm thick. 

 Deep in the white matter (lower trace) only the D 

 wave was evoked; as the stimulating electrode was 

 pulled into and through the gray matter, I waves ap- 

 peared and were maximum at a depth of 2.0 mm. 

 Incidentally, this experiment eliminates the possi- 

 bility that I waves recorded under barbiturate anes- 

 thesia result from re-excitation via the recurrent col- 

 laterals of Betz cells (23, 25, 74). Stimulation in the 

 white matter should invade such collaterals anti- 

 dromically, producing I acti\ity, but the lower trace 



shows little or no late activity. More pertinent argu- 

 ments against the participation of recurrent collaterals 

 in the re-excitation of Betz cells are given below. 



Thus, the I waves of a cortically evoked, pyramidal 

 discharge result from relayed excitation of Betz cells 

 via cortical interneurons, and the size of the waves 

 provides a convenient and reliable measure of cortical 

 excitability (12, 21, 80, 105, 114). The question arises 

 whether the units synaptically fired during I activity 

 are those which were directly fired to produce the D 

 wave. Inspection of figure 3 provides a partial answer; 

 the area of the third I wave in the fourth trace from 

 the top is considerably greater than that of the D 

 wave; hence, even assuming some sharing of units in 



