6lO BRAIN MECHANISMS AND LEARNING 



frequency and (2) 20-45 /seconds. The first appeared with closed eyes in all 

 subjects and/or with open eyes in subjects who in these conditions 

 presented an alpha rhythm. Its frequency was the same as that of the 

 spontaneous alpha rhythm. The fast rhythm was present with open eyes in 

 subjects not presenting the alpha rhythm and it had the same frequency as 

 that of the spontaneous rhythm. 



The slow after-discharge of alpha frequency is similar to that described 

 by Ciganek (1958b) who, however, noted it as being inconstant. This is 

 explained by the fact that the after-discharge develops only in the course 

 of habituation and this temporal evolution was not taken into account by 

 this author. Hernandez-Peon (personal communication) also observed the 

 development of a slow after-discharge in the cat through habituation. 



Bremer and Bonnet (1950) studying the response evoked by auditory 

 stimulation in the awake or anaesthetized cat ('encephale isolc' preparation) 

 described two types of after-discharge: the slow one of 8-12 seconds 

 observed during sleep and the fast one which appeared in wakefulness or 

 under light ether anaesthesia. The after-discharges which appeared by 

 effect of habituation in man are comparable to those described by these 

 authors. Their frequency varied with the background rhythm to the 

 EEG: the subjects showing a moderately synchronized tracing (alpha 

 rhythm) presented a slow discharge. In other activated record presented a 

 rapid after-discharge. In other word's, the spontaneous rhythm tends to be 

 synchronous with the photic stimulation as a consequence of being 

 triggered by it. 



Topo(^rapliic distribution. In the course of habituation the evoked 

 responses of O2, C4, T2 and T6 experienced a similar evolution. How- 

 ever, the evoked responses of the central and temporal regions underwent 

 greater reductions in amplitude. In some instances the central and temporal 

 responses disappeared whilst the occipital response persisted (Fig. 3). 

 Since the amplitude of the evoked potential in the central and temporal 

 regions was less than in the occipital region, it might be thought that a 

 reduction in the amplitude oi^ the response would cause the earlier 

 disappearance in those regions where originally the amplitude had been 

 less. Nevertheless a decrease in amplitude and even the disappearance of 

 the central and or temporal responses was observed without any change 

 appearing in the amplitude of the occipital potential. This fact indicates 

 that during habituation the evoked response undergoes a true reduction in 

 its topographic distribution, and from being diffuse tends to become 

 localized in the occipital region. 



The fast or slow after-discharges produced by habituation are recorded 



