CENTRAL MECHANISMS OF VISION 



715 



ceived as appearing and disappearing. \'ision is also 

 response to spatial rclationsiiips of radiation origi- 

 nating in various directions from the eye. When these 

 relationships are experienced, we see objects at various 

 locations, manifesting various inovements and assum- 

 ing various directions from us. We are also visually 

 responsive to manipulations in timing of radiation 

 coming from various parts of the field. When this 

 response is in the form of experience, events are seen 

 as occurring in succession, or together in time, and 

 lasting for various durations. Photic radiation may 

 also be differentially responded to in terms of its 

 wavelength. The experience then is of color, hue, 

 saturation and brightness. 



It is meant to be clear to the reader that vision, 

 i. e. perceptual response, may occur in the form of 

 clear immediate experience, in the form of inarticulate 

 gross incipient reactions or in the form of articulate 

 differential motor responses. In the last analysis, the 

 sharp dichotomizing between the experiential and the 

 motor is only one of the possible ways of dealing with 

 beha\ior. It would seem that when one looks carefully 

 at the kinds of behavior which the human manifests, 

 it is more appropriate to postulate a kind of spectrum 

 of kinds rather than two opposite kinds with no form 

 intervening between them. 



The following are some of the essential types of 

 problems which we must handle as best we can with 

 our present information and interpretations, a) The 

 first problem is that of seeing various brightnesses, 

 i) Next we have the problem of seeing continuously. 

 It is known that many neurophysiological processes 

 are discontinuous. Were these the only processes we 

 could discover in the nervous system, the problem of 

 getting continuity from discontinuity would seem to 

 be a cardinal one. We are beginning to learn of sus- 

 tained activity or sustained state of potential, and 

 this may help a great deal, c) This is followed by the 

 problem of differential response to various parts of the 

 space field, since response to the field as a whole is 

 not what would be expected were the response to iso- 

 lated parts either independent or summative. d) The 

 problem of fine resolution is the question of how 

 closely adjacent parts of the field are seen as separate. 

 i) Finally, we have the problem of differential re- 

 sponse to various parts of the spectrum — both local 

 response to an isolated part of the field and response 

 to the whole field. 



While the problems that ha\e just been listed are 

 fundamental, they are in the form of generalizations 

 and cannot be dealt with as directly and as concretely 

 as is the case when particular visual phenomena are 



chosen. For most of our considerations, we have chosen 

 phenomena that are fairly specific but stand for the 

 broader classes to which they belong. 



The first general phenomenon appropriate for men- 

 tion is that of gross response to the simplest major 

 intensity differentiations in the field. This is the 

 response merely to one large part of the field as more 

 intense than the others. 



W'hereas the foregoing item may be thought to 

 have to do with brightness, brightness is an experi- 

 ence. It and any overt response that seems to be 

 related to it in an experimentally approachable way 

 had better be put in a class by themselves. Hence, in 

 the present category, we refer to the higher order re- 

 sponses to intensity relations in the stimulus field. 

 These responses would be expected to be based on 

 cortical function, whereas those in category above may 

 be subcortical. 



The observer experiences undulations of darkness 

 and lightness in temporal sequence. This is flicker. At 

 high rates of intermittent stimulation this experience 

 is lost, a fact implying that the neural or some other 

 mechanism is unable to keep pace. Differential re- 

 sponse to the intermittent stimulation of the optic 

 mechanisms of subhuman species also are quite 

 common and manifest many close parallels or simi- 

 larities to the quantitative features of the experiential 

 responses of the human subject. 



Brightness enhancement is another feature of 

 human experiential response. It is the case in which 

 intermittent stimulation results in a higher brightness 

 than continuous stimulation of the same intensity. 



Bilateral functions involving the use of the two eyes 

 result, of course, in a different input into the central 

 nervous system than the involvement of one eye alone. 

 It has been found that both the experiential and the 

 oculomotor outcomes differ in the two cases. 



Brightness contrast occurs when fields made up of 

 certain patterns and intensities of radiation are pre- 

 sented. They are reacted to in ways not predictable 

 from the separate local intensities of the parts of the 

 field. These parts are not independent in effect nor 

 are the results simply additive when they are interde- 

 pendent. The major phenomena in this category are 

 often known as brightness-contrast phenomena. 



Visual movement creates complexities. Portions of 

 the visual field are not stable. They quickly appear 

 and disappear as segregated portions that may or may 

 not undergo spatial displacement. When movement 

 is seen under conditions where no visual target ele- 

 ments are displaced, it has been customary to call the 

 movement 'apparent movement.' When displace- 



