ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Endocrine Abstracts (2011) 25 SE1.3

An exploration of colour theory

Paul Belchetz

Leeds Nuffield Hospital, Leeds, UK.

Colour perception depends on light, photoreception and central processing in the nervous system. Newton, in 1666, showed the coloured spectrum formed from white light passed through a prism underwent no further splitting on passing through a second prism. Newton favoured the corpuscular theory of light, opposing the wave theory of Huygens, which was taken up about 1800 by Thomas Young to explain experiments on diffraction and later formulated the trichromatic theory of human colour perception, independently rediscovered by Helmholtz and supported by Maxwell’s work on electromagnetic radiation. Modern physics encapsulates both elements, post Einstein and Planck.

Landmarks in understanding colour vision include description of retinal topography and physiology, especially elucidation of rod and cone function. Humans have three types of cones with peak light absorption at 450 nm (blue receptor), 530 nm (green), 560 nm (yellow). These properties depend on small variations in cone pigments, which single chains of about 350 amino acids containing seven transmembrane helices – the super family including many hormone receptors. Embedded in the pigment proteins of rods and cones is the same chromophore 11 cis-retinal, which on light absorption straightens the side chain, altering the protein shape, which in turn catalyses the downstream biochemical and electrical events signalling on to the bipolar cells and then the ganglion cells of the retina. These early stages encode topographically defined information about colour, contrast and provide substrates for phenomena such as after-image and shadow colours. The ganglion cells axons pass along the optic nerves, via the optic chiasm (cave pituitary adenomas!) to the lateral geniculate bodies, whence further connections lead to the occipital cortex. Centrally there are less well understood phenomena: colour constancy and relativism discussed by Monge, and yellow–blue, red–green antagonistic systems postulated during the mid-19th-century by Hering.

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