Response of the human visual system to variable illuminant conditions: an analysis of opponent-colour mechanisms in colour constancy.

Illuminant changes in a scene generally cause displacements in the chromaticity and luminance of the objects of which the scene is composed; however, the visual system is capable of maintaining a constant object colour appearance independently of the illuminant. This is the phenomenon traditionally known as colour constancy. A classical asymmetric colour-matching experiment is reported to address two aspects of colour constancy: successive colour constancy and the role of opponent mechanisms in it. To this end, colour matches are made with chromatically complex backgrounds under different illuminants and with an equal-energy light source as standard. The results are analysed in the cone-excitation space, separately along the L-2M and S dimensions. Data are presented showing that colour-vision mechanisms respond differently to illuminant changes when colour constancy is considered at both receptoral and post-receptoral levels. The L- and M-cones tend to adapt so as to support colour constancy, whereas S-cones are strongly influenced by the illuminant changes. In addition, the data suggest good approaches to colour constancy linked particularly to the yellow-blue mechanism.