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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



wane or be entirely absent in random order. Finally, 

 all pulses would be responded to by equal waves but 

 of a reduced size, the amplitude in keeping with the 

 rate. If the rate was high the amplitude would be low, 

 and vice versa. This irregular initial period was 

 looked upon as a reorganization period during which 

 redistribution of the various retina-to-cortex channels 

 responsive to successive stimuli was brought about so 

 that all stimuli were responded to. 



Hartley (9) obtained a very definite cortical off- 

 response as well as an on-response in the rabbit when 

 using a .slow photic pulse rate and a pulse-to-cycle- 

 fraction of I to 4. The results are pictured in figure 1 1 . 

 In it, the shape and temporal characteristic of the 

 wave following the off-response would suggest that it 

 is the usual final slow component of a typical cortical 

 response or, in other words, an alpha wave. If this be 

 the case, then it is suggested that the off-response may 

 institute an alpha series in the same sense as it can be 

 said that a brief electric stimulus to the optic nerve 

 mav do so. The fact that an off-response is at all 

 discernible in the cortical record makes the supposi- 

 tion that the off-response plays a role in controlling 

 critical flicker frequency all the more plausible. 



Brightness Enhancement 



When intermittent photic stimulation is used at 

 rates below those producing the experience of steady 

 light (i.e. at subfusional pulse frequencies), it may 

 become more effective than steady stimulation in pro- 

 ducing brightness. This increased effectiveness we call 

 'brightness enhancement' which is pictured in figure 

 12. With intense pulses, effects such as shown in 



MSEC 



200 400 



INTERVAL BETWEEN SHOCKS TO OPTIC NERVE 



FIG. 13. The cycle of rcsponsixcnt'ss ot the optic cortex of 

 the rabbit as determined by paired stimulation. [From Hartley 

 (4)-] 



curve A will occur. With weaker photic radiation, 

 results shown in curve B will occur. While it is to be 

 taken for granted that photochemical processes in 

 sense cells play their usual roles in determining the 

 magnitude of afferent input over the optic nerve, 

 they do not account for the nature of brightness en- 

 hancement. We must look to neurophysiological proc- 

 esses for this. 



It will be seen from the diagram in figure 12 that 

 the effectiveness of intermittent stimulation increases 

 as pulse rate is reduced, and that under some condi- 

 tions it becomes maximum in the human in the region 

 of 10 pulses per sec. This region is the peak and still 

 slower rates result in reduced effectivenesses. One 

 might well start off with these findings and make 

 various manipulations of pulse rate, pulse-to-cycle- 

 fraction, pulse intensity, etc., to further one's under- 

 standing of brightness enhancement in general. The 

 study of Ijrightness enhancement has not proceeded 

 on this ba.sis. The work that has provided the impetus 

 for brightness enhancement investigation lay in the 

 findings of neurophysiology of the optic pathway. On 

 this account, it may well seem much clearer to the 

 reader were we to describe behavior of the visual re- 

 sponse apparatus before continuing to deal with 

 brightness enhancement. 



Bishop and Hartley, in their study of cortical re- 

 sponse to precise stimulation of the optic nerve in the 

 rabbit, disclosed a number of temporal and intensive 

 features of the behavior of the cortex. Bishop (18) first 

 demonstrated the rhythmicity for the cortex in rela- 

 tion to peripheral stimulation. Stimuli presented to 

 the optic nerve at intervals without regard to cortical 

 events produced random-sized responses. He showed 

 that stimuli could be tuned to the cortex, so that all 

 responses would be essentially the same; either all 

 small, all large or all medium-sized, depending upon 

 the phase to which the input was tuned. He showed 

 that if the first stimulus in a train was maximal, that 

 it would, in effect, 'drive' the cortex. This is to say, 

 it would be able to start oflT a sequence of cortical 

 consequences having the properties of the natural 

 rhythm but shifted somewhat in time from it. Subse- 

 quent closely-following stimuli would obey the laws 

 of the rhythmicity but according to the shifted timing. 

 Hartley (4) mapped the nature of the rh\thm by 

 using paired stimuli systematically varied in their 

 separation. He found that the size of a second maximal 

 stimulus to the optic nerve did not produce a cortical 

 response the same size as the first until the temporal 

 interval became equal to the cortical period found by 

 the means earlier discovered. The findings of Bartley 



