738 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



in the human subject, namely the exposure of two 

 restricted local targets separated by an interspace and 

 presented in sequence. To go with this target arrange- 

 ment from conditions that are not productive of 

 apparent movement to those that are is simply to 

 adjust timing and spatial separation for the given 

 intensity used (Korte's laws). In this investigation 

 timing and spatial separation were manipulated, and 

 it was demonstrated that cortical responses to two 

 separate targets could be recorded at two separate 

 cortical locations in the rabbit and that various inter- 

 action effects were obtainable when space separations 

 were reduced and when the delivery of the stimuli 

 was made close together in time. The elements of a 

 study of apparent movement were demonstrated. The 

 in\estigation did not go far enough to determine the 

 conditions under which the rabbit responds to two 

 stimuli as to a single moving target. A conditioning 

 experiment would ha\e Ijeen necessary for this. Thus, 

 if the rabbit could be 'conditioned to apparent move- 

 ment,' the cortical experiments, carried further than 

 Bartley (4) was able to do, could possibly have given 

 a picture of some of the cortical e\ents involved in 

 seeing movement. 



Color Vision 



No consideration of vision should bypass what is 

 called color vision, the difTerential response to the 

 spectrum. In discussing color vision, there is very 

 often some confusion as to what is really meant, owing 

 to the fact that the stimulus differentiators may lie 

 not only at the periphery but also in the central ner- 

 vous system, and owing to the possibility of setting up 

 diflferent criteria for color response. There are actually 

 si.x items to keep in mind and make clear in a dis- 

 cussion of color vision. They will be mentioned here 

 to set matters straight, a) There is the question of the 

 existence of color sense cells and the number of kinds 

 of such cells in the species in question, i) Often this 

 discus.sion takes the form of whether some of the cells 

 are differentially sensitive to the spectrum and some 

 not sensitive (cones and rods). All of these matters 

 have been studied on an anatomical basis, c) There is 

 the question of directly or indirectly recording elec- 

 trical responses to answer the questions in a and h. 

 d) There is the problem of obtaining differential con- 

 ditioning of overt responses to the spectrum in the 

 species in question, e) In human subjects, there is the 

 study of color experience. /) There is the realization 

 of the possibility that any species might possess a 

 well-developed spectral analyzer of which it can make 



little or no use. For example, the eye of a rabbit or a 

 cat or a monkey may be quite like that of a human, 

 but this does not mean that in any or all of these cases 

 there is the same color experience. In fact, we know 

 nothing of subhuman experience in any case. 



For our purposes here, we want to know the role 

 played by central mechanisms in either muscular dif- 

 ferential response to the spectrum or in the production 

 of various color experiences. Obviously, even though 

 we credit the retina in both its photochemical and 

 neural mechanisms as being a keen analyzer and thus 

 providing the central nervous system with a differen- 

 tiated message, the central apparatus must also, in a 

 way, be an analyzer, else it cannot make differential 

 use of the message. The requirement of an analyzer 

 applies both to the center and the periphery. This is 

 made apparent to those possibly more difficult to 

 convince by the fact that color experiences can be 

 predictably elicited by nonspectral stimuli. Certain 

 alternations in intensity of stimulation as produced by 

 a rotating disk with high- and low-reflecting ('white' 

 and 'black') portions are sufficient to produce color 

 experience. The central apparatus responds to this 

 nonspectral presentation in the same fashion as to 

 certain spectral presentations. 



The foregoing phenomena taken together, or many 

 of them taken alone, lead us to the conclusion that 

 for much of what we call vision we must include the 

 central mechanisms that are not visual, else we have 

 nothing that can be called vision. It is customary to 

 call the surrounding areas association areas, but we 

 see that their function is not to associate rigid units of 

 activity each of which plays a single role but rather 

 to participate in the overall differentiation of activity 

 we call response. 



Cortical as well as retinal responses to spectral stim- 

 ulation (200 msec, in length) have been recorded by 

 Lennox & Madsen (52). Simultaneous records from 

 the cortex and retina were compared in wa\e form, 

 amplitude and latency. The spectral points involved 

 were 'blue' (445 m^i); 'green' (560 m^); 'yellow' (575 

 mti); and 'red' (620 mti). 



The recordable threshold of the cortex lay about 

 one logarithm below the retinal threshold. The on- 

 response of the cortical potential consisted in a di- 

 phasic wave, initially surface-positive. At low and 

 moderate intensities, the positive response was double. 

 At high intensities the initial phases contained four 

 or five spikelets. 



Increasing stimulus intensity decreased the latency 

 of both the retinal and cortical responses and in- 



