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HANDBOOK OF PHYSIOLOGY 



NEUROFHYSIOLOGY 1 



apparatus, can not distinguish between a steady 

 source and one the intermittency of which is as low as 

 4 cps, just so long as the total flux per unit time is the 

 same in the two cases. Hence, although much has 

 been said about the photochemical basis for flicker 

 and its elimination, the ultimate crucial point of de- 

 termination of critical flicker frequency (c.f.f.) appears 

 to be in the cortex. 



Durinc flicker, the activity in the optic nerve waxes 

 and wanes with sufficient amplitude and at such rates 

 that the cortical activity may also vary in its temporal 

 aspects in significant ways. It has been noted that 

 whereas the response of the optic pathway up to and 

 including the postsynaptic elements in the lateral 

 geniculate body are ijrief and spike-like, response 

 beyond this is somewhat extended in time and in- 

 volves certain complexities absent in its precursors. 

 This in itself would be a kind of e\'idence for believing 

 that the cortex cannot respond at the same high rate 

 as the peripheral mechanisms. 



Be this as it may, a rate can be attained that re.sults 

 in the perception of uniform continuous light. The 

 point at which this is reached (c.f.f.) is also known as 

 the fusion point. All rates above this maintain fusion. 

 This means that at the fusion point any temporal 

 undulations in cortical activity that may occur are so 

 slight as to be of no ultimate effect. 



Talbot found that when fusion was reached, the 

 level of perceived brightness of the light field was less 

 than for a continuous and uniform stimulus of the 

 same intensity. The effect is as if the input instead of 

 being intermittent were uniform and spread evenly 

 throughout the cycle. Thus, if the PCF (pulse-to- 

 c)cle-fraction) is one-half, the level of brightness is 

 one-half. Whereas those devoted to photochemistry 

 have shown how this effect might be attributed to the 

 manner in which photochemical systems react to 

 photic impingements, certain features of the behavior 

 of the optic pathway have been overlooked. One of 

 these is the way the neuroretina behaves. It rearranges 

 the temporal distribution of the sense-cell discharge 

 effects of the retina. Since we are not dealing pri- 

 marily with peripheral respon.ses, we cannot go into 

 this matter further. Needless to say, the cortex must 

 take a hand in even the determination of critical 

 flicker frequency and the Talbot effect (7, 8). 



Since the Talbot effect represents the simplest 

 possible smoothing-out result from a waxing and 

 waning stimulus, we can suppose that the cortex 

 operates on the simplest principle in that respect. 



The following investigations in which cortical re- 

 sponse was elicited by stimulation of the retina rather 



than electrical stimulation of ilic optic nerve was used 

 to give some information relative to the mechanisms 

 at work in flicker and fusion. Bartley (3-5) measured 

 the latency of the cortical response to various forms of 

 photic stimulation. One of the factors varied was the 

 duration of a "dark' interval. When these intervals 

 were very short, the off-response to the termination of 

 the photic pulse and the on-response to the beginning 

 of the succeeding pulse were both evident in the 

 record when the interval was as short as 1 2 msec. 

 When this interval was shorter than the implicit time 

 of the ofl'-response, the resumption of stimulation 

 did not preclude the appearance of the off-response, 

 nor the appearance of the on-response to the begin- 

 ning of the next pulse. Since 12 msec, compare to the 

 interval between pul.ses when pulse frequency is 40 

 per sec, if the pulse-to-cycle fraction is one-half, it 

 would seem as though under the conditions dealt 

 with, Bartley was reaching the point called critical 

 flicker frequency in hmnan flicker experiments. 



The implicit times of the on- and off-responses are 

 not equivalent. It would .seem from the results (4, 5) 

 that for similar conditions the implicit times of the 

 on-response are shorter than those for the off-response. 

 Thus, as the 'dark' interval in the cycle is made 

 shorter and shorter, the off-response to the termina- 

 tion of the one pulse and the on-response to the be- 

 ginning of the succeeding pulse finally becomes con- 

 current. This might be one factor iit ijringing about 

 fusion in flicker experiments, since in some way these 

 two responses might counteract each other at some 

 final level in the cortex. 



That the two forms of response (on and off) could 

 be concurrent is to be understood from the finding of 

 Bartley that the two responses occupy separate chan- 

 nels all the way from the retina to the cortex. One of 

 the evidences for this was the finding that an on- 

 response can follow an off-response as clo.sely as 1 2 

 or fewer msec, whereas an on-response to a second 

 stimulus cannot follow unless the two are at least 80 

 msec, apart. Electrograms of the retina have been 

 interpreted as showing that a second pulse presented 

 shortly following the termination of the first will 

 inhibit the off-response to the first. Bartley (4) showed 

 in a number of ways that phenomena that were de- 

 tectable in the cortical record are not discernible in 

 the electroretinogram recorded under the same con- 

 ditions. It would thus seem logical to rely on the 

 cortical record in cases where differential responses in 

 the electroretinogram fail to show up. 



Bartley also measured the implicit time of cortical 

 on-re.sponse when duration was the variable (2) and 



