Why fish don’t drift away when they swim

No matter whether we are walking, turning around, or sitting in a car – the world passes by us and leaves traces of this motion on the retinas of our eyes. The brain uses this so-called optic flow to calculate our movements, and if necessary to adjust them.

 The Freiburg neurobiologist Dr. Aristides Arrenberg from the department of Prof. Dr. Wolfgang Driever at the Institute of Biology I and Prof. Dr. Herwig Baier and Dr. Fumi Kubo from the Max Planck Institute of Neurobiology in Martinsried near Munich have described new nerve cell types in the brain of fish responsible for perceiving and compensating for self-motion.

Arrenberg is supported by the Cluster of Excellence BIOSS Centre for Biological Signalling Studies as well as the “Elite Program for Postdoctoral Researchers” of the Baden-Württemberg Foundation. The findings were published in the journal . They improve our understanding of how motion is processed in the brain of vertebrates.

When we walk, an image of our surroundings appears to move backwards over our retina. This happens in the same direction for both our eyes. When we turn around, the image appears to rotate around us. The brain processes these large-scale motions from the visual environment, the optic flow. Fish in flowing water also use this mechanism, for instance to prevent themselves from drifting away in the current. The research team determined where in the fish’s brain these compensatory motions originate and what nerve cells are responsible for triggering them. Until now, four so-called direction-selective cell types in the retina were known. Scientists assumed that these cell types and the nerve cells they activate process the eye movements and pass on the commands for the fish to hold its position. Now the research team has succeeded in demonstrating that such nerve connections actually exist. In addition, they found seven further, previously unknown cell types with more complex responses to the signals received by both of the eyes. These more complex responses can be explained by combinational circuitry of the signals flowing in from both of the eyes. The research team demonstrated how the neural activity of all the nerve cells in a particular area of the brain can be recorded and how the activity patterns can be used to make inferences about the circuit connectivity. The scientists used striped patterns in their experiments to delude fish larvae into believing that they were moving. Then they observed the nerve cells’ reaction to the direction of movement of the stripe patterns. The brains of fish larvae are nearly transparent and consist of roughly one hundred thousand nerve cells. At the laboratory in Freiburg the team applied fluorescent dyes that light up when a nerve cell becomes active to trace the neural activity.

Original publication:

Functional architecture of an optic flow-responsive area that drives horizontal eye movements in Zebrafish.
Kubo F, Hablitzel B, Dal Maschio M, Driever W, Baier H, Arrenberg AB.
Neuron. 2014 Mar 19;81(6):1344-59

http://www.sciencedirect.com/science/article/pii/S0896627314001937