Colourfulness as a possible measure of object proximity in the larval zebrafish brain

The encoding of light increments and decrements by separate On- and Off- systems is a fundamental ingredient of vision, which supports the detection of edges in space and time and makes efficient use of limited dynamic range of visual neurons [1]. Theory predicts that the neural representation of On- and Off-signals should be approximately balanced, including across an animal's full visible spectrum. Here we find that larval zebrafish violate this textbook expectation: in the fish brain, UV-stimulation near exclusively gives On-responses, blue/green-stimulation mostly Off-responses, and red-light alone elicits approximately balanced On- and Off-responses (see also [2,3,4]). We link these findings to zebrafish visual ecology, and suggest that the observed spectral tuning boosts the encoding of object ''colourfulness'', which correlates with object proximity in their underwater world [5].