  Pierons and Chocholles seminal psychophysical work predicts that human
response time to information relative to visual contrast and sound frequency
decreases when contrast intensity or sound frequency increases. The goal of
this study is to bring to the fore the ability of individuals to use visual
contrast intensity and sound frequency in combination for faster perceptual
decisions of relative depth in planar object configurations on the basis of
physical variations in luminance contrast. Computer controlled images with two
abstract patterns of varying contrast intensity, one on the left and one on the
right, preceded or not by a pure tone of varying frequency, were shown to
healthy young humans in controlled experimental sequences. Their task was to
decide as quickly as possible which of two patterns, the left or the right one,
in a given image appeared to stand out as if it were nearer in terms of
apparent or subjective visual depth. The results show that the combinations of
varying relative visual contrast with sounds of varying frequency exploited
here produced an additive effect on choice response times in terms of
facilitation, where a stronger visual contrast combined with a higher sound
frequency produced shorter forced choice response times. This new effect is
predicted by crossmodal audiovisual probability summation.
