Alan Gilchrist | PubMed Health

PubMed2006

Lightness of an object under two illumination levels.

Anchoring theory (Gilchrist et al, 1999 Psychological Review 106 795-834) predicts a wide range of lightness errors, including failures of constancy in multi-illumination scenes and a long list of well-known lightness illusions seen under homogeneous illumination. Lightness values are computed both locally and globally and then averaged together. Local values are computed within a given region of homogeneous illumination. Thus, for an object that extends through two different illumination levels, anchoring theory produces two values, one for the patch in brighter illumination and one for the patch in dimmer illumination. Observers can give matches for these patches separately, but they can also give a single match for the whole object. Anchoring theory in its current form is unable to predict these object matches. We report eight experiments in which we studied the relationship between patch matches and object matches. The results show that the object match represents a compromise between the match for the patch in the field of highest illumination and the patch in the largest field of illumination. These two principles are parallel to the rules found for anchoring lightness: highest luminance rule and area rule.

Analysis of Variance↗

PubMed2004

Lightness depends on immediately prior experience.

The lightness hangover illusion is an unusually robust, long-lasting, prior-experience-based lightness effect. The effect occurs in the Mondrian world, a miniature chamber with interior walls covered with dark gray to black patches. The lightest patch in this scene, physically dark gray, looks white. When real whites and light grays are added to the scene, all the patches darken, but at an unusually slow rate. For several seconds, the white patches look self-luminous and the other patches continue to look very light. The luminosity fades and the other patches darken only after 2 min. We tested three possible explanations for this illusion: retinal adaptation, lightness persistence, and anchor persistence. The results clearly support anchor persistence, which is caused by the presence of steady patches, surfaces that retain their luminance values across scenes. The data also show that the size of the illusion varies directly with the number of these steady patches.

Adaptation, Ocular↗