Part III  Chromatic rod vision: a historical account

The finding that stimulation of rods alone may give rise to qualitatively different colour sensations came as a surprise, since it challenged the fundamental Principle of Univariance. This principle follows from Helmholtz’s (1896) specific fibre-energy doctrine and implies that a given receptor or nerve fibre does not discriminate between variation in intensity and wavelength of a test light and hence mediates only one sensory quality. Accordingly, when only the rod receptor system is stimulated, variation in wavelength can be simulated by variation in intensity – they both produce variation in brightness only.

Indeed, the Principle of Univariance had been directly demonstrated by Graham and Hartline (1935). By analyzing the nerve impulses arising in the retina of the Limulus (horseshoe crab), where each photoreceptor is linked with a separate nerve fibre, they found that the variation in the response of the single fibres with wavelength could be simulated by suitably adjustment of the incident light energy. Thus, when the intensity was suitably adjusted, any test wavelength could be made to evoke the same frequency of impulses from a given receptor cell. Hence, it appeared that single retinal receptors alone had no power to discriminate between wavelength and intensity.

How, then, could the Principle of Univariance be reconciled with the new discovery that test stimulation of rods may give rise to all the principle hues of the spectrum? An answer to this question became apparent when it was discovered that the scotopic hues were due to rod-cone interactions. It followed that the Principle of ­Univariance and the specific fibre-energy doctrine of Helmholtz (1896) should only be applied to functionally independent receptor systems.

11  Night vision may appear bluish

The origin of the idea that rods may give rise to chromatic sensations may be traced back to the hypothesis of Ebbinghaus (1893) that the photopigment of rods (rhodopsin) was the yellow-blue see-substance postulated by Hering (1878), and the suggestion of König (1894) that the rod receptors represented the primary ‘blue’ receptor system of photopic vision. Substantial evidence in favour of the idea was provided by von Kries and Nagel (1896) who found that twilight vision contained a tint of blue.

In order to identify more precisely this blue colour quality of rods, von Kries (1896) attempted to find the spectral light that did not change its hue when test intensity was reduced from photopic to scotopic levels, i.e. when the rod component increased. He used Nagel (a deutranope) as his subject and instructed him first to make a colour match at a photopic intensity level between a homogeneous spectral test light from the short-wave region of the spectrum (and hence with a relatively high potential scotopic value) and a mixture of spectrum red (670 nm) and violet (435 nm) lights (and hence with a relatively low potential scotopic value). Thereafter, the intensity of the test and comparison fields was reduced in the same proportion­ from the photopic level until the homogeneous light strongly activated the rod component. His results showed that the spectral light that did not change in colour quality with intensity reduction was situated between 480–485 nm, i.e. in the green-blue part of the spectrum.

Dreher (1912) made a somewhat similar investigation using two trichromats as subjects and confirmed the results of von Kries. G. E. Müller (1923), in his review on the colour quality of night vision, also concluded that, in addition to the dominant achromatic component, it entailed a tint of green-blue. He presumed that this green-blue colour

110chromatic rod vision: a historical account

was generated when light activated rhodopsin, but could not decide with certainty whether the rhodopsin molecules generating the green-blue colour were situated in the outer segment of rods or cones, although he thought it most likely that the green-blue colour had a rod origin.

It should be noted, however, that there was no general agreement as to the colour quality of night vision. Most subjects described night vision as achromatic. Even von Kries (1896, p. 87) found the word ‘farblos’ (‘colourless’) an appropriate description. Thus, in a footnote he remarked,

Es wäre also vielleicht richtiger, nicht zu sagen, dass die Stäbchen farblose Empfindungen, sondern dass sie einen nur einsinnig veränderlichen Empfindungseffekt liefern. Da indessen der Stäbcheneffekt sich schwerlich in erheblichem Masse von der Farblosigkeit im gewöhnlichen Sinne unterscheidet, so schien es mir besser, den obigen, seine Bedeutung jedenfalls sehr viel anschaulicher kennzeichnenden Ausdruck beizubehalten.

(Thus it might be more correct not to say that the rods give rise to colourless sensations, but that they give rise to one quality only. Yet, since the rod effect does not differ much from what is generally considered as achromatic sensation, it appears to me better to retain the above expression, which, at any rate, gives a far more elucidating characterization.)

The discrepancy between the reports of different subjects with regard to the colour quality of night vision could most easily be explained by individual differences. Yet, an alternative and more fruitful hypothesis was offered by Nagel (1911). He assumed that the colour quality of twilight vision, for each subject, may vary from colourless to blue. Thus, he wrote, ‘Incidentally, it is quite conceivable that the light sensations that occur under the condition of pure twilight vision have a certain range of fluctuation as to their quality, varying from absolute colourlessness to a cyan-blue of no little saturation’ (Nagel, 1911, p. 350). He assumed that this change in colour quality was due to some unknown adaptation mechanism.

night vision may appear bluish 111

The important hypothesis of Nagel (1911) that the colour quality of scotopic vision may vary with conditions of adaptation was also suggested by the results of a simultaneous contrast experiment performed by Willmer (1950). In this experiment he had found that the colour quality of a test field presented at an intensity level below the specific-hue threshold (but not necessarily below the absolute cone threshold) changed from achromatic to bluish when a chromatic long-wave inducing field was applied. Presupposing that the test field activated rods only, he assumed that the change from achromatic to blue resulted from interacting rod and cone activities, supporting his suggestion that the rods constituted the ‘primary’ blue receptor of photopic vision. Thus, he speculated that the blue colour observed in daylight was due to rod activity that escaped inhibition from cones (Willmer, 1961).

As we have seen, this speculation of Willmer was disproved byspectrophotometrical measurements that demonstrated the existence of ‘blue’ cone receptors. Yet, the more general presumption of Willmer, that rods and cones may interact to produce hue sensations, was supported by a preliminary study of Stabell and Stabell designed to determine the first appearance of rod activity following substantial bleaching. Thus, the subject should decide when the colour of the test field presented at a mesopic intensity level first changed during the long-term dark-adaptation period. On the basis of Lie’s (1963) results, it was expected that the rod intrusion would be marked by a sudden desaturation change (Stabell, 1967b). Surprisingly, however, it was found that a test colour that appeared green during the cone-plateau period suddenly changed markedly towards blue-violet in addition to being desaturated. To ascertain that this blue-violet colour obtained was actually generated by rods, the test intensity was dimmed to a scotopic intensity level. The results obtained confirmed the suggestion, and hence revealed that rods could trigger both blue-violet and achromatic colours depending on the chromatic state of adaptation.

This finding raised the question of whether the mechanisms underlying achromatic colour in scotopic and photopic vision actually

112chromatic rod vision: a historical account

were analogous. The question was whether the achromatic aspect of rod vision could be accounted for by the suggestion that opponent chromatic colour processes activated by rod signals antagonized each other completely when the eye was stimulated at scotopic intensities in a chromatically neutral state of adaptation. If this suggestion was correct, one would expect that by selectively chromatic adaptation of the eye, it would be possible to obtain every principle hue of the spectrum when test stimulating at scotopic intensities.