Colour Vision

      • Humans have 3 types of cones (3 cone opsins): a 3-colour vision
        • Other mammals have a two-factor colour vision with middle (black and white) and short (blue-yellow) wavelength cones

        • Primates developed high resolution M cones which added long wavelength sensitivity, creating red-green colour vision

Clinical Correlate

Most colour vision defects affect red-green colour vision through defects in the genes for L and M cone opsins on the X chromosome

      • Trichromatic theory: a colour is the totality of inputs from the three photoreceptor receptor types: three cone types responding in their own way to the same wavelength (with different sensitivities)

        • Long wavelength sensitive: red cones (570)
        • Medium wavelength sensitive: green cones (545)
        • Short wavelength sensitive: blue cones (445)


Rods do not contribute to colour detection (peak sensitivity: 500)

      • The colour perceived depends on:
        • Wavelength/hue
        • Background composition/saturation
        • Luminance/light adaptation: with increasing brightness, eventually all hues appear yellow-white (the Bezold-Brucke effect) and with low light levels, all hues appear colourless (Purkinje shift).

      • Colorimetry: measure of visual function at the photoreceptor level
      • Different colours have different brightnesses
        • Dark-adapted: blue-green (500nm) has peak luminosity
        • Photopic conditions: yellow-green (555nm) has peak
        • Photochromatic interval: difference between the threshold brightness at which light is detected and the brightness at which it is seen as a colour

      • The ability to distinguish colour depends on having different photoreceptor types.

Neural processing of colour

      • Occurs at ganglion cell level with inputs from amacrine and horizontal cells
      • Two types of colour ganglion cells:
        • Red-green opponent colour cells: detect red/green contrast
        • Blue-yellow opponent colour cells: detect ‘yellow’ signal from red and green cones and contrast it with info from blue cones

      • Parvocellular pathway (slow fibres from fovea/parafovea) conveys information to areas V1, V2 and V4 of the cortex.

Testing colour vision

      • Ishihara plates are designed so that the numbers cannot be seen by contrast differences alone. Plates are not visible to subjects with red/green colour deficits

        • Patients who cannot read numbers can be tested using the pattern plates at the back of the book

        • Essentially a screening test for congenital red-green defects
      • Farnsworth Munsell “100” hue test: 84 coloured tiles are graded by colour
        • Can detect mild colour-vision abnormalities
        • Most comprehensive test of colour vision
      • D-15 test: quick test that cannot detect mild issues
      • Lanthony New Colour test: designed for children

Deficiencies of colour vision

      • Anomalous trichromatism eg. deuteranomaly: relative deficiency in a cone population.
        • Genes for medium (green) and long (red) wavelength photopigments are found on the X chromosome

        • Therefore defects are inherited in an X-linked recessive pattern. Red-green colour deficiency affects 10% of males

          • Deuteranomalous: colour-weak: can distinguish pure red and green. This is the common form

          • Deuteranopia: colour-deficient: cannot distinguish between red and green

        • Short (blue) wavelength pigments are encoded by genes on chromosome 7 (autosomal dominant inheritance)

      • Rod monochromatism (1 in 30,000): true achromatic vision, low acuity, photosensitive, nystagmus and signs of macular dystrophy

        • Autosomal recessive
      • Cone monochromatism (1 in 100,000): normal acuity, cannot discriminate coloured lights of equal brightness. All three cone types are normal but have defective neural processing

      • Dichromatism: colours are matched with only two primaries (anopias as above). The missing spectrum of secondary colours are seen as white

        • Protan (red) deficiency
        • Deutan (green) deficiency
        • Tritan (blue) deficiency 
        • Anopia eg. tritanopia: total absence of blue cones (autosomal dominant)
      • Achromatopsia: defects in V4 affecting cortical processing

        • Congenital or acquired lesions of the lingual or fusiform gyrus cause cerebral achromatopsia associated with prosopagnosia (failure to recognise faces)

Kollner’s rule

    • Macular disorders give rise to blue-yellow colour vision defects
    • Optic neuropathies cause red-green defects
    • Glaucoma, demyelinating optic neuritis and cone dystrophies are exceptions

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Visual Acuity