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How We See:
The First Steps of Human Vision
By: Diane M. Szaflarski, Ph.D.
Photograph by Kristin Borgeson
Take a look around the room that you are in. Notice how the various images and colors that you see update constantly as you turn your head and re-direct your attention. Although the images appear to be seamless, each blending imperceptibly into the next, they are in reality being updated almost continuously by the vision apparatus of your eyes and brain. The seamless quality in the images that you see is possible because human vision updates images, including the details of motion and color, on a time scale so rapid that a "break in the action" is almost never perceived. The range of color, the perception of seamless motion, the contrast and the quality, along with the minute details, that most people can perceive make "real-life" images clearer and more detailed than any seen on a television or movie screen. The efficiency and completeness of your eyes and brain is unparalleled in comparison with any piece of apparatus or instrumentation ever invented. We know this amazing function of the eyes and brain as the sense of vision.
From the beginning of time humans have tried to explain the complex process of vision. It is interesting to examine a few of the ancient theories and to compare them to our modern knowledge. Recorded studies of human vision date back at least to the time of Aristotle. Aristotle was a prominent philosopher, scientist, and scholar who lived in ancient Greece around the 4th century BC. Aristotle studied many scientific issues and published his thoughts in a variety of texts. Aristotle's explanation of the process of human vision was that the object being looked at somehow altered the "medium" (now known to be air) between the object itself and the viewer's eye. This alteration of the medium was thought to propagate to the eye, allowing the object to be seen. During the Middle Ages Aristotle's theory was reversed. Instead of postulating that the object itself had innate properties which allowed vision, popular theory of the time suggested that the viewer's eyes sent out emissions to the object and that those emissions enabled vision to occur. These theories may seem strange, unsatisfying or illogical today, but remember that the theories of long ago were not based on today's extensive experimental scientific data. Instead they were based on the conjecture and observation of scholars. Thanks to the knowledge generated by countless generations of scholars and scientists using increasingly sophisticated tools in pursuit of scientific knowledge our understanding of vision has come a very long way!
Vision is a complicated process that requires numerous components of the human eye and brain to work together. The initial step of this fascinating and powerful sense is carried out in the retina of the eye. Specifically, the photoreceptor neurons (called photoreceptors) in the retina collect the light and send signals to a network of neurons that then generate electrical impulses that go to the brain. The brain then processes those impulses and gives information about what we are seeing. In this unit we will investigate the initial steps in the process of vision. We will discover how the photoreceptors work, and will specifically examine at the photoreceptor proteins to learn how light energy is converted into electrical energy. Additionally, we will examine some of the current studies that are helping to further our understanding of the proteins involved in the vision process.
Eye Anatomy and Function
Human anatomy has been studied since ancient times. For over 1400 years our understanding of anatomy was based on theories of the Greek physician, Galen of Pergamum (130-200 AD). However an accurate and comprehensive understanding of human anatomy was delayed until the Renaissance period, primarily because dissections and autopsies were forbidden by most religions. One of the first systematic studies of human anatomy which involved actual examination and dissection of the human body, was carried out by Andreas Vesalius (1514-1564). As a result of his extensive work, many of the previous misconceptions of Galenic medicine were corrected. The accumulated research of scientists over many hundreds of years has led to an excellent understanding of human anatomy .
A sketch of the anatomical components of the human eye , as we now know it, is shown in Figure 1. The main structures are the iris, lens, pupil, cornea, retina, vitreous humor, optic disk and optic nerve. A discussion of the role of each component will not be presented here. These details are covered in most high school biology books and even in many sites on the World Wide Web. For example, try
Instead, we will examine the growth of the understanding of the eye's function.
A realistic understanding of the function of the components of the eye began around the 17th century, after the gross anatomy of the eye had been firmly established. It was realized in the 17th century that the retina, not the cornea as was previously thought, was responsible for the detection of light. Johannes Kepler of Germany and Renee Descartes of France, both prominent physicists of their time, made many advances in understanding vision. Much of their work applied the physical concepts of light rays and geometric optics to the vision process. Kepler first proposed that the lens of the eye focuses images onto the retina. A few decades later Descartes demonstrated that Kepler was correct. In a landmark experiment, Descartes surgically removed an eye from an ox and scraped the back of the eye to make it transparent. He then placed the eye on a window ledge as if the ox were looking out of the window. He looked at the back of the eye he and saw an inverted image of the scenery outside! Descartes correctly postulated that the image was inverted as a result of being focused onto the retina by the eye's lens.
Around the beginning of the 19th century Thomas Young, a prominent physicist and physician, carried out a number of studies on the eye that resulted in an understanding of how the lens focuses images onto the retina. He also showed that astigmatism results from an improperly curved cornea. We now understand that a number of vision disorders, including both near- and far-sightedness, also result from an improperly curved cornea. The lenses in eyeglasses function by correcting for the improper corneal curve.
We now know the basic function of the components of the human eye and how they participate in the vision process. Light that reflects off of objects around us is imaged onto the retina by the lens. The retina, which consists of three layers of neurons (photoreceptor, bipolar and ganglion) is responsible for detecting the light from these images and then causing impulses to be sent to the brain along the optic nerve. The brain decodes these images into information that we know as vision.
Microscopic Anatomy: Rod Cells and Cone Cells of the Retina
Although the microscope was first used in scientific observation in the late 16th and early 17th centuries, both the tool and the techniques of its use reached a sufficient level of sophistication by the 19th century to make it invaluable in examination of the structures of the eye. It was in the 1830's that several German scientists used the microscope to closely examine the retina. During this time that two different cells were discovered in the retina, the rod cells and cone cells. These cells were named because of their shape as viewed in the microscope.
A microscopic view of the rod cells of a zebrafish shows us how these cells actually look in an animal. Additional research showed that the rod and cone cells were responsive to light. Max Schultze (1825-1874) discovered that the retinal cones are the color receptors of the eye and the retinal rod cells while not sensitive to color, are very sensitive to light at low levels. Selig Hecht showed, in 1938, the exquisite sensitivity of rod cells when he showed that a single photon can initiate a response in a rod cell. Cone cells on the other hand are less sensitive to light but show great sensitivity to different colors. It is the cone cells that allow us to see in color. It is because cone cells remain unstimulated in low light environments that we do not see color in dimly lit places. Try this for yourself. Go into a closet and decrease the light level. Soon you will see only shades of gray. Slowly increase the light levels until you can begin to see color. This demonstration usually works well in a closet because of the many different colors of your clothes.
In the human eye, there are many more rod cells in the retina than there are cone cells. The number of rod cells and cone cells in animals is often related to the animal's instincts and habits. For example, birds such as hawks have a significantly higher number of cones than do humans. This let them to see small animals from a long distance away, allowing them to hunt for food. Nocturnal animals, on the other hand, have relatively higher numbers of rod cells to allow them better night vision.
A schematic drawing of rod and cone cells are shown in Figure 2. The cells are divided into two sections. The bottom portion is called the inner segment. It contains the nucleus and the synaptic ending. The synaptic ending attaches to the neurons which produce signals that go to the brain. The top portion is called the outer segment. The outer segment is comprised of a membrane which is folded into several layers of disks. The disks are comprised of cells that contain the molecules that absorb the light.
During the 1800's the visual pigments were discovered in the retina. Scientists, working by candlelight, dissected the retinas from frog eyes. When the retinas were exposed to day light they changed color. These scientists had discovered that the retina is photosensitive. They realized that the color they were observing was due to presence of a visual pigment, which was given the name rhodopsin. Later studies showed that rhodopsin is a protein that is found in the disks of the rod cell membrane.
Pigments are also found in cone cells. There are three types of cone cells, each of which contains a visual pigment. These pigments are called the red, blue or green visual pigment. The cone cells detect the primary colors, and the brain mixes these colors in seemingly infinitely variable proportions so that we can perceive a wide range of colors. Prolonged exposure to colors, for example when staring at a particular object, can cause fatigue in cone cells. This results in a change in the way that you perceive the color of the object that you are viewing. You will find a demonstration of the color fatigue effect on the Exploratorium's "Bird in a Cage" Web page.
The original theory of color vision was introduced by Thomas Young around 1790, prior to the discovery of the cone cells in the retina. Young was the first to propose that the human eye sees only the three primary colors, red, blue and yellow and that all of the other visible colors are combinations of these. It is now known that color vision is more complicated than this, but Young's work formed the foundation of color vision theory for the scientists that followed. The photoreceptor proteins of the cone cells have not yet been isolated. This may possibly be due to the difficulty in obtaining them. There are many fewer cone cells than rod cells in the retina. Also many animals do not have cone cells and hence do not see in color.
An Important Protein in the Rod Cell: Rhodopsin
George Wald and his coworkers at Harvard University pioneered our understanding of the molecules responsible for the first steps in the vision process. For this and other work on vision he was the recipient of the 1967 Nobel Prize in Medicine and Physiology. Wald's group was the first to elucidate the molecular components of the rod cell's functional protein rhodopsin. Prior to his work, rhodopsin was thought to be a chunk of molecular material. Wald and his co-workers determined that the protein consists of two molecular parts: a colorless amino acid sequence called opsin and a yellow organic chromophore called retinal.
It is now known that the rhodopsin protein has a molecular weight of ~40 kDa. The protein spans the membrane of the rod cell, and is therefore called a trans-membrane protein. The exact structure of rhodopsin has never been determined, however experimental data lead scientist to predict that it contains seven helices or turns. A schematic drawing of rhodopsin in the rod cell membrane is shown in Figure 3. About half of the protein is contained within the membrane with approximately 25% of the protein lying both above and below the membrane.
It is the rhodopsin protein in the retina that absorbs the light that enters the eye. Specifically, it is known that the retinal molecule, which is embedded inside rhodopsin, undergoes photo-excitation by absorbing light. In the photo-excitation process, the rhodopsin absorbs light and is excited to a higher electronic state. Numerous studies have been carried out to try to understand what happens after the rhodopsin absorbs light. Research has shown that upon photo-excitation the retinal part of rhodopsin undergoes a twisting around one of its double bonds (see Figure 4). The retinal then dissociates from the opsin. The change in geometry initiates a series of events that eventually cause electrical impulses to be sent to the brain along the optic nerve. Further research is needed to fully understand this complex process.
Vitamin A and Retinal
During the early part of the 20th century work continued on the frontier of research aimed at understanding vision. It was also around this time that the relationship between vision and proper nutrition began being studied at universities and agricultural schools. It had been shown during World War I that a vitamin A deficiency caused night blindness. The link between vitamin A and night blindness, however, did not become clear until George Wald and his coworkers isolated vitamin A from the retina in 1933. Prior to this finding the importance of vitamins was poorly understood. Additionally, the complete role of vitamins in physiological processes was unknown.
It is now understood that the human body makes retinal from vitamin A. A picture of retinal and vitamin A is shown in Figure 5. Both the retinal and vitamin A molecules contain a long chain of double bonds. When retinal dissociates from opsin, some of the retinal is destroyed. To replenish the destroyed retinal, it is important to have a source of vitamin A in your diet. Without this source of vitamin A, night blindness can develop as the rods can not function effectively without sufficient sources of retinal.
Recent Reports on Photoreceptors and Retinal
Scientists continue to study the role and mechanisms of photoreceptors in vision both to better understand the mechanism of human vision and to try to understand and remedy eye disease and blindness. Additionally, studies on photoreceptors can lead to the development of better electronic and optical devices, as well as improvements in the field of robotics and artificial sensing. Some of the recent publications from the journals, "Science" and "Nature" in the field of photoreceptors are summarized below.
1) Laser Experiments to Elucidate the Twisting of the Retinal Bond Following
Photon Absorption, Science, Volume 254, October 18, 1991, p 412-415.
Since the discovery of the laser in the 1960's numerous studies have been carried out on biological molecules, like rhodopsin, in order to try to understand the molecular and atomic action of the molecules. Since rhodopsin's function is to absorb light it is not surprising that scientists should use light to elucidate the behavior of the molecule. Even prior to the discovery of lasers, many spectroscopic studies were performed using lamps and other light sources. Recent efforts in laser experimentation to look at the twisting of the molecular double bond following photon absorption include the time measurements for the molecule to twist. Scientists have been examining this question for several years but with recent advancements in laser technology and instrumentation the rate of the twist has now been determined.
2) Determination of the Structure of Bovine Rhodopsin, Nature, Volume 362,
April 22, 1993, pp. 770-772.
As mentioned above, information on the structure of bovine rhodopsin has recently been obtained. Although many aspects of rhodopsin are known, its structure has never been measured due to the difficulty in producing rhodopsin in crystalline form. Recently the projection map of bovine rhodopsin has been determined from electron micrographs. The results presented show the configuration of the helices. These results confirm the proposal that rhodopsin has seven helices.
3) Making Mutated Forms of Rhodopsin in order to Elucidate its Structure and
Function, Science, Volume 250, October 5, 1990, pp. 123-124.
Currently, the exact role ( the binding sites, the structure, etc.) of rhodopsin in the generation of the neural signals is not completely understood. Parts of the mechanism are still in the hypothesis stage. In order to further elucidate the mechanisms and to prove or disprove the hypothesis, studies on mutated forms of rhodopsin are being carried out. An example of this was recently published. In this study the binding sites of rhodopsin with its G-protein were examined. The authors of this paper mutated rhodopsin at the sites that were thought to bind to the G protein. The scientist then tested the see if the mutated rhodopsin behaved similarly to its unmutated counterpart. The authors identified a portion of the rhodopsin that was required for binding and aiding in the formation of the neural signal.
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