The brightness of coloured flashes on backgrounds of various colours and luminances.

Abstract Using interocular brightness-matching, experiments on superthreshold flashes analogous to some of Stiles's two-colour increment threshold experiments have been carried out. The results were similar to those of Stiles, except that the high intensity mechanismsπ 4 ′andπ 5 ′were more prominent, and the blue mechanismπ 1 absent, superthreshold. It is argued that up to two log units above threshold, brightness is determined by the most active mechanism (ofπ 4 ,π 5 ,π 4 ′,π 5 ′), and that the attenuating effect of a background depends on processes occurring within and not between π mechanisms. For each mechanism separately, the effect of a background is to attenuate all inputs, whatever their strength, by the same factor.