To distinguish colors, the brain compares signals from sensory cells that are activated by light of different wavelengths. But how does color information reach a fly’s brain? Insects possess compound eyes, which are composed of many individual eye units, so-called ommatidia. A single ommatidium consists of eight light-sensitive photoreceptor cells. Six of them are arranged in a ring, with the two remaining cells located in its center. In flies, the six outer receptor cells respond to light over a broad range of wavelengths. Since the perception of color depends on the processing of specific wavelength regions, researchers assumed that these receptors are mainly responsible for motion perception. In contrast, the two inner photoreceptors are sensitive to light of single narrow wavelength regions—and may therefore pass on precise information about colors. So far, they have been considered to be the exclusive source of color vision in flies.
Neuroscientists at the Bernstein Center Munich, the Max Planck Institute of Neurobiology in Martinsried, and the LMU Munich now discovered that this assumption needs to be revised. „The outer photoreceptors contribute to color discrimination of the fly as well,“ explains Thomas Wachtler, one of the researchers involved in the study. Using a computer model, the biologists mimicked the processing of photoreceptor signals in the fly eye—and realized that the wavelength sensitivity of the outer photoreceptors must be taken into account in order to obtain the fly’s color discrimination abilities.
To support their theoretical results with experimental data, the researchers selectively controlled photoreceptor function in genetically manipulated flies. In this way, they obtained flies that possessed only one of the two color-specific inner receptor cells besides the outer receptor cells. Yet, the flies were able to distinguish two colors. „This indicates that the brain draws on information from both inner and outer photoreceptors for the color comparison,“ explains Christopher Schnaitmann, first author of the study. The assumption was confirmed when the scientists inhibited the activity of nerve cells that convey signals from the outer receptor cells to the brain: the flies‘ ability to perceive color differences was severely impaired.
The outer photoreceptors thus seem to be true multitaskers, contributing to both motion and color vision in the fly. Their dual role makes sense in small animals: it ensures that despite a limited number of neurons, flies still have complex visual skills—and may easily distinguish a bread crumb from a dust grain.
The Bernstein Center Munich is part of the National Bernstein Network Computational Neuroscience in Germany. With this funding initiative, the German Federal Ministry of Education and Research (BMBF) has supported the new discipline of Computational Neuroscience since 2004 with over 170 million Euros. The network is named after the German physiologist Julius Bernstein (1835-1917).
Dr. Hiromu Tanimoto
Max-Planck-Institut für Neurobiologie
Am Klopferspitz 18
Tel: +49 (0)89 8578 3492
PD Dr. Thomas Wachtler
Department Biology II
Tel: +49 (0)89 2180 74810
C. Schnaitmann, C. Garbers, T. Wachtler & H. Tanimoto (2013): Color discrimination with broadband photoreceptors. Current Biology, 23(23): 2375-82.