7o6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



a large range of intensities the on-off ratio will undergo 

 considerable variation. This variation has been held 

 to support a mechanism of discrimination based on 

 the overlapping receptive fields. Overlapping recep- 

 tive fields, less elaborated than in the retina, also 

 occur in other sense organs. These principles have 

 been discussed at some length by Granit (73). 



Many problems of retinal neurology have been 

 clarified by Hartline's work on the much simpler 

 Limulus eye, to which students of the verteljrate retina 

 are advised to give careful attention. This in par- 

 ticular applies to the recent important analysis of the 

 lateral inhibition (90) in the neural network below 

 the ommatidia, because the suppression of the dis- 

 charge of one ommatidium by its illuminated neigh- 

 bor is clearly a mechanism of contrast. In the frog 

 retina Barlow (15, 16) has found the discharge from 

 a receptive field to be inhibited by illuminating retinal 

 regions just outside it. 



There are other aspects to the problem of inhibition 

 than those connected with the organization ot the 

 receptive fields. There is, for example postexcitatory 

 inhibition (73) and the generalized inhibition recently 

 described Ijy Dodt (43). 



STIMULUS CORRELATES 



The average electroretinographic response is 

 roughly proportional to the logarithm of stimulus 

 intensity. However, since most retinae contain two 

 organs in one (rods and cones) and the ERG's of 

 rods and cones do not relate similarly to stimulus 

 intensity in addition to differing in latency and rate 

 of rise (see above), too much should not be made out 

 of this general logarithmic relationship. It does, how- 

 ever, suggest that the elementary generator potentials 

 of which the total response is made up tend to be 

 logarithmically related to stimulus intensity, and this, 

 whenever they have been recorded, actually is the 

 case [cf. the discussion by Granit (73)]. In Limulus 

 the spike frequency emanating from the excentric cell 

 of a single ommatidium is logarithmically related to 

 stimulus intensity meaning that the level of om- 

 matidial generator potential, at least when stabilized, 

 is directly proportional to spike frequency [the cor- 

 responding relation for muscle receptors has been 

 described (97)]. Many single fiber preparations rep- 

 resenting receptive fields in frogs (86) and mammals 

 (67) have shown a general logarithmic relation to 

 stimulus intensity, superimposed, as it were, upon 

 on-off complexities. The overall effect of an assembly 



of cells is likely to follow this rule which, of course, is 

 nothing but the well-known Fechner law looked at in 

 a diflferent way [for a full discussion, see Granit (73)]. 



The retina is a detector of so-called visible light 

 which is visible because photochemical substances 

 within the rods and cones absorb energy and trans- 

 form it into a form appropriate for stimulation. Most 

 color theories have assumed that different kinds of 

 cones are provided with photochemical substances ad- 

 justed for absorption of light within different parts of 

 the spectrum but the only substance known until 

 fairly recently was Boll's visual purple or rhodopsin 

 in the rod outer limbs, and these organs, on the 

 duplicity theory, were assumed to be color blind. 

 This was the situation until some experiments with 

 the electroretinogram (78, 84) definitely proved that 

 the frog light-adapted eye showed distributions of 

 spectral sensitivity that required a minimum of three 

 cone substances to be intelligible. This has since been 

 confirmed in many other types of experiments with 

 different eyes. In light-adapted frogs and in the cone 

 eyes of the turtle, Forbes et al. (55) showed that, if 

 they were illuminated by two 'white' lights, these 

 could be exchanged without influencing the ERG but 

 that certain pairs of colored lights never could be 

 exchanged, whatever their intensity ratio, without a 

 specific electroretinographic color response. Japanese 

 workers (62, 63, 140) using frogs studied the multiple 

 off-effects and wavelets on top of the off-effect, men- 

 tioned above, and found evidence for a representation 

 of differential spectral sensitivity in the different crest 

 times of such wavelets. There was a minimum of three 

 humps appearing in the order red, green and blue, 

 as also shown in the recent work of Heck & Rendahl 

 (93). A similar order had previously been observed 

 bv Donner (51) working with the spike frequency- 

 time differentials of single ganglion cells in the cat 

 retina and by Motokawa and his group (108, 109, 

 143). There is a critical review of Motokawas work 

 by Gebhard (59). Motokawa's measurements were 

 based on the rate of rise of retinal sensitivity to a 

 brief polarizing current after preillumination with 

 different wavelengths. 



Recent work with the human electroretinogram is 

 definite in showing that the ERG contains com- 

 ponents of different color sensitivity, even in the light- 

 adapted state (10, II, 93, 130). HowcNcr, all work 

 with a mass response such as the ERG suff"ers from 

 the difficulty of isolating the spectral components in a 

 quantitative way. 



Color vision as an electrophysiological problem 

 contains two different aspects: a) the primary sensi- 



