1458 



M WDHIIIlK Hi PHYSIOLOGY 



NKIROI'HVSIOI.OGV III 



drawn from this work is that information about the 

 spectral composition of the stimulus is coded by the 

 modulator mechanisms into both spatial and tem- 

 poral patterns of optic nerve discharge. 



None ol the modulators in the cat has a separate 

 retinal pathway. I lie high degree of neural con- 

 vergence precludes the possibility that spectral in- 

 formation is coded on the basis of different pathways 

 for different wavelength ranges. The experiments ol 

 Donner (55) support the notion that the coding is 

 based largely on the temporal distribution of impulses 

 in the optic nerve. Man) 'on-ofF libers in the cat 

 retina exhibit maximal spiking at one or another of 

 three places in the time course of their discharge, the 

 position of the maximum depending upon the wave- 

 length, but not the intensity, of the stimulating light. 

 In the cat, then, the modulator mechanisms utilize 

 primarily the frequency code of the optic nerve dis- 

 charge. "In animals well supplied with modulators, 

 such as snakes, pigeons, and to some extent frogs, color 

 reception is likely to be based on both "topography" 

 ('place' or local sign') carried bv modulators and on 

 specific frequency patterns" (8j, p. ->Bg). Unfor- 

 tunately, similar experiments have not been done at 

 the primate level, but the peripheral mechanisms are 

 probably not essentially different from those of the 

 forms studied. 



( )n the basis ol anatomical observations, LeGros 

 Claris (39—41) proposed that color discrimination 

 might be accounted lor in pari by a place or topo- 

 graphical principle. The lateral geniculate nucleus ol 

 primates is divided into six layers, three receiving 

 libers from the contralateral nasal hemiretina and 

 three from (he ipsilateral temporal hemiretina. Clark 

 suggested that each Layer transmits impulses from 

 die modulators related to one of the three primary 

 c omponents of the trichromatic theory of color vision, 



six layers being needed to eonvev (he impulses from 

 (he two e\es. I he e\ idence presented by ( 'lark to SUp- 

 poi I his h\ pothesis has been criticized b\ Walls U 1 j I 



The proposal must be regarded as a tentative one, 

 needing further experimental confirmation. In other 

 recenl studies, a search has been made loi electro- 

 physiological correlates ol color vision. Some e\ idence 

 has been reported indicating that amplitude, latenc) 

 and shape oi responses evoked hum die visual cortex 



varv with (he color Used in Stimulating (he e\ e 1 ;-, 



1 j-', 1 _•■•;, 135). The results reported to date are only 



jestivc II these characteristics of central nervous 



system response do have .1 relation to coloi vision, it 



would appe.11 advisable 10 explore them in the 



primate or in another mammal known to have good 



color vision rather than in the cat which has been 

 shown to be deficient in color discrimination (44, 51, 



143)- 



Studies of behavioral discriminations of color be- 

 fore and after the production of lesions in neural 

 pathways and centers of the visual system provide 

 almost no evidence that will help in formulating a 

 hypothesis as to a neural basis of color vision. In the 

 monkey, there is evidence that discrimination on the 

 basis of color cannot be made after bilateral ablation 

 of the visual cortex (1 15). In other animals such as the 

 dog and cat, which have been used most often in be- 

 havioral studies of vision, experiments indicate that 

 at best the intact animal has a very low order of color 

 vision. As pointed out above in discussing; electro- 

 physiological investigations, such animals are ques- 

 tionable subjects for color discrimination studies. 



Reports from the clinic, likewise, furnish very little 

 additional information on the role of the visual cortex 

 or of lower centers in color discrimination. Electrical 

 stimulation of the striate areas in man snves rise to 

 reports of visual sensations which include that of color 

 (163, p. 143). Ablation of parts of the visual areas in 

 man produces scotomas in which complete absence of 

 color vision is usually reported. 



The evidence, though it is limited, docs not lend 

 much support to an explanation in which color dis- 

 crimination is accounted for on the basis of separate 

 sense-organ and neural units for the separately dis- 

 criminable colors. On the other hand, it still remains 

 a possibility that there are a number of separate color 

 receptors in the retina and that they maintain to some 

 extent independent central nervous system connec- 

 tions. It is more probable that different colors are 

 represented in the central nervous system by patterns 

 of activitv which vary with respect to temporal and 

 Spatial arrangement of neural activitv in groups of 

 neural units. 



I Ielmholt/'s extension of the specific nerve energy 



theorv to account for pitch discrimination in hearing 



lias met with greater success than the similar Sugges- 

 tion lor color vision, lbs resonance theorv of hearing 

 proposed that sounds are analyzed bv the cochlea in 

 such 111. inner that for eacdi discriminablc pitch, there 

 is a separate receptor unit which is excited bv a given 



frequency or narrow band ol frequencies. (Cochlear 



processes are considered bv Davis in Chapter XXIII 

 and central auditor) mechanisms bv Ades in Chapter 

 XXIV ol this Handbook. > Although the original simple 

 teson.mce principle ol analvsis has been proved inade- 

 quate, it is generall) accepted that peripheral analvsis 

 does lake place, that high tones excite receptors in the 



