6o8 



BRAIN MECHANISMS AND LEARNING 



man. Whilst the cat requires hours to become habituated to an inter- 

 mittent photic stimuhis, man did this in a few minutes. It should be borne 

 in mind ni this respect that in man, as a consequence of the liigher develop- 

 ment ot his nervous system, learning is much faster than in the carnivores. 

 Furthermore, the significance of a flash is much greater to a caged animal 

 on the defensive than to a man who, although he has no previous know- 

 ledge of the experiment, knows what a flash is and also knows that he will 

 not have to face anything unpleasant or dangerous during the experiment. 



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Fig. I 



Changes of the visual evoked response by effect oj habituation. Continuous flicker 

 frequency 2/sec. Stroboscope at 3 m. from the retina. Eyes open. Fifty super- 

 imposed sweeps. Dawson's technique. Arrow indicates the stimulus. Lead 

 02-right mastoid (10/20 system), (i) control without stimulation; (2) flashes 

 1-50, three waves are observed surface-negative-positive-negative; (3) flashes 

 101-150, the first negative wave tends to disappear and the others are re- 

 duced in amplitude; (4) flashes 301-350, the first negative wave has dis- 

 appeared; (5) flashes 401-450, amplitude even more reduced. Calibrations, 

 25/sec., 50 microvolts. 



To the cat, the flash signifies danger, to man it is simply light. Conse- 

 quently, it is possible that the latter soon ceases to pay attention to it and 

 becomes habituated. 



Persons subjected to the action ot a repetitive photic stimulus, on being 

 questioned, stated that after a tew hundred flashes their interest decreased 

 on realizing that the stimulus was monotonously repeated. 



Some subjects with previous experience showed a lesser amplitude in 

 their response to the first flashes. For example the tirst 100-150 flashes 

 evoked responses of lesser amplitude than the 150-250 following ones. 

 Subsequently the amplitude was again reduced, the habituation which was 

 to persist throughout the whole experiment becoming apparent. In these 

 subjects the factor of initial surprise was lacking, their attention becoming 



