Does the octave illusion evoke the interaural tempo illusion?

Abstract

Many observers experience an auditory illusion when presented with a repeating dichotic chord (consisting of a 400-Hz and an 800-Hz tone) that alternates between the ears. Listeners state that they hear a sequence of single tones, alternating between a low and a high pitch and at the same time alternating between the ears. This is called the octave illusion (Deutsch, 1974, 1980). The aim of the present study was to measure objectively the subjective angle over which the illusory single tone alternates. Informal listening in our laboratory suggested that the subjective tones were not fully alternating over 180°. In order to measure the precise angular separation in the octave illusion, we made use of another illusion. If sounds are alternated between locations, the time intervals between the sounds appear to be longer, and when the angle of alternation becomes larger, the rate of the sequence appears to be slower (interaural tempo illusion). By means of a reaction-time technique, it is possible to infer the angle of alternation from the apparent rate. Listeners who indeed perceived the octave illusion had to perform the reaction-time task with the dichotic tone sequences (giving rise to the octave illusion) and several types of controltone sequences. The reaction times showed that the time intervals between the subjectively alternating tones in the octave illusion were not longer than the intervals between nonalternating tones. Hence, we can conclude that the output of the “what-where” mechanism is not operated on by the interaural tempo processor.