J. p. SEGUNDO, C. GALEANO, J. A. SOMMER-SMITH AND J. A. ROIG 29I 



perhaps your two separate waves represent the arrival ot the initial peripheral 

 stimulus and the secondary attending to it. 



Segundo. I think there are two main points in Dr Buser's comment. As regards 

 the first, we cannot know for certain what the cat felt. As to the second, it is 

 indeed true that EEG 'arousal' is associated usually with decreased cortical poten- 

 tials. I would like to mention, however, recent experiments by Bremer and 

 Stoupel and Dumont and Dell, in which stimulation of: mesencephalic reticular 

 formation produced EEG 'arousal' and simultaneously 'facilitation' ot responses in 

 primary receiving cortices (Bremer and Stoupel, 1959; Dumont and Dell, 1959). 

 Hence 'desynchronization' is not associated necessarily with reduction of sensory 

 cortical potentials. 



Olds. I think it relevant to the problem of whether pain is reduced is the question 

 of whether you ever get adaptation to this subcutaneous stimulus by itself How 

 long is the series of shocks before the tone is introduced? And I would like also to 

 ask : Is there ever any change in responding during the course, either recruitment 

 or readaptation? 



Segundo. We stimulated between seconds and hours and an average of 3-4 

 minutes was used currently. There were marked spontaneous variations of evoked 

 potentials: for instance, and since you mentioned 'recruitment', in certain cats 

 8 per second stimulation produced an effect that was entirely comparable to 

 'augmentation'. 



Hernandez-Peon. I would like to remind Dr Segundo ot the experiments of 

 Bremer and Stoupel. Electrical stimulation oi the mesencephalic reticular forma- 

 tion produces striking facilitations of the cortical potentials evoked by electric 

 shocks applied to the optic nerve. But when they used a flash ot light instead 

 (which I think is more physiological than an electric shock applied to the optic 

 nerve) they observed a striking redtiction of the evoked potential. With reference 

 to Dr Magoun's question about subcortical changes in the somatic afterent path- 

 ways, specially where pain is concerned, I can say that we have recorded evoked 

 potentials both at the spinal sensory nucleus in the bulb and in the lateral column of 

 the spinal cord in conscious cats. Experiments have shown at both levels that the 

 potentials evoked by nociceptive stimuli are reduced when the cat focuses its 

 attention upon some other stimulus of greater significance. Of course, this reduc- 

 tion is more difficult to obtain on the potentials evoked by nociceptive stimuli 

 than on the potentials evoked by tactile stimuli. 



Segundo. A priori, one would tend to think that also for sensory control, the 

 nervous system has two types of mechanisms, one facilitatory and another in- 

 hibitory: in terms of unit responses this is true. When evoked potentials have been 

 controlled, facilitation has not been frequent but we must remain receptive to the 

 idea that under certain conditions, sensory evoked potentials may be facilitated. 



