Spontaneous otoacoustic emissions in the guinea pig

Measurement of spontaneous otoacoustic emission (SOAE) in a large number of animals was conducted in the guinea pig. A remarkably high incidence of SOAE (51 ears out of 248 ears; 20.6%) was observed in the animals that were raised under unexceptional conditions. The frequencies of SOAE were distributed between 0.7 kHz and 2.3 kHz (average: 1.16 kHz). The emission signals disappeared with hypoxia and with furosemide injection, and recovered after certain periods with some frequency fluctuations. Their electrical correlates that behaved in exactly the same manner as SOAE were clearly distinguished by FFT analysis of the AC potential from the cochlear surface. The signals could be divided into 4 types: (1) emission with pure and stable frequency spectrum and steady level, (2) emission with stable frequency and fluctuating level, (3) emission alternating between two closely situated frequency points, (4) emission having an irregular, wide spectrum band. Light microscopy revealed no abnormalities in the organ of Corti that have been widely accepted to be SOAE-related, i.e., patchy lesions of the organ of Corti and/or irregularities in hair cell arrangement. This suggests that the spontaneous oscillation within the guinea pig cochlea can be generated by some minor, functional, and probably reversible disturbances of the active cochlear transduction mechanism, without major structural derangement of the organ of Corti.