CENTRAL MECHANISMS OF VISION 



737 



It is as if one can be seen through the other. This con- 

 current existence of the two fields is best brought out 

 when one uses a temporal alternation of the targets for 

 the two fields. Thus when the over-all binocular tar- 

 get is made up of a steady annulus surrounding an 

 intermittent disk, luster eventuates when the intensity 

 of the annulus is one-half that of the positive phase of 

 the disk. Whether it shows up or whether one sees 

 simply light and dark alternating is dependent partly 

 upon the rate of stimulus intermittency. As one slowly 

 shifts the rate, one can watch the phenomenon 

 emerge. 



Rivalry can occur even when the target \iewed as 

 first described constitutes only a small part of the 

 visual field. Ri\alry in this case occurs when the field 

 surrounding the target is of medium brightness and 

 the target seen via one eye is white and via the other is 

 black. Apparently the neural contour processes for 

 the two targets interact in some way that involves some 

 sort of alternation, thus bringing about the rivalry (8). 



It has been shown by Graham (43) that the abso- 

 lute light threshold is no lower for the two eyes than 

 for one. This finding, though it might be unexpected, 

 is in line with the supposition of summation at a 

 common central region, as found by Fry & Hartley 

 (41). The latter pointed out that two thresholds must 

 be recognized : a) the minimal radiation required to 

 activate either of the two converging pathways, and 

 A) the minimal frequency of impulses reaching a com- 

 mon central region to produce postsynaptic activity. 

 The perceptual end result does not occur unless one 

 or the other of the two pathways delivers the threshold 

 frequency. If in either one of the two the minimum is 

 reached, there is no lowering of the first threshold by- 

 adding a stream of impulses via the other converging 

 pathway. 



Brightness Contrast 



Whereas the foregoing illustration (Fechner's para- 

 dox and its pupillary analogue) was meant to demon- 

 strate intensive effects based upon the interaction of 

 the two sides of the visual apparatus, it also exemplifies 

 brightness contrast inasmuch as it has to do with 

 adjacent as well as corresponding areas of the two 

 retinas and with adjacent portions of a single retina. 

 Brightness contrast pertains to adjacent portions of 

 the \isual target, but to explain it relevant adjacent 

 portions of the visual apparatus must be dealt with. 

 It would seem that whatever neural mechanism will 

 account for Fechner's paradox will go a long way in 

 accounting for brightness contrast. 



I isual Movement 



Brightness contrast, a spatial phenomenon, is a con- 

 figurational one. The same principle would seem to 

 apply to both perceptual and neurophysiological 

 phenomena described in temporal terms. One order 

 of temporal phenomena in perception is the experi- 

 ence of movement. Very often the crucial neural con- 

 ditions underlying movement have been thought to 

 be retinal and neuroretinal. These cannot be given 

 space here. Be it sufficient to say that in this category 

 lie .some of the conditions for apparent visual move- 

 ment. Apparent movement is defined as phenomenal 

 (experienced) movement that is elicited by visual 

 targets that do not undergo displacement. Real move- 

 ment is the movement stemming from targets that do 

 undergo displacement. 



Despite all the patterning produced in the retina, 

 there is still much left for the cortex to do. The cortex 

 probably plays a part in making the end product 

 resulting from optic nerve discharge under conditions 

 of target displacement often very similar to that ob- 

 tained with target fixity. We know that the perceptual 

 end results, in some cases, are indistinguishable. 



Since in beta movement (the form of apparent 

 movement in which two spatially discrete targets are 

 used) there is a temporal gap between the two portions 

 of stimulation, the cortex was \ery definitely Ijrought 

 in by early workers to account for it. Supposedly some 

 sort of spatial-temporal coalescence of the afferent 

 discharge into the cortex was finally achieved in cor- 

 tical activity much like that produced by the periph- 

 eral input from real-mo\'ement targets. An early form 

 was called a short-circuit theory, but never has a 

 theory been worked out to the point of being con- 

 N'incing. It would seem that in the recent establishment 

 of the nature of dendrite acti\ity, we have an essential 

 tool for this purpose. Until recently, any theorist 

 wishing to account for certain more persistent effects 

 (those lasting up to seconds, minutes or longer) had 

 to rely upon purely hypothetical processes such as 

 those described by Kohler (51) or upon reverberative 

 circuits. Now it would seem that with the demonstra- 

 tion that some tissue does maintain potential and not 

 merely conduct potential \ia a fleeting impulse, cer- 

 tain more slowly changing active relations between 

 tissue elements or central nervous regions are made 

 more concretely thinkable. 



The investigation (4) described in an earlier section 

 on cortical localization is relevant here. Actually, the 

 stimulus conditions used in this investigation were 

 the very ones that produce apparent visual movement 



