06/18/2001 Minutes - Laserfiche WebLink

Home Browse Search

Webvision: Color Perception Page 9 of 18 .4 1964). 3. Identification of the genetic code for L, M and S cones (Nathans et al, 1986a,b). 4. Colour matching functions. 5. Isolating photoreceptors and measuring their physiological repsonses as a function of wavelength (Baylor et al, 1984). 6. Spectral sensitivity measurements (Wald -Marne spectral sensitivity functions and Stiles' 7c - mechanisms) However, the trichomatic theory fails to account for the four unique colours: red,. green, yellow and blue, and also fails to explain why dichromats can perceive white and yellow. It also fails to fully explain colour discrimination functions and opponent colour percepts. The opponent colour .theory was first proposed by Hering in 1872. At the time, this theory rivalled the well accepted trichromatic theory which explains the trichromasy of vision and predicts colour matches. Hering s opponent colour theory suggests that there are three channels: red -green, blue -yellow and black -white, with each responding in an antagonist way. That is, either red .or green is perceived and never greenish -red. Hering, however, never'challenged the initial stages of processing expressed by the trichromatic theory. He simply argued that any colour vision theory should explain our perception, that is, colour opponency as revealed by coloured after images. Hurvich and Jameson (1957) provided quantitative data for colour opponency. Using hue cancellation paradigms, the psychophysical colour opponent channels were isolated. The VX function was used to brightness discrimination to describe the perception of blackness and whiteness. Therefore, by adjusting the amount of blue or yellow AND red or green, any sample wavelength can be matched (figure 15). Complementary wavelengths can be used to cancel each other for all wavelengths except the four unique hues (blue, green, yellow and red). Fiunrel5. Hurvich and _Jam eson c%lieriment using blue or v w AND red or green io mich alawavcleneths of the � isiirle srrectrum (Hurykh and 7ameson's data !]X157) from Bcniamin, .T. (Ed). P.,orislr's Clinical Refraction. Philadel )Ha: W. B. Saunders Comnant; 199fi)_L� rr snaue1 Other evidence supporting the opponent colour theory include: 1. Electrical recordings of horizontal cells from fish retina show blue -yellow opponent process and red -green opponent (Svaetichin, 1956). 2. Electrical recording from the lateral geniculate nucleus showing opponent colour processes http://www.webvision.med.utah.edu/]KallColor.html 6/1-8/01