370: Color Perception Pt2

1. Why Three Cones
1. ROYGBIV
1. R - 700 nm
2. O -
3. Y -
4. G -
5. B -
6. I -
7. V - 400 nm
2. One receptor type produces no color vision
3. Two cones produces some color but it is incomplete
4. More cones produce more colors
1. Many animals have 4+ cones
5. All you brain knows is the firing rate of action potentials.  So if a receptor just transmitted 100 units of electrical activity can you tell anything about the wavelength of the photons just absorbed?
1. We can't get color vision from this.
2. Principle of Univariance
1. Example
1. Say we have one pigment of medium wavelength that is maximally sensitive to wavelengths of 550 nm.  If you have 1000 photons at 550 or 590 you will have the best response at 550 but you will also respond at 590 even though it is slightly off and not your preferred wavelength
2. If however you have 2000 photons hit 590 instead of 1000 then the firing rate at 590 will be at the same amount as 550 where it was only half of the rate of 550 in the first example with only 1000 photons hitting it.
3. Color Deficiency
1. Monochromat
1. A person with only rods (or occasionally they may have one cone type)
2. Needs only one lambda to match any color. Truly "color blind"
3. If you are a monochromat then you will only see in black and white
4. Why would a monochromat like to hang out in the dark?
1. Because they see better
2. Dichromat
1. A person with two cone types.  Need only two lambda to match any color.  Poor color differentiation
3. Anomalous Trichromat
1. Needs three lambda in different proportions than normal trichromat.  Good color vision, just a bit different
2. How do we find a person like this?  They would technically see something different but it would be labelled the same so they would be extremely difficult to identify.
1. You can find them behaviorally based on the color matching experiment.  Most people will be the same but an anomalous trichromat will color match slightly differently.
3. A possible reason for this could be diet
4. Unilateral Dichromat
1. Trichromatic vision in one eye and dichromatic in the other.  Very rare.  Useful for figuring out subjective color experiences of dichromats
2. This is very interesting because the same person can explain how the world looks for both trichromatic and dichromatic vision
4. Monochromacy
1. Rod Monochromacy
1. Typical or complete achromatopsia, total color blindness, day blindness
2. Blue cone Monochromacy
3. Cone Monochromacy
1. Normal visual acuity and complete ability to see color.  May or may not exist
4. Cerebral Achromatopsia
1. Disorder where you use to be able to see color but after damage, from a stroke for example, you can no longer see color.  Atypical achromatopsia.
5. Definition of Monochromacy:
1. This is a very rare hereditary condition where you have only rods and no cones and you are sensitive to bright lights because of your reliance on rods.
5. Dichromatic
1. Three Types