Two-photon calcium recordings of cones

For colour vision, retinal circuits separate information about intensity and wavelength. This requires circuit-level comparison of at least two spectrally distinct photoreceptors. However, many vertebrates use all four ‘ancestral’ photoreceptors (‘red’, ‘green’, ‘blue’, ‘UV’), and in those cases the nature and implementation of this computation remains poorly understood. Here, we establish the complete circuit architecture of outer retinal circuits underlying colour processing in the tetrachromatic larval zebrafish, which involves all four ancestral cone types and three types of horizontal cells. Our findings reveal that the synaptic outputs of red- and green-cones efficiently rotate the encoding of natural daylight in a principal component analysis (PCA)-like manner to yield primary achromatic and spectrally-opponent axes, respectively. Together, these two cones capture 91.3% of the spectral variance in natural light. Next, blue-cones are tuned so as to capture most remaining variance ...