(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.
