Vision and the Eye Part 2 Main Menu Restart Ready to start? This tutorial will take you through the basic structure and function of the back part of the eye. It contains both information and questions to test yourself. Questions appear in the box at the bottom of the slide, and answer options appear to the right. Just click on the answer to see feedback; then go to the next slide to continue. Use the forward and backward arrows to navigate. Hit ‘Esc’ to stop the tutorial at any point. Front Back You bet! Not right now

Vision and the Eye Part 2 Main Menu Restart The part of your eye behind the iris is the _____ Image modified from Wikipedia, used under a Creative Commons license Good work! You figured it out. Check again! The pupil‘s in the middle of the iris. Does that word really refer to the back ? Posterior chamber Pupil Anterior chamber

Vision and the Eye Part 2 Main Menu Restart The white layer covering the outside of the eye is the: Image modified from Wikipedia, used under a Creative Commons license That’s right! The sclera is the white of your eye. The cornea’s completely transparent. The iris is inside your eye! Sclera Cornea Iris

Vision and the Eye Part 2 Main Menu Restart Light rays are detected when they hit the: Image modified from Wikipedia, used under a Creative Commons license Right! That’s where the neurons are. The iris is mostly muscle, not neurons. The lens focuses light, but doesn’t detect it. Retina Iris Lens

Vision and the Eye Part 2 Main Menu Restart The layer of blood vessels between the sclera and retina is the ________ These are in front of the retina, not behind it! The cornea is the clear layer in the front of the eye Right! Retinal arteries Cornea Choroid Image modified from Wikipedia, used under a Creative Commons licensehttps://en.wikipedia.org/wiki/Mammalian_eye

Vision and the Eye Part 2 Main Menu Restart The jelly-like substance that fills the posterior chamber, pushing the retina against the choroid, is the _______ Nice try, but this is in the anterior chamber. Good work! Retinal arteries don’t fill the chamber. Aqueous humor Vitreous humor Retinal arteries Image modified from Wikipedia, used under a Creative Commons licensehttps://en.wikipedia.org/wiki/Mammalian_eye

Vision and the Eye Part 2 Main Menu Restart Good work on the anatomy! The posterior chamber of your eye is all about REPORTING light. The photoreceptors in the retina change their firing in response to light, and transmit that information to the brain. Image modified from Rev. Xanatos Satanicos Bombasticos (ClintJCL) : used under a Creative Commons licenseRev. Xanatos Satanicos Bombasticos (ClintJCL)

Vision and the Eye Part 2 Main Menu Restart Let’s look at the cells that do this work. The retinas develop as outpouchings of your brain, so they are actually brain tissue! This is the brain and retinas of an early chick embryo, but humans develop the same way. Image from Wikipedia, Used under a Creative Commons license

Vision and the Eye Part 2 Main Menu Restart Let’s look at a close- up of the retina. The photoreceptors are in the posterior side of the retina, right next to the choroid blood vessels so they can get lots of nutrients and Oxygen. choroid photoreceptors

Vision and the Eye Part 2 Main Menu Restart The photoreceptors synapse with the bipolar cells. The bipolar cells synapse with the ganglion cells. The ganglion cells send their axons out through the optic nerve, all the way into the brain. choroid photoreceptors Bipolar cell Ganglion cells Optic nerve

Vision and the Eye Part 2 Main Menu Restart What’s their name? The photoreceptors in this picture are named for their shape. I drew them in different colors to indicate that they respond to light of different colors- red, green, and blue. choroid photoreceptors Bipolar cell Ganglion cells Optic nerve Rods are rod- shaped That’s right! An easy way to remember is ‘c for color.’ They are photosensors, but you can be more specific. Rods Cones Photo- sensors

Vision and the Eye Part 2 Main Menu Restart Can your brain distinguish them? Now I’ll take some of them away so you can see more clearly. Suppose you look at two objects. The light from one focuses and hits cone A, and the light from the other hits cone B. Can your brain tell which cone the light hit? choroid Optic nerve They do both detect red light, but in different places! The brain doesn’t know what they are, only if they sent it a message. That’s right! The brain gets a different message from each cell! No, because they both detect red light No, because they’re both cones Yes, because they have separate connections to the brain A B

Vision and the Eye Part 2 Main Menu Restart Good! Each cone sends its own message to the brain. Because each cone sends its own message, the dots are distinct Oops - Each cone detects only one color. If light hits all these cones at once, what will your brain detect?

Vision and the Eye Part 2 Main Menu Restart Each of your cones has its very own bipolar cell and ganglion cell – its own hotline to the brain! So your brain can tell what each individual cone is seeing. That’s how you can distinguish small objects from one another. That ability is called ACUITY. Your cones give you very ACUTE vision. So cones give you acute color vision. The down side is – they need a lot of light to work. They are not very SENSITIVE in low light.

Vision and the Eye Part 2 Main Menu Restart What will the brain detect? Before we go away from this diagram of the back of your retina, answer another question. What will happen if light focuses on point C of this retina? choroid Optic nerve The only cells that fire in response to light are photo- receptors. Right! There are no photo- receptors here to detect the light. True – but if light doesn’t hit photo-receptors, your brain will never know about it. A big blur of light, because all the axons are stimulated Nothing, because it won’t hit any photo- receptors White light, because it won’t hit any cones A B C

Vision and the Eye Part 2 Main Menu Restart When you look around, do you see a big blank spot? That’s right! Your brain fills the blind spot in with whatever is hitting the cones near it. If light hits all these cones at once, what will your brain detect? Where the axons exit your retina into the optic nerve, you have a BLIND SPOT – a place where there are no photoreceptors to detect light.

Vision and the Eye Part 2 Main Menu Restart What do you think these photoreceptors are called? Let’s look at a close- up of a different part of the retina – toward the edge. Just like before, the photoreceptors are right next to the choroid blood vessels so they can get lots of nutrients and Oxygen. But they look different. choroid photoreceptors That’s right! They are rod- shaped. But you saw cones had a different shape… Can you be more specific? Which kind of photosensor? Rods Cones Photo- sensors

Vision and the Eye Part 2 Main Menu Restart What’s different? Just like cones, rods synapse with bipolar cells. The bipolar cells then synapse with ganglion cells. But it’s not quite the same, is it? choroid photoreceptors That’s right! Rods just distinguish light and dark. What else? That’s right! Many rods share one bipolar cell. What else? That’s right! Many rods share one ganglion cell. What else? They all see the same color There aren’t enough bipolar cells There aren’t enough ganglion cells photoreceptors Bipolar cell Ganglion cell

Vision and the Eye Part 2 Main Menu Restart What’s different? If light hits cell A on this part of the retina, can your brain tell it apart from light hitting cell B? choroid photoreceptors But the brain only knows what the ganglion cell tells it! Right! No matter which of these rods fires, the same ganglion cell sends a message to the brain. Yes, they’re different cells No, they use the same axon to the brain photoreceptors Bipolar cell Ganglion cell A B

Vision and the Eye Part 2 Main Menu Restart What’s different? If light hits a cell on this part of your retina, what will your brain detect? choroid photoreceptors But the brain only knows what the ganglion cell tells it! Does it know which rod fired? Right! The brain just knows one of them fired, but not which one. photoreceptors Bipolar cell Ganglion cell A B

Vision and the Eye Part 2 Main Menu Restart The cones are located in the back of your eye, in the central portion of your retina. The rods are located in the peripheral parts of your retina. That means cones see what’s right in front of you, while rods see the things that are off to the sides. Image modified from Wikipedia, used under a Creative Commons license

Vision and the Eye Part 2 Main Menu Restart So when you watch the beautiful balloons… what your rods and cones actually detect is more like this. Image modified from Wikipedia, used under a Creative Commons license

Vision and the Eye Part 2 Main Menu Restart How do you see the whole scene in color? You keep moving your eyes back and forth, and your brain remembers the colors you just looked at. It fills them in, the same way it fills in your blind spot. Image modified from Wikipedia, used under a Creative Commons license

Vision and the Eye Part 2 Main Menu Restart Which cells in the retina see the ‘V’ in the title above? You’re looking down on someone’s eyes and brain. The two sides of the retina and their nerve tracts have been color-coded. Check again. Light from that ‘V’ has to enter the eye through the pupil – where will it hit the retina? Right! The light from the ‘V’ will hit the blue part of the retina on both eyes. The orange ones The blue ones Image from Wikipedia Commons, e:1204_Optic_Nerve_vs_Optic_Tract.jpg used under a Creative Commons license e:1204_Optic_Nerve_vs_Optic_Tract.jpg

Vision and the Eye Part 2 Main Menu Restart Which cells in the retina see the ‘2’ in the title above? Check again. Light from that ‘2’ has to enter the eye through the pupil – where will it hit the retina? Right! The light from the ‘2’ will hit the orange part of the retina on both eyes. The blue ones The orange ones Image from Wikipedia Commons, e:1204_Optic_Nerve_vs_Optic_Tract.jpg used under a Creative Commons license e:1204_Optic_Nerve_vs_Optic_Tract.jpg

Vision and the Eye Part 2 Main Menu Restart Which side of the BRAIN will detect the ‘2’? You got it! The retina that sees the 2 sends its fibers to the left side of the brain. Check again. Which side do the orange fibers go to? The left  The right  Image from Wikipedia Commons, e:1204_Optic_Nerve_vs_Optic_Tract.jpg used under a Creative Commons license e:1204_Optic_Nerve_vs_Optic_Tract.jpg

Vision and the Eye Part 2 Main Menu Restart Image modified from Wikipedia Commons, used under a Creative Commons license What’s the point of this? Why should fibers from your eye cross over to the other side of the brain? Here’s a clue; it only happens in animals with binocular vision – animals that can look at one object with both eyes at once.

Vision and the Eye Part 2 Main Menu Restart Image modified from Wikipedia Commons, used under a Creative Commons license When both eyes look at the ‘E’, the nerve impulses caused by that image are sent to the same place in the brain. The brain can compare them. Look at where those light rays hit the retina – almost the same place in each eye.

Vision and the Eye Part 2 Main Menu Restart Image modified from Wikipedia Commons, used under a Creative Commons license Now let’s move the E closer to the eyes. Where does the light hit the retinas now? It’s a quite different spot on each retina, isn’t it? That tells your brain the object is closer to your eyes. E

Vision and the Eye Part 2 Main Menu Restart Image modified from Wikipedia Commons, e:1204_Optic_Nerve_vs_Optic_Tract.jpg used under a Creative Commons license e:1204_Optic_Nerve_vs_Optic_Tract.jpg One last thing to notice in this diagram is the branch each optic tract sends out to the superior colliculi. This area controls reflex responses to sight. So you sometimes react right away, even before you know what you saw!

Vision and the Eye Part 2 Main Menu Restart You’ve finished the second part of the Vision tutorials. Now you should be comfortable with: Rods Cones How the visual impulse reaches your brain. If you’re not confident you understand these topics, just click on them to go back to that part of the tutorial.