426 BRAIN MECHANISMS AND LEARNING 



primary responses. In Fig. 7 (CRO recording with bipolar electrodes) 

 some of these modifications can be observed, i.e. a slight change in the 

 first spike and a marked increase of amplitude in the following deflections 

 which are precisely those which arc most changed by habituation and 

 conditioned learning (Guzman, Alcaraz and Fernandez-Guardiola, 1957; 

 Hernandez-Peon, Guzman, Alcaraz and Fernandez-Guardiola, 1958). 



Studies made with monopolar electrodes demonstrated that the ampli- 

 tude of the first positive deflection, currently designed wave i, remained 

 unmodified or increased slightly. Conversely, positive waves 3 and 4 and 

 negative wave 5 showed a constant and appreciable increase. We wish to 



NORMAL CAT 



PRETRIGEMINAL CAT 



NORMAL CAT 



Fig. 6 



Oscillographic recording of secondary cortical photic responses 



Stimulus: flash i /second. Monopolar record in the same cat before (normal) and after mid- 



pontine section. Five responses in each trace. Downward deflection negative. Time calibration 



40 msec. Amplitude calibration 100 \iV. 



Stress once again that these results were obtained with a desynchronized 

 EEG pattern. In rostropontine animals exactly the same changes were 

 seen, but cortical spindles intcrfereci with the regularity observed in the 

 midpontine cat. 



Since the potentiation described was achieved after total transsection of 

 the brain stem, we assume that it is the result of the elimination of a tonic 

 inhibitory influence originated behind the lesion and as a consequence of 

 the exaltation of facilitating influences situated above the level of the 

 lesion. 



Regarding the site of action of these ascending inhibitory influences of 

 bulbar origin, nothing definite can be said at the present time. Neverthe- 

 less, on the basis of some anatomical and neurophysiological data, we may 



