Perceiving surface colour 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 colour, 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 this perceptual process: whether it requires attention. We measured visual search times for both surface colour (requiring separation of surface and illuminant properties) and raw, colorimetric colour (which does not). We found a clear difference between the two: visual search for colorimetric colour was fast and near-parallel, while search for surface colour was slow and consistent with the serial deployment of attention. That is, search times suggest that the perceptual separation of surface and illuminant properties in colour constancy may require an attention-based process analogous to perceiving conjunctions of simple features in feature binding. Colour discrimination thresholds suggested that while colorimetric colour detection is fast and parallel, once attention was directed to these stimuli and perceptual scission occurs, colorimetric colour information was discarded by the visual system. These results offer important new insights into the sequence of processes the brain uses to accomplish colour constancy. Public Significance StatementThis work sheds new light on how the human visual system recognises the colour properties of different materials under varying lighting conditions. We find evidence that although material and lighting properties are perceptually separated automatically when attended, that this process requires attention.