Behavioural evidence of spectral opponent processing in the visual system of stomatopod crustaceans
Stomatopods, commonly known as mantis shrimps, possess an intricate colour vision with up to 12 photoreceptor classes organised in four specialised ommatidia rows (rows 1-4 in the midband region of the eye) for colour perception. While 2-4 spectral sensitivities suffice for most visual systems, the mechanism behind stomatopods 12-channel colour vision remains unclear. Based on neuroarchitecture, it was initially suggested that rows 1-4 may function as four parallel dichromatic channels allowing fine spectral discrimination and strong colour constancy in narrow spectral zones. Subsequently, unexpectedly low resolution in behavioural experiments indicated that a binning processing system may operate instead of or in addition to the normal opponent processing system, categorising information into separate channels to create an activation pattern for rapid colour recognition. Previous anatomical and behavioural studies have speculated on the potential coexistence of these two systems in stomatopods colour vision. However, no behavioural study has specifically investigated the potential for colour opponency in their colour vision. Our findings provide the first direct behavioural evidence for spectral opponency in stomatopods visual system, showing that rows 1-4 operate, at least some of the time, as multiple dichromatic channels.