M. PALESTINI AND W. LIFSCHITZ 425 



learning has already been discussed (Hernandez-Peon, Jouvet and Scherrer, 

 1957). It would seem useful to consider from this point of view the role of 

 the low bulbo-pontine structures ni the origni anci development of the 

 processes of internal inhibition, taking into account the Pavlovian view of 

 the intimate relationship between the latter and sleep (Pavlov, 1928). 



E. MIDPONTINE PRETRIGEMINAL PREPARATION AND CORTICAL POTENTIALS 

 EVOKED BY PHOTIC STIMULATION 



It is an accepted fact that the cortical potentials evoked by sensory 

 stimuli diminish in amplitude during spontaneous desynchronization 

 periods. Electrical stimulation of the reticular formation provokes an 

 arousal reaction and reduces the cortical potentials evoked by physio- 

 logical stimulation of the specihc sensory pathways. On the other hand it 

 has been observed that during EEG synchronization the evoked responses 

 increase in amphtude (Bremer, 1953). 



The midpontine animal, with its permanent state of wakefulness, and 

 the rostropontine cat, with a synchronized EEG pattern, seemed to be two 

 suitable preparations for studying these problems. In addition, we have 

 assumed that inhibitory influences which facilitate habituation originate in 

 the medulla. If this were so, midpontine section, separating this structure, 

 might reveal a facilitation of the cortical evoked responses. 



Cats with electrodes permanently implanted in the gyrus lateralis and 

 gyrus lateralis anterior were used in these experiments. Primary and 

 secondary photic cortical potentials were recorded (EEG anci CRO) 

 before and after midpontine lesion. 



The secondary potentials recorded in normal cats showed an initial 

 negative deflection. Latency, which was difficult to measure in such condi- 

 tions, had an average value of 20 msec, and its amplitude was about 

 80 (iV. Iterative photic stimulation with a frequency of 1 per second 

 produced rapid habituation and the secondary response became masked by 

 cortical activity. 



The midpontine transsection performed in the same animals brought 

 about some clear changes. Latency (20 msec.) became very regular and 

 easy to measure and the amplitude increased notably, reaching sometimes 

 150 [xV. or more. These results were obtained with a dcsynchronized 

 EEG background (Figs. 4, 6). Similar changes to those c^escribed in the 

 midpontine cat were also noticed in rostropontine lesions, accompanied 

 in this case with cortical synchronization. 



Pretrigeminal section performed in these animals also modified the 



