Auditory Illusions

The Precedence Effect

Our hearing mechanism has evolved an ingenious mechanism for minimizing problems caused by echoes in the environment. Instead of correctly perceiving a set of overlapping sounds, each coming from a different location in space, we obtain the illusion of a single sound that appears to be coming from its original source. This phenomenon was first discovered by the 19th-century physicist Joseph Henry. To demonstrate this effect, the listener is seated in front of two loudspeakers, with one to his left and the other to his right. A single stream of speech is presented through both loudspeakers; however, the signal at the right loudspeaker is delayed relative to the left one. When the sounds coming from the speakers differ in onset by less than about 30 milliseconds (ms), the listener perceives the sound as coming only from the left loudspeaker. The right loudspeaker contributes to the loudness and liveliness of the sound, but appears to be completely silent. When the offset exceeds the critical time limit, two distinct streams of sound are correctly heard as coming from separate loudspeakers.

The Octave Illusion

The octave illusion was discovered by Diana Deutsch in 1973 and is experienced with tones that are presented via stereo headphones. Two tones an octave apart are alternated repeatedly at a rate of four tones per second. The identical sequence is presented to both ears simultaneously; however, the tones are offset in time such that when the right ear receives the high tone the left ear receives the low tone; and vice versa (see Figure 1, left panel).

Despite its simplicity, this pattern is almost never heard correctly, and instead produces a number of illusions. Many people hear a single tone that alternates from ear to ear, while its pitch simultaneously switches back and forth between high and low. So it appears that one ear is receiving the sequence “high tone—silence—high tone—silence” while the other ear is receiving the sequence “silence—low tone—silence—low tone.” Even more curiously, when the earphone positions are reversed most people hear exactly the same thing: The tone that had appeared in the right ear still appears in the right ear, and the tone that had appeared in the left ear still appears in the left ear. As a further surprise, right-handers tend to hear the high tone as on the right and the low tone as on the left, regardless of how the earphones are positioned; yet lefthanders vary considerably in [Page 161]where the high and low tones appear to be coming from. The illusion is hypothesized to result from the use of incompatible cues to determine what sounds are being presented, and where each sound is coming from. The strong association with handedness indicates that the ways in which the octave illusion is perceived reflect differing patterns of brain organization.

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