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Lecture 17

01:830:301 Lecture 17: Achromatic System
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Department
Psychology
Course
01:830:301
Professor
Singh
Semester
Spring

Description
Lecture 17:Achromatic System Review: Opponency vs. Tricromacy -trichromacy is correct at the leve of the retina (3 cone types) -experimental prediction: humanobserversshould beableto matchthe appearanceof anycolor just by mixing in R,G,B -oponency theory is correct at the level LGN and V1 -cell exhibit color antagonism -R to G -B to Y “First trichromacy then opponency” -building opponent axes from the 3 cone types TheAchromatic System -take the sum of L and M cone responses -red-green channel  what should we do with the three responses to tell us where we are along the red-green spectrum? If I looked only at S and M cones, it would only tell me if I had more red or green  compare the strength of the response of L and M cones (talk about ratio or difference) -key point is just compare (take the difference) of L cone responses and the M cone responses (suppose the response difference is 0/equal, it would mean that it is achromatic/gray) L L+M M S THE CHROMATIC SYSTEM -The “Red-green” channel -take the difference between L and M cone responses. -if the L and M cones are responding equally  gray patch (achromatic) L L - M -if the L - M cones are positive M  more red S -if the L– M cones are negative  more green. -if it is M-Lit is going to be opposite -that comparison is going to show which part of the axes the color is. Rather than simply combining L and M responses, we are comparing them and saying which one is bigger. The “Blue-yellow” channel/system L (red) M (blue) S (green) -we want to know if there’s more blue or more yellow. Step1: Pull the L+M response (not caring about what’s red or green) Step 2: compare it to the S response -take the difference between the L and M response and the S response. (L+M-S) -by comparing L+M vs. S (the minus is simply a way of asking is L+M bigger or is S bigger – responding more strongly) -if it is more blue  the S response is larger -if more yellow  L+M is larger -when we talk about yellow, we’re not concerned about red and green  it’s ok to pull L+M responses because you care about the overall spectrum (we don’t need to know the stronger L response or stronger M response) -you can very easily build up the three opponent color axes by combining the three cone responses in difference ways. COLOR CONSTANCY Constancyin perception: your perception remains the same even though the image of the retina is getting smaller/bigger. 3D shape seems constant even if our point of view changes Color Constancy: changing light conditions change the composition of light reflected from a surface. -the retina sees color and shadow different colors, but we perceive it the same color. -when the light levels change, when the color composition of light changes, we still perceive red as red and blue as blue. -comparing how red color looks with different wavelength composition is different (sunlight, fluorescent light, neon light, etc) -perceived surface color remains constant. -in order to have constancy, it has to compare “The Dress” -the reason why you see the dress in two different ways -blue and black dress  bright, slightly yellow light -white and yellow  under a shadow (dark, slightly blue) Wavelength depend on colorof surface+wavelength composition of light thatis illuminating on the scene. -the blue and yellow is the same on the retina. -RGB on the image  RGB on the retina Adelson’s checkboard illusion (the same concept as the dress) – this is light constancy. The Color Constancy The light Lthat is hitting our retina - reachingour eyes is a combination of two different things: 1. Spectral reflectance of the surface (S) – it is the physical property; for each wavelength, we ask, what proportion of light in the wavelength does it balance out. -for each wavelength, how much light is reflected back by the surface. -graph  what color would this it be?  purplish (high wavelength in blue and red) type of surface -the surface reflects back certain wavelengths more than it does with other wavelengths -but the light that hits our eye also depends on: 2. Spectral composition of the illuminant (the light source) (I) -graph  corresponds to sunlight- it has approximately equal wavelength (except for the blue) -the reflectant of gray surface  let’s say that the reflectants of gray surface is 60% (0.6)  let’s say the light has 200 photons hitting the surface. How
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