6l4 BRAIN MECHANISMS AND LEARNING 



There were differences in the temporal course ot changes in different 

 subjects (Fig. 7). 



These modifications oi the evoked response caused by discontinuity of 



7 A/V^^^v^^^ 



8 



4 ''■\l\p-''''^''-^ '0 



6 -y^l^p^^y"^-^^ 12 AvA/^A/V^ 



t 



t 



Fig. 5 



Changes of the visual evoked response by effect of discoiitiniioiis flicker stiiiiiilatioii. 

 Stroboscope at distance of 3 in. from the retina. Eyes open. Forty superimposed 

 sweeps. Flicker frequency 2/sec. during 20 seconds, interrupted during 20 seconds, 

 Lead C4-right mastoid, 10/20 system. Arrows indicate the stimulus, (i) control 

 without stimulation; (2) after habituation to a continuous flicker frequency, 

 after 1200 flashes; (3) first trial after darkness-flicker association, the response is 

 comparable to the previous one; (4-6) 3rd, 4th and 5th trials respectively, the 

 responses increase progressively (dishabituation, time conditioning); (7-10) 7th, 

 15th, 17th and lyth trials respectively, the response starts to decrease until in (10) 

 it attains amplitude and form comparable to those of (2) (rehabituation) ; (11 and 

 12) 2 1st and 22nd trials respectively, the rehabituation is accentuated and an after- 

 discharge of 20-22/sec. develops. Calibration, 100 msec. 



the flicker are an example of electrocortical time conditioning where the 

 period of darkness acts as a conditioned stimulus and the flicker as an 

 unconditioned one. 



