The effect of the nature of the surround on the perceived lightness of grey bars within square-wave test gratings.

The perceived lightness of grey bars within grey-white and grey-black square-wave test gratings was measured in the presence of different surrounding regions (plain black, plain white, or black-white square-wave gratings). The results for the grey-white test gratings are explained in terms of three separate processes: (i) lightness contrast; (ii) lightness assimilation resulting from the limited ability of the visual system to deal with grating contrast at higher spatial frequencies; and (iii) lightness assimilation resulting from lateral inhibition between pattern detecting channels mediating the perception of the gratings in the test regions and the surrounds. The results for the grey-black test gratings were explained without reference to the third process. It is concluded that techniques involving lightness assimilation could provide investigators with a sensitive new method of investigating the specificities of the inhibitory interactions underlying pattern perception.