ABSTRACT
To appreciate why it is mistaken to regard coloured shadows as the products of reflection or contrast, it is necessary to recognise how they – like object colours – are generated by constancy mechanisms operating at different retinal and cortical levels in the brain. Constancy is so-named because it ensures that the colours of surfaces remain broadly the same in absolute terms across small changes in the colour or spectral composition of the illumination, and remain the same relative to one another across more significant changes. Several recent commentators have pointed out that mechanisms exist in the higher reaches of the visual cortex which can implement this kind of analysis of the radiant energy reflected by a scene. Adaptation also explains why white surfaces are of especial importance to constancy, and shadow colour, with the idea that they act as a benchmark against which people can compute the colours around them.
