Colour in the eye of the beholder: receptor sensitivities and neural circuits underlying colour opponency and colour perception

Colour vision — the ability to discriminate spectral differences irrespective of variations in intensity — has two basic requirements: (1) photoreceptors with different spectral sensitivities, and (2) neural comparison of signals from these photoreceptors. Major progress has been made understanding the evolution of the basic stages of colour vision–opsin pigments, screening pigments, and the first neurons coding chromatic opponency, and similarities between mammals and insects point to general mechanisms. However, much work is still needed to unravel full colour pathways in various animals. While primates may have brain regions entirely dedicated to colour coding, animals with small brains, such as insects, likely combine colour information directly in parallel multisensory pathways controlling various behaviours.