Most vertebrates, including humans, cannot ‘see colour in the dark’ because the retinal rod photoreceptors that mediate vision in dim light usually come in a single spectral flavour (“green”), precluding spectral comparisons. But frogs and some salamanders have an additional ”blue”-rod type, potentially allowing for purely rod-based colour discrimination. Using behaviour, I recently demonstrated that this is indeed the case: frogs do make spectral comparisons down to the absolute visual sensitivity threshold. However, the ecological relevance of nocturnal colour vision and the structure and function of the retinal circuitry underlying rod-rod spectral comparisons, and enabling the simultaneous preservation of sensitivity and spectral resolution, have not been explored.
Thus, this proposal sought to establish the purpose as well as the underlying retinal physiology and circuit implementation of frog colour discrimination near the visual threshold. For this, we proposed to combine natural scene imaging, high-throughput multi-electrode array recordings from 1,000s of retinal ganglion cells and synaptic resolution serial section electron microscopy of the blue-sensitive rod circuitry.
This would enable unravelling which visual information is available to be used, and how it is processed, to reach the low-light limits of colour discrimination performance available to the vertebrate eye. It would also provide a link between the more intensely studied retinal circuits of fish and mammals, providing important insights about the evolution of vertebrate retinal networks’ architecture and computations. Finally, the topic of ‘colour vision in the dark’ is one that attracts considerable public interest, so this project would create valuable opportunities for public engagement with the research supported by the European Commission.
From the results obtained so far, we can conclude that in dim illumination frog retinas use a computational strategy for encoding colour that is different from the ‘classical’ colour discrimination mechanism, with input from the blue-sensitive rods apparently having a suppressive role over input from the green-sensitive rods. From the electron microscopy dataset we have also been able to conclude that the internal anatomy of the blue-sensitive rod differs between frog species.
