Evidence for a Dual Colour Code for ‘Object Colour’ and ‘Illumination Colour’ in Centre-Surround Configurations with Inhomogeneous Surrounds

Abstract

We provide evidence that the surround-dependent change in appearance of an infield in a centre - surround configuration is not to be understood as an elementary recoding of channels by a simple surround-dependent gain control but in fact mirrors the triggering of a much more complex mechanism for establishing a dual code for ‘object colour’ and ‘illumination colour’.

We constructed centre - surround configurations—referred to as Seurat configurations—whose surrounds are given by a random structure of overlapping circles of a fixed diameter. These surrounds have been spatially modulated along the red - green axis only (isoluminance condition), along the luminance axis only (isochromatic condition), and along both axes, while keeping the space average fixed. Unique yellow settings were made for the infield. Our results suggest that the visual system seems to interpret a reduced variance of receptor codes for the surround as an indication of an illuminant that has to be discounted at the location of the infield. Furthermore, for the case of an isoluminant surround with strong spatial red - green modulation only, the unique yellow settings strongly tend towards the ones of an achromatic surround, ie the surround seems to be almost ineffective with respect to a differential gain control.

While our data on Seurat configurations with same space-averaged surrounds refute simple notions of functional equivalence, they suggest an interpretation in terms of a triggering of elementary perceptual categories related to ‘object colour’ and ‘illumination colour’.