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Body balance Why do human beings and some other species find symmetrical patterns more appealing than asymmetrical ones? Two research teams working independently offer an explanation: the sensory bias towards symmetry is part of the development of the perceptual ability of the brain and nervous system (Nature, Vol 372, No 6502).

While the first team of scientists -- Magnus Enquist of University of Stockholm, Sweden, and Anthony Arak of Archway Engineering Ltd, UK -- implicate the evolution of the nervous system and the mental faculties in prefering the symmetrical visual patterns, Rufus Johnstone of Cambridge University, UK, looked anew at why the females of some animal species favour males with symmetrical traits.

Both studies used artificial neural networks -- computers designed to mimic the way the brain works -- and digitally simulated a visual system for recognising pictures or patterns. The scientists adapted a process -- known as training the neural networks -- which mimics the natural selection process followed by the brain. Several networks that varied slightly in their pattern recognition were chosen and fed with a set of images of varying symmetry -- pictures of different bird tails, for instance -- together with some random patterns. The network that was more successful in identifying the symmetry images among the random ones was retained and the rest were deleted.

In the process, the scientists found that the network unexpectedly developed a preference for the most symmetrical one among the bird tails. Thus the symmetry preference of humans and certain other species grow out of the brain's efforts to make sense of a world where symmetrical things are mixed up with asymmetrical ones.

Enquist and Arak manipulated the image in a plane by moving and rotating it, in a way similar to the eye projecting different images of any object on the retina. They found that the network also evolves along with the image and chooses only the most symmetrical variation.

Johnstone's neural network -- trained to mimic the recognition system of a female bird -- favoured the most symmetrical ornaments of the males. Interestingly, it also preferred longer tails in response to a set of tail patterns with different tail lengths confirming the earlier observation of evolutionary biologists (Nature, Vol 361, No 6411). On occasions when there was a conflict between the symmetry and the length of tails, the network compared the differences in the levels of symmetry and length and opted for one which outweighed the other.

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