Perception of Bilateral Symmetry

Bilateral or mirror symmetry is an important property for many animals to perceive, in that asymmetry in an animal is a sign of development gone wrong. Many animals thus prefer bilaterally symmetric characteristics in potential mates.

Most research on the perception of bilateral symmetry has been conducted on humans with patterns composed of dots. Our program is to extend the range of stimuli to more naturalistic ones such as the insect below, and to test nonhuman animals as well.

In this example, the asymmetrical stimulus in the top right is a natural animal containing slight asymmetries. The symmetrical insect (top left) is digitally generated to have perfect bilateral symmetry. The two dot patterns are created from the animals by picking out key points on the periphery of the images of the animals.

With these stimuli, it is a hard task to tell the symmetric ones from the asymmetrical ones. One property that humans exhibit in judging symmetry is that the task is done best with the axis of symmetry vertical. We tested if this held with complex stimuli such as the figures above, and the results showed a resounding vertical advantage in both accuracy and reaction time.

The research is published in

Evans, C.S., Wenderoth, P.M., & Cheng, K. (2000). Detection of bilateral symmetry in complex biological images. Perception, 29, 31-42.

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