Evolution of Two-eyed Vision

Two Eyes to the Side
Nature has given animals the physical attributes necessary for survival. Lateral placement of the eyes is essential to the survival of hunted animals or herbivorous animals (e.g., horse, rabbit, cow) as it allows them to increase side or peripheral vision.

Side vision (increased by lateral placement) is a sensitive detector for motion or movement. Peripheral vision allows creatures to effectively scan for danger. The rabbit must be constantly aware of its natural enemies while it eats your garden greens. At the first sign of danger, peripheral vision, the motion detector system, alerts the rabbit that there is danger. The immediate reflexive response is for the rabbit to run.

Two Eyes in Front
Faster moving carnivorous hunters do not need as much peripheral vision as the hunted. It is more important for hunters to locate their prey and accurately determine the distance from themselves to that prey. Therefore, animals that hunt (carnivorous or meat eating animals, e.g. lion, cat) as well as humans have frontal placement of the two eyes in order to determine the exact location of their prey. The hunters sacrifice the large peripheral motion detection system afforded by side placement of the eyes in favor of the incredibly accurate depth perception system created by frontal placement of the eyes. To make up for the loss of peripheral vision, most carnivorous animals have also developed a sophisticated, pivoting system which extends the range of side vision...that is, the neck.

The Benefits of Two Eyes in Front
Frontal placement of the eyes allows for a remarkable visual phenomenon called stereopsis. Stereopsis is the 3D perception that occurs as a result of both eyes working together to create relative depth perception.

Many of you have experienced exaggerated demonstrations of stereoscopic depth by viewing I-Max 3D movies or old stereoscopes. Or, perhaps, you have seen photos of theatergoers in the 1950's wearing special Polaroid glasses in order to view 3D movies.

What is Stereopsis?
Stereopsis results from the combination of the two images received by the brain from each eye. Each eye views the world from a slightly different vantage point (See Fig 1).

Figure 1

The fusion of these two slightly different pictures from our two "cameras" (the eyes) gives us the sensation of strong three-dimensionality or relative depth.

At near, there is a greater difference in what the two eyes view as compared to far. Thus, stereopsis is strongest and most important at near distances. At near is where man uses accurate hand-eye coordination to make tools and other items!

The Benefits of Stereopsis
Stereopsis has been very important in human development. Keen and accurate two-eyed depth perception has allowed man to develop tools and the manufacture of goods, a central aspect of modern civilization. Stereopsis plays a role in many other human activities, such as, catching a ball, parking a car, threading a needle, performing surgery, or any other activity that requires accurate depth perception at close distances.

Animals that have lateral position of the eyes and individuals who have reduce binocular vision lack stereopsis. This does not mean that they have absolutely no depth perception.  There are many one-eyed (monocular) depth perception cues that allow us to make reasonably accurate depth judgments.  These monocular depth perception cues may be familiar to you and include: perspective, overlay, shadowing, aerial perspective (color of the sky), relative motion, relative size, etc.

Binocular vision cues (from two eyes), such as stereopsis and parallax, are dependent on accurate alignment of the eyes and appropriate unification of the two images by the brain. People with only monocular or one-eye depth perception skills can do fine in many situations. However, they are not allowed to fly a rocket ship, drive the trains in New York city subways, and they definitely should not be surgeons. They may have trouble catching a fly ball or becoming a NBA point guard. However, many jobs do not require stereopsis and thus the lack of stereopsis does not preclude a successful life.

Stereopsis does enhance quality of life and life choices, however!  Some eye doctors might tell you that it is a luxury, but it is part and parcel of our evolution and human potential. 3D vision is a human skill we all want and deserve. Every attempt should be made to develop this visual skill in a child [and it's not too late for many adults!]

What is the "critical period?"
In the early 1960's, two Nobel Prize winners from Harvard , Hubel and Weisel, did research on the development of vision. They studied monkeys and cats who have stereoscopic vision similar to humans. This led to conclusions regarding a "critical period" of development for stereopsis.

We now know that the development of keen binocular vision with resultant stereopsis is a result of genetics and appropriate development of the binocular system during the early formative years. The ability to see 20/20, focusing ability (accommodation), eye muscle coordination (aiming or alignment) and stereopsis are all developed by 6 mos. of age. in humans. A tremendous amount of research over the last 40 years using monkeys and cats, which have stereoscopic vision similar to ours, has improved our understanding of how the eyes work.

In the early 1960's two Nobel Prize winners, Hubel and Weisel from Harvard, recorded activity of cortical cells. In a group of eloquent studies they covered one of the cat's eyes, stabilized the movement of the viewing eye. Then they inserted an electrode in a cell in the visual cortex of the brain of a cat, amplified the signal, and recorded the output of various cells that they tested. Lastly, they moved a light around until the cell responded. Specifically, they studied cells in the area in the occipital cortex that was known to be associated with vision. Each cell responded to a different location in space.

In addition, the cells responded to different types of light, e.g. some cells responded to a bar of light moving left to right while others responded to light moving up and down. First they recorded from one eye and then the other. They found that 80% of the cells responded to the input from one eye (binocular cells) while the other 20% of the cells only responded to the input from either eye (monocular cells). Binocular cells are necessary for the two eyes to work together and are the basis for depth perception or stereopsis. This was an important breakthrough since they demonstrated the location and characteristic of stereopsis in the brain.

Then they altered the cat's visual experience. They patched one eye for weeks on end, blurred an eye with contact lenses, and/or made the cat artificially strabismic (eye turn). Afterwards they recorded the effect of these procedures by measuring the responses of the cells in the visual cortex. These altered visual experiences changed how the cells fired. Cells that use to respond to the input from either eye now only responded to the input from one eye. Thus all the cells became monocular. Actually, the cells of the "good eye" inhibited the responses of the "bad eye". With special techniques neuro-physiologists also developed techniques to measure visual acuity, color vision, depth perception, etc. in the cats and monkeys. Vision was reduced and there was a loss in depth perception. Autopsies of the animals demonstrated the cells associated with seeing and binocular vision became atrophic (smaller in size). These results only occurred if the disruption in the visual experience happened early in life. Thus, early visual experience changed how the cells responded, what the animals saw, and how the cells looked. Altering visual experience via patching, blurring of vision or surgery did not effect older animals as much. The period of time in which the cells changed from alteration of visual experience is known as the critical period. It is of important interest to note that intense vision therapy after the end of the critical period still resulted in improvement in vision and binocularity in these animals. Thus, the critical period is only the time of maximum neurological plasticity.

These animal studies teach us how the visual system develops and works. They show that altering the visual experience of the cat or monkey's life during the first few years of life, critical period, has a great impact on future development. This same phenomenon happens in humans. Strabismus (eye turn) and amblyopia (reduction in vision because one eye is deprived clear single vision during the critical period) could be experimentally created in animals and studied. These models help us learn how amblyopia and strabismus develop and must be treated. Research suggests that the maximum critical period in humans is from just after birth to 2 years of age. Any disruption of binocular vision from 6 mos. to approximately 4 years will result in strabismus and/or amblyopia. Thus, every infant without an apparent problem should have their first examination between 9 mos. to one year of age. Up to the first 6 mos. of age intermittent strabismus is a normal developmental milestone. By 9 mos. of age the system is in place. Young babies are also easy to exam. Age 2 is neurologically late and a difficult time to examine the young totter. If everything is normal at that 9 mos. examination, the next examination should be in kindergarten.

The best chance of success of eliminating the effects of amblyopia or constant strabismus occurs before the age of two. However, this does not preclude excellent success in older patients and partial success in most patients older than 6 years of age. There are numerous studies that demonstrate that treatment after the age of 6 is very successful. One study compared treatment before age 6 to treatment after age 6. They found no statistical difference between the two groups. As a matter of fact, loss of an eye in patients over the age of 65 who were never treated for their amblyopia experienced a spontaneous improvement in vision in over one-third of the cases. Thus, every attempt should be made to improve visual problems though treatment might not be as effective after the age of six and definitely requires more work. Also, remember that if an eye turn occurs only some of the time (intermittent), the cells of the brains do not develop the changes associated with constant eye turns. It doesn't take much stimulation to maintain function.

An analogy to understanding the relationship of treatment of eye muscle anomalies would be learning to speak a second language. During the period of neurological development, around the first year of life, language development is natural and spontaneous. Children raised in families that speak two languages from birth automatically learn both languages. However, if the second language is introduced in later school years language development is longer and more arduous. Remember, people learn languages in their sixties and seventies. The same is true of visual development. Its easier to develop normal vision during the critical period, but with work many people can develop normal binocular vision in later years. It is never too late to try.

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