E. GARCIA-AUSTT, J. BOGACZ AND A. VANZULLI 605 



a slot of a given width. On the other hand this method has the advantage 

 of having a greater sensibihty, for potentials are added, unlike the other 

 method in which they are superposed only. Their greater sensibility made 

 it possible to record evoked responses with the stroboscope placed at a 

 distance of over 3 m. from the retina. 



In all experiments the frequency of the flicker stimulation was of 0.5- 

 5/sec. With both techniques, twenty to fifty sweeps were superimposed. 

 The stroboscope was situated at a distance of 3 m. from the eyes, but in 

 some experiments it was placed nearer to check the eftect ot the distance. 



The visual evoked response was recorded by these procedures in the 

 following regions: occipital (O2 electrode of 10/20 system), parietal 

 (P4), central (C4 and C6), frontal (F4) and temporal (T2 and T6). C4, T2 

 and O2 were the positions most often used. No response was recorded in 

 the nose and frequently not in the prefrontal region (Fp2). 



The evoked potential was constituted by a succession of regular naono- 

 morphous waves of 4-5 or more hemicycles. The usual sequence of polarity 

 was as follows : surface-negative, positive, negative, positive and negative. 

 The peaks of the three fu-st negative waves developed at 80-120, 180-240, 

 270-300 msec, and the peaks of the two first positive waves developed at 

 120-170 and 230-250 msec. In most cases the last waves were of longer 

 duration. 



The latency of the first negative wave was 15-40 msec. Sometimes the 

 latency was higher — from 80 to 100 msec, — because the first negative 

 wave was missing. Its absence was noted on various occasions when a 

 common reference electrode was used on the mastoid. On the other hand 

 It was never absent when the nose was used as a reference. The latency of 

 the first negative wave was at its minimum on O2. On C4 and T2 the 

 peak of the first negative wave had a delay of 15-30 msec, with respect to 

 O2. The duration of the waves was longer on C4 and T2 than on O2. 



The amplitude of the response was related to the duration of the waves: 

 the longer the duration, the greater the amplitude. With Dawson's 

 method the values reached were between 30-40 microvolts. The maximum 

 amplitude of the response was obtained on O2, the minimum on F4 and 

 in the mastoid, when the nose was the reference electrode. 



In general a slight increase in amplitude was observed in all regions with 

 eyes shut, although the superposition was better with the eyes open 

 because the alpha rhythm was then eliminated. All cases were studied in 

 wakefulness. The waking state was controlled by analysing the spon- 

 taneous electrical activity recorded by means of an electroencephalo- 

 graph. On bringing the stroboscope nearer to the retina, and thus increasing 



