Colored visual stimuli evoke spectrally tuned neuronal responses across the central nervous system of zebrafish larvae

Abstract

Background Visually guided behaviors such as optomotor and optokinetic responses, phototaxis, and prey capture are crucial for survival in zebrafish and become apparent after just a few days of development. Color vision, which in zebrafish is based on a spatially anisotropic tetrachromatic retina, provides an additional important component of world representation driving fundamental larval behaviors. However, little is known about the central nervous system (CNS) circuitry underlying color vision processing downstream of the retina, and its activity correlates with behavior. Here, we used the transparent larva of zebrafish to image CNS neurons and their activity in response to colored visual stimuli. Results To investigate the processing of chromatic information in the zebrafish larva brain, we mapped with cellular resolution, spectrally responsive neurons in the larva encephalon and spinal cord. We employed the genetically encoded calcium indicator GCaMP6s and two-photon microscopy to image the neuronal activity while performing visual stimulation with spectrally distinct stimuli at wavelengths matching the absorption peaks of the four zebrafish cone types. We observed the presence of a high number of wavelength-selective neurons not only in the optic tectum, but also in all other regions of the CNS, demonstrating that the circuitry involved in processing spectral information and producing color-selective responses extends to the whole CNS. Conclusions Our measurements provide a map of neurons involved in color-driven responses, revealing that spectral information spreads in all regions of the CNS. This suggests the underlying complexity of the circuits involved and opens the way to their detailed future investigation.

Ancestral circuits for vertebrate colour vision emerge at the first retinal synapse

“…Most notably, the retina's acute zone ( 25) is dominated by UV-sensitive circuits (13). Here, most bipolar cell terminals respond primarily to UV-stimulation, and only some in addition respond to other wavelengths ( 13)a general pattern that is recapitulated also at the level of the retinal ganglion cells (24) to drive a strong UV-response in the brain (47,55,56) which filters all the way to spinal circuits (56,57). Nevertheless, despite this profound functional dominance, no anatomical study has reported the presence of UV-cone-dedicated bipolar cells, as for example in the case of greencones (49) (see above).…”

Circuit mechanisms for colour vision in zebrafish

“…Until very recently, spectral processing in the larval zebrafish brain had received little attention -that is, until 2020, which saw three independent imaging studies appear quasi-simultaneously 32,88,92 , all asking the same key question: how do neurons in the tectum, and the rest of the brain, respond to spectrally distinct stimuli? The results from these studies, which are largely in agreement with each other, confirmed and extended our previous understanding from diverse species of adult cyprinids: Sensitivity is far from uniform over the fish-visible spectrum, instead showing a clear dominance of UV-and long-wavelength responses over mid-wavelengths (Figure 5E,F).…”

“…This link was already suggested based on the dominant presence of UV-On circuits in the retina's acute zone 33,35 and the demonstration that the corresponding upper-frontal part of visual space is critical for behavioural performance [104][105][106] . In fact, this acute-zone UV-dominance begins as early as the development of cones and is one of the most pervasive features of zebrafish spectral processing throughout the entire visual system 32,33,35,40,43,88,92 including the spinal cord 92,107 .…”

“…(E,F) Summary of responsive neurons in the larval zebrafish brain to upper-frontal (approximately aligned with acute zone) wide-field stimulation at different wavelengths as indicated. Note the different duration flashes used in ((E) modified from Fornetto et al92 ) and ((F) modified from Bartel et al32 ) which meant that On and Off components could not be discerned in the former, and which likely masked some slower responses at mid-wavelengths. (G) From left, summary of spectral tuning of the larval brain's bulk On-, Off-and On-Off responses (grey lines, based on F), with cones (from Figure2K) and/or cone-fractions (from Figure3E) superimposed.…”

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

“…Recordings revealed that, despite some expected variation 2 , 3 , 4 (for example, Figure S1 B), neural responses in all major visual centres of the brain had a common, overarching spectral sensitivity profile: UV-On, Blue/Green Off, Red On-Off ( Figure 1 B). This organisation into three spectral processing zones (UV, Blue/Green, Red) can be linked to visual ecology.…”