The Phantom Gradient: A variation in lightness without a perceived variation in the surface.

The so-called phantom gradient (PG) can be observed when a surface is filled by a linear luminance gradient whose variation is under threshold; under these conditions the surface appears homogeneous. It is known that a perceived luminance gradient affects the lightness of a target superimposed on its centre, in the direction of contrast (Agostini and Galmonte, 2002 Psychonomic Bulletin & Review 9 264 - 269). Under PG conditions the gradient is not noticeable but still it affects the lightness of a superimposed patch. In a series of experiments we show that the direction of the PG lightness induction varies according to the gradient width: for wide and medium widths invisible gradients produce an effect in agreement with simultaneous contrast, while narrow ones result in an effect in the direction of assimilation. Target lightness is always significantly different from its physical value. The gradients were never detected by the observers. Therefore, it seems that the visual system assigns lightness values according to the global spatial distribution of luminance gradients.