Detection of bilateral symmetry in complex biological images

The recognition of bilateral symmetry in simple dot patterns is reliably in fluenced by orientation. Performance is best when the axis of symmetry is v ertical. We conducted two experiments to determine whether stimulus orienta tion also affects detection of the low levels of naturally occurring asymme try in complex biological images. University students judged whether colour images displayed on a computer monitor possessed perfect bilateral symmetr y. Stimuli were generated from high-resolution plan-view images of crabs an d insects. In experiment 1, the asymmetric stimuli were the original animal s, displayed on a standard black background. Symmetrical versions of each n atural image were generated by sectioning the shape at the midline, copying and reflecting one side, and then fusing the two halves together. To facil itate comparison of results with those obtained in earlier studies, we also presented dot patterns based upon both the slightly asymmetric and perfect ly symmetrical natural images. Experiment 2 was designed to assess whether symmetry detection was dependent upon the markings and patterns on the body and appendages of the animals. The natural images were converted to silhou ettes and tested against matched dot patterns. In both studies, images were presented in a random sequence with the axis of symmetry vertical, horizon tal, oblique left, and oblique right. Performance with the biological image s was consistently better than with the dot patterns. Abolishing fine detai l did not appreciably reduce this effect. A pronounced vertical advantage w as apparent with all stimuli, demonstrating that this phenomenon is robust despite considerable variation in image complexity. The implications of ori entation effects for perception of natural structures are discussed.