COLOUR VISION Colour: trichromatic colour, opponent colour ...
COLOUR VISION Colour: trichromatic colour, opponent colour processes, retinal mechanisms Additive and Subtractive Colour • Paint reflects something and absorbs everything else – subtractive • Red + blue = absorbs what each both absorb = dimmer = subtractive • Additive colour = cellophane on torch – red + green = yellow Three different cone absorbances • Long L = Red • Medium M = Green • Short S = Blue
Trichromatic Theory of Colour Vision • 3 different receptor mechanisms (Young and von Helmholtz) • based on colour matching • light of a particular wavelength stimulates the 3 receptor mechanisms to different degrees and the pattern of activity in the 3 receptors is then interpreted as colour • with hue, luminance, saturation and contrast information we can discriminate >1 million different colours Biological Basis of Trichromatic Colour Vision • 3 cone pigments found in 1960s: short 419nm; medium 531 nm; long 558 nm • S and M opsins have only 44% genetic homology • M and L opsins have 96% homology and peak absorption differs by only 27 nm à more recent evolutionary divergence? • Blue cones extremely sparse, especially in fovea – yellow dots are blue cones
Opponent Processes Theory of Colour Vision Retinal Ganglion Cell Receptive Field Types • P cells (L-cone ON, L-cone OFF) receive input only from L- and M-cones (red and green) • K cells (S-cone ON, S-cone OFF) also receive input from S-cones • Yellow created by mixing L and M cones (red and green - + input from both = yellow) Colour opponency varies in indvidiual ganglion cells • Depending on the ‘flavours’ of the surrounding cones • R and G cones are ‘clumped’ and red:green about 1.6:1 in number • R and G cones are clumped but S cones are regular but sparse in distribution Opponent Neurons • There are neurons in the LGN which respond with an increase in firing to one wavelength (i.e. red or blue) and inhibit the opponent (i.e. green or yellow)
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Integration of Trichromatic and Opponent Process Theories: • First, cones of each pigment respond differentially to wavelengths in order to determine red/green/blue, then neurons integrate the inhibitory and excitatory signals from the receptors • Trichromatic ratio information; opponent difference information Colour Blindness • Congenital type usually from loss of one opsin • Ishihara plates a rapid test
Trichromatic Theory of Colour Vision • 3 different receptor mechanisms (Young and von Helmholtz) • based on colour matching • light of a particular wavelength stimulates the 3 receptor mechanisms to different degrees and the pattern of activity in the 3 receptors is then interpreted as colour • with hue, luminance, saturation and contrast information we can discriminate >1 million different colours Biological Basis of Trichromatic Colour Vision • 3 cone pigments found in 1960s: short 419nm; medium 531 nm; long 558 nm • S and M opsins have only 44% genetic homology • M and L opsins have 96% homology and peak absorption differs by only 27 nm à more recent evolutionary divergence? • Blue cones extremely sparse, especially in fovea – yellow dots are blue cones
Opponent Processes Theory of Colour Vision Retinal Ganglion Cell Receptive Field Types • P cells (L-cone ON, L-cone OFF) receive input only from L- and M-cones (red and green) • K cells (S-cone ON, S-cone OFF) also receive input from S-cones • Yellow created by mixing L and M cones (red and green - + input from both = yellow) Colour opponency varies in indvidiual ganglion cells • Depending on the ‘flavours’ of the surrounding cones • R and G cones are ‘clumped’ and red:green about 1.6:1 in number • R and G cones are clumped but S cones are regular but sparse in distribution Opponent Neurons • There are neurons in the LGN which respond with an increase in firing to one wavelength (i.e. red or blue) and inhibit the opponent (i.e. green or yellow)
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Integration of Trichromatic and Opponent Process Theories: • First, cones of each pigment respond differentially to wavelengths in order to determine red/green/blue, then neurons integrate the inhibitory and excitatory signals from the receptors • Trichromatic ratio information; opponent difference information Colour Blindness • Congenital type usually from loss of one opsin • Ishihara plates a rapid test
