6l2 BRAIN MECHANISMS AND LEARNING 



for 20 minutes. On the other hand the same subjects showed habituation 

 to this response with the stroboscope at a distance of 3 m. or more. This is 

 not surprising since it is well known that the more intense the stimulation, 

 the harder it is to achieve habituation. Even when, on increasing the 

 intensity oi die stimulus no decrease in amplitude was observed during 

 habituation, a slow after-discharge appeared in experiments with closed 

 eves (Fig. 2, C). It is interesting to emphasize this point since it proves that 

 modifications in the amplitude and form ot the response during habitua- 

 tion may take place independently. 



Summarizing, the changes in the evoked visual response observed dur- 

 ing habituation to a continuous flicker stimulation were as follows: 

 (i) reduction of amplitude, (2) multiplication of the waves, (3) appearance 

 of a fast or slow after-discharge depending on the background rhythm 

 and (4) reduction m diftiision. The changes observed during dishabituation 

 provoked by attention or by modifications in the quality of stimuli, were 

 opposite. 



/ 



Fig. 4 

 Change:: of the visual evoked response by effeit of attention. Continuous 

 flicker frequency 0.5/sec. Stroboscope at 3 ni. from the retina. Eyes 

 open. Forty superimposed sweeps. Lead 02-right mastoid 10/20 

 system. Arrow indicates the stimulus, (i) control without stimulation; 

 (2) flashes 1-40, response formed by three successive oscillations; (3) 

 flashes 41-80, throughout this period an intense tone is associated, the 

 response tends to disappear, the first wave only persisting with a very 

 low amplitude; (4) flashes 81-120, in the same conditions as in (2) 

 without the associated tone, the response reappears. Calibration, 

 200 msec. 



Habituation was a persistent phenomenon. If after habituation had 

 become established the stimulus was suspended for 15-30 minutes, on 

 applying it again the response maintained the same characteristics. In 

 previously trained subjects fewer flashes were needed to establish habitua- 

 tion. Similar results have been obtained in animals by Hernandez-Peon, 

 Jouvet and Scherrer (1957). 



If during the continuous flicker stimulation, before habituation was 

 established, the attention of the subject was withdrawn from the repetitive 



