color vision
TRANSCRIPT
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Science of the Eye: Vision How do we see?
Have you ever thought about how we see the world around us? What is the science behind this sense? In this unit, we will discuss how vision works, how we see color and some of the diseases that affect vision. Let’s start with the general concept of vision. We will start with a few questions that will help you understand how you think vision works. Unit Objectives:
1. Understand the molecular basis of vision 2. Recognize the complexity of the retinal network of neurons (Massachusetts Biology Frameworks 4.7) 3. Practice Science Inquiry Skills (SIS)
Comments for Teachers are in italics (i.e., different activities to perform in the classroom and directions on how to perform these activities). Mirror, Mirror, on the Wall: If a mirror is mounted flat against the wall, how big does it have to be in order for you to see your entire body? Does distance play a role?
What is your prediction? (Sketch your description)
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Let’s think about how we see objects. Using the images below, show how you think we see the apple.
How long does the mirror have to be?
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The Digital Camera and the Eye: You may have heard that the eye is like a camera. The digital camera is even a better analogy of how the vertebrate eye works. A traditional camera uses a chemical process that “burns” the image on a thin film. On the other hand, a digital camera uses a sensor that absorbs light and converts it to a signal more analogous to what happens in our eyes.
Digital Camera Questions: What is a digital camera? Do you own a digital camera? If so, what kind (i.e., what’s the zoom, how many pixels, etc.)? How do you think it works?
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What do you need to make a digital camera? Using the diagram below, observe the parts of the camera and label the analogous parts in the eye. Show the pathway of light including the image.
Lenses – focuses light onto the sensor Sensor (e.g. CCD) – contains pixels (photosensors) that capture light (color and intensity) and convert it to an electrical signal and then to a digital signal Image Processing Unit ‐ takes signal from sensor, processes the information, and creates an image (e.g. jpeg)
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In Living Color: Constructing Color Vision
Why is the sky blue? How can we distinguish between a purple flower and a yellow flower? Why are there so many shades of gray? Color vision is essential to how we “see” our everyday world. It is sometimes considered a construction project because it requires many components to function completely. How we see color is still a subject of investigation for scientists. Scientists do not know all of the details of color processing; that is, how we see or perceive colors. Have you ever looked at a picture and thought that it did not exactly capture all of the contrast and hues of colors that you observed. Even the most expensive camera does not have the same resolution as our visual system. PreDiscussion Questions: 1. What are your favorite colors? 2. What organ(s) of the body do you think are responsible for color vision? 3. What is light and how is it useful? 4. Look at these two photographs. Describe three differences between the two photos.
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Discussion: Visible Light Spectrum Most animals and plants can detect and respond to light. Animals detect a narrow region of the electromagnetic radiation spectrum called the visible light region because it is visible to the human eye. Light reflects from objects and is refracted by the cornea and the lens onto a photosensitive (light detecting) region of the eye called the retina. Photosensitive cells in the retina can detect the visible light region of the spectrum ranging from short wavelengths, starting at 390 nm, to longer wavelengths, ending at 750 nm. How we see or perceive the colors of this visible light spectrum is the next topic of this unit.
Diagram of the Electromagnetic Radiation Spectrum and the Visible Light Spectrum
Other animals are able to see outside of the visible light spectrum. For example, insects such as bees can see in the ultraviolet region of the electromagnetic spectrum. If you look at an ultraviolet picture of a flower, in some cases you can almost see a pattern reminiscent of a landing strip that directs bees to the flower’s pollen (see picture below). We will see why bees can detect UV later in this unit.
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Marsh Marigold Flower – A. Visible Light (how humans see the flower) B. UV light (how bees see the flower) (Image from http://www.eso.org/~rfosbury/home/natural_colour/biochromes/UV_flowers/nc_bio_flower_uv.html)
Discussion: Retina and Phototransduction Introductory activities to introduce color vision in the classroom:
1. Where’s Waldo? Use a Where’s Waldo picture that is printed in color and in black and white. Give half the class the color photo and half the class the black and white photo. Each student records how long it takes to find Waldo. Average the time it takes students to find Waldo and create a comparison chart (you can include in this analysis standard deviation and create bar graphs or other graphs). (At the end of the activity, ask students what are some problems with this analysis, e.g., there is no real control, some people may have more experience with this type of puzzle than others, which can affect the outcome.)
2. Obtain paint samples (swatches) from the store that are within the same color family and test
students’ ability to tell the different colors apart.
How the eye uses signals to communicate what we see? PreDiscussion Questions:
1. Give an example of a signal. How do we use this signal? 2. Name one example of how signals are important in your everyday life?
3. What are some examples of signals in the cell or human body?
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What do signals mean to us? We use signals in our everyday lives. Signals provide order, a way to communicate a message, and a way to pass on information. The definition of a signal is anything that serves to indicate, warn, direct, command or act. In electronics, a signal is an electrical quantity or effect, as current, voltage, or electromagnetic waves that can be varied in such a way as to convey information (Webster’s Dictionary). Imagine you want to pass a note to someone at the back of your class, but you can’t use your cell phone in class!! To deliver the message, you might write something on a piece of paper and pass it to a number of individuals in your class until the person receives it.
In the cell or in the body, there are messages that are being passed along as a way of communicating from one cell to another or from one region of the body to another. For example, when you put on your earphones from your iPod, the hair cells in your ears move to waves of sound. The sound waves move your hair cells and send a signal to the brain with the message of what is playing on your iPod. The mechanical force of the moving hair cells turns into an electrical signal that goes to the brain. Light can also be converted to electrical signals to send messages to the brain via the retina. How the retina signals to the brain: The retina contains several layers of cells that are responsible for translating light refracted from the front of the eye (cornea and lens) into signals to send to the brain. Here is a simplified close‐up of the retinal layers. The photoreceptor cells respond to light first and translate or convert it into an electrical signal. Phototransduction is the process of converting light to electrical signals. When light is absorbed, it sends a message or signal that triggers a change in the photoreceptor. There are two types of photoreceptors, cones and rods. The cone photoreceptors are named as such because part of the cell is shaped like a cone. Rod photoreceptors have rod‐shaped parts. The parts of the cell that are cone‐shaped or rod‐shaped are called the outer segments. The cone photoreceptors are responsible for color vision and respond in bright light. The rod photoreceptors respond under dim light conditions.
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The outer segment regions of the photoreceptors are membranous disc structures. Proteins that respond to light are embedded in these structures.
Cartoon diagram of a rod and a cone
There are three major parts to the photoreceptors: the outer segment, the cell body, and the synaptic terminal. The outer segment regions of the photoreceptors are disk‐like structures made up of the cell membrane. The proteins that respond to light are embedded in these structures. The cell body is where the nucleus is located. The synaptic terminal is where the neurotransmitter gets released.
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Under a high‐powered microscope (Scanning Electron Microscopy), the cones and rods look like the following. (Only the outer segments are visible in this image.)
Image from http://www.chm.bris.ac.uk/webprojects2003/white/functional_anatomy_of_files/image004.jpg
Compare the sizes of the photoreceptor outer segments. Which one is bigger? Photosensitive Proteins: Rhodopsin and Cone Opsins Rhodopsin and the cone opsins are receptor proteins that span the membrane of disks in the outer segments.
Rhodopsin and cone opsin are situated in the membrane of the discs in the photoreceptor outer segments. Since the rod photoreceptors have larger outer segments, they have more opsin protein (rhodopsin). This is part of the reason they can respond in lower light levels.
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The wavelengths of light that maximally initiate the response of the three types of cone
photoreceptors and rod photoreceptor I can see in UV!! Some animals have an additional type of cone photoreceptor, which maximally responds to UV light. This photoreceptor contains an opsin protein that responds to UV light (< 380 nm). There is a huge amount of research ongoing that addresses how animals see in the UV and how seeing in the UV affects their behavior. For example, in the picture of the flower shown above, bees are thought to be able to see a contrast in the area of the flower’s pollen. It almost looks like a landing strip for the bee! Phototransduction involves three steps: 1. In the dark, photoreceptors are constantly releasing neurotransmitter. They are depolarized or they have a negative membrane potential (just like other neurons). The photoreceptors respond to light through the rhodopsin (rod) and cone opsin (cones) proteins. 2. The absorption of light initiates a conformational change in the protein. This change sets off a cascade of events (a signal) that closes Na+ ion channels in the cell. The Na+ ion channels are gated by binding cGMP (cyclic GMP, a molecule in the cell). “Gated ion‐channel” means that the channel can only open if something is bound to it. The signal initiated by light breaks up the cGMP molecule (an enzyme is activated that breaks down cGMP to GMP) so that it no longer can bind to the Na+ ion channel. As a result, the channel closes. Closing the channel prevents Na+ molecules from coming in the cell, which makes the photoreceptor more negative or hyperpolarized (membrane potential). 3. The change in membrane potential reduces the release of a signal (neurotransmitter) in the synaptic vesicle from the synaptic terminal. Fewer signals are then transmitted to the next layer of cells, the bipolar cells. Light is acting as a signal to turn down the amount of neurotransmitter that gets released from the cell. The next cell, the bipolar cell, is a neuron that responds to both stimuli, lights on and lights off. There are bipolar cells that are called ON‐bipolar cells that respond (send a signal) when the lights are ON and there are OFF‐bipolar cells that respond (send a signal) when the lights are OFF. Also there are horizontal cells that connect to photoreceptors and receive input from them. Although much is known about these pathways, researchers are still uncovering information of how all of these neurons coordinate with one another to pass along information about what we see to our brains.
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A diagram representing Phototransduction It may seem strange that the photoreceptor releases less transmitter upon light and that the next level of neurons, the bipolar cells, can respond to the less transmitter. One way to think about it is that, like other neurons, the photoreceptors release neurotransmitter when depolarized (membrane potential is slightly negative). Dark in this case does not have to be complete darkness. It can be a shadow that passes over your eye, a change in light intensity. Activity: What can you see in the dark? This exercise demonstrates how the two types of photoreceptors in the eye, the rods and cones, adapt in dim light conditions. PreActivity Questions:
1. When you walk into a dark room, can you immediately see objects around you?
2. Are you able to distinguish colors in dim light?
3. Do you think that you can see certain colors in the dark? If so, which colors?
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4. When you walk out of a dark theatre after a movie, what can you see? For this activity, students will first separate balls that have the same shape but are in three different colors. You can do this with any object that feels the same but has different color or the writing on the object is different. Have students place a pirate’s patch on one eye. Ask students to separate the balls into categories in bright light. Mix up the balls again and turn off the lights. Immediately ask the students to separate the balls in the dim light. This can be a quantitative exercise by counting the number of errors per group and making charts with averages and standard deviations. Then have students switch the patch to the other eye, repeat the sorting of the objects and count the number of errors among student groups. Other classroom activities: 1. You can try to separate cups of water with different concentrations of food coloring and ask students to sort the cups of water by increasing degree of color. 2. You can try magazine pictures or different colors of construction paper. 3. You can test variables, such as time, (e.g., how long does it take for the rod photoreceptors to adapt to dim light?) 4. After being in the dark for several minutes, turn on the light and have students stare at different colored items. Then, determine how long it takes for them to distinguish the colors. (This activity is testing how long it takes cone photoreceptors to adapt to bright light.) Is it difficult to distinguish color in the dark? The cone photoreceptors are responsible for color vision. A certain amount of light energy (threshold) has to be present for the cone photoreceptors to respond. However, in dark conditions, in humans and other animals, the cone photoreceptors do not receive enough light to give a response. Light Adaptation Activity: What is the minimum amount of light (threshold) required for the cone photoreceptors to respond? Students can measure how much light is required for the cone receptors to respond by performing the aforementioned activities and increasing the level of light. The amount of light can be measured in Watts by a basic light meter (check with your school’s physics lab). Why does it take a long time to adapt in dim light? The rod photoreceptors are the photoreceptors responsible for vision in dim light. A single photon of light is enough energy for the rod photoreceptors to respond. In bright light, the rod photoreceptors are saturated with light and are bleached, which means that the visual pigment has converted to another form. In this form, it can no longer respond to light and has to go through a set of reactions to return to its original form. When you first turn off the lights, the rod photoreceptors are still saturated and it takes a few minutes for the pigment molecules that respond to light to regenerate. It usually takes 20 ‐25 minutes to complete dark adaptation of the rod photoreceptors. When you turn the lights on, the rods again will saturate (the visual pigment can no longer respond to light), but the light is now at a threshold in which cone photoreceptors can respond.
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Activity: Acting it OUT!! This provides an opportunity to get students outside! Have students act out one part and then act out another part in this demonstration. Parts: Light (students will hold light bulbs) Outer segment disks – outline with students in the shape of the outer segments (students are kneeling down) Synaptic vesicles (two students holding hands with one student in the middle who will get released) Opsin protein (students can wear different colors) IonChannel (two students stand armlength apart in membrane) cGMP (student is “bound” to channel) Na+ (student holds salt shaker) In the Dark: The channel is open with cGMP student bound to it. Na+ student shakes salt into the cell through the channel. Synaptic vesicles are releasing neurotransmitter. Students can demonstrate this by picking up another student and releasing him or her. In the Light: Students with light bulbs touch the Opsin protein student. This initiates a chain reaction and the cGMP student moves away from the channel. The students who are acting as the channel close by moving side by side to each other. The student with the salt shaker stops shaking the salt. The synaptic vesicle students reduce the release of neurotansmitter. Discussion: Central and Peripheral Vision The Macula: The macula is the small sensitive region of the retina that gives central vision. In this central region, there are all cones and no rods. The fovea is the center part of the macula, which contributes to the sharpest vision. As shown in the figure, the fovea is a thinning of the retinal tissue and shaped like a pit. This thinning of the retinal layer allows for more light to directly contact the cone photoreceptors, avoiding other cells in the retina. In humans, there are about 120 million rods and 6 million cones. The majority of the cone photoreceptors are in the fovea region.
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Crosssection of the retina
Central and Peripheral Vision Activity: Hold this image paper a few inches from your face. Then, stare at the black cross in this picture for 10 – 20 seconds.
What did you observe? What happened to the dots after staring at the cross for 10 ‐20 seconds?
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When you are staring at the cross, where are the dots located? Based on your observations, do you think your peripheral vision is better or your central vision is better? Measuring your central vision (adapted from The Cheshire Cat and other eyepopping experiments on how we see the world)
PreActivity Questions: How much do you think you can see out of the corner of your eye? Shapes? Colors? A persons face? Materials: Poster board pushpin String about 2 feet long a pencil scissors 2 plastic cups white index cards glue stickers of different colors and shapes Assembly: For assembly, you basically want to make a large protractor to measure the angle of your central vision. Make a circle with a radius of 1 foot, using the pushpin in the middle and tying the string around the pushpin and the pencil. Cut out the circle and cut it down the middle to make a half circle. Cut a small circle in the middle of the board to place your nose at the base of the giant protractor. Cut strips of index cards and place the stickers with the colored shapes at the tip of the strip.
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Procedure: Before the Procedure, predict where you think you would see color or shapes on your giant protractor.
1. Using the cups as the handles, hold the poster board base up to your face and put your nose in the center hole.
2. Place a pushpin directly in front of your eyes. 3. Stare at the pushpin, while another person in your group slowly moves the index card with the
colored shape inward toward the pushpin. 4. When you see the color or the shape, let the person in your group know. 5. Make a mark at that point. 6. Using the protractor and ruler, measure the angle at which you recognized the shape and color.
For more advanced classrooms: Carrots are good for your eyes?
Rhodopsin and the cone opsins are located in the photoreceptor outer segment disks. A small
molecule called retinal binds to the rhodopsin and cone opsin proteins. The absorption of light causes a change in the retinal molecule. Retinal changes conformation from its cis form to its trans form. This change is like a domino effect in the photoreceptor cell. When retinal changes, rhodopsin also changes, leading to a cascade of events. The last event is that the Na+ channels close and the cell becomes more negative because now Na+ ions which are positive cannot flow into the cell. This change in membrane potential causes a reduced amount of neurotransmitter to be released.
Carrots contain β ‐carotene, a molecule that the body uses to make retinal. We do not produce β‐
carotene, therefore we must obtain it from our diets. Besides carrots, olives are another good source of β‐carotene. However, don’t worry too much that you’ll need to eat like a rabbit. As long as you are eating sensibly, your body will make enough retinal for proper vision. However, in countries where vitamin A deficiency due to lack of nutritious foods is a serious issue, vision problems, such as night blindness, do occur.
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Colored Shadows: (Adapted from Exploratopia, from the Exploratorium) The basic principle of this exercise is to remind students that there is a difference between subtractive and additive color mixing. They probably learned this in elementary school or middle school. However, we will know relate this to color vision. Preactivity Questions: What color to you get when you mix all colors together? Procedure: Using flashlights or lamps with color filters, arrange each light so that they converge on a whilte surface. Try to make the room as dark as possible. Observe the colors that you see once you begin to overlap the light. Now, place your hand in front of the light. Explorations Questions:
1. How many shadows do you see?
2. What are the colors of the shadows?
3. What are the colors of the shadows when they overlap?
4. Now, block one of the bulbs or turn off the light.
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5. How does this change the colors that you see in the shadows? Other Activities for the classroom: 1. If you have a light source or a lamp in your class, you can look at the effects of light when you use different color filters. 2. Using three flashlights you can place blue, red, and green color filters in front of each flashlights and project it on a white board or white poster board. Discussion: How do we detect different colors?
If you mix red, green, and blue light, you will equally activate all three cone photoreceptors resulting in the perception of white color. White is an additive effect of all three colors, which activate all three cone photoreceptors. Other colors are perceived by mixing the proper ratio from the red, green, and blue light. The contributions of red, green, and blue cones largely determine the colors that we perceive. The contribution of all three cones is processed in part by a direct path from photoreceptors to bipolar cells to ganglion cells. Other retinal neurons influence the flow of this information indirectly. The horizontal cells receive input from photoreceptors and influence the surrounding bipolar cells. Amacrine cells contribute to the signal by receiving input from bipolar cells and sending signals to ganglion, bipolar, and other amacrine cells. In a sense, the horizontal and amacrine cells form a web of information contributing to the overall signal sent to the ganglion cells. Basically, information from 126 million photoreceptors has to be funneled into 1 million ganglion cells to be sent to the brain. The ganglion cells then pass on the message to the brain via its axons that make up the optic nerve (see picture below).
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How information flows through the retina (Image modified from Purves et al. (Ed.), "Neuroscience", Sinauer Asociates Inc. Publisher)
Retinal Processing: the Flow of Information
Picture the retina as a train station to think about how it processes information. Passengers get to the train station via different mechanisms; for example, some walked, some took the bus, some drove, or some rode their bike. Next, passengers board the train to get to the next station. In this analogy, the input of the photoreceptors, bipolar, amacrine, and horizontal cells come together to activate the ganglion cells (the train). The train traveling to the next station is analogous to the output from the ganglion cells to the brain. The axons of the ganglion cells make up the optic nerve, which travels from the retina (one station) to the brain (next station). Activity: What a weird American Flag!
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What are the colors represented in this version of the American Flag? Stare at the white cross in the center of this image of the American Flag for 1 minute. Then move your eyes to this blank box.
Do you see anything? If so, what colors do you see? Discussion IV: Why does this happen?
So far we have discussed the initial stages of color vision, specifically how light can be converted (transduced) into an electrical signal to be sent eventually to the brain. However, there is more to color vision than the processing from the cone and rod photoreceptors. The signal gets processed in the retina by other neurons and it gets processed in the brain. That is why we are often fooled by optical illusions!! The ganglion cells respond to all of the input from multiple cone photoreceptors. Part of the retinal processing of color is described by the color‐opponent theory.
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The ColorOpponent Theory (For more advanced classrooms): Have you ever seen a color that is red‐green or blue‐yellow?
Certain colors are not seen in combination. These are called opponents, or antagonists. White and Black are also opponents in the sense of light illumination (the measure of light and dark or whiteness and blackness). The theory is that the information passed down to the some of the ganglion cells gets sorted into three different channels before it goes to your brain. A channel represents a field of ganglion cells or a group of cells responding together. Let’s say you are watching TV and you can only switch to three different channels. However, those three different channels have all the shows categorized into three different topics, sports, news, and reality TV.
In the retina, information about color and light intensity is sorted into the three channels. One
channel is devoted to information from the group of ganglion cells that responded to red and green light. One channel is devoted to information from the group of ganglion cells that responded to blue and yellow light. One channel is devoted the group of ganglion cells that respond to light intensity measured by the degree of blackness or whiteness. Specific types of ganglion cells gather these three different processes together (light versus dark, red versus green, yellow versus blue) to send to the brain. The ganglion cells that respond in this fashion are called color‐opponent cells. These ganglion cells are sensitive to differences in the wavelength of light. For each of the channels, if both types of stimuli stimulate the ganglion cells, only one output gets read: red or green, blue or yellow, and light or dark. For example, if a concentric field of ganglion cells is stimulated by red light (via mainly red cone photoreceptors) and green light (via mainly green cone photoreceptors), the center ganglion cell might have a direct pathway from the red photoreceptor cell while the surrounding ganglion cells are receiving signals mainly from green photoreceptors. The channel from this field of cells will cancel each other out and the ganglion cell will not fire (send an electrical pulse). The same would happen if the center ganglion cell is illuminated by green light and the surrounding cells are illuminated by red light. The same is true for blue and yellow light responses. Thus, you never get both responses from both red and green light or blue and yellow light sending their signal at the same time. That is why you never see colors that are red‐green or blue‐yellow.
Analogy: Imagine you have entered an arm wresting contest. In this scenario, you are the input from a red light stimulus and your opponent is the input from a green light stimulus. Your first opponent is someone who is stronger than you so he or she wins the match. In this case, the ganglion cell fires in
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response to the green light. Your next opponent is someone who is weaker than you so you win the match. Thus, the ganglion cell fires in response to the red light. You and your next opponent have an equal amount of strength. In this case, the ganglion cell does not fire because the input from red light stimulus and the input from the green light stimulus cancel each other out. Afterimages: The picture of the American flag above is an example of an afterimage. Afterimages occur because after staring at one color for a prolonged time, the corresponding cone photoreceptor becomes saturated. Just like other sensory cells, the cell will stop responding if bombarded with the same stimulus for long periods of time. The cone photoreceptors in a sense become “fatigued.” The reason you see the opposite color can be partly explained by the opponent‐color theory. Remember red and green are not seen at the same time, blue and yellow, and white and black (light illumination) are also antagonistic (they cancel each other out). Remember white is an addition of all colors and requires all of the cone photoreceptors input. Thus in order to “see” white (light reflected from the background), both red and green cones (which due to the color‐opponent theory cancel each other out) have to send signals. The cone photoreceptor that is “fatigued” will not respond when you switch to the white background. Thus, you are left with the response from the opposite cone photoreceptor. For example, what if you were to stare at the red box below for a minute or so then switch to the white square? Your red photoreceptors are “fatigued” from the prolonged stimulation, so you perceive red’s opponent color, green. Analogy: Let’s continue with the arm wrestling analogy. If your opponent has the same strength as you, no one wins. The ganglion cell will not fire, so the signals cancel each other out. However, if you stopped during the match and performed 20 push‐ups, your arm is fatigued, now your opponent wins the match. Since you are staring at the red square for long periods of time, your red photoreceptors become over stimulated and now its opponent, green can be perceived.
Stare at the cross in the middle of the blue square for at least a minute. Based on the color‐opponent theory what do you predict you should see? What color do you see?
Switch a Roo: Stare at these images for 1 minute and then look at the white box.
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Questions that can be investigated in classroom activities:
1. Repeat the exercise by altering the time that you stare at the boxes. How does the time you stare at the box make a difference in the color that you see?
2. Repeat exercise this time with your left eye closed, then open your left eye and stare at the box with your left eye open but right eye closed. How does this make a difference in the color that you see? What is your explanation for your results?
3. Make your own afterimage using opponent colors.
Research Focus: A “Bionic” Eye? Note: I would like to include a comparison of the digital camera and eye first in this section and maybe incorporate an activity with cameras, etc? Questions: Understanding what you know now about the retina, what would happen if you no longer had functional photoreceptors? Looking at the pictures below, describe how the vision of an individual with retinitis pigmentosa is affected? An individual with age‐related macular regeneration? (Hint: What part of your vision is affected, central or peripheral?)
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Images Courtesy of the National Eye Institute (http://www.nei.nih.gov/health/examples/)
Have you ever heard of the TV shows, “The Six Million Dollar Man” and “The Bionic Woman”? The shows were about two individuals who were in terrible accidents and had parts of their bodies replaced with prosthetic limbs. However these prosthetic limbs actually improved their ability to run, hear, and see. For example, the six million dollar man had prosthetic legs that allowed him to run at the speed of 60 mph (miles per hour)! Sounds like science fiction, right? Well, maybe not! What if you could implant a prosthetic retina in someone to restore eyesight? Last year, researchers at multiple labs funded by the US Department of Energy performed clinical trials in which they implanted an artificial retina or what is being called a “bionic” eye in blind individuals. This “bionic” eye will not give the blind individual better vision than non‐blind individuals, but it is still a remarkable advance in the field of vision science.
There are two major retinal diseases, Retinitis Pigmentosa and Age‐related Macular Degeneration, which can cause blindness due to loss of photoreceptor function (see pictures above). Remember the photoreceptors are a part of the retina and respond to light sending electrochemical signals to the rest of the retina and brain. In these diseases, the photoreceptors degenerate, but in many cases, other retinal neurons, such as the bipolar, horizontal, amacrine, and ganglion cells are still present and functional. One of these diseases causes photoreceptors primarily in the fovea to be destroyed. What is the function of the fovea? Which disease affects the photoreceptors of the fovea?
There are no known cures for these retinal diseases. Millions of people have lost their sight due to these diseases. Two million Americans have Age‐related Macular Degeneration; while hundreds of thousands of people suffer from Retinitis Pigmentosa. Engineers and scientists are now trying to improve this device to assist millions of people who have lost their sight due to these retinal diseases. How does it work? If the retina contains photoreceptors that are not functional, how is vision affected? What could you do to simulate other retina cells? An external miniature digital camera is placed on a pair of sunglasses worn by the patient. The digital camera gathers the image and transmits the information to a miniprocessor. This miniprocessor
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converts the data from the image to electrical signals that stimulate the implanted microelectrode array on the surface of the retina. The microelectrodes will send out electrical pulses to stimulate the other retinal cells. So instead of a release of transmitter from the photoreceptors to the bipolar cells, this electrical signal directly stimulates any remaining retinal cells. This signal goes through the optic nerve and to the brain. The brain can now perceive light and dark spots, which corresponds to the microelectrodes that were stimulated due to the image received from the digital camera.
(From: http://www.sciencedaily.com/releases/2009/10/091021012847.htm, Credit: Jessy Dorn / Second Sight Medical Products, Inc. and Department of Energy Artificial Retina Project: http://artificialretina.energy.gov/) Not Perfect Vision, Yet! Do you think that individuals with the “bionic” eye implant will be able to perceive images immediately? Why or why not?
Remember that the brain is a crucial component of the visual system. After receiving the implant, individuals have to learn what they are “seeing.” Their eye and brain has to be in a sense “retrained” to be able to interpret the images that they now are able to see. Most individuals with the implant can see light and contrasts between objects. The picture below shows what researchers think is possible for individuals with the implant to see when they increase the number of electrodes and improve the processing software of the digital camera (pre‐processing the image before it is sent to the electrode).
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(Image courtesy of California Institute of Technology, http://artificialretina.energy.gov)
Further Discussion Questions: Do you think that with the “bionic” eye implant, individuals will be able to see colors? If not, what do you think would be necessary for them to see colors? You are the scientist working for the artificial retina project. What are some things you would like to know before making the implant available to any blind patient? If you knew someone who was thinking about receiving the “bionic eye” implant, what would you tell him or her? Would you have any concerns?
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Resources: 1. Neuroscience: Exploring the Brain, 3rd edition, Bear M., Connors B., Paradiso, M. 2. The thinking eye, the seeing brain: Explorations in visual cognition, James T. Enns 3. Eye and Brain: the Psychology of Seeing 5th edition, Richard Gregory, Oxford University Press 1998 4. Neuroscience for kids: http://faculty.washington.edu/chudler/neurok.html 5. The Exploratorium: http://www.exploratorium.edu/, http://www.exploratorium.edu/exhibits/mix_n_match/ 6. How a Digital Camera works: http://electronics.howstuffworks.com/digital‐camera.htm 7. http://colour.australianmuseum.net.au/ 8. Webvision: http://webvision.med.utah.edu/ 9. More about the “Bionic” Eye: http://artificialretina.energy.gov http://www.cnn.com/2009/HEALTH/12/11/bionic.eye/index.html http://www.youtube.com/watch?v=iUz1ScDKslk 10. More about “mirror on the wall” exercise: Harvard Smithsonian Center for Astrophysics 1995. Vision: Can we believe our own eyes? http://www.learner.org/workshops/privuniv/pup05.html 11. Video acting out Neurons Firing Action Potentials: http://www.youtube.com/watch?v=TQ7NIvXEpVY