Ординатура / Офтальмология / Английские материалы / Relearning To See_Quackenbush_2000

"Blind Spot" will disappear. No peeking! The light rays from the words "Blind Spot" land in the optic disc, where there are no light receptors. When the page is moved closer to or farther away from your eye, the light rays from the words "Blind Spot" fall onto the rods (and cones) outside the optic disc.

PartB:

Sketch the letter "v" in the word "Fovea" in Part A When you have found the distance at which the words "Blind Spot" disappear the best, then sketch the letter "l" in the sentence "Fill it in, Brain!" Magic! The brain fills in the empty space in the line to the left with what logically should be there—a "continuous" line. Moving this page closer and farther away reveals the empty space once again.

If you would like to experience the blind spot with your right eye, turn this page upsidedown, and repeat the same instructions.

If you have difficulty with these experiences, rotate this page a couple of degrees counterclockwise (clockwise when using the right eye.) Since the optic disc is a couple of degrees above the horizon relative to the fovea, this may help "Blind Spot" disappear. (Most students do not need to do this.)

The brain filling in the empty space, when it is at the blind spot, is an indication of how mental the process of seeing is. How much of the picture we see all day long is "filled in" between

the spaces of the cones and rods in the retina?

T H E B I R D S ' A N D T H E B E E S ' V I S I O N

See Plate 50: Animal Vision.

Most animals are either diurnal (daytime) or nocturnal (nighttime) creatures. As such, each has developed a visual system that best suits its purposes.

The eyes of land and air predators are placed forward in the head for excellent depth perception. (Predator fish must move their heads left and right to spot prey that is straight ahead.)

Along with humans, some primates, reptiles (including alligators, crocodiles, lizards), and almost all diurnal birds have at least one fovea in each eye.

Among animals that have cones, not all have all three types of cones. Many have only one or two types, which means their color vision is limited. Among mammals, only apes and most monkeys have color vision close to humans.

NICTITANS, THE THIR D EYELI D

Most nocturnal birds and some reptiles, like lizards, crocodiles and alligators, and camels, have a functioning third membrane called a nictitans (from the Latin nictatus, meaning "to wink"). This special thin, transparent membrane is located near the inner angle of the eye or beneath the lower eyelid.

Even when closed, the transparent nictitans allows the animal to continue seeing— a decided advantage. The nictitans helps protect animals from dirt and debris. By closing the nictitans, birds can protect their eyes while in flight. Nocturnal birds, like the owl, can use the nictitans to protect their eyes from bright sunlight during the day.

BIRDS

Birds have very large eyes relative to the size of their head and body. Some eyes are so large, they touch each other inside the bird's head! (Thus, the phrase "bird brain.") A larger eye has more cones, which provides sharper vision. Birds have the highest quantity and density of light receptors of all animals. A sparrow has a cone density of 400,000 mm2, which is more than twice that in the human eye.

As mentioned above, nearly all birds have a fovea. About half of all birds have two foveas in each eye.

For all birds, the lower eyelid moves upward for protection from dirt and other particles and to close the eyes during sleep; the owl is the only bird that can lower its upper eyelid.

Daytime Birds

Some daytime birds, including eagles, hawks, chickens and canaries, have all cones, but no rods Unlike humans, these animals see everything sharp and colorful simultaneously, similar to the picture from the camera in Plate 44: How We See, a. Unfortunately, "pure" cone vision makes a daytime bird unable to see in true nighttime.

Raptor (predatory) birds such as eagles,

hawks, and falcons have the best distance vision of all creatures on Earth. The buteo hawk has the highest density of cones at 1,000,000 mm2. This is more than five times that of humans. An eagle can spot a mouse on the ground from high in the sky, while a vulture can spot a carcass from nearly two and a half miles away.

All raptors have two deeply pitted foveas in each eye. These specialized foveas produce magnified "telescopic" sight both straight ahead and to the sides. One fovea picks up a point of clarity straight ahead, and the other picks up a point of clarity in the peripheral vision.

Due to its very high concentration of cones and specialized foveas, the red-tailed hawk can see the raccoon across the river with greater clarity than humans. (See Plate 50.)

A chicken has all cones. The entire visual field is equally sharp and colorful in the daytime. However, because it lacks rods, the chicken is essentially "blind" at night and needs to hide from the rod-equipped fox.The eyes of chickens and ducks are placed on the sides of their heads, typical of prey animals.

Nighttime Birds

Pronounced nighttime birds, like night owls and some bats, have only rods and no cones. Owls see only indistinct, gray shapes. (Some diurnal owls have some cones, and therefore some color vision.) Nocturnal owls have more rods than humans. For example, the barn owl has four times as many rods as humans.

Additionally, the rods in owls are in larger groups (association cells), resulting in even greater sensitivity to light at night—up to 100 times more sensitive than the darknessadapted vision of humans.

One advantage of not having a fovea centralis is that a tiny, dim object does not disappear from the central vision at night. The central object will be "grainy" and colorless, but at least it can be seen.

Most nocturnal birds are not able to accommodate. Even if they could, the best vision would still be much less than 20/20, since the rods do not pick up sharp detail. Movement and very low-level light perception are more important for nocturnal birds.

The owl compensates somewhat for its immobile eyes by having twice as many vertebrae in its neck. With the extra vertebrae, an owl can turn its head almost 1800 in each direction. This gives some children the impression the owl can rotate its head all the way around, 3600! The owl can also bend its head all the way back.

Contrary to their reputation, bats do use their eyes to see at night. In fact, the fruit bat has excellent night vision. Many bats (but not fruit bats) also use a form of sonar, sending out chirps that echo off of insects and other prey. The time it takes for the echo to return helps the bat gauge the prey's distance.

OTHER A N I M A L S

The color vision of many mammals is limited to faded yellows and blues. Red colors appear dark brown, while greens appear white or gray.

Other Daytime Animals

Within the large, wild cat families, only the cheetah is a diurnal predator. Notice in Plate 50 the black stripe extending from the eye to the mouth. This helps prevent reflection from the bright sunlight. Football players have

copied the cheetah's ways! Some lizards ha\e a black circle around their eyes which serves the same purpose.

Although their retinas contain mostly rods, cheetahs, leopards, and seabirds have a thin, horizontal band of concentrated cones in the middle of their retinas—a semi-fovea.This gives cheetahs sharper vision to spot gazelles and antelope along the flat plains of Africa. But gazelles and antelopes also have horizontal foveas to spot their predators.

Notice the forward position of the cheetah's eyes, typical of land predators. This provides excellent depth perception. The cheetah is the world's fastest land mammal, accelerating from a stand-still to over 70 miles per hour within a few seconds. Only the three birds, the golden eagle, peregrine falcon, and Indian swift, can travel faster. Cheetahs are currently an endangered species.

The native black tail prairie dog (actually a squirrel that sounds like a small dog!), squirrels, and turtles have nearly all cones. They have no night vision. Note the location of the eye on the side of its head, allowing excellent peripheral vision, like chickens and rabbits This is typical of prey animals.

The vision of the mandrill monkey is very similar to human trichromatic (RGB) vision, whereas some other monkeys have more limited color vision.

The South American squirrel monkey has very good cone vision, but is shghtly deficient in the red region.

The chameleon, a type of lizard, has an extremely high density of cones in its fovea— nearly 800,000 mm2, almost matching that of the buteo hawk. Turtles have almost completely cone vision.

Other Nighttime Animals

Nocturnal animals have virtually all rods, with few or no cones. Since the rods are very poor at picking up red light, a red flashlight can be used at night to find night animals without frightening them away. At night, human eyes can detect red better than most night animals.

Cats have both rods and cones; however, the rods far outnumber the cones. The density of cones in cats is only 24,000 mm2, about lA that of humans. Cats do not have a fovea. The night vision of a cat is about eight times better than human night vision. Part of this better night vision is due to the tapetum which is described below.

Cats, including lions, are primarily nighttime animals. A cat's pupil is round when large but becomes a narrow vertical slit when closed. This helps protect their night-adapted rods from sudden bright light.

Research on the vision of dogs is conflicting. About half of the studies suggest dogs do not see color, and the other half suggest they do. Some think dogs have very limited color vision in the blue region. In any case, the majority of seeing by dogs is with the rods, and therefore the picture is gray. The fovea is absent in dogs.

Other nocturnal animals include rats, mice, lemurs, flying squirrels, raccoons, rabbits, crocodiles, dolphins, alligators, raccoons, skunks, and badgers. These animals have essentially all rods.

Dolphins, being nighttime hunters, have 7,000 times more rod association cells than humans; cats have 2,000 times more.

Deep-sea fish have the most light-sensitive retinas of all creatures. Their retinas are particularly designed to pick up blue wavelengths of light, as most red light is absorbed near the surface of the water.

Rabbits have "wide-angle" eyes at the sides of their head, typical of land prey animals. Rabbits have almost 3600 wrap-around vision. With almost no defense from predators except freezing and running, their panoramic vision is important for survival. Since rods are movement detectors, "freezing" can be an effective method of avoiding detection. Rabbits have no cones.

T H E T A P E T U M — D O U B L I N G SENSITIVITY

Many animals including owls, cats, lions, cows, sheep, oxen, rabbits, raccoons, some dogs, elephants, bears, wolves, dolphins, whales, and some deep-water fish have eyes that glow at night. This is due to an extra highly reflective layer called the tapetum lucidum. The tapetum (from the Latin tapete, meaning "carpet," and lucere, meaning "to shine"; literally "the shining carpet"), which is not present in humans, lies in the choroid, just beyond the pigment epithelium in the retina.

In animals with tapetums, light that misses the light receptors passes through a relatively transparent epithelium layer and reflects from the mirror-like tapetum back into the rods (and cones, if any) layer. The tapetum gives the light ray a second chance of hitting a rod or cone, giving these animals better nighttime vision. Some of the light reflected from the tapetum exits the eyes again, creating a cat's "glowing" eyes at night.

Some animals even have a "super tapetum" made of guanine crystals. These crystals reflect light even better than the common tapetum.

Notice in Plate 50 how the eyes of the rabbit "glow" from the camera flash.This is due to the rabbit's tapetum. The pupils are large because the rabbit is in the shade at sunset.

ULTRAVIOLET A N D I N F R A R E D VISION

Bees, butterflies, deer, and many lizards can see UV light. UV light patterns on flowers and insects are important for feeding on nectar and for mating.

Some snakes, e.g. the green tree python, pit vipers, and rattlesnakes, some owls, and some bats can see in the infrared region. At night, a mouse is not as protected as it thinks, as the viper snake can see the mouse's "heat" image.

Special "thermographic" camera technology now allows us to take infrared "heat" pictures. For example, a heat picture can be taken of a hand-print on a wall just after the hand has moved away.

Goldfish can see both infrared and UV light.

There are several books listed in the Bibliography that describe the vision of animals. How Animals See, by Sandra Sinclair, is a standout.

CHAPTER s u m m a r y

I once asked a group of students how they would design the Ught receptors in the retina. One student said, "The retina should have all cones during the day, and then transform into all rods at night."

There are many types of visual systems, only a few of which have been discussed in this chapter. For humans the cones are maximized in the center of the visual field, for

Chapter Seventeen: The Retina

excellent acuity and color. Rods pick up indistinct gray images, motion, and very low levels of light in the periphery.

In the movie Tombstone an observer of the famous Wild West lawman Wyatt Earp states, "He's got the eyes of both predator and prey." Basically, nature has given us "eagle-eyes" in the center of the visual field, and "night-owl" vision in the periphery.

N O T E S

1Sandra Sinclair, How Animals See (New York: Facts on File Publications, 1985), p. xv.

2Steve Richards, "How to Extend Your Sight," Invisibility (Wellingborough, Northamptonshire, England: The Aquarian Press, 1982), p. 52.

3Natalie Angier, "New Clues to Vision: People Whose Glasses Must Be Rose-Colored; When the retina has no cones, scientists can see the

rods at work," The New York Times (Novem-

ber 17,1992), p. B6.

4Clara A. Hackett and Lawrence Galton, Relax and See (London: Faber and Faber, Limited,

1957), PP-196-97-

5Charles H. May, Diseases of the Eye (Baltimore, Maryland: William Wood and Company, 1943),

p. 294.

6John Downer, Supersense: Perception in the Ani- mal World (New York: Henry Holt and Com-

pany, 1988), p. 52.

7R. L. Gregory, Eye and Brain: The Psychology

of Seeing (New York: McGraw-Hill Co., 1966),

p. 48.

The angle upward from the center of the visual field to the eyebrow is 45°; downward from the center to the cheek is 65° The total central vertical angle is 1100 .

Blind spots are eliminated in the Combined Visual Fields. However, the two optic disc areas are monocular, because each area is only seen with one eye.

T H E FUSE D FINGER

See Plate §1: Binocular Vision, The Fused

Finger.

Examine your right forefinger carefully while holding it about eight inches in front of your head. When asked how many fingers there are, we say there is only one finger, and we think we see this finger like a camera sees it, as in Camera Finger.

A closer examination reveals there are actually two fingers that are "fused" into one. Up close, the right eye sees a significantly different view of the finger than the left eye does Close or cover each eye alternately to discover this fact. Note the fingernail images seen with each eye separately.

The brain merges the two different images, the best it can, into "one" finger. The finger we see is actually a composite of the two separate images from each eye, as in Fused Fingers. This is not the same as a camera's view.

This demonstration is more dramatic if you hold your hand vertically about four inches in front of your head. Position the thumb so it is close to the head and the small finger is farther away. Alternate closing each eyelid. The two views of the hand are very different. The right eye can see the right sides of all the fingers, but not the left sides, and vice versa. With both eyes open, you see both sides simultaneously.

Now hold a finger or hand out at arm's length. Examine the finger or hand carefully again. Close each eye alternately to discover there is less difference between the two sides.

The closer an object is to a person, the greater the difference between the two pictures of that object. The brain uses the amount of difference between the two images, along with the size of the object, to determine its distance. The difference between the images in each eye changes the most from directly in front of the head out to a distance of about twenty feet.

Theoretically, a finger or hand positioned at infinity would be seen the same with each eye. With our visual system, however, objects at twenty feet and beyond are essentially at infinity as far as fusion and depth perception are concerned.

B I N O C U L A R VISIO N FOR DEPTH

P E R C E P T I O N

The main advantage of having two eyes in the front of the head with overlapping visual fields is better depth perception. Binocular vision helps us determine an object's distance.

As discussed above, the difference between the two images is one way the brain gauges depth.

Another way the brain judges an object's distance is by its size. A rabbit one foot away appears much larger than the same rabbit twenty feet away. But what if the rabbit twenty feet away was so large it appeared to be the same size as the rabbit only one foot away? Flow can the brain tell the difference? This is where the difference in the images in the two eyes comes into play. The two images on the right and left retinas of a smaller rabbit one foot away would have a

greater difference between them than the two images on the retina of a far-away, larger rab­ bit. Also, by experience, we know that rab­ bits are not huge!

Two other ways the brain can judge the dis­ tance of an object is by convergence and accommodation. Convergence, discussed more below, is alignment of the two eyes so that the object of interest (centralization) lands in the foveas for sharp acuity. The more the eyes turn inward, the greater the con­ vergence, and the closer that image must be. The accommodation of the eye to see a near object is another clue the brain has to gauge the distance of an object. As mentioned in Chapter 3, "Understanding Lenses and Pre­ scriptions," accommodation occurs basically within the first twenty feet.

JUDGING RELATIVE D I S T A N C E S

Refer to Plate 52: Judging Relative Distances.

The model in a-i is the same model as in a-2;b-i is the same as b-2; and c-i is the same as

a-i, b-i, and c-i are views of the models from the top.

a-2, b-2, and c-2 are views from behind the eyeballs, looking toward the letters L, C, and R. These models show the different images created on the retinas when two objects (L and R) are at different distances from the eyes.

The eyes in these models are directed toward the center of the letter C, as indicated by the gray lines extending from the fovea centralis of each eye out to and through the center of the letter C.

In examining these models, there are sev­ eral discoveries we can make about how the retina "sees" multiple objects. The brain

Chapter Eighteen: Stereoscopic VLuon

processes different retinal images and then draws conclusions regarding the relative dis­ tances of those objects.

First, notice how all letters on the retina are flipped upside-down and leftside-right. Next, notice that L is to the left of C, but on the retina the image of L is to the right of C. R is to the right of C, but on the retina the image of R is to the left of CThe image of the world is completely reversed on the retina.

In a-i/a-2, L, C, and R are the same dis­ tance from the eyes. Also, L is the same dis­ tance to line AB as R is to line CD. AB and CD represent the lines of sight from the fovea to letter C. In a-2, the images of L, C, and R on the retina are the same size, and the images of L and R are the same distance to the image of C.

In b-ilb-2, L has been moved closer to the eyes, while R has been moved farther away. In b-i, notice how the letters L and R are now closer to line AB, while they are farther away from line CD. This may be easier to see if you rotate the illustration counterclockwise so that AB is vertical; then, rotate the page clockwise to make CD vertical.

In b-2, in the left eye, the images of L and R are now closer to C, while in the right eye they are farther away. The positions of the letters on the two retinas are now quite dif­ ferent from each other, and from the images on the retina in a-2. In b-2, how does the brain fuse these two different pictures from the retinas?

The image of С does not change its loca­ tion or size on the retina because С has not been moved from its original location and the eyes are still centralizing on C.

In c-ilc-2, L has been moved farther away from the eyes, and R closer. Compare all of the images on the six retinas. There are only

two that are identical. Which ones are they? Along with the other methods described above, the brain uses the difference in dis­ tances between images of L and R on the two retinas to make judgments about the relative and absolute distances of these letters. The main differences occur when the two objects

are within the first twenty feet.

If the distances between the images of L and R on the retinas are the same, as in a-2, the brain may assume L and R are at the same distance from us. If the distance between the images of L and R is smaller on the retina of the left eye than on the right eye, as in b-2, the brain may conclude L is closer than R. Finally, if the distance between images of L and R is greater on the retina of the left eye than on the right eye, as in c-2, the brain may conclude L is farther away than R.

Notice in b-2 that the images of L are larger on the retinas because L is closer to the eyes; the image of R is smaller because it is farther away. In these models, the brain may assume a larger letter is closer, and a smaller letter is farther away.

These letters are the same size. If they were different sizes, the brain might need more information to judge their distances. Learn­ ing the size of objects by experience, like the rabbit example above, is an important part of learning to gauge distances of objects.

The images in the above models are rela­ tively large, and close to the eyes. If these same three letters were placed too feet away, and the letters L and R were moved the same distances closer and farther away, the differ­ ences between their images on the two reti­ nas would be much less. This is one reason it is easier to gauge the relative distances of objects within the first twenty feet.

A TOPSY-TURVY WORLD

Some studies have been conducted in which a person wears special goggles which make objects appear upside down, and the right side on the left. After a certain period of time, while still wearing these goggles with prisms, some objects are seen right-side up in their correct orientations!1

3-D VISION, MORE THAN JUST

STEREOSCOPIC VISION

The sense of depth created by binocular vision is called stereoscopic vision. The brain merges both the two-dimensional images, one from the right eye and one from the left eye, into one three-dimensional image. Stereopsis (from Greek, meaning "solid sight") means the appearance of depth when both eyes are used.

In addition to stereoscopic vision, the right hemisphere of the brain creates an even more 3-D image.

In natural vision classes, students sketch scenic pictures with their imaginary nose-pen­ cils. At the end of the second class, most vision students experience a marked feeling of depth in these scenic pictures—as if the stu­ dent could actually walk into the scene, sens­ ing the edges around objects.

This sense of 3-D depth perception is not the same depth perception experienced by stereoscopic vision, as this "extra sense" of 3-D can be experienced with only one eye. This 3-D experience is one of the qualities of vision that resurfaces when the visual sys­ tem begins to relax and returns to a natural balance.

During his initial telephone call, one of my students told me he had perfect sight for гЬЪ years, and that he had been wearing glasses

for only six months. He told me that as much as he wanted to improve his acuity, he also wanted his 3-D vision back. He said that the more he wore his glasses, the "flatter" the world seemed to get, especially while wearing the glasses. Unfortunately, most people who have worn glasses for many years do not remember natural 3-D vision. It can be startling to many students when it begins to return.

So, a person may have stereoscopic vision, due to the proper convergence of the two eyes, but he may not have right hemisphere, 3-D vision. Left-brain, mechanical glasses, the "machines of seeing," switch off many right-hemisphere qualities of vision. Sight through glasses is not similar to the experiences of true natural vision.

The ophthalmologist Dr. R. Agarwal, in his book, Mind and Vision,1 describes this extra sense of 3-D and some other aspects of natural vision:

So I gave a picture of the Taj Mahal to this girl student. She was asked to look at the people right in front and at those who were on the floor of the Taj Mahal as if they were a mile away, very far. By alternate shifting of sight she could feel the distance between the people in front and the Taj

Mahal. Taj Mahal appeared as if a real monument before her. Instantly the girl observed that the sun had also come there, as if from nowhere, and was shining brilliantly on the golden dresses of the girls in the photograph. Suddenly she cried out, "Ah, it is really beautiful." She saw the depth in the windows of the Taj Mahal. The shadows of the front walls of the Taj falling on the walls behind added to its threedimensional appearance. The four minars, the conical trees, the water canal and the carpet of green grass, all appeared quite real to her. The mind [began] to see everything in its true perspective, as if Taj was visible with its length, breadth and height. The people in front seemed to be walking in reality... The coloured saree of the lady became hundred-fold beautiful and magnificent with all its sober designs. When the mind got completely relaxed, the visual cells of the retina began to function with their full capacity.3

Since the retina is part of the brain, the brain began to function with its full capacity.

Later he writes of another student:

... she was taught centralization and was given a picture of the Taj Mahal. At first glance the view-card appeared to be a flat coloured picture but by looking at it in a particular way without effort or strain she was able to enjoy it The Taj appeared in all its glory in a bright light. The effect of the sun could be seen on the building, the shadows could be seen behind the persons walking in front of Taj. The three-dimensional effect could be produced easily with each eye separately.4

This story shows that this extra sense of • • - three-dimensional vision is not dependent upon stereoscopic vision. There is a definite, extra quality of 3-D vision, difficult to describe