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

• 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)

• 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