The Unfolding of an Auditory Illusion

PERSPECTIVES IN BIOLOGY AND MEDICINE Volume 42 ¦ Number 2 ¦ Winter 1999 THE UNFOLDING OF AN AUDITORY ILLUSION ROBERTA. BUTLER* The illusion as first reported is this: a tonal stimulus of high pitch when presented straight ahead appears to originate above a tonal stimulus lower in pitch. Its actual elevation matters not; your judgment is based on the frequency of the auditory stimulus. The illusion is compelling even when you know that the sound source is fixed in space. The unequivocal conclusion is that stimulus frequency has a spatial referent. The first formal study ofwhat has been termed the "spatial characteristics of auditory frequency" was conducted by Pratt [I]. He concealed a telephone receiver behind a screen divided into 14 equal parts and numbered accordingly. Five different tones, ranging in octave steps from 256 to 4096 Hertz (Hz) (Hz being equivalent to cycles per second) , were delivered by the receiver from five different vertical locations in the median sagittal plane—the plane that bisects the head longitudinally. Pratt asked listeners to report the vertical position of each tone, and they reported that the higher pitched sound seemed to come from above, and the lower pitched sound seemed to come from below. Some kind ofstrong associative process must be operating here [2] . We regularly use the words high and low to describe the pitch ofa tone: in musical passages high-pitched tones are light and airy, whereas lowpitched tones are voluminous and massive. Perhaps Pratt's subjects, without consciousness awareness, thought that the 4096 Hz tone originated from near the upper sector of the vertical screen because it had a higher pitch than the other tones. Suzanne Roffler, a graduate student in psychology, and I agreed that this auditory illusion was an interesting problem and decided to devote time to it. We essentially replicated Pratt's study and extended the frequency This research was funded in large part by the National Institutes of Health. *Departments of Surgery and Psychology, University of Chicago, 5841 S. Maryland, Chicago , IL 60637.© 1999 by The University of Chicago. All rights reserved. 0031-5982/99/4202-1090$01.00 Perspectives in Biology and Medicine, 42, 2 ¦ Winter 1999 157 range from 0.25 kHz to 7.2 kHz. [3] . Similar to Pratt's findings, our listeners showed a strong tendency to report the higher frequencies as coming from the upper regions of space and the converse for the lower frequencies. But how does one test the hypothesis that associating the words high and low with the frequency of a tone is responsible for its perceived elevation? We chose to work with four- and five-year-old children. They were old enough to follow instructions, but based on brief questioning, they had not yet developed a linguistic facility which would have enabled them to describe the psychological correlate of stimulus frequency, i.e., the pitch as being "low" or "high." Seated on the lap of a caretaker, they faced a white screen containing orange circles arranged vertically. In the context ofgame playing, they were asked to point to the "sun" that was making the sound. As in the adult group, they indicated the higher tonal frequencies originated from above and the lower tonal frequencies originated from below eye level. Actually, the loudspeaker was concealed behind the screen and placed at eye level. To generate the illusion in its most striking form, a restricted set of listening conditions must be adhered to. Interaural difference cues—cues available when localizing sound binaurally—have to be eliminated. The most dominant of these cues are the interaural temporal differences that arise when the incident sound wave reaches one ear before it reaches the other, with the result that we perceive the sound as coming from the side first stimulated. Our central auditory nervous system is exquisitely designed to discriminate between interaural temporal differences as small as 30 microseconds . The other important cues for localization are the interaural level differences. The sound wave that reaches one ear first is more intense at that ear because the head serves as a sound shield, thereby attenuating the sound level when it reaches the opposite ear. One way to abolish these interaural cues is that followed...