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'PHOTIC PULSE 



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FIG. II. The cortical response of the rabbit to intermittent stimulation of the eye in which the 

 photic pulse occupied one quarter of the cycle. Note responses both to onset and to termination 

 of the photic pulse. [From Bartley (9).] 



FIG. 12. Brightness enhancement, the greater relative effectiveness of intermittent stimulation 

 than of steady stimulation at lovkf photic pulse rates. This is shown in curve A but not in curie B. 

 B shows the effect under some conditions of weak stimulation. [From Bartley (7).] 



when area of the target was a variable (3). For varia- 

 tion in duration of photic pulse to affect implicit time, 

 it must be as short as 4 or 5 msec, for targets with 

 luminosities as great as 2400 candles per square foot, 

 and which subtend 6 or 7 degrees. Increasing area 

 reduced implicit time and thus would be expected 

 to work in the direction of raising critical flicker 

 frequency. 



Jasper (46) recorded potentials from the occipital 

 cortex in man following pulse rates of 55 to 60 per sec, 

 and thus was near the critical flicker frequency under 

 the conditions. This was not interpreted as being a 

 demonstration of driving the cortical alpha rhythm 

 beyond its normal 8 to 1 3 per sec. frequency. He found 

 that the amplitude of the waves at 20 per sec. was 

 about one-half of what it was at 10 per sec, and waves 

 at 40 per sec. were about one-fourth as high as those 

 at 10 per sec. A further \ery crucial observation also 

 supporting his interpretation of the impossibility of 

 driving the cortex was the following. As the stimulus 

 rate was slowlv increased, there were stages at which 

 the waves would undergo what he called desynchroni- 

 zation. For example, at frequencies of from 14 to 15 

 per sec, this would happen and the result would in- 



clude a shift in amplitude so that at from 18 to 20 

 per sec, the amplitude would drop to one-half the 

 height up to that time. 



Halstead and colleagues (44, 45, 67) reported that 

 although the dominant brain waves (alpha) in the 

 monkey could be 'driven' up to rates comparable to 

 critical flicker frequency only, the pathway prior to 

 the cortex could follow input intermittencies beyond 

 the c.f.f. Obviously, the records of Halstead and col- 

 leagues manifest waves at the rates indicated and 

 thus, in essence, tally with those of Jasper. Whether 

 this is driving depends upon one's definition of the 

 term. A standard definition has not yet been put into 

 the literature. 



The foregoing tallies with Hartley's observations 

 and his alternation of response theor\ (which is con- 

 sidered in the section on brightness enhancement). 

 For example, one observation (3) was that if the rate 

 of intermittent retinal stimulation suddenly delivered 

 to the retina was definitely above 5 per sec. (the rab- 

 bit's alpha rate), the following would occur. A large 

 cortical response to the first pulse would appear. No 

 response to the second pulse would result. Then, the 

 responses to the following few pulses would wax and 



