Frequency variation in spontaneous sound emissions from guinea pig and human ears

Spontaneous acoustic emissions were recorded from 7 of 15 human ears at frequencies between 900 and 4000 Hz and just above 1000 Hz in a guinea pig. The constancy or changes in the frequency composition of these emissions is of particular interest in the light of current models which implicate outer hair cell activity in their generation. In our present study, most emissions underwent continuous fluctuations in frequency over a bandwidth of 0.5-10 Hz in humans and more than 20 Hz in a young guinea pig. Jumps in frequency of emission by as much as 20 Hz occurred during test sessions in some of the human ears. Changes in emission frequency were also observed during suppression by an externally delivered tone. The frequency of the emissions at successive test sessions also varied in some human ears and in the guinea pig between the ages of 6 and 9 weeks. The guinea pig emission also underwent an increase of 50 Hz in frequency and a reduction of 25 dB in level during recovery from anaesthesia. Neely's model for the role of hair cells in the generation of spontaneous emissions suggests that changes in hair cell conductance could influence the frequency of spontaneous oscillation. The small, but continuous changes in spontaneous emission frequency seen in all ears may reflect changes in conductance at the hair cell level due to synaptic and transducer channel activity.