CENTRAL MECHANISMS OF VISION 



729 



experiments in which the target was increased beyond 

 about 20 degrees, a break in the curve appeared. The 

 question of how far across the retina spatial summa- 

 tion may operate has been dealt with by Adrian and 

 others. To say the least, spatial summation has come 

 to be considered to possess definite limitations. The 

 limiting subtense in this investigation seemed to be in 

 the region of 20 degrees. The curve in each experi- 

 ment shows that when targets of broad angular sub- 

 tense are reduced in size, the implicit time is length- 

 ened. This continues until the target is reduced to 

 about 20 degrees and then, fairly quickly, implicit 

 time becomes much shorter than expected for further 

 target area reductions. 



While the present investigation offered no way to 

 eliminate the stimulation out.side the retinal image, it 

 did present evidence of the operation of two stimulus 

 components (increase in image area and increase in 

 stray radiation intensity). The same evidence demon- 

 strated that the two factors operated at different rates. 

 In the demonstration that two factors were in opera- 

 tion, the author showed intensity per unit area of 

 retinal image was involved in determining implicit 

 time. Thus it can be said it was not merely flux as 

 such that produced the response as recorded but that 

 the response was in a way a brightness response. 

 While it was not doubted that the rabbit has bright- 

 ness \ision, it was a question from the beginning as 

 to whether it could be demonstrated by the neuro- 

 physiological experiments of the kind being per- 

 formed. 



Bnglitness 



The experience of brightne.ss or a motor response 

 based on the same principle is something different 

 than the response to mere flux differences in two 

 major portions of the photic field. Brightness is the 

 result of manipulation of intensity per unit area of 

 visual target. Hence an area can be seen as brighter 

 than another even though the total flux of the first 

 area is less than that of the second. This would be the 

 case if the flux per unit area were greater. Kliiver's 

 monkeys, devoid of the geniculostriate system, gave 

 no evidence of being able to do this. They could learn 

 to distinguish between two equal-sized and equally 

 intense targets when one was removed to a greater 

 distance than the other, in which case its retinal 

 image was smaller. A lesser total flux on the retina 

 for it than for the near target was thus invoked. 

 Kliiver states that he does not see any evidence in the 

 behavior of cats and other animals that would indicate 



that they can distinguish brightness at subcortical 

 levels. 



Electrophysiological experimentation upon the re- 

 sponse of the optic pathway has not been of such a 

 nature as to make the needed distinctions between 

 response to total flux and to flux per unit area. One 

 way to extract evidence on this point is to try to com- 

 pare certain perceptual responses with the electro- 

 physiological ones and make what deductions we can. 



It now seems as though certain comparisons be- 

 tween brightness in perception and the amplitude of 

 the response to specific brief stimulation can be made. 

 In a later section we deal with brightness enhance- 

 ment. In it we are comparing the experience that is 

 evoked by steady continuous stimulation with one 

 that is evoked by intermittent stimulation. So long as 

 we keep the two areas equal, certain justifiable com- 

 parisons between the intensity needed in both cases 

 to produce equally bright surfaces can be made. 

 Many of the conditions for producing the various 

 levels of effectiveness of the intermittent stimuli seem 

 to be the very same as tho.se similarly varying the 

 amplitude of the cortical response to such stimuli. 



Since continuous steady stimulation produces noth- 

 ing in the extended record of cortical activity, the 

 amplitude of which we can measure, we are prevented 

 from making the same amplitude-brightness compari- 

 sons for steady photic impingements. With the con- 

 crete evidence at present available, we seem unable to 

 go beyond the gross comparison just described. Per- 

 haps we do not know enough regarding the measure- 

 ment of steady states and the relation of steady states 

 in one part of the cortex to those in others. Steady 

 states seem, at the present state of our knowledge, to 

 be quite dead and processless. To explain some things, 

 however, they seem to be just what is required. (For 

 further discussion of cortical response to continued 

 peripheral stimulation, see the later section on bright- 

 ness enhancement.) 



Flicker and Fusion 



When a series of photic pulses is delivered to the 

 retina, the experience is flicker, except when the rate 

 of delivery reaches a critical value. Obviously, the 

 intact human and even certain subhuman species can 

 distinguish between an intermittent and a steady 

 photic source. This is true at least down to arthropods 

 and crustaceans. How the discrimination is accom- 

 plished must differ in detail at the various phylo- 

 genetic levels. It would seem from Kliiver's observa- 

 tions that a monkey, clepri\-ed of the geniculocortical 



