Collision avoidance using optic flow

The news that driverless cars guided by sophisticated cameras and sensors are likely to appear on our roads in the not too distant future set me thinking about some of the methods that animals use to avoid bumping into each other and surrounding objects. That thought happened to coincide with the results of some research from Lund University in Sweden that compared how bumblebees (Bombus terrestris) and zebrafish (Danio rerio) use their in-built systems to avoid collisions.

Being able to regulate speed and judge distance accurately is a particular problem for creatures that fly or swim as they have to factor in the effects of variable wind or water currents as well as their own speed and direction. The researchers used tunnels containing air or water to compare the behaviour of the bumblebees or zebrafish. Inside the tunnels they employed black and white patterns to produce either strong horizontal optic flow cues on both walls, weak horizontal optic flow cues on both walls or a mixture of strong optic flow cues on some walls and weak optic flow cues on others.

Optic flow can be explained as the feeling we might have in a fast-moving vehicle whereupon we seem to be sitting still while the scenery appears to race past the window. The researchers showed that the closer the bumblebee or zebrafish came to an object, the faster that object seemed to move and that this apparent increase in speed strengthened the optic flow effect in the creature’s field of vision. They also found that although both bumblebees and zebrafish use optic flow to avoid collisions they use it in quite different ways.

Both species seem to be able to balance differing strengths of the optic flow registered by each of its eyes. If the optic flow suddenly became stronger in one eye compared to the other the creature reacted. However, the researchers were surprised to find that whereas the bees turned away from a stronger optic flow to avoid the risk of colliding with the object, the zebrafish moved closer to the strongest optic flow signal.

One explanation for the difference in behaviour might be that zebrafish live in a more turbid environment where the visibility may be limited to a few centimetres. Therefore although they need to avoid hitting the object, swimming too far away from it risks losing sight of a point of reference in their surroundings which would actually make it more difficult for them to decide on an appropriate speed and direction.

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Citation
The Pharmaceutical Journal, PJ, 18 October 2014, Vol 293, No 7832;293(7832):DOI:10.1211/PJ.2014.20066547