E. GARCIA-AUSTT, J. BOGACZ AND A. VANZULLI 621 



When the stiniuhis is significant and therefore attention is paid to it, the 

 response is relatively simple and widespread. When on the other 

 hand, the stimulus is not significant and no great attention is paid to it, 

 the response is reduced, complex and localized. It is interesting to point 

 out that during the state of attention not only has the evoked potential a 

 greater amplitude but also a much larger area of the brain receives the 

 'information'. 



These changes in the response are closely related to the state ot sensorial 

 receptivity of the subject, i.e. the "attention level' towards the stimulus. 



Between these two phenomena, the electrical and the psychological, 

 there may exist: (i) a cause and effect relation, (2) a common cause. 

 Although the first possibility cannot be discarded, it is probable that both 

 are the consequence of processes taking place in the subcortical systems ot 

 unspecific projection (Jouvet and Hernandez-Peon, 1957; Buser and 

 Roger, 1957; Hernandez-Peon, Brust-Carmona, Eckhaus, Lopez- 

 Mendoza and Alcocer-Cuaron, 1958; Gastaut, 1958). 



Directing, blocking and switching of the sensorial messages would 

 appear to take place on the level of the reticular formation and or thalamic 

 reticular system. As a consequence oi the analysis and integration taking 

 place in these structures, the cerebral cortex would receive information of 

 correct magnitude and proper distribution for immediate needs. The 

 modifications of the 'level' and 'focus' of attention would be the con- 

 sequence of this selective activity. 



Whatever the explanation is, the electrical changes and the variatii^ns of 

 attention run side by side. Therefore, the electrical changes provide an 

 idea and may even go as far as to constitute a measure of the concomitant 

 psychological phenomena. If this should be so, with the equipment avail- 

 able in every neurophysiological laboratory and with a simple and rapid 

 technique it would be possible to obtain an objective appreciation of the 

 time pattern of attention, habituation and other forms of learning in man. 



GROUP DISCUSSION 



JouvET. In connection with the significance ot the amplitude ot the evoked 

 responses in man, I would like to show you some recordings made during stereo- 

 taxic surgery on the thalamus. In three patients suffering from intractable pain 

 before the thalamotomy, we recorded from the V.P.L. with a depth electrode 

 somestiietic potentials evoked by mechanical stimulation of the skin (usually the 

 contralateral forearm). When the patients, who were conscious, were paying 

 attention to the stimuli, the evoked responses had a much higher amplitude than 

 when the patients were not (mental calculation, auditory attention ... ) In some 

 cases, the evoked responses could disappear almost completely. 



