Perceptual consequences of the interactions between spontaneous otoacoustic emissions and external tones. I. Monaural diplacusis and aftertones.

Research into monaural diplacusis has led to the concept of idiotones (tone-like stimuli of cochlea origin). Spontaneous otoacoustic emissions (SOAEs) are tone-like stimuli generated by the cochlea and detected in the ear canal. In diplacusis, the existence of idiotones is inferred from disturbances of the perception of single tones. Spontaneous otoacoustic emissions are measured by placing a small microphone at the entrance to the ear canal. Many of the puzzling properties of the hypothesized idiotones are consistent with measurements of the interaction of SOAEs with external tones. The interactions of the SOAEs with external tones were analyzed acoustically. The perceptual properties evoked by 250 ms pulses (presented twice a second) of the acoustic stimuli used in the OAE experiments were systematically investigated. At some stimulus levels, all subjects reported the perception of a second tone alternating with the external tone. The relative pitch of this percept was consistent with the frequency of the SOAE. The frequency dependence of the signal levels needed for the percept had many aspects in common with the suppression tuning curves of the SOAEs. At lower levels of the external tone the subjects sometimes reported a perception of two simultaneous tones. This would be consistent with the subject detecting SOAEs when they are frequency shifted, but not suppressed. The consumption of aspirin by one subject reduced the SOAE into the noise floor and eliminated the monaural diplacusis.