Perceiving surface reflectance requires attention

Colour constancy refers to our ability to distinguish changes in surface properties from changes in the properties of light illuminating the scene. The apparent ease with which we can tell that objects do not change in their surface properties, for example, when moving from sunlight to the shade, belies the complexity of solving this ill-constrained problem. Although there is a substantial body of work testing which image cues might be used to accomplish this, there is surprisingly little known of how the brain performs this computation. Here, we tested a fundamental aspect of the perceptual scission of surface reflectance from illuminant/filter properties: whether it requires attention. We measured visual search times for both surface reflectance (requiring separation of surface and illuminant properties) and local tristimulus value (which does not). We found a clear difference between the two: visual search for colour defined by tristimulus value was fast and near-parallel, while search for a particular surface reflectance was slow and consistent with the serial deployment of attention. That is, search times suggest that the perceptual separation of surface and illuminant or filter properties in colour constancy may require an attention-based process analogous to perceiving conjunctions of simple features in feature binding. These results offer important new insights into the sequence of processes the brain uses to accomplish colour constancy.