How Vision Works

THE FUTURE OF:

NIGHT VISION

"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree."

-Charles Darwin in On the Origin of Species

KS

ENGS 5

April 16, 2003

Over 67% of rapes occur at night[1] and burglaries are 40% more likely to occur at night than in the day.[2] In general, the clandestine darkness of the night seems to facilitate criminal action. Pulling the trigger of a gun is much easier when the individual is protected by the disguise of night than in broad daylight. Psychological studies show that darkness leads to a sense of anonymity. This sense of anonymity leads to decreased self-awareness in individuals, and facilitates increased tendency to commit crimes. Thus, augmented night vision in humans could help to reduce crime.

CURRENT APPROACHES TO NIGHT VISON

Currently, the main approach to night vision is the development of night vision goggles. Night-vision equipment works by electronically illuminating and magnifying surroundings in the dark or using infrared sensors which detect light that humans cannot. Little to no research has been done on how to actually transform the human eye into a device with night-vision capabilities.

I propose an entirely new biological solution to night vision: A third eye, bioengineered and grown in-vitro, with specialized night vision capabilities, that will then be implanted on the foreheads of individuals. With the advent of this night vision eye, the propensity for violent crimes at night will be reduced.

BASICS OF EYE STRUCTURE AND VISION

To understand how to develop the human eye, one must first understand how it works. Although small in size, the eye - approximately 1 inch wide, 1 inch deep and 0.9 inches tall - is a very complex organ. The basic structure of the eye is shown in Figure 1. The anterior portion of the eye is essentially a lens system, made up of the cornea and crystalline lens, whose primary purpose is to focus light onto the retina. The retina contains receptor cells, rods and cones, which, when stimulated by light, send signals to the brain. These signals are subsequently interpreted as vision.

Figure 1: Figure 2:

When light enters the eye, it first passes through the cornea, then the lens, and ultimately reaches the retina, which is the light-sensing structure of the eye. The retina contains two types of cells: rods and cones (see Figure 2). Rods handle vision in low light – scotopic vision, and cones handle color vision and detail – photopic vision. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed creates electrical impulses in the optic nerve. The retina contains 100 million rods and 7 million cones.[3] This dual-receptor system allows the human eye to maintain sensitivity over an impressively large range of ambient light levels. The dimmest light in which the rods can function is equivalent to conditions of an overcast night with no moonlight. The dimmest light in which the cones can function is roughly equivalent to a night with 50% moonlight.

VISION AT NIGHT: DARK ADAPTATION

Consideration of the eye’s current ability to see at night will present the areas in which vision can be enhanced. In the process of dark adaptation each eye adjusts from a high-luminance setting to a low-luminance setting. Rods and cones differ markedly in their rate of dark adaptation. Cones attain maximum sensitivity in 5-7 minutes. Depending on the pre-adaptation to light, dark adaptation is about 80% complete within 30 minutes, but it may take hours, or even days, to acquire total dark adaptation. The rods adapt slowly, but eventually achieve a much greater sensitivity than the cones. As such, rods are the key players in night vision. Thus, the artificial third eye will have an extremely high ratio of rods to cones, though some cones will be kept to aid in visual acuity. Note that the lack of cones will cause night vision to be in shades of gray, black, and white.

The advantages of an excess of rods (as compared to they typical human) can be seen in considering the eye of a cat. Cats have a higher ratio of rods to cones, as compared with humans, and have a much greater ability to see at night, as shown by Figure 3[4]:

Cat’s vision at night: Human’s vision at night:

Moreover, the third eye will be closed during the daytime and in bright lights. Thus, the period of dark adaptation will not be necessary. The third eye will be fully dark adapted at all times.

In addition to adaptation caused by chemical reactions, the eye has other mechanisms for adapting to changing light conditions. The size of the pupil plays a large role in determining the amount of light that is permitted to hit the retina. The diameter of the pupil can contract to 1.5mm and expand to 8mm. If this range were even greater, as in an owl’s vision, a greater sensitivity could be achieved. Many species of Owls can actually see better than humans in bright light because the larger pupil allows more light to pass through the lens and onto the retina. [5] The artificial third eye will thus have a pupil that can open to 15mm and thus allow more eye to reach the retina.

Another light-adaptive mechanism is neural adaptation. In the retina the photoreceptor sends an electrical signal down successive stages of neurons. A change in "neural gain" occurs in seconds and can improve night vision by a factor of 10 or more.[6] Thus, more neurons in the eye will increase the speed and level of signals being sent to the brain. Unlike the photoreceptor chemical changes, these mechanisms occur instantaneously.

THE OPTIMAL NIGHT VISION EYE IN SUM

One final transformation involves the addition of a new non-human feature that will enhance night vision. The tapetum is a mirror-like membrane in the back of a cat’s eye that reflects light back through the rods, a second time, this time in the opposite direction. The result is a double exposure of the light, which permit cats to see in almost complete darkness.[7] In sum, the optimal night vision eye will be unique in several ways: a higher rod to cone ratio, a larger pupil, a greater number of neurons in the eye sending signals to the brain via the optic nerve, and a tapetum membrane.

GENETIC ENGINEERING

At this point you may be wondering how an eye with these qualities will be developed. A solution is genetic engineering. With genetic engineering the desired genes can be removed from cats and owls and introduced into the human body. First the desired gene must be separated out using a chemical that dissolves the cell membrane. Restriction enzymes, chemical compounds that act like "DNA scissors", are used to cut out the section of DNA corresponding to the particular gene. The gene can then be inserted into the eye cells using plasmids. Plasmids, circlets of DNA, can enter living cells, where cells take up the DNA and treat it as their own.[8] The difficulty of this procedure will be in identifying the exact genes in cats and owls that correspond to the exact features desired.

GROWING THE ARTIFICIAL EYE

The entire night-vision-eye, with all the above stated desired qualities, will be grown outside the human body, through tissue engineering. This neo-organ will be a hybrid of living cells and natural or artificial polymers. In the current state of biotechnology organs such as the eye, which are made up of more than one type of cell, have not been fully developed. However, evidence abounds that it is at least theoretically possible.[9]

First, a reliable source of human eye cells is paramount. One possibility is the use of human stem cells. Stem cells have not yet differentiated into specific cell types and so can be induced to develop into particular eye cells. Note that the details of this process are in the approaching future. There are two possible types of stem cells: adult human stem cells and embryonic stem cells. These cells can be specialized through genetic engineering into the optimal night vision eye cells.

These harvested eye cells would then be grown in-vitro using scaffolding or matrix to allow the tissue to take the shape of the eye. The matrix creates and maintains space for the formation of tissue and allows for structural development. The three-dimensional shape will be formed using scaffolds of biodegradable polymers. Since the biodegradable polymers will degrade over time once incorporated into the body, the scaffold will be entirely compatible with the human body. A biocompatible glue will bind the cells together. Due to the differences in the lens and iris from the rest of the eye, these will be grown on different scaffolds, then assembled into the complete eye.

An important thing to remember in this synthesis is growing cells must be supplied with adequate nutrients. Angiogenesis, blood vessel growth, can be stimulated. Molecules to promote angiogenesis will be used to ensure that blood will supply the growing eye with nutrients. The eyes will grow in the labs until they are fully developed and ready to be implanted.

IMPLANTATION

The third eye will be placed in the center of the forehead. This location was deemed most appropriate because it allows the range of view most similar to the day vision. Of course, with only one eye the individual will lack binocular perception at night. However, due to the use of two eyes during the day the individual should be able to adequately account for this difference.

So that the eye does not protrude abnormally from the skull, a half-circle shaped piece of bone will be removed from the center of the forehead. The size and shape of this piece will correspond to the size of the eye socket in each individual. The eye socket will be lined with fatty tissues, as in the normal eye.

At this point the most difficult challenge must be surmounted: the artificial eye must be connected to the optical cortex of the brain to facilitate the complex processing of vision. Recall the aforementioned optic nerve; This nerve is what sends the messages from the eye to the brain. Thus, another optic nerve must be grown and directed towards the sight at the forehead where the eye is to be implanted. It has been shown in goldfish that the optic nerve can in fact regenerate in such a manner.[10] A hole will be drilled in the back of the skull to get access to the optic cortex. At the sight of the optic cortex a brand new optic nerve could then be multiplexed from the original one.[11] At the sight in the optic cortex where the surgeon has drilled to, an artificial peripheral nerve graft will be placed to begin the growth of the new optic nerve. The growth of the optic nerve would then be directed to the implantation sight at the forehead. Growth factors cause a patient’s own cells to migrate to the site where the growth factors are placed, turn into the right type of cell, and regenerate. A tract of targets – nerve growth factors - would have to be laid down by drilling more holes in the skull, and inserting cannulas containing nerve growth factors on the surface of the brain. The growth factors would then induce the optic nerve to regenerate, leading it to the forehead where the eye would be implanted. A sufficient period of time would be necessary to allow the eye to form a strong attachment to both the socket and the optic nerve. Details of this process require further investigation. However, I suggest that this be done with the eye covered by an attached artificial eyelid so as to prevent damage.

ETHICS

As in any situation in which the human body is altered, ethical issues come to the forefront. Embryonic stem cells are one of the most intensely debated issues today. Opponent of the use of stem cells believe that life begins at conception, whether conception occurs naturally or in a petri dish in a lab. The issue transcends the field of science, broaching religious and moral sentiments. Adult stem cells could be used, but these are more difficult to obtain, especially in large quantities. Another concern is animal welfare with cats and owls being a source of genes.

Finally, the question would arise as to who should have night vision. Should infants be given a third eye at birth so that everyone will eventually be the same? With this arises ethical concerns about altering humans without their consent. Perhaps criminals could be given the surgery to lessen their likelihood of committing more crimes. But would it be fair to have tax-payers fund the enhancement of the physical capabilities of criminals? Deterring who would have the third eye would pose a difficult problem.

This surgery cannot occur today, nor in the near future, and should not be considered in the context of current times. However, researchers in biotechnology are currently making advancements at an impressive rate. Consider the future. If this surgery became a simple process, would you want to be able to see at night?

1

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8] Mooney, David J. and Antonios Mikos. Scientific American Presents: “Your New Body- Growing New Organs.”12

[9] Mooney, 10

[10] E. Antonian, G.W. Perry and B. Grafstein, 1987. Fast axonally transported proteins in regenerating goldfish optic nerves: Effects of abolishing electrophysiological activity with TTX. Brain Research 400, 403-408.

[11] Professor Sabelman assisted with the development of this idea