(Underwater vision / Binocular vision / Inverted vision)
In
this section, Feynman discusses neural machinery, optical components, and
neural connections of human eyes. However, the discussions could be related to
underwater vision, binocular vision, and inverted
vision.
1. Underwater
vision:
“This is why we cannot see clearly under water,
because we then do not have enough difference between the index of the cornea,
which is 1.37, and that of the water, which is 1.33. Behind the
cornea is water, practically, with an index of 1.33, and behind that is a
lens which has a very interesting structure: it is a series of layers, like an
onion, except that it is all transparent, and it has an index
of 1.40 in the middle and 1.38 at the outside (Feynman et al., 1963, p. 36–3).”
Feynman explains that we cannot see clearly under
water because the difference between the refractive index of the cornea and
water is not enough. One may clarify the ability of cornea, humor, and lens of
the human eye to focus images (bend light rays) on the retina, but the clarity
of the images is dependent on the ratio of refractive index of the cornea and
that of the medium where the object resides. Based on research data over the
recent 20 years, the refractive index of cornea changes from about 1.400 at the
epithelium to 1.373 at the endothelium (Patel &
Tutchenko, 2019). However, the unclear underwater vision is also due to the
light absorption in water whereby the attenuation is dependent on the
wavelength of light. Furthermore, the absorption and attenuation are dependent
on the depth and turbidity of the water.
It should be worth mentioning that the human eye is
adapted for viewing in air because the ratio of the refractive index of cornea
to that of the air (Snell’s law of refraction) is relatively larger. On the
other hand, fishes have a better underwater vision because their corneas are
more spherical and thus, they can bend light rays better to achieve a better
focus. A disadvantage of fishes’ corneas is that it causes them to be
short-sighted when viewing in air. Interestingly, penguins are able to focus more
clearly on land and underwater, but their corneas are relatively flatter than other
birds (Hadden & Zhang, 2023). Essentially, the penguins’ lenses are softer and
their eye muscles can adjust the shape of the lens more than other animals to
view in air and underwater.
2. Binocular vision:
“This
is the manner in which the information from each of the two eyes is put
together in order to tell how far away things are. This is the system of binocular
vision (Feynman et al., 1963, p. 36–4).”
Binocular
vision is the integration of monocular sensory and motor visual information
into a combined percept of the surrounding physical space (Stidwill & Fletcher, 2017).
There are at least three conditions for binocular vision: (1) Interocular distance: Two eyes and a separation between the eyes (interocular
distance), which is about 65 mm in adult humans. (2) Neural pathway: A neural pathway (or neural connections) to transfer the two images
to the brain. (3) Neural machinery: Neural
processing systems to integrate the different types of visual information, such
as luminosity, size, color and contrast. According to Feynman, the binocular
vision tells how far away things are, but one may add that it is the ability of
human eyes to perceive a three-dimensional image by combining the two images
from the left and right eyes simultaneously.
“… it
is impossible consciously or unconsciously to turn both eyes out at
the same time, not because there are no muscles, but because there
is no way to send a signal to turn both eyes out, unless we have had an
accident or there is something the matter, for instance if a nerve has been
cut. Although the muscles of one eye can certainly steer that eye about, not
even a Yogi is able to move both eyes out freely under
voluntary control, because there does not seem to be any way to do it (Feynman et al., 1963, p. 36–5).”
There are two aspects of binocular vision,
convergent vision and divergent vision, which is the ability of both eyes to move together
to perceive depth and distance. In general, vergence eye movements consist of simultaneous
movement of both eyes in opposite directions that maintain binocular vision. Convergent vision (or convergence
eye movement) occurs when both eyes focus on a nearby object, i.e., the eyes
rotate inward, towards each other, to align their lines of sight onto the
object. On the contrary, divergent vision (or divergence eye movement) occurs
when both eyes focus on a distant object, whereby the eyes rotate outward, away
from each other, to align their lines of sight onto the object. Interestingly,
a person (non-Yoji) may have partial divergence eye movement, e.g., right eye
diverging while left eye remains relatively stable as shown below.
Source: Vergence - Wikipedia
3. Inverted vision:
“Believe
it or not, the up-and-down line in our visual field as we look at something is
of such a nature that the information from all the points on the right side
of that line is going into the left side of the brain, and information from
the points on the left side is going into the right side of the brain,
and the way this area is made, there is a cut right down through the middle, so
that the things that are very close together right in the middle are very far
apart in the brain! (Feynman et
al., 1963, p. 36–4).”
One may adopt the term inverted vision (or left-right
reversed
vision) that refers
to a vision related to the interpretations of the eye-brain system. In short,
the inverted image formed on the retina of the eye is “re-inverted” by the
brain. In other words, the visual information is processed and inverted by the
brain to present a coherent and upright perception of the world observed by the
eyes. On the contrary, a lady by the name of Marietta Everett had
up-down inverted vision and see the world upside down (Davis, 1969). Due to the image is inverted on the
retina and abnormalities in the visual cortex, the brain would receive a
reversed representation of the visual scene. While most people perceive the
world upright through perceptual adaptation, Everett’s brain was unable
to perform this adaptation, resulting in her perception of the world as
inverted.
“This is an
important point, because most of the earlier books on anatomy and psychology,
and so on, do not appreciate or do not emphasize the fact that we are so
completely wired already—they say that everything is just learned (Feynman et al., 1963, p. 36–6).”
Perceptual adaptation
refers to the brain’s ability to adapt to distorted sensory inputs over time,
allowing individuals to perceive these inputs as normal or familiar. In
Erismann and Kohler’s Innsbruck Goggle experiments, perceptual adaptation is
demonstrated through the subjects’ ability to adapt to wearing upside down
goggles (e.g., flipping top and bottom or flipping left and right). Despite
initially perceiving a distorted world through the goggles, over time the
subjects were able to interpret the transposed visual information as normal. In other words, the perceptual experience of the
reversed image may become normalized, and individuals may perceive it as if it
were in its original orientation. The orientation of visual information in the
brain is determined by the arrangement of neural connections from the eyes to
the visual cortex.
Review
Questions:
1.
How would you explain underwater
vision?
2. How would you explain binocular vision?
3. How
would you explain inverted vision?
The
moral of the lesson: Underwater vision,
binocular vision,
and inverted vision are related to refractive index, eye movements (or ocular
motility), and perceptual adaptation.
References:
1. Davis, B. (1969). Upside-down girl baffles
scientist, Jet, 36(26), 17-22.
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. Hadden, P. W., & Zhang, J. (2023). An overview of the penguin visual system. Vision, 7(1), 6.
4.
Patel, S., & Tutchenko, L. (2019). The refractive index of the human
cornea: A review. Contact Lens and Anterior Eye, 42(5),
575-580.
5. Stidwill, D., & Fletcher, R.
(2017). Normal binocular vision: Theory, investigation and practical
aspects. John Wiley & Sons.
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