(Functions of eye / Structure of retina / Eye-brain system)
In this section, Feynman discusses functions of human eyes,
the structure of human retina, and the eye-brain system.
1. Functions of
eye:
“Light enters the eye
through the cornea; we have already discussed how it is bent and is
imaged on a layer called the retina in the back of the eye, so
that different parts of the retina receive light from different parts of the
visual field outside (Feynman et
al., 1963, p. 35–1).”
Perhaps Feynman could have explained the human eye, a
biological organ, using more physics-oriented explanations, physical principles,
or physical analogies. For example, the human eye functions in a manner similar
to a camera, both collecting, detecting, and processing light from the
surrounding environment. In addition, light enters the eye through the cornea,
the transparent front part of the eye. Similarly, light also enters the camera
through the camera lens, which is analogous to the cornea in the human eye. On
the other hand, the retina contains two types of photosensitive cells or
photoreceptors: rods (responsible for night vision) and cones (responsible for
color vision). It is analogous to the camera’s sensor that acts like tiny
photosensitive receptors, detecting the intensity of light that falls on it.
In his autobiography, Feynman shared how he viewed the
explosion of atomic bomb using the truck windshield. In his words, “They gave out dark glasses that you
could watch it with. Dark glasses! Twenty miles away, you couldn't see a damn
thing through dark glasses. So I figured the only thing that could really hurt
your eyes (bright light can never hurt your eyes) is ultraviolet light. I got
behind a truck windshield, because the ultraviolet can't go through glass, so
that would be safe, and so I could see the damn thing (Feynman, 1997, p. 134).”
It is worthwhile to have some knowledge of human
eye functions and how to prevent damage in the retina. For example, the lens of
the eye can absorb some ultraviolet (UV) radiation and provide a certain degree
of protection to the retina, but this natural UV filtering capacity is limited.
2. Structure
of retina:
“The retina is not absolutely uniform: there is a place, a spot, in the
center of our field of view which we use when we are trying to see things very
carefully, and at which we have the greatest acuity of vision; it is called
the fovea or macula (Feynman et al., 1963, p. 35–1).”
In the Audio Recordings* [5 min: 25 sec] of this lecture,
Feynman says: “it is called the fovea and that’s here”
instead of “it is called the fovea or macula.” The macula
is the central portion of the retina with a high concentration of cones,
contributing to central vision and color perception. The fovea is a specialized
pit structure within the macula that contains only cones and provides the greatest
acuity of vision, making it responsible for the highest detailed and sharp
visual perception. We may rephrase the above-statement (highlighted in yellow)
as: “The retina is not uniform, and there is a specialized region in the center
of our field of view known as the fovea, which is part of the larger macula.
The fovea is the area where we have the greatest acuity of vision and use for
tasks that require precise and detailed visual perception.”
*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html
The
retina can be distinguished into at least 3 regions: fovea, macula, and
periphery (see figure below). In the fovea (0.35 mm in diameter), the density
of cones is highest, while the rods are completely absent. Moving away from the
fovea, the density of cones is lower in the macula, but it is relatively high
compared to the peripheral retina. This allows for good visual detail and color
perception in the central part of our vision. As we move towards the periphery,
the density of cones decreases, but the density of rods increases. The
periphery of the retina is more sensitive to low intensity light and is better
suited for detecting motion and objects in the dark. Specifically, rods are
more numerous in the peripheral retina, enhancing our ability to perceive
movement and objects in our side vision.
![]() |
Source: Why don't we get color or detail
information from our peripheral vision? | Socratic
3. Eye-brain system:
“Now the interesting thing is that in the retina each of the cells which
is sensitive to light is not connected by a fiber directly to the optic nerve,
but is connected to many other cells, which are themselves connected to each
other. There are several kinds of cells: there are cells that carry the
information toward the optic nerve, but there are others that are mainly
interconnected “horizontally.” There are essentially four kinds of
cells... (Feynman et
al., 1963, p. 35–2).”
Feynman mentions that there are essentially four kinds of cells and
there are cells that are interconnected “horizontally.” Perhaps he
could have explained that the horizontal cells are interconnected laterally instead
of horizontally. The term “horizontal cells” is derived from their horizontal (side-to-side)
connections with the neighboring retinal cells, emphasizing their function in
integrating and regulating signals across the retina. Instead of saying
four kinds of cells, some may emphasize that there are six kinds of retinal
cells, namely, horizontal, bipolar, amacrine, interplexiform, ganglion, and
photoreceptor cell (Dowling, 2012). For example, amacrine
cells are involved in fine-tuning the signals transmitted between bipolar cells
and ganglion cells, i.e., they play a role in modulating contrast and
sensitivity to different light intensities.
![]() |
Source: Human eye - Retina, Optic Nerve, Vision | Britannica |
“…the
information from the various cells does not immediately go to the brain, spot
for spot, but in the retina a certain amount of the information has already
been digested, by a combining of the information from several visual receptors.
It is important to understand that some brain-function phenomena occur in the
eye itself (Feynman et
al., 1963, p. 35–2).”
Although
the human retina is physically located within the eye, its role extends beyond
simple light detection. The retina’s functions and connections are closely tied
to the brain, which processes the visual information collected by the retina. In
essence, the retina is an integral part of the eye-brain system, where the eye
captures light and initial visual information, whereas the brain processes and
translates this information into the visual experiences that humans perceive.
The processing and interpretation of visual information occurs in the brain,
making the retina and the brain tightly interconnected. In a sense, photoreceptor cells can be compared to the pixels on a
camera sensor. Just as pixels on a camera sensor detect light and convert it
into electrical signals, photoreceptor cells in the retina (rods and cones)
detect light and convert it into electrical signals, initiating the process of
visual information.
Note:
In Chapter 36, Feynman explains: “As a matter of fact, people who study anatomy and the development of the
eye have shown that the retina is, in fact, the brain: in the development of
the embryo, a piece of the brain comes out in front, and long fibers grow back,
connecting the eyes to the brain. The retina is organized in just the way the
brain is organized and, as someone has beautifully put it, ‘The brain has
developed a way to look out upon the world.” The eye is a piece of brain that
is touching light, so to speak, on the outside. So it is not at all unlikely
that some analysis of the color has already been made in the retina’ (Feynman
et al., 1963, p. 36–2).”
Review
Questions:
1.
How would you explain the functions of human eyes?
2. How would you explain the structure of the
retina (fovea, macula, and periphery)?
3. How
would you explain whether the eye is a part of the brain?
The
moral of the lesson: the eye-brain system contains two types of
photoreceptors: rods (for night vision) and cones (for color vision) that are related
to six kinds of retinal cells (horizontal,
bipolar, amacrine, interplexiform, ganglion, and photoreceptor).
References:
1. Dowling, J. E. (2012). The
retina: An approachable part of the brain. Cambridge, MA: Harvard
University Press.
2. Feynman, R. P. (1997). Surely You’re Joking, Mr. Feynman! :
Adventures of a Curious Character. New York: Norton.
3. 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.
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