The neural basis of color vision: (I) photoreceptor interactions and origin of color opponent processing.

• Applicant: Professor Dr. Dierk F. Reiff
• Subject Area: Cognitive, Systems and Behavioural Neurobiology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 408330929

Color vision is an important sensory capability that enables many animals to differentiate spectral content independent of intensity. It provides animals with additional power for the detection of objects and interactions with the environment. Most of our knowledge on its neural basis comes from vertebrate research, in particular on the retina, where many of the underlying neuronal circuits, cell types, synaptic interactions, and processing mechanisms have been revealed. We know that signals of photoreceptors with different spectral sensitivities are combined antagonistically by the nervous system, employing a mechanism akin subtraction. Humans and other trichromatic primates for instance combine short (S), middle (M) and long (L) wavelength sensitive cone photoreceptors to create L–M and S–(L+M) color opponent pathways. This color-opponent processing overcomes the shortcoming of photoreceptors that inherently confound wavelength and intensity and enables the detection and enhancement of spectral contrast. Little is known about the neural basis of color vision in none-vertebrates. Combined behavioral and computational studies suggest that color-opponent processing is implemented in the nervous system of Drosophila (and likely ‘all’ animals with color vision). However, physiological recordings from cells of the color detection system in Drosophila have not been feasible so far. ln our recent study we overcame this major roadblock and we recorded the signals of ~3500 photoreceptors in about 40 different genotypes. Contradicting the common thought in the field, our recordings showed that color-opponency is already implemented in the terminals of inner photoreceptors R7/R8 in the medulla. We revealed detailed insights into the underlying synaptic pale- and yellow-specific circuit mechanisms and we deduced a simplified circuit model that now allows us to predicting ‘missing’ processing steps. Based on these predictions and our technical achievements I am here presenting a comprehensive research plan to reveal the immediate next processing mechanisms, in particular the neural basis of spectral feedback inhibition of R7p/y and R8p/y, respectively. We will reach this goal by applying a combined genetic, anatomical, and physiological approach that represents the state-of-the-art in the field. We will characterize the receptive field properties of identified neurons with an emphasis on their chromatic and spatiotemporal organization. Finally, we will use the obtained information to extend our model and to identify novel candidate neurons of the color vision system in Drosophila. By this work we will contribute to a better understanding of color vision in the ecologically important group of insects and we will provide insights into the variability of the circuit implementation of color vision across taxa.

• DFG Programme: Research Grants