Chapter 8

Chapter summary

There are three classes of cone photoreceptor responsive in the short, medium and long wavelength regions of the spectrum (trichromacy). Lights containing additive mixtures of different light wavelengths can appear identical in colour (metamers). Mixtures of no more than three colours are needed to match any colour.

Colour descriptions indicate that there are two opposing pairs of colours (red-green, and blue-yellow) which map onto ganglion and LGN cell properties. Cortical cells have more complex colour responses; evidence for a specialized colour centre in the human cortex is not convincing.

Spatial context and adaptation can influence colour appearance and are thought to be important for colour constancy, in which the colour of an object remains stable even in the face of changes in the spectral composition of the illuminating light.

Observers with deficient colour vision fall into three categories: Anomalous trichromats, who require three primaries for metameric matches but in abnormal proportions; dichromats who require only two primaries to achieve metameric matches; and monochromats, who require only one primary and therefore cannot discriminate colour at all. Colour deficiency is inherited genetically via a recessive gene on the X chromosome, so its incidence is much higher in males than in females.