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Bilateral symmetry
is ubiquitous in nature and important for many processes, including mate choice.
Females of several species, including swallows , zebra finches, scorpionflies
and humans, prefer symmetrical males. Such effects are particularly marked
in animals that possess elaborate ornaments such as the elongated tails of
some birds and fish. Similarly, insects prefer symmetrical flowers, which
have a higher nectar yield. Given the biological significance of bilateral
symmetry, surprisingly little is known about its perception in complex naturalistic
stimuli. Most previous work on mechanisms of symmetry detection has been conducted
with simple geometric patterns (left). Human performance is best when the
axis of symmetry in such stimuli is vertical, but there are no studies testing
whether this effect might also be apparent with more complex images, or with
the low levels of asymmetry characteristic of most organisms. It is also not
clear whether symmetry detection in humans reflects special perceptual or
cognitive properties. We wish to discover whether the vertical advantage found
with simple stimuli in humans is also apparent with other kinds of stimuli,
and in other species. Recently completed experiments address the first of
these questions. We examined the detection of mirror symmetry by human observers
who were shown insects and crustaceans that varied in orientation (right).
Some of these images had low levels of natural asymmetry, while others had
been edited so that they were perfectly symmetrical. Our results suggest that
the vertical advantage, characteristic of tasks with synthetic stimuli, also
predicts performance with complex natural images. Follow the links below to
try this experiment for yourself.
We are currently conducting comparative studies with pigeons and bees to
explore symmetry detection in other organisms.
Participants: Chris Evans
Collaborators: Ken
Cheng, Peter Wenderoth
& Marcia Spetch
(University of Alberta)
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Click
on the still images to see sample stimulus sets.
Once the QuickTime file is visible in your browser, use the arrow keys to
play the sequence frame-by-frame and see whether you can recognise
perfect bilateral symmetry. Note that images have been reduced to 1/4 of
normal size to reduce download time. |
Original
images kindly supplied by: Dr. Forrest L. Mitchell, Texas A&M University (The
Digital Dragonfly); Ms Joanne Taylor, Museum of Victoria, Australia (Crustaceans
of South Australia); and Dr. Richard Vogt, University of South Carolina (Richard
B. Dominick Moth and Butterfly Collection).
Get QuickTime
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