N I G H T V I S I O N AS CHROMATIC V I S I O N

Bj0m STABELL and ULF STABELL

Institute of Psychology, University of Oslo, Norway

STABELL, B. & STABELL, U. Night vision as chromatic vision. Scand. J . Psychol., 1967, 8, 145-14g.-Assuming that night vision is an exclusive function of the rods, and that it is colorless, the duplicity theory states that rod vision is achromatic vision. Studies relevant to color in night vision are reviewed. It is concluded that color may be observed well below the break- point of the dark adaptation curve, and that the duplicity theory therefore needs revision.

Schultze (1866) discovered that the rod structure is dominant in nocturnal animals, while the cone structure dominates in the diurnal animals. He concluded that night vision is an exclusive function of the rods. Assuming that night vision is colorless, he stated that rod vision is achromatic vision. Thus, the psychophysical hypothesis forms a basis on which the duplicity theory rests. Other possible evidence of an anatomical, biochemical, or electro- physiological nature in support of rods as mediators of achromatic vision presupposes that the psychophysical hypothesis is correct. The present paper attempts to show that the hypothesis is not in accord with experimental facts, and that the duplicity theory, therefore, needs revision.

In stating that rods mediate achromatic vision, Schultze was probably influenced by the observations of Purkinje (1825, p. 109) and Aubert (1865, p. 126). Purkinje reported that colored papers appear grey or black at dawn. Aubert accomplished the first thorough experimental study of this phenomenon. He noted that all the pigment colors, after dark adaptation and at low levels of illumination, appear colorless, differing only in brightness. Chodin (1877, pp. 394-407) made observations both with pigment and spectral colors and confirmed the results of Aubert. Later, the observation of an achromatic interval has been repeatedly demonstrated (Cohn, 1882; Hillebrand, 1890; Tschermak, 1929; Kohlrausch, 1931; Hecht, Haig & Chase, 1937; Mandelbaum, 1941; Lie, 1963). At the present time there exists a nearly universal agreement that night vision is achromatic vision.

However, different lines of evidence converge to support the conclusion that night vision is bluish. The methods employed are generally of two kinds: following (a) dark adaptation, or (b) pre-stimulation, the eye is stimulated at low levels of illumination. Von Kries & Nagel (1896), apparently influenced by Ebbinghaus (1893) and Konig (1894), started the first line of investigation. A homogeneous light of 495 mp was made to match a color mixture of red and blue light by Nagel, a deuteranope. After dark adaptation and at lower levels of intensity, he reported that the homogeneous light appeared too bright and too blue. The result was interpreted as indicating that rods mediate a sensation of blue. Some support for this assumption was found in that many subjects describe their sen- sation as bluish when stimulated at low levels of intensity. In a footnote von Kries (1896, p. 87) remarks: ‘Es wZre also vielleicht richtiger, nicht zu sagen, dass die Stabchen furblose Empfindungen, sondern dass sie einen nur einsinnig veranderlichen Empfindungseffekt

I 0 - 671941 S c a d J . Psychol., Vol. 8,1967 145

146 BJ0RN STABELL AND ULF STABELL

liefern. Da indessen der Stabcheneffekt sich schwerlich in erheblichem Masse von der Farblosigkeit im gewohnlichen Sinne unterscheidet, so schien es mir besser, den obigen, seine Bedeutung jedenfalls sehr vie1 anschaulicher kennzeichnenden Ausdruck beizube- halten.’ Thus, von Kries asserted that night vision is somewhat bluish, but continued to use the word ‘farblos’. This may have been of particular significance for the one-sided emphasis attributed to the achromatic aspect of night vision.

Both Loeser (1904, p. 17) and SivCn (1905, 1912) have mentioned the blue aspect of night vision, although SivCn (1905, pp. 379-384) stated that at the lowest intensity levels no hues are observable. Also Miiller (1923, p. 128) reported that the sensation, at the lowest intensity levels, appeared colorless. However, since the evidence in favor of a colorless interval had been inconclusive, he induced Kroh (1922) to investigate the blue aspect of night vision. Kroh demonstrated that a field which in a light-adapted state appeared achro- matic, was observed as bluish after 30 minutes of dark adaptation. The blue color was observed for several minutes. However, the intensity of the field does not preclude an explanation where cones are involved. To exclude this interpretation, he demonstrated that a field of relatively long wave lengths, which in a dark-adapted state and at low levels of intensity appeared achromatic when the fovea was stimulated, appeared blue-white in the periphery. In conclusion, then, the first line of evidence indicates that night vision may be bluish, at least for some subjects. Thus, the duplicity theory seems contradicted either the rods initiate color sensation or the cones function in night vision.

The other line of evidence was apparently started by Dittler & Satake (1914) and Hauer (1914). Under the direction of Hering, who wrote the introduction to the paper, Dittler & Satake attempted to determine the ‘opponent’ wave lengths, i.e. wave lengths which, when mixed in proper proportions, give an achromatic sensation to the chromatically neutral eye. Such wave lengths were found for the extra-macular region of the retina, dark-adapted for 5 to 10 min, and it was checked whether they really were tone-free by dimming the mixture after pre-exposure for about 30 sec. The experiment, however, is beset with the difficulty of finding absolutely toneless results upon dimming. Actually, even with the most favorable choice of both lights, a weak bluishness often remains. The authors did not attempt an interpretation of this fact. It should be noted that, under similar conditions of experimentation, Schubert (1928) failed to reveal the chromatic component when the fovea was stimulated. He seems to have taken for granted that Dittler & Satake had not controlled the effect of previous retinal stimulation and thus had allowed a yellow valence to enter into their experiment.

The observation of Dittler & Satake is, however, in conformity with the data of Hauer (1914). He stimulated the eye with a bright, white field for 10 to 20 sec. Subsequent to this light adaptation the intensity was reduced. All subjects reported seeing blue at the low intensity level. To account for the result, it was suggested that light adaptation changes the balance of sensitivity of the cone and rod mechanisms, leading to a relative enhance- ment of sensitivity in the rod mechanism. Consequently, the observance of blue at the lower intensity levels was attributed to the natural light of the rods. In support of this view, Hauer points out that the phenomenon is lacking when the fovea centralis is stimulated. The evidence can hardly be regarded as conclusive, since neither the adaptation level nor the intensity levels of the pre- and post-stimulation were controlled. Hauer is therefore

Scasd. J. Psychol., Vol. 8,1967

NIGHT VISION AS CHROMATIC VISION ‘47

not in a position to conclude that the rods mediate the color observed. Actually there is no basis for inferring a relative enhancement of sensitivity in the rod mechanism.

Broer (1929) extended the observations of Hauer and Dittler & Satake. Sitting in a room flooded with daylight, the subject fixated one of 14 different color disks covering a range from purple to violet. The disks were large enough to extend beyond the rod-free region. After about 45 seconds of pre-stimulation, the after-image was observed while the eye was stimulated with a white disk. The procedure was repeated with the difference that the after-image was observed while the eye was stimulated at lower intensity levels. Comparing the after-images of the two different intensity levels, it was found that the after-images of the low level were shifted towards the blue-violet part of the spectrum. Both pigment and

I spectral colors yielded this effect (Broer, 1932). Caution is needed in the interpretation of the results, since cones were probably involved when stimulating at the lowest intensity level.

To conclude, the results of these early studies may be interpreted in different ways. Since none of the investigators has ascertained that the stimulation was below the threshold of dark-adapted cones, the question remains open as to whether rods or cones mediate the blue color.

Two assumptions are generally accepted. (I) The photochromtic interval indicates the range of intensity within which rods alone

function (von Kries, 1929, p. 698; Hecht, 1937, p. 248; Mandelbaum, 1941, p. 206). (2) The break-point of the dark adaptation curve represents the level on the intensity

scale at which the absolute threshold of the dark-adapted cones is nearly reached (Kohl- rausch, 1931, p. 1584; Hecht, 1937, pp. 250-252).

Since Lie (1963) found the specific threshold to rise at about the break-point of the dark adaptation curve, it is not to be inferred that both assumptions are correct. However, they would both agree with the assumption that rods alone function below the break-point. It seems therefore reasonable to confine the term ‘night vision’ to the intensity range between the break-point and the absolute threshold of the dark-adapted eye.

A few studies satisfy the claim of stimulating rods only. Hecht, Haig & Chase (1937) light-adapted the subject to one of five different intensities for two minutes. The pre-expo- sure light was white, and to secure the largest range of rod adaptation, violet light was used for measurement of dark adaptation. Following the adaptation to one of the lowest inten- sities, a color of blue was definitely associated with threshold intensities well below the break-point of the dark adaptation curve. Using a Hecht & Schlaer (1938) adaptometer, Haig (1941) and Mote & Riopelle (1951, 1953) have confirmed this observation. The pre- exposure light was white and the threshold stimulus violet. Apparently none of these authors has made a systematic attempt to explain the observation.

I t is well known that, in general, the higher the light adaptation, the slower the dark adaptation of the rods. Visual purple may be synthesized by at least two processes: a rapid ‘anagenesis’ directly from photoproducts, and a slow ‘neogenesis’ more indirectly from the vitamin A stage (Winsor & Clark, 1936; Wald & Clark, 1938; Wald, 1954). An inspection of the data of Hecht, Haig & Chase (1937), Haig (1941), and Mote & Riopelle (1951, 1953) indicates that the color at low intensity levels is associated with rapid rod recovery. A neces-

Scand. I. Psychol., Vol, 8,1967

BJ0R.N STABELL AND ULF STABELL 148

sary condition for experience of blue color below the break-point seems therefore to be: Stimulation of a photoproduct formed upon bleaching rhodopsin. This being the case, the problem is far from solved on the molecular level. One may, for instance, ask which of the rodopsin intermediates is involved. Furthermore, subsequent to selective chromatic adap- tation, all the principal colors have been observed at intensity levels below the break-point (Stabell, 1967). Since all the photoproducts have their maximum of absorption in the short wave lengths, the problem becomes one of explaining the colors related to long wave lengths. Finally, using the method of simultaneous contrast, blue and green colors have been ob- served below the break-point level (Stabell, 1967). It is possible, therefore, that the colors in night vision may originate more centrally than at the photochemical level.

CONCLUSION

In view of the evidence presented, there can be no doubt that color may be observed well below the break-point of the dark adaptation curve. It is therefore hardly possible to maintain the conception that night vision is always achromatic vision. The conclusion must be drawn that the basic psychophysical hypothesis, on which the duplicity theory rests, is wrong.

REFERENCES

AUBERT, H. (1865). Physiologie der Netzhaut. Breslau: Morgenstern.

BROER, F. (1929). uber das Purkinjesche Phino- men im Nachbild. 2. Psychol. Physiol. Sinnes- o w , 113, 9-70.

BROER, F. (1932). uber das Purkinjesche Phino- men im Nachbild vom Spektralfarben. 2. Psychol. Physiol. Sinnesorg., 125, 71-89.

CHODIN, A. (1877). uber die Abhiingigkeit der Farbenempfindungen von der Lichtstarke. In W. PREYER, (Ed.), Sammlung physiologischer Abhundlungen. Jena: D e t . Pp. 375-444.

COEN, H. (1882). uber Farbenempfindungen bei schwacher kiinstlicher Beleuchtung. Arch.

DITTLER, R. & SATAKE, Y. (1914). Eine Methode zur Bestimmung der gegenfarbig wirkenden Wellenrigen des Spectrums. 2. Sinnesphy- SiOl., 48, 240-25 I .

EBBINGHAUS, H. (I 893). Theorie des Farbenseh- ens. 2. Psychol. Physiol. Sinnesorg., 5, 145- 238.

HAIG, C. (1941). The course of rod dark adapta- tion as influenced by the intensity and dura- tion of pre-adaptation to light. J. gen. Physiol.,

Augenhklk., II, 283-302.

24,735-751. HAUER, F. (1914). Beitrage zur Theorie der Farb-

emofinduneen. SitzBer. Akad. Wiss.. Wien. Math.-nahcnuiss. Kl., Abt. IIa, 123, 629-651:

HECHT, S. (1937). Rods, cones, and the chemical basis of vision. Physiol. Rev., 17, 239-290.

The influence of light adaptation on subse- quent dark adaptation of the eye. J. gen. Phy-

HECHT, S., HAIG, C . & CHASE, A. M. (1937).

SioL, 20, 831-850.

HECHT, S. & SCHLAER, S. (1938). An adaptometer for measuring human dark adaptation. J. opt.

HILLEBRAND, F. (1890). Uber die specssche Helligkeit der Farben. Beitrgge zur Psycho- logie der Gesichtsempfindungen. SitzBer. Akad. WiSs., Wint, Math.-natunuiss. Kl., Abt. 111, 98, 70-120.

KOHLRAUSCH, A. (193 I). Adaptation, Tagessehen und Diimmerungssehen. In A. BETHE, G. BWGMANN, G. EMBDEN & A. ELLINGER (Eds.), Handbuch der n o m k und pathologischm Physiologie. Bd. 12 (2). Berlin: Springer. Pp.

KRIES, J. VON (1896). Uber die Funktion der Netzhautstiibchen. 2. Psychol. Physiol. Sin- nesorg., 9 , 81-123.

KRIES, J. VON (1929). Zur Theorie des Tages- und Dammerungssehens. In A. BETHE, G. BERGMA", G. EMBDEN & A. ELLINGER (Eds.), Huna'buch der normalen und patholo- gischen Physiologie. Bd. 12 ( I ) . Berlin: Springer.

SOC. AM., 28, 269-275.

1499-1594-

PP. 679-713. KRIES, J. VON & NAGEL, W. (1896). uber den

Ekfluss von Lichtstiirke &d Adaptation a d das Sehen des Dichromaten (Griinblinden). 2. Psychol. Physiol. Sinnesorg., ra, 1-38.

&OH, 0. (1922). Die Weissempfindung des Stjibchenauges. 2. Sinnesphysiol., 53, 187- 196.

KONIG, A. (1894). uber den menschlichen Seh- purpur und seine Bedeutung fiir das Sehen. SitzBer. preuss. Akad. Wiss. 2. Halbbd. Pp. 577-598.

Scad. J . Psychol., Vol. 8,1967

NIGHT VISION AS CHROMATIC VISION '49

LIE, I. (1963). Dark adaptation and the photo- chromatic interval. Documenta ophthalmol., 17,411-510.

LOESER, L. (1904). Uber den Einfluss der Dunkel- adaptation auf die spezifische Farbenschwelle. Z . Psychol. Physiol. Sinnesorg., 36, 1-18.

MANDELBAUM, J. (1941). Dark adaptation. Arch. Opthalmol., 26, 203-239.

MOTE, F. A. & RIOPELLE, A. J. (1951). The effect of varying the light-dark ratio of intermittent pre-exposure upon subsequent dark adapta- tion in the human eye. J . opt. SOC. Amer., 41, 120-124.

MOTE, F. A. & RIOPELLE, A. J. (1953). The effect of varying the intensity and the duration of pre-exposure upon subsequent dark adapta- tion in the human eye. J. comp. physiol. Psy-

MOLLER, G. E. (1923). Zur Theorie des Stab- chenapparates und der Zapfenblindheit. Z. Sinnesphysiol., 54, 102-145.

PURKINJE, J. (1825). Beobachtungen und Versuche zur Physiologie der Sinne. Neue Beitrage zur Kenntniss des Sehens in subjectiver Hinsicht. 2. Bd. Berlin: Reimer.

SCHUBERT, G. (1928). Sind urfarbige Lichter- paare komplementar? Pfiig. Arch. ges. Physiol., 220, 82-100.

cho[., 46949-55.

SCHULTZE, M. (1866). Zur Anatomie und Physio- logic der Retina. Arch. mikrosk. Anat. Enfw. Mech., 2, 175-286.

S I ~ N , V. 0. (1905). Studien iiber die Stabchen und Zapfen der Netzhaut als Vermittler von Farbenempfindungen. Skand. A7ch. Physiol., 17, 306-388.

S I V ~ N , V. 0. 11912). Die Stabchen als farben- perzipierende- Oigane. Arch. Augenheilk., 71,

STABELL, B. (1967). Rods as color receptors in scotopic vision. Scaizd. J. Psychol., 8, 132-138.

TSCHERMAK, A. (1929). Licht- und Farbensinn. In A. BETHE, G. BERGMANN, G. EMBDEN & A. ELLINGER (Eds.), Hundbuch der normalen und pathologisclzen Physiologie. Bd. I Z ( I ) . Berlin: Springer. E'p. 679-713.

WALD, G. & CLARK, A. B. (1938). Visual adapta- tion and chemistry of the r0ds.J. gen. Physiol.,

WALD, G. (1954). On the mechanism of the visual threshold and visual adaptation. Science, New York, 119, 887-892.

WINSOR, C. P. & CLARK, A. B. (1936). Dark adaptation after varying degrees of light adap- tation. Proc. natn. Acad. Sci., U . S . A . , 22, 400-404.

157-166.

21, 93-105.

Scad. J. Psychol., Val. 8,1967