  Symmetry contributes to processes of perceptual organization in biological
vision and influences the quality and time of goal directed decision making in
animals and humans, as discussed in recent work on the examples of symmetry of
things in a thing and bilateral shape symmetry. The present study was designed
to show that selective chromatic variations in geometric shape configurations
with mirror symmetry can be exploited to highlight functional properties of
symmetry of things in a thing in human vision. The experimental procedure uses
a psychophysical two alternative forced choice technique, where human observers
have to decide as swiftly as possible whether two shapes presented
simultaneously on a computer screen are symmetrical or not. The stimuli are
computer generated 2D shape configurations consisting of multiple elements,
with and without systematic variations in local color, color saturation, or
achromatic contrast producing variations in symmetry of things in a thing. All
stimulus pairs presented had perfect geometric mirror symmetry. The results
show that varying the color of local shape elements selectively in
multichromatic and monochromatic shapes significantly slows down perceptual
response times, which are a direct measure of stimulus uncertainty. It is
concluded that local variations in hue or contrast produce functionally
meaningful variations in symmetry of things in thing, revealed here as a
relevant perceptual variable in symmetry detection. Disturbance of the latter
increases stimulus uncertainty and thereby affects the perceptual salience of
mirror symmetry in the time course for goal relevant human decisions.
