Colour Vision I: The re0nal basis of colour vision and the inherited colour vision deficiencies Prof. Kathy T. ullen What is colour? What physical aspect of the world does our sense of colour inform us about? cgill Vision Research (H4.14) Dept. of Ophthalmology kathy.mullen@mcgill.ca h>p://www.mcgill.ca/mvr/ 28 th Sept 2011 Spectral colors Reflectance curves of some common foods Reflectance (percent) emon Orange Tomato Cabbage Violet Indigo Blue Green Yellow Orange Red Wavelength (nm) 1
The colour circle What is colour? Colour vision allows us to distinguish between surfaces with different spectral reflectances White light is produced by mixing three colours How do we see colour? 2
11/4/11 ixing red and green lights to match yellow: Principle of Trichromacy ixing together three coloured lights in suitable proportions enables us to make an exact match to any other colour The 3 mixing lights are called primaries A B C A and B. Green and red lights on the top are mixed by the subject to match the yellow light presented on the bottom. C. The red-green mixture perfectly matches the yellow. The match is called metameric - meaning that identical colour sensations are produced even though the stimuli are physically different 1 + 2 + 3 3 mixing lights The mixture in C as it appears to a deuteranomalous observer. test light to be matched JW Three cones types of human retina: AN JW 4 The Human Cone osaic False color images of the arrangement of human cones taken 1 deg from the living fovea using an Adaptive Optics Scanning aser Ophthalmoscope. The bottom right image is from acaque monkey. From the lab of Austin Roorda UC Berkeley. http://vision.berkeley.edu/roordalab/ acaque monkey 3
The distribution of rods and cones across the retina: the receptor mosaic Fovea Periphery Distribution of rods and cones (120 million rods and 5 million cones) visual eccentricity (deg) spatial density (cells/square mm) macula lutea cones rods Figure 2.16 (Right) The mosaic of rods and cones in the peripheral retina of a monkey. The small circles are rods and the larger ones, cones. The cones appear larger because the retina has been sliced across the receptor s inner segments, which are fatter for the cones compared to the rods, in the peripheral retina. The cones that are stained yellow are the S-cones. (From deonasterio et al., 1981.) retinal eccentricity (mm) Spectral sensitivities of, & S cones A single type of photoreceptor cannot signal colour 100 og relative sensitivity edium Short ong Relative absorbance % 50 1 2 Wavelength (nm) 450 550 (nm) 4
Response curve for a single receptor Principle of Univariance Relative absorbance % 1 2 1 = 2 (2) The response of a photoreceptor to any wavelength can be matched to any other wavelength simply by adjusng the relave intensies of the two smuli Therefore: any single receptor type is colour blind Wavelength (nm) Response curve for a two receptor system How is colour coded? 100 relative absorbance % Cone 1 Cone 2 540 565 Wavelength Each colour produces a unique pattern of relative activities in the three cone types 5
Relative absorbancy 100 50 0 The basis of colour mixing in a two receptor (dichromatic) system ights 1 3 2 W (nm) Each light is absorbed by the and cones in a certain proportion. Relative absorbancy 100 50 0 The basis of colour mixing in a two receptor (dichromatic) system ights 1 3 2 W (nm) Each light is absorbed by the and cones in a certain proportion. Receptors : 1 2 1+2 3 90 50 Receptors : 1 2 1+2 3 90 55 50 95 A dichromatic system requires 2 mixing lights A trichromatic (three receptor) system requires 3 mixing lights (primaries) Relative absorbancy Receptors 100 50 0 The basis of colour mixing in a two receptor (dichromatic) system : ights 1 3 2 W (nm) 1 2 1+2 3 90 55 145 95 50 95 145 1:1 95 1:1 The mixture of red and green light looks the same as the yellow light because the red-green mixture and the yellow the same proportional absorptions in the and cones A dichromatic system requires 2 mixing lights A trichromatic (three receptor) system requires 3 mixing lights (primaries) Colours with different wavelength distributions look identical if they produce the same ratio of absorptions in the, and S cone types Colours with different wavelength distributions look different if they produce different absorptions and hence different patterns of activity in the three cone types 6
Question: How does the brain work, in 5 words or less? Answer: Brain cells fire in patterns Inherited color vision deficiencies Quote from Harvard (and former cgill) neuroscientist Stephen Pinker interviewed on the Colbert Report. Colour vision is computed from the pattern of activity in the three cone types and is one of the best understood examples of a pattern code. etameric (matched) colour pairs for colour deficient observers ixing red and green lights to match yellow (Raleigh match): A B C A and B. Green and red lights on the top are mixed by the subject to match the yellow light presented on the bottom. C. The red-green mixture perfectly matches the yellow. The mixture in C as it appears to a deuteranomalous observer. 7
45 or spots Ishihara test for RG color blindness 29 or spots Inherited color vision deficiencies 6 or spots 56 in both Systematic and predictable losses Both eyes affected ale - sex linked for & (red-green) deficiencies Genetic S cone deficiencies are autosomal and rare - many are undetected or are acquired disorders Color vision tests may not diagnose achromats http://www.toledo-bend.com/colorblind/ishihara http://www.vischeck.com/daltonize/ Trichromats Trichromats One of the three cone types is anomalous Three colours are required to match any other See a full range of colours, but with poorer discrimination in some regions Types Protanomalous = anomalous cones 1 % (m) Deuteranomalous = anomalous cones 5 %(m) Tritanomalous = incidence unknown 8
Dichromats One of the three cone types is missing Dichromats Only need two colours to match any other Sees a much reduced range of colours Types Protanope = lacks cones 1% (male) Deuteranope = lacks cones 1% (male) Tritanope = lacks S cones 0.002%? Genes for the (OPN1W) & (OPN1W) cone pigments lie nose to tail on the Q arm of the X chromosome. Frequent mutations occur making these the most rapidly mutating genes in the human genome onochromats No colour vision: any colour matched with any other Rod monochromat (0.003%) (complete achromatopsia) All cones are functionally absent: low acuity, photophobia and nystagmus Blue cone monochromat (incomplete achromatopsia or atypical monochromat) Only S cones are present (0.001%): low acuity, no photophobia, no nystagmus. Worse with artificial illumination. http://ghr.nlm.nih.gov/condition/color-vision-deficiency 9
11/4/11 onochromats Difficult to differentiate complete vs incomplete types Incomplete may use colour names effectively Incomplete may perform OK on some standard colour tests Autosomal mutations CNGA3 (incomplete achromatopsia), CNGB3, and GNAT2 Original Deuteranope Protanope Tritanope Original Deuteranope Protanope Tritanope 10