Distributed chromatic processing at the interface between retina and brain in the larval zebrafish.

Larval zebrafish (Danio rerio) are an ideal organism for studying color vision, as their retina possesses four types of cone photoreceptors, covering most of the visible range and into the UV.1,2 Additionally, their eye and nervous systems are accessible to imaging, given that they are naturally transparent.3-5 Recent studies have found that, through a set of wavelength-range-specific horizontal, bipolar, and retinal ganglion cells (RGCs),6-9 the eye relays tetrachromatic information to several retinorecipient areas (RAs).10-13 The main RA is the optic tectum, receiving 97% of the RGC axons via the neuropil mass termed arborization field 10 (AF10).14,15 Here, we aim to understand the processing of chromatic signals at the interface between RGCs and their major brain targets. We used 2-photon calcium imaging to separately measure the responses of RGCs and neurons in the brain to four different chromatic stimuli in awake animals. We find that chromatic information is widespread throughout the brain, with a large variety of responses among RGCs, and an even greater diversity in their targets. Specific combinations of response types are enriched in specific nuclei, but there is no single color processing structure. In the main interface in this pathway, the connection between AF10 and tectum, we observe key elements of neural processing, such as enhanced signal decorrelation and improved chromatic decoding.16,17 A richer stimulus set revealed that these enhancements occur in the context of a more distributed code in tectum, facilitating chromatic signal association in this small vertebrate brain.