Benham's top

   Also known as Benham's disk, Benham's wheel, and artificial spectrum top. The eponym Benham's top refers to the British amateur scientist and toymaker Charles Edwin Benham (18601929), who in 1894 published an account in Nature describing a device which he called the artificial spectrum top, with the aid of which illusory colours can be created out of flickering monochromatic light. The device consists of a disc which is half black and half white, and bears three patterns of black concentric lines of increasing size. When the disc is rotated at a speed of around 5-10 revolutions/s, four concentric rings of colour appear. When the disc revolves clockwise, the colours red, green, pale blue, and dark purple can be discerned, running from the centre towards the periphery. When it revolves counter-clockwise, the colours appear in reverse order. The resulting illusory colours are referred to as pattern-induced flicker colours (PFICs). The underlying effect is known as the Prévost-Fechner-Benham effect. The neurophys-iological correlates of this effect are not entirely understood, but it is believed that both the retina andtheprimaryvisualcortexplayanactivepart in its mediation. Benham's top was marketed as a toy for children. A variant of the device, of which Benham was apparently unaware, was described in 1838 by the German psychologist Gustav Theodor Fechner (1801-1887). The illusory colours created by the two devices are therefore generally known as *Fechner's colours. This phenomenon is classified as a * physiological illusion.

   References

   Benham, C.E. (1894). The artificial spectrum top. Nature, 51, 113-114.

   Fechner, G.T. (1838). Ueber eine Scheibe zur Erzeugung subjectiver Farben.In: Annalen der Physik und Chemie. Edited by Poggendorff, J.C. Leipzig: Verlag von Johann Ambrosious Barth.

   Kenyon, G.T., Hill, D., Theiler, J., George, J.S., Marshak, D.W. (2004). A theory of the Ben-ham top based on center-surround interactions in the parvocellular pathway. Neural Networks, 17, 773-786.