Triggerfish uses chromaticity and lightness for object segregation

Humans group components of visual patterns according to their colour, and perceive colours separately from shape. To find out if fish use colour for object segregation in a similar manner to humans, we used stimuli inspired by the Ishihara test for colour deficiency. In this test, observer is asked to detect a pattern made up of dots of similar colour with variable lightness against a background of dots made from different colour(s) and lightness. We demonstrate that fish segregate shape on the basis of common colour and/or lightness. Similarly to humans, fish generalise between stimuli of different colours, suggesting that colour and shape are processed by fish independently.