Colors Underwater: A Behavioral and Neural Systems Study on Color Vision in the Larval Zebrafish

Abstract

Being able to see color is such a natural and intrinsic component of our vision that we rarely pay it mind. In fact, color vision is a very widespread capability of visual systems across species, but despite its intuitive nature for humans, its neural implementation is not well understood in any organism. The zebrafish, Danio rerio, has potential as a color vision model organism, given its optical accessibility, established genetic toolset and four types of cone photoreceptors, all in a vertebrate system. Despite these advantages, very little is known about the color-related computations taking place in the zebrafish brain. Hence this work aims to shed light on these processes, both from a behavioral and neural perspectives. In the first study, a phototaxis assay was used to assess the wavelength preferences of the zebrafish larva. Stimuli were projected from below following the animal in a closed loop, and forcing it to make a choice at every turn between the stimulus on the left or the right. This study showed that zebrafish larva avoid UV light, especially if combined with visible light. This UV avoidance behavior is dependent on the eyes, and in particular on the presence of the UV cone in the retina. The true evolutionary drive for such a behavior is unknown, but it is suggested that its role is to balance sun exposure with enough light to find food and shelter. The second study delves into the brain of the larva, and asks whether there is any indication of color computations taking place there. In particular, the study focuses on the interaction between the output of the eye, the retinal ganglion cells (RGCs) and their main target brain region, the optic tectum (OT). The study found that the responses at both regions seem to be UV dominated, as expected from previous reports, and that some of the cone types are correlated in generating RGC or Tectal responses. This is an indication of RGC color types, a staple of the known color vision systems. Additionally, there are clear signs of chromatic information processing, as there is input decorrelation between the two cell populations. This is observed more markedly in terms of chromaticity of the stimulus than in terms of its spatiotemporal pattern. In sum, it appears the zebrafish has the potential to become a color circuitry model organism, given it performs wavelength specific behaviors and it shows indicators of color information processing, even in the absence of learned behaviors. This study sets the first steps in the establishment of this model, and paves the way for the next stages in fully elucidating the color vision system of a vertebrate.