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 edge detection and makes efficient use of the limited dynamic range of visual neurons¹. Theory predicts that the neural representation of On- and Off-signals should be balanced, including across an animal's visible spectrum. Here we find that larval zebrafish violate this textbook expectation: in the zebrafish 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 references^2-4). 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 world⁵.