Section 35–1 The human eye

 (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.