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How is the most acute colour vision created?

Photo by Kazuo Unno (

Publish Date: 27.06.2022

Category: Interdisciplinary research, Our contribution to sustainable development goals

Sustainable development goals: 4 Quality education, 15 Life on land (Indicators)

The remarkable colour vision of a butterfly is based on a neural network, described by anatomical and numerical model

Butterflies are the most colourful insects and have the most acute colour vision of any animal species studied. The Japanese swallowtail butterfly, Papilio xuthus, is able to detect 1 nm differences in light wavelength across the spectrum from ultraviolet to red. Its retina contains six classes of light-sensing cells, tuned to different wavelengths and arranged in distinctive clusters across three types of ommatidia, which are randomly distributed throughout the large compound eye. What is the neuronal basis of such extraordinary colour vision? In the broader sense, this is the central question of neuroethology, which studies the neural basis of animal behaviour.

The swallowtail is the main experimental organism in the laboratory of Prof. Arikawa in Hayama, near Tokyo, with which the Group for Animal Physiology of the Department of Biology at the Biotechnical Faculty in Ljubljana has been collaborating for the last ten years. During this time, the Faculty’s laboratory has developed a method for microelectrode recording from single nerve fibres in insect ganglia, and demonstrated, together with Japanese colleagues, that the light-sensing cells of the swallowtail form pairs, connected with inhibitory chemical synapses. In these colour opponent pairs, cells with different colour sensitivities mutually inhibit each other’s activity, and thus sharpen their colour detection. This takes place in the lamina, the optic ganglion just below the retina. The Japanese researchers analysed the lamina using a special scanning electron microscope that can simultaneously image the samples and cut them into ultra-thin sections. The samples comprised columns of 100 cells from seven ommatidia, and 100,000 chemical synapses were identified and located. Dr Marko Ilić from Biotechnical Faculty of University of Ljubljana then developed a mathematical model that simulates the effects of synapses on the changes in sensitivity of nerve fibres along the pathway from the retina to the brain. He fed the model with a matrix of all synaptic connections identified and made a prediction which he compared with the actual records of how these cells responded to light stimuli. It turned out that the model based on anatomical data, or connectome, could provide a very good explanation of the effect of the neural network on the processing of sensory information in the visual pathway of the butterfly. The researchers thus shed light on how signals are processed at the earliest stage of the visual pathway, which then allows the swallowtail to have accurate colour vision.

The results were reported in a paper entitled “Connectome of the lamina reveals the circuit for early color processing in the visual pathway of a butterfly,” published in Current Biology, a leading scientific journal in neuroethology.

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