The experience of seeing a lightning bolt before hearing its associated thunder some seconds later provides a fairly obvious example of the differential speeds of light and sound. But most intervals between linked visual and auditory stimuli are so brief as to be imperceptible. A new study has found that we can glean distance information from these minimally discrepant arrival times nonetheless.
In a pair of experiments at the University of Rochester, 12 subjects were shown projected clusters of dots. When a sound was played about 40 or 60 milliseconds after the dots appeared (too short to be detected consciously), participants judged the clusters to be farther away than clusters with simultaneous or preceding sounds. Philip Jaekl, the lead author of the study and a postdoctoral fellow in cognitive neuroscience, says it makes sense that the brain would use all available sensory information for calculating distance. “Distance is something that's very difficult to compute,” he explains. The study was recently published in the journal PLOS ONE.
Speed of sound in dry air at 68 degrees Fahrenheit.
Speed of light in a vacuum.
SOURCES: HYPERPHYSICS http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html (top stat); NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY REFERENCE ON CONSTANTS, UNITS, AND UNCERTAINTY http://physics.nist.gov/cgi-bin/cuu/Value?c (bottom stat)
Aaron Seitz, a professor of psychology and neuroscience at the University of California, Riverside, who was not involved in the work, says the results may be useful clinically, such as by helping people with amblyopia (lazy eye) improve their performance when training to see with both eyes. And there might be other practical applications, including making virtual-reality environments more realistic. “Adding in a delay,” says Nick Whiting, a VR engineer for Epic Games, “can be another technique in our repertoire in creating believable experiences.”