35–6 Physiochemistry of color vision

 (Rod visual pigment / Cone visual pigment / Color sensation)

 

In this section, Feynman discusses the rod visual pigment, cone visual pigment, and ends the chapter by demonstrating the subjectivity of color sensation. The two demonstrations are sometimes known as the Land effect and Fechner color effect.

 

1. Rod visual pigment:

“The pigments that can be obtained from a retina consist mainly of a pigment called visual purple... This fact was discovered in 1877 (Feynman et al., 1963, p. 35–9).”

 

Feynman adopts the archaic term visual purple instead of rhodopsin and mentions that the fact was discovered in 1877. Historically, the discovery of rhodopsin are attributed to two German physiologists Franz Christian Boll and Wilhelm Kühne. In Jan 1876, Boll described the color of rods dissected from dark adapted frog retinas as “Sehrot” (Wade, 2008). Kühne coined the term rhodopsin, in which “rhodo” and “opsis” refer to rose and sight respectively. Furthermore, Boll’s “Sehrot” was translated as vision red and Kühne's “Sehpurpur” as vision purple. Instead of saying a retina consists mainly of a pigment called visual purple, one may clarify that the numerical difference between rods and cones in the retina is substantial: about 120 million rod cells and 6 to 7 million cone cells (having different visual pigments). Remarkably, Feynman suggests that this is due to the need of more visual purple to see at low lighting conditions.

 

Note: In section 36–3 The rod cells, Feynman clarifies that “[t]here are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods. The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene, which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light.”

 

“The most remarkable features of this are, first, that it is in the eye of almost every vertebrate animal, and second, that its response curve fits beautifully with the sensitivity of the eye, as seen in Fig. 35–9, in which are plotted on the same scale the absorption of visual purple and the sensitivity of the dark-adapted eye... (Feynman et al., 1963, p. 35–9).”

 

Feynman explains that the response curve fits beautifully with the sensitivity of the eye in Fig. 35–9, but it oversimplifies their relationship. However, most researchers in the field of color vision had produced widely differing sensitivity curves for the mechanisms of the trichromatic theory (Davson, 1962). The dark-adapted sensitivity curve is influenced by the combined responses of rod cells containing rhodopsin, and the relationship is more complex than the beautiful fit as shown in the graph. On the other hand, a remarkable feature of the rhodopsin is composed of a protein called opsin and a light-sensitive molecule known as retinal, which is derived from vitamin A. Ensuring an adequate intake of vitamin A through a balanced diet is crucial for preventing night blindness. George Wald, a New Yorker, was awarded the 1967 Nobel Prize in Physiology or Medicine for his discoveries about chemical and physiological visual processes in the eye (including vitamin A is found in rhodopsin).

 

Note: In the next chapter, Feynman mentions that if we do not eat enough of Vitamin A, we do not get a supply of retinene, and the eye becomes what we call night blind. (see section 36-3)

 

2. Cone visual pigments:

“The light goes right down into the sensitive point, bounces at the bottom and comes back out again, having traversed a considerable amount of the color-vision pigment; also, by looking at the fovea, where there are no rods, one is not confused by visual purple. But the color of the retina has been seen a long time ago: it is a sort of orangey pink; then there are all the blood vessels, and the color of the material at the back, and so on (Feynman et al., 1963, p. 35–9).”

 

Feynman says that the color of the retina is a sort of orangey pink, however, the light goes right down into the sensitive point, and it traverses a considerable amount of the color-vision pigment (or cone visual pigments). Furthermore, the description of the color of the retina as orangey pink oversimplifies the complex composition of the retina, i.e., the retina does not have a uniform color. When we look at the retina using an ophthalmoscope (see figure below), we can see an orange background, also known as the fundus, with a network of arteries (Valberg, 2007). The fundus refers to the interior surface of the eye, including the retina, optic disc, and blood vessels. In the context of an eye examination, it is more appropriate to describe the “color of the fundus” rather than the “color of the retina.” The orange color of the normal fundus is a result of complex interactions among the visual pigments, vascularization, and the optical properties of the eye.

 

Source: Fundus photography - Wikipedia

“How do we know when we are looking at the pigment? Answer: First we take a color-blind person, who has fewer pigments and for whom it is therefore easier to make the analysis. Second, the various pigments, like visual purple, have an intensity change when they are bleached by light; when we shine light on them they change their concentration (Feynman et al., 1963, p. 35–9).”

 

Bleaching occurs in both rod and cone visual pigments, although the process may differ slightly between them. Perhaps Feynman could have clarified that the term bleaching in this context whether it simply means a change in color to white, almost transparent, or other colors with a deeper meaning. In early 1876, Boll noted that the frog retina is paler after light exposure and can become completely colorless in direct sunlight. On the other hand, Kühne established the notion of “visual cycle”, i.e., visual purple in the rods is bleached by light to form visual yellow which is later transformed into visual white (Wade, 2008). Kühne also showed that the rate of bleaching was dependent not only on the intensity of light but also on its wavelength.

 

In his Nobel Lecture, Wald (1968) explains: “We have been in the habit of saying that light bleaches visual pigments. What it does however is to isomerize the chromophore. The end of this process, if it is allowed to go to completion, is a steady-state mixture of isomers of the chromophore, in proportions that depend upon the wavelength of irradiation and the relative quantum efficiencies of the photoreactions.” In short, bleaching refers to the photodecomposition of the pigment molecules triggered by light absorption.

 

3. Color sensation:

“Color is not a question of the physics of the light itself. Color is a sensation, and the sensation for different colors is different in different circumstances (Feynman et al., 1963, p. 35–10).”

Colors are a result of the complex interaction between light, objects, and the human visual system. The concept of color can be defined from the perspective of cone cells, visual spectrum, and human observer: (1) Cone cells: Color perception is subjective and varies from person to person, i.e., influenced by individual differences in the sensitivity of the eye's cones; (2) Color is the perceptual result of light incident upon the retina in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm; (3) Color is a product of human perception, and it doesn’t exist in the same way without an observer with the ability to perceive and interpret the visual stimuli. To show that color is a sensation and not simply the effect of physical light, Feynman ends the lecture by demonstrating the Land effect and Fechner color effect.

 

In the Audio Recordings* [57 min: 00 sec] of this lecture, Feynman says something like: “One of the possible explanations is this: that the three different color receptors have different timing of response and so because of the flashings, the red and green and so on information come to the head at different times and so you get different effective colors. But look if in this region here, the eye if it held in one spot would see black, black, black, and white and in this region black, black, black, and white, and so on the same proportion. In other words, in coming to the eye here and here are the same flashings exactly (three parts black and one part white) and nevertheless the colors are different. Why? The only difference in the two cases is the characteristics of the background and neighborhood of the bar and therefore there is an integration of the information at the background and the bar (not just the bar). It is very important to appreciate that the retina is already thinking about the light. It is comparing what it sees in one region with another not in the conscious way, but already in retinal level and this is demonstrated by this crazy color phenomenon known as the Fechner colors. Thank you very much.” After the lecture, one undergraduate approached Feynman and mentioned that he had read about Land’s theory some time ago, however, the Land effect is elaborated in the beginning of the next lecture.

*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

Review Questions:

1. How would you explain the most remarkable features of rod visual pigments?

2. What is the meaning of bleaching in the context of cone visual pigments?

3. How would you explain the sensation for different colors is different in different circumstances?

 

The moral of the lesson: The absorption curve of visual pigments in the eye aligns well with the sensitivity curve of the dark adapted eye, based on Rushton’s ophthalmoscope method to detect changes in pigment concentration, which provide insights on visual pigments and color perception.

 

References:

1. Davson, H. (Ed.). (2014). The Visual Process: The Eye. Academic Press.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

3. Valberg, A. (2007). Light vision color. Hoboken, NJ: John Wiley & Sons.

4. Wade, N. J. (2008). Visual purple (Sehpurpur). Perception, 37(11), 1617-1620.

5. Wald, G. (1968). Molecular basis of visual excitation. Science, 162(3850), 230-239.