736 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Bilateral Fiinclions 



In viewing a single visual target, both sides of the 

 optic pathway are generally involved. When corre- 

 sponding portions of the two retinas receive photic 

 radiation from this target, the results are as follows. 

 If the photic flux is unequal to the two retinas, the 

 surface seen will, of course, be singular but will not 

 look as bright as though viewed by the eye receiving 

 the greater radiation. That is, summation does not 

 take place. This is Fechner's paradox. If, instead, 

 equal radiation is received by both eyes, the result is 

 summative. The brightness is greater than when one 

 or the other eye views the target alone. Fechner 

 did not make a full study of this matter. DeSilva & 

 Hartley C39)> snd Fry & Hartley (41) manipulated 

 stimulation so as to provide curves showing this func- 

 tion under a wide range of conditions (see fig. 15). 

 Hartley (lo) later studied a correlate phenomenon of 

 Fechner's paradox, namely the way the pupil behaves 

 under comparable conditions of stimulation. The 

 same pattern of quantitative response was demon- 

 strated. This is to say that if one eye alone is presented 

 successive increments of photic radiation, the pupil 

 constricts step by step. The same occurs in the un- 

 stimulated eye. Then, if the flux to the first eye is held 

 at the final high level and step by step the flux to the 

 second eye is increased from zero upward to the level 

 for the first eye, at first, the paradoxical reversal of 

 effect occurs. Instead of constricting further, the two 

 pupils begin to dilate. They continue to do so as 

 further steps of increment are added to the second eye. 

 Finally, the paradox reaches a peak and further incre- 

 ments begin to cause constriction. When finallv both 

 eyes are receiving equal amounts of radiation, the two 



pupils have constricted more than when the final level 

 of radiation was directed to the one eye alone. This is, 

 in quantitative pattern, the very thing that happens 

 in perception. 



It was also shown that the essentials of the paradox 

 are manifested when noncorresponding points of the 

 two eyes are involved and finally when two targets of 

 differing inten.sity are imaged on two parts of a 

 single retina. 



It is not too startling to find the parallel between 

 the perceptual and the motor phenomena when once 

 it is remembered that the two end results stem from 

 the same input, namely the discharge of the optic 

 nerve. The same pattern of input must go to the 

 geniculate and to the superior coUiculus. In our illus- 

 tration we have two simple aspects of the respective 

 categories of response. Were we to try to compare 

 other aspects of motion and perception (experience), 

 we would be hard put to find modes of quantification. 

 That is to say, it would be very difficult, if not impos- 

 sible, to find convincing quantitative parallels in limb 

 movement and in the concurrent perception of color 

 or position of a seen object. Vet there must be rational 

 (lawful) relations between what an organism sees and 

 where it reaches to grasp the object seen. 



Binocular relations of other sorts are Ijroughi out in 

 other perceptual phenomena. If two fields differing in 

 texture or color as seen separately are presented, one 

 to the one eye and the other to the second eye simul- 

 taneously, the result may not be a fused single stable 

 field but rather a single field that alternates in texture 

 or color. This is called l^inocular ri\alry. Some times 

 this ri\alry in lightness is replaced by a curious effect 

 called 'luster.' Further analysis shows this luster to be 

 a transparent light field behind which is a dark field. 



FIG. 15. Binocular summation and 

 subtraction in response to photic stim- 

 ulation. The subtractive effect is Fech- 

 ner's paradox. Top line indicates the seen 

 brightness when both eyes are presented 

 equal intensities. Horizontal line ('one 

 eye alone') indicates the seen brightness 

 when only one eye is exposed to the 

 target. The curve shows the relative seen 

 Ijri^htncsscs when one eye is exposed to 

 the full intensity of the target and the 

 second eye is exposed to various frac- 

 tions of full intensity shown on the hor- 

 izontal axis. [From Da Silva & Bartlcy 

 (39).] 



1/16 1/8 1/4 1/2 3/4 7/8 15/16 



RELATIVE AMOUNT OF ILLUMINATION PRESENTED SECOND EYE 



