E. GARCIA-AUSTT, J. BOGACZ AND A. VANZULLI 613 



Stimuli and focused on some other interesting stimulus, the amplitude of 

 the response decreased. This reduction was seen for example when the 

 subject was performing a mental calculation or was made to listen to an 

 intense continuous tone during the flicker stimulation (Fig. 4). Hernandez- 

 Peon et al. (1957) reported the same phenomenon in the optic pathways of 

 the cat, Buser and Rougcul (1956) found it in the cerebral cortex, and 

 Jouvet and Courjon (1958) studied it in man by recording from the optic 

 radiations. 



Inversely it was possible to provoke a state of dishabituation by increas- 

 ing the attention directed to the flicker. For example, when the subject 

 counted the flashes (Fig. 3) or simply checked a possible variation in them, 

 the amplitude of the response increased, the after-discharge disappeared 

 the waves became simplified and the response was recorded in other 

 regions as well as in the occipital region. In other words the response 

 regained its prc-habituation characteristics. 



When, alter habituation, a click or a brief tactile stimulus was associated 

 with each flash, dishabituation was again provoked. If this association was 

 prolonged for a time habituation again set in. On suspending the associa- 

 tion and continuing with the pure flicker, dishabituation again took place. 

 That is to say, that habituation was observed whenever the stimulus, 

 either pure or in its associated form, was sufficiently repeated, and dis- 

 habituation took place whenever the stimulus was modified by association 

 or disassociation. 



EFFECT OF DISCONTINUOUS FLICKER STIMULATION 



When, having achieved habituation of the response by continuous 

 flicker stimulation, the stimuli were interrupted for regular and constant 

 periods, after three to seven trials dishabituation was achieved (Fig. 5). The 

 response regained all its initial characteristics. The most remarkable feature 

 was the increase in amplitude. 



If a discontinuous flicker stimulation was used initially without previous 

 habituation having been established, an increase in amplitude was again 

 obtained, but this time of lesser magnitude. In either case if stimulation by 

 a discontinuous flicker was continued, after ten to fifteen trials the response 

 became habituated (Figs. 5 and 6). The general evolution of the response 

 was not the same in the different regions explored. Whilst in O2 the 

 increase in amplitude was only discrete and the response was rapidly 

 rehabituated, in C4 and T2 the increase was greater and more persistent. 



