Seeing Color. Muller (1896) The Psychophysical Axioms. Brindley (1960) Psychophysical Linking Hypotheses

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1 Muller (1896) The Psychophysical Axioms The ground of every state of consciousness is a material process, a psychophysical process so-called, to whose occurrence the state of consciousness is joined To an equality, similarity, or difference in the constitution of sensations there corresponds an equality, similarity or difference in the constitution of the psychophysical process, and inversely. Moreover, to a greater or lessor similarity of sensations, there also corresponds respectively, a greater or lessor similarity of psychophysical processes, and conversely. If the changes through which a sensation passes have the same direction, or if the differences which exist between series of given sensations are of a like direction, then the direction then the changes through which the psychophysical process passes have a like direction. Moreover, if a sensation is variable in n directions, the psychophysical process lying at the basis of it must be variable in n directions. Brindley (1960) Psychophysical Linking Hypotheses problem - phenomenological and physiological terms are from different realms of discourse if used together a bridging statement is needed Only 1 bridge whenever 2 stimuli cause physically indistinguishable signals from the sense organs to the brain, the sensations produced by those stimuli, as reported by S, in words, symbols, or actions, must also be indistinguishable. metamers most of Muller s axioms not acceptable Seeing Color interesting test case in cognitive science example of the psychology preceding the neurophysiology by nearly 100 years psychology, clinical observation, & reason. 1

2 definition Color spectral composition of light non-spectral colors???? brown dimensions hue saturation - amount of white light added brightness - quanta/unit time/unit area A few statistics We can discriminate (Gouras, 1989) 200 steps of wavelength 500 brightness steps 20 levels of saturation = 2,000,000 Color names = 7500 Colored surfaces amount of light reflected gray, white, black at which wavelengths Additive and Subtractive Color Mixtures lights -> all colors together = white pigments -> all colors together = black Trichromatic Theory Helmholtz - Young (1852) Evidence Color matching experiments (Maxwell & Young) human color perception is based on activities of 3 independent mechanisms that are differentially sensitive to different wavelengths monochromatic light 3 monochromatic lights Task - adjust intensities of 3 lights to make a metamer 2

3 Metamer definition - two lights of different spectral compositions that are perceptually indistinguishable failure of the perceptual system to sense the difference Results Most people could do the task with 3 other monochromatic lights Few people could do it with only two lights Therefore, color vision based on three mechanisms a/b/c Why? Every wavelength has a detector We see 502 nanometers when this detector fires and at no other time implications??? only one mechanism that fires to all wavelengths with different efficiency implications??? Hypothetical Question If we had more than 3, would our color vision be better? How? Opponent Processes Theory Hering (1878) Hypothesis - color vision based on processes that oppose red and green yellow and blue dark and light Qualitative data Afterimages green -> red red -> green blue -> yellow yellow -> blue Evidence 3

4 Simultaneous contrast White square in the center of red looks green in an afterimage Visualizing colors CAN Blue-green Reddish-yellow CANNOT Bluish-yellow reddish-green Quantitative Evidence Hurvich and Jameson (1957) Measured the strength of the opposition Asked- how much red needed to remove any green from the perception? (and vice versa) how much yellow needed to remove any blue from the perception? Who was right? Helmholtz Hering 4

5 Both Trichromatic Theory Wald (1964) microspectometry cone types 419 nm 531 nm 558 nm Opponent Processes Svaetichin (1956) gold fish retina Opponent cells excitation -inhibition Neural fatigue? DeValois (.60) red+/green- blue+/yellow- 90% 10% LGN Cells 5

6 Magnocellular Receptive Fields LGN Parvocellular Receptive Fields LGN G- R+ spatially opponent and chromatically opponent R+G- R-G+ G+R- G-R+ spatially opponent B+ Y-- 90% from L-M 10% from S and L-M Double Opponent Receptive Fields V1 Clinical Evidence R+G- G+R- Color blindness dicromatic color perception 2 of the 3 cone systems chromatically opponent B+Y- Y+B- What is it good for??? Color blindness protonopia - missing L 1% males See blues and yellows Neutral point 490 nanometers deuteranopia - missing M 1% males See blues and yellows Neutral point 498 nanometers tritanopia - missing S.002% males and.001% females See red and green Neutral point 570 nanometers Funny things about blue cones... 6

7 Color Constancy So how do we see a yellow chair when it is illuminated by a green light? 7