Sound travels more slowly than light. This is apparent in a thunderstorm. We can often see lightening seconds before we hear the thunder. Sound can also interact with vision. For instance, in the stream bounce illusion two dots move toward one another, become superimposed, and then move away from one another. The dots can be seen to either pass through, or to bounce off, one another. However, if a brief tone is sounded at an appropritae moment oberservers are biased to see the two dots as bouncing off one another.
Click here to see the stream bounce illusion without sound
Click here to see the stream bounce illusion with an appropriate sound
Click here to see the stream bounce illusion with an inappropriate sound
We wanted to determine if, when sound and sight interact, there is a perceptual compensation for the slower speed of sound. If there is a compensation, the appropriate timing for the tone should not vary as a function of viewing distance when sight and sound have a common source. Instead, we found that when we presented the tone over a speaker placed near a visual display, the appropriate timing for the tone varied in a manner that closely approximated the difference between the speeds of light and sound. In contrast, when we presented the tone over headphones, the appropriate timing for the tone did not vary with viewing distance.
These findings suggest that auditory and visual signals of an event that reach an observer at the same time tend to interact with one another, even when the signal sources could not have been coincident.