From Guy Inchbald, Upton-upon-Severn, Worcestershire, UK
Readers have discussed the effects of hallucinogens on people with colour-blindness (Letters, 30 September). A person I know was once prescribed a drug, a side effect of which turned out to be hallucinations. These included colours that he had never seen before. He was not colour-blind: these were genuinely new colours.
The opponent-process model of colour vision holds that the brain converts red, green and blue signals from the eye into a red-green and a blue-yellow difference and brightness – much as old analogue TV broadcasts did. The “space” of colours that this can describe is larger than that filled by vision. An over-excited brain area can presumably stimulate signals outside normal vision.
