White's effect: a dual mechanism.

White (1979) has described a phenomenon in which grey bars replacing segments of the white phase of a square-wave grating appear darker than identical grey bars replacing segments of the black phase of the grating. We have investigated the properties of this effect with a view to discovering the underlying mechanisms. Four experiments are reported which reveal the effects of the heights and widths of both the flank and coaxial inducing bars upon the brightness of the grey bars. The results show that two processes, one the local corner effect, and one a spatially extensive process (possibly involving filters with elongated end-zones) operate to produce the effect. The implications of the findings for models of brightness perception are discussed and suggestions are made for further experiments.