Any of several visual illusions that are created if a cord made by twisting dark and light strands together, or graphical representations of thin strips of diagonal stripes resembling such a twisted cord, are displayed against a chequered background, the alignment of the cords or strips appearing distorted and disordered (see illustration). The illusion is caused by the responses of orientation-specific neurons in the visual cortex. The Scottish physician and psychologist James Fraser (1863–1936) discovered it in 1908, allegedly after observing wool made from strands of different colours lying on tartan fabric, and called it the ‘twisted cord on chequer-work background illusion’. Also called a Fraser illusion. See also Fraser spiral.
Twisted-cord illusion. The lines outlining the letters are perfectly vertical or horizontal.