Structure of the eye and retina

Transcription

1 1 of 10 9/19/ :53 AM Syllabus pdf file Course Schedule Structure of the eye and retina In-class demo: do Virtual Lab activity 3-6 (Visual Path in the Eyeball)

2 2 of 10 9/19/ :53 AM Focusing, changes with age and correction Some important terms: hyperopia/presbyopia (farsightedness because of age...image is focused behind the retina) myopia (nearsightedness...image is focused in front of the retina) LASIK...surgery on the cornea, not the lens. Cataract...clouded lens Glaucoma...increased intra-ocular pressure in aqueous humor Visual receptors and transduction

3 3 of 10 9/19/ :53 AM What does the work of Hecht et al (1942) tell us about the diagram above? Dark adaptation Be ready to describe the general procedure for producing the blue curve above. How would you have to modify the procedure to get the yellow curve? Rods & cones Some basic facts about the retina: each retina has a blind spot, located degrees below and medial to the fovea.

4 4 of 10 9/19/ :53 AM ~120M rods ~6 M cones rods have much lower absolute thresholds than cones fovea contains only cones periphery has mostly rods, some cones concentration of rods is greatest ~15 degrees from fovea three types of cones, each maximally sensitive to a different wavelength (R, G, B) the overall spectral sensitivites of rod and cone vision are different: How does the information above explain the following: It's a dark, clear summer night and you've been outside long enough to be fully dark adapted. As you look at the sky, you see a dim star in the corner of your eye. When you look directly at it to see it better, it disappears, but reappears when you look away again. Even though each retina has a blind spot, we don't continually see two blank areas in our visual field. Highway workers wear those ugly yellow-green vests. You are sitting on your porch one summer evening and, as the sun goes down, you notice that the red roses, which were so beautiful a few minutes ago, are quickly losing their color and now just look dark. The green grass and blue flowers, however, seem to be staying bright and colorful a bit longer When measuring your visual acuity (ability to see fine details), your eye doctor asks you to look directly at the letters on the wall. Neural convergence in the retina The diagrams below are highly schematized versions of Fig in the text. In-class demo: do Virtual Lab activities 3-7 & 3-8

5 5 of 10 9/19/ :53 AM A couple of addional facts: each retina contains ~126M receptors (rods + cones) each optive nerve contains ~1M fibers (axons from ganglion cells) thus, on average, convergence must be ~ 126:1 convergence is minimal in fovea but increases significantly in periphery to a max of ~5000:1 the cones, highly concentrated in each fovea, giving rise to about 40% of the 1M fibers (axons) the the optic nerve. This means the fovea has disproportionately large representation in the brain: Based on the information above, be ready to explain the following: When measuring your visual acuity (ability to see fine details), your eye doctor asks you to look directly at the letters on the wall. Complete the demonstration found HERE. Were the letters easier to read in trial 1 or trial 2? Why? Color Vision: a two-part story. Part 1: three types of cones (trichromatic processing) Can you use the figure above to explain additive color mixing?

6 6 of 10 9/19/ :53 AM Exploring additive mixing a bit further. Exploring Abnormal Color Vision For this activity, you should: 1. Download the Colorblind Vision app to your ipad 2. Search the web to learn about the following terms: deuteranotpe deuteranomaly protanope protanomaly monochromacy 3. Use the spectral luminosity curves above to predict how each of the color vision abnormalities above would see the colors in the 4x3 matrix at the bottom of this screen. OK, but...how about my color vision problems? I have lots of trouble with red and green, but do fine with yellow! How can that be? The answer? Opponent-process color processing in ganglion cells and LGN (thalamus).

7 7 of 10 9/19/ :53 AM How to resolve this: Lateral Inhibition Lateral inhibiton simply refers to the fact that when one receptor in the retina fires, it tends to inhibit the firing of other nearby receptors. Here are a couple of additional bits of info: The faster a receptor fires, the greater the inhibition on nearby receptors Inhibition decreases with distance. Here's how the text explains this phenomenon: What should happen if we were to move the light from receptor B to C? In-class demo: Virtual Lab activity 3-9 Remember the image below, from the text? What do you suppose it has to do w/ lateral inhibition?

8 8 of 10 9/19/ :53 AM Look carefully at the two center gray bands and describe how they appear. Does each band look uniformly gray across it's entire width? Lateral inhibition and mach bands produce distortions in the way we see the world. Can you think of why this might be useful? [Hint: think about seeing edges.] Here, on the other hand, is an example of the distortions from lateral inhibition interfering with perception:

9 9 of 10 9/19/ :53 AM And how about this example of simultaneous contrast:

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