Blind and yet not blind
Flies use different nerve cell circuits to process motion and position information
If a mosquito approaches a human ear or a bee heads for the next flower, two things are important: the insects must be able to locate their destination and correct course deviations, caused by a gust of wind for example. How does the brain process these different situations so that both behaviours are possible? Scientists at the Max Planck Institute of Neurobiology in Martinsried have demonstrated in behavioural experiments that both behaviours are controlled by separate circuits in the brain of the fruit fly (Drosophila). One of these neural networks processes motion information in the surrounding environment and helps the fly to stabilise its course. The other is responsible for determining the position of an object and is used for object fixation.
If a drum with vertical stripes rotates around an insect, the animal will rotate in the same direction as the stripes. This innate behaviour is known as an optomotor reaction. The experiment replicates a natural phenomenon: if, for example, a gust of wind moves a flying fly to the right, from the fly’s perspective, the surroundings move to the left by its eyes. The optomotor reaction consequently leads to a compensation for the gust of wind and brings the fly back on course. Scientists have long suspected that the nerve cells controlling this behaviour are located in the lobula plate of the fly’s brain. Up until now, however, it was not clear whether these cells are necessary to control the observed behaviour.
Alexander Borst and his department at the Max Planck Institute of Neurobiology are investigating how motion information is processed in the brain of the fly. To find out whether the lobula plate plays a role in the optomotor reaction, the neurobiologists developed a behavioural testing apparatus: in a virtual environment, they presented flies with a rotating striped pattern to which the flies displayed a clear optomotor reaction. However, when the scientists blocked the nerve cells from which the lobula plate receives its information, the behaviours disappeared completely. The flies were thus motion-blind. The experiments show that the lobula plate is a necessary element in stabilising the course of the fly.
In nature, however, flies must also be able to process information about other things than motion. Was this still possible? The next thing that the neurobiologists concentrated on was another, well-documented behaviour of insects: object fixation. If a single vertical stripe is displayed during the experiment, flies will turn to the stripe and try to keep it in front of them. This object fixation enables the animals to approach an object or to “keep an eye” on it. In the experiment, the scientists allowed a vertical stripe to appear slowly at different locations in the flies' field of vision and then disappear again. If the stripe appeared on the right side of the fly, the animals turned to the right, if it appeared on the left, they turned to the left. If the motion perception system controls this behaviour, then motion-blind animals should no longer be able locate the stripes. Interestingly, motion-blind flies and control flies responded in exactly the same way.
The scientists concluded from these experiments that an independent position perception system must co-exist with the motion perception system. If a small object moves in the space, local changes in brightness occur. These are recorded by the position perception system. Motion-blind flies can therefore still recognise the position of an object even if they can no longer see it moving.
“It was a very complicated process to set up the experiment in a way that solid results could be obtained,” explains Armin Bahl, the lead author of the study. It was previously assumed that cells in the lobula plate are responsible for motion perception, as well as for object fixation. The scientists have now refuted this assumption and already described important properties of the fixation behaviour. “We do not yet know exactly where the cells of the position perception system are located in the fly’s brain, but we have a few good candidates,” says Armin Bahl, indicating the direction that the research will now take.