370: Light & the Eye

1. Rods and Cones
1. Rods
1. specialized for night vision and are more sensitive
2. Cones
1. specialized for day vision and provide better acuity
2. most people have short, medium, and long wavelength cones
3. Produce color sense
2. Fovea
1. There are layers in the retina
2. consists solely of cones
3. There are no rods in the fovea but they are all distributed throughout the periphery
4. Periphery
1. Poor color vision but good motion detection
2. Demonstration using crayons and a volunteer where the crayon started behind her head and slowly moved outwards till she could see the color
5. When you know what a color is and you look away, but you can still see it in the periphery, your brain will assume that the color is the same.
3. Blindspot
1. A spot on your eye that has no receptor fields
2. Your brain fills it in so that you never notice
1. Our brain is very adept at ignoring things that are constant and detecting change
4. Transduction
1. Taking one form of energy and changing it into another form of energy
2. Transduction of Light
1. Visual: receptors that are sensitive to light
1. Rods and cones have outer segments with two components
1. Opsin: large protein
2. Retinal: a light sensitive molecule
2. Occurs when retinal absorbs one photon and changes its shape
5. Isomerization
6. Just one photon needed to change the firing rate
7. Transduction of Light
1. Isomerization triggers an enzyme cascade
1. Cascade means single reaction leads to increasing number of chemical reactions
2. Enzymes facilitate chemical reactions
3. Dark Adaptation
1. Moving from photopic to scotopic vision
2. Test both rods and cones 
1. by having you fixate we can test adaptation of cones
2. periphery starts to get at rods but not quite exclusively
1. There is not a way to get at just rods for normally developing people
2. However, there are rod monochromats that you could test (they lack all 3 types of cones)
3. Photopic
1. daylight vision
4. Scotopic
1. nighttime vision
5. Dark adaptation curve
1. Sensitivity increases in 2 stages
4. Sacod
1. Our eyes constantly move so our photoreceptors don't get tired