370: Color Perception

1.  Pink Does Not Exist
1. True in that there is not one wavelength that corresponds to pink
2. False in that we perceive it.  
3. Color
1. Color does not exist in the physical world it only exists in perception of waves of light.  In other words it only exists in our head.
1. So if it only exists in our head then because we perceive pink it exists
2. Color Perception
1. Carrots
1. Carrots are orange in honor of William of Orange :)
2. Functions of Color
1. They help us classify and identify objects
1. Segmentation
1. With color we are able to identify objects where it is hard to dissociate objects in grayscale
3. How Can We Organize Color Experience?
1. Color Circle
1. Shows the perceptual relationship between different colors
2. HSV
1. Hue (red vs blue)
2. Saturation (red vs pink)
3. Value (red vs brown)
3. Color Wheel
1. Complimentary colors are on opposite sides of the wheel
3. Color Perception
1. Colors are determined by wavelengths
2. objects reflect a broad range of wavelengths
4. How Is Light Reflected
1. It can be absorbed (black) 
2. Specular
3. Lambertian
4. Transmission
5. Refraction
6. Scatter
5. Theories of Color Vision
1. Trichromatic Theory
1. there are three color mechanisms
1. From those three you can make all different colors (red, yellow, blue)
2. Behavioral Evidence
1. Young & Helmholtz
1. Suggested 3 receptors
2. Color Matching Experiment
1. Participants had to mix colors to match a test field color
1.  Always needed at least 3 colors
2. Unless a person was color blind (dichromats with only two cones)
1. Color blind people could do this task with only 2 colors
3. Physiological Evidence
1. Measured absorption spectra of cone visual pigments in receptors
2. Color-opponent Theory
1. There are four color mechanisms
1. Four colors make all different colors (red, green, blue, yellow)
2. Proposed by Hering
1. When you stare at an x between red and green or blue and yellow then when the colors disappear the colors still are perceived but in opposite positions
3. Physiological Evidence
1. Researchers performing single-cell recordings found color-opponent neurons...He changes slides too fast!
3. Theories Combined
1. Both theories are correct as each describes physiological mechanisms in the visual system:
1. Trichromatic theory explains the responses of the cones in the retina
2. Color-opponent theory explains neural response for cels connected to the cones further in the brain
6. Color Mixing
1. Additive 
1. Mixing different wavelengths of light
2. All wavelengths are available for the observer to see
3. Superimposing blue and yellow lights leads to white.  Why?
1. Because yellow is a mixture of red and greed so adding red and green to blue results in white.  When you mix more colors you get white.
2. Subtractive color mixing
1. Mixing paints with deff pigments
1. Blue and Yellow will result in green.
7. Why Three Cones
1. One receptor type produces no color vision
2. Two cones produces some color but it is incomplete
3. More cones produce more colors
1. Many animals have 4+ cones
4. 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.