Unit 2- Week 3

1. Unit 2- Week 3 Color & Exam 2 Review

2. Topics Covered • Detection • Cone types/sensitivities • Discrimination • Problem of univariance • Color matching & mixing • Metamers • Appearance • Hue, saturation & brightness • Afterimages • Color constancy & spectral power distribution • Theories • Trichromacy theory • Opponent-process theory • Colorblindness • Deuteranope, Protanope, Tritanope, Cone monochromat, Rod monochromat • Achromatopsia (vsAnomia)

3. Detection

4. Cone Types • Photopic – day-light light levels • Scotopic – dimmer, night-time light levels • M cones – for “medium” • S cones – for “short” • L cones – for “long” • Short, medium, long what?

5. Cone Types • Photopic – day-light light levels • Scotopic – dimmer, night-time light levels • M cones – for “medium” • S cones – for “short” • L cones – for “long” • Short, medium, long what? Wavelength

6. Spectral sensitivity Curves

7. Spectral Sensitivities • As the professor noted, it is important to distinguish what wavelength is being ABSORBED (the photons with a given wavelength visualized as the probability they will be transduced based on each type of cones’ sensitivity for the wavelength spectrum) versus what wavelength is being REFLECTED • For example, it may be the case that an L cone is busy transducing its favorite wavelength of 565, but what color do we perceive?

8. Discrimination

9. The Problem of Univariance • An infinite set of different wavelength:intensity combinations can cause a given photoreceptor to respond just as strongly • Implications: 1-dimensional color vision is colorblind

10. Why Univariant Systems Cant Discriminate Color vv In a univariant system, we only have one spectral sensitivity curve, which means any given neuron can respond equally to any given color.

11. Trichromacy Theory • Tries to speak to the problem of univariance • How does it?

12. Trichromacy Theory • Tries to speak to the problem of univariance • How does it? By adding 3D color vision, showing that at least 2D color vision is necessary to discriminate colors, and that the majority of us in fact have this capacity. • Trichromacy theory describes color as a set of 3 numbers relating to each photoreceptor type (S, M, L)

13. Why Trichromacy Theory Solves the Problem of Univariance vv With 3D (and 2D) color vision, our neurons respond differently to different wavlengths, because there is more than one spectral sensitivity curve., meaning different sets of neurons are more responsive to what they “like”

14. Metamers • When two colors appear to be the same, but their spectral power distributions are not. The long “red” wavelength and short “green” wavelength when mixed produce the same neuronal response as the “yellow” on the right. These lights are metamers, because they are physically different but perceptually elected to be equivalent.

15. Additive & Subtractive Color Mixing • Light color mixing: additive • Paint color mixing: subtractive

16. Additive (Light Mixing) In additive color mixing the rays from both light sources are added together so that the reflected rays are a mixture of the rays in both of the original light sources

17. Subtractive (Paint Mixing) Subtractive color mixing takes a pigment source and outputs only a fraction of the rays bouncing off of it. When you mix another pigment in, it filters out or “subtracts” some of the rays that previously would have been reflected, because the added pigment changes which rays are absorbed by the paint/pigment it mixes with.

18. Opponent Color/Process Theory • The color purple looks both reddish and bluish. The color orange looks both reddish and yellowish. Turquoise both bluish and greenish. But you've never seen a color that looks both green and red. Nor have you ever seen a color that looks both yellow and blue.

19. Cone-Opponent Cells • Retinal lateral inhibition also speaks to color antagonistic center-surround receptive fields. Starting in the retina, cone-opponent cells pit different chromatic information against each other. • Single-opponent cell: Found in LGN, center-surround neurons whose output is based on a difference between sets of cones • Collect info about the color of a broad area • Double-opponent cells : Found in V1, these cells combine the properties of two color opponent cells from LGN • Conveys info about chromatic edges (where color changes)

20. Cone-Opponent Cells A single-opponent cell is excited by red and inhibited by green. • Double-opponent cells are excited by redder hues in its center/greener hues in surround & inhibited by green hues in center and redder hues in surround

21. Trichromacyvs Opponent-Process Theory • For many years, the notion of opponent colors was viewed as a competing/alternate theory to trichromacy. Today, we understand how the two theories fit together. • Trichromacy comes from the fact that you have three cone types with different spectral sensitivities. • Opponent processing occurs in the retina when the cone signals get recombined into opponent mechanisms: • White/black: adds signals from all three cones types, L+M+S. • Red/green: L-M & M-L • Yellow/blue: (L+M)-S & S-(L+M)

22. Appearance

23. Hue, Saturation & Brightness • Hue: The chromatic (color) aspect of light • Saturation: The chromatic strength of a hue • Brightness: The distance from black in color space • Unique hue: Any of four colors that can be described with only a single color term: Red, yellow, green, blue • For instance, unique blue is a blue that has no red or green tint

24. Achromatopsia • Cortical color blindness • Eyes are FINE. Deficits in cortex responsible for processing color vision

25. Anomia versus Agnosia • We know that someone with an agnosia means they struggle with what?

26. Anomia versus Agnosia • We know that someone with an agnosia means they struggle with what? • Object recognition • Someone with anomia struggles with naming objects to begin with, specifically with naming colors • They say it “looks right”

27. Adaptation & Afterimages • We already know that adaptation to spatial frequencies desensitizes neurons most tuned for the given spatial frequency adapted to • When we adapt to color and then stare at a white piece of paper, what do we see? • The opponent colors replacing the colors we adapted to • Yellow opposes Blue • Red opposes Green

28. Color Blindness • Protanope – someone who has no L cones • Deuteranope – someone who has no M cones • Tritanope – someone who has no S cones • Cone monochromat – only 1 cone type • Rod monochromat – no cones, just rods, cannot see in fovea because there are no rods in the fovea, suffers with day vision due to bleaching effects

29. Color constancy • The phenomena which ensures that the perceived color of objects remains relatively constant under varying illumination conditions.

30. Spectral Power Distribution Spectral power distribution (SPD) is a plot of energy versus wavelength. Most lights contain energy at many wavelengths. A light that contains only one wavelength is called a monochromatic light. Any light can be characterized as the sum of a bunch of monochromatic lights, and that is what is plotted in the SPD graph.

31. Color – the “Big Picture” • Trichromacy theory predicts when two lights will look the same, only when they are presented in the same context (same surround, same state of adaptation). • Trichromacy theory does NOT predict what the two colors will look like, it only predicts whether or not they will look identical to one another. • A full theory of color appearance also involves color opponency and color constancy in addition to trichromacy. • Under most circumstances, the visual system manages to get a consistent percept of surface color, despite changes in the illuminant, based on the three cone responses across the scene.