290 BRAIN MECHANISMS AND LEARNING 



effect. In the EEG sphere, I am atraid we did not search for possible effects of 

 somatic stimuU in auditory regions. 



Magoun. IDid your conditioned reduction oi potentials occur in the thalamus as 

 in the cortex? 



Segundo. Thalamic potentials were not studied. 



Gerard. I would have assumed that you have evidence against any serious 

 decrease in the input from the fiict that your early waves often did not change at all. 

 Is that unsound? 



Segundo. The classical mterpretation of somatic sensory evoked potentials 

 considers that the early wave of the cortical response is due to afferent volleys and 

 late waves to intracortical processes. I find it difficult to admit, however, that no 

 afferent activity at all reaches the cortex during those delayed oscillations. 



Magoun. My interest in subcortical potentials is related to the question of: 

 whether activity evoked in the sensory cortex is to be correlated with pain percep- 

 tion. Might some feature of the evoked cortical potential later than the early spike 

 or some activity which never reached the cortex at all, be of importance here? The 

 reduction or block of pain perception might not be correlated with what you are 

 recording in the sensory cortex. Conceivably you might fmd potentials subcorti- 

 cally that would be more directly related to the perception of pain. Could you give 

 us your impression of what potentials are evoked in the CNS by a peripheral pain 

 stimulus? 



Segundo. Various potentials were evoked bv subcutaneous stimuli in our 

 'chronic' cats : though we did not explore extensively, it was obvious that, in these 

 animals as in certain 'acute' preparations, subcutaneous stimuli evoked at least 

 'cortical primary', 'cortical associative' and 'reticular' responses. A number o{ 

 dubious issues is involved, however, in the question of which electrograpliic 

 phenomenon was correlated with perception of pain: correlation of pain probably 

 experienced by our cats on the one hand, with potentials evoked in different areas 

 or individual waves on the other is extremely hazardous (see Discussion in 

 text). 



Lundberg. I would like to ask to what extent you think it is likely that some of 

 the evoked potentials are due not to stimulation of pain fibres but to stimulation of 

 touch fibres. If so, these evoked potentials may not have any relation to the 

 nociceptive behaviour of the animal. 



Segundo. As mentioned above, we cannot correlate individual waves pertaining 

 to cortical evoked potential with cat behaviour or pain sensations. Other types of 

 peripheral fibres were probably activated as well and we do not know to what 

 extent each type participated in determination of each wave. 



BusER. I would like to ask whether in Dr Segundo's opinion, the cat felt pain 

 during the period it submitted to sound; in trying to correlate the amplitude of an 

 evoked potential to the 'conscious perception' of the corresponding stimulus, we 

 are used to the fact that a higher level of wakefulness may correspond to a smaller 

 amplitude of the cortical response (at least with peripheral stimuli) ; in the case of Dr 

 Segundo's experiments, could it not be the opposite? 



Gerard. I think you are talking about the differences between sensing something 

 and attending to the sensation. We know morphine does not raise the threshold to 

 pain stimulation, it just makes the person not worry about it so much; and frontal 

 lobe operations do the same thing. I was wondering, along the same lines, whether 



