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 neurons. 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). 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.