Development of transient evoked otoacoustic emissions in the neonatal rat

Otoacoustic emissions (OAEs) represent acoustic energy generated by the cochlear amplifier which contributes to auditory sensitivity and frequency discrimination. Therefore the OAEs can serve as a noninvasive tool to study the cochlear amplifier. While transient evoked OAEs (TEOAEs) are generally recorded clinically in man, it has been difficult to record them in animals and instead cubic distortion product OAEs (DPOAEs) have been experimentally studied in animals. In a previous study, we perfected a method of recording TEOAEs routinely in rats and this technique was used here to study the development of OAEs in neonatal rats. TEOAEs were recorded and compared to the DPOAEs on several postnatal days. With increasing postnatal age, TEOAE peak-to-peak amplitude and spectral energy in the 2- to 4-kHz band increased, their threshold decreased and their input-output functions became less monotonic with a change in slope (notch and/or plateau) in the mid-intensity region. The DPOAEs to higher frequencies appeared first, then the TEOAEs, followed by the DPOAEs to lower frequencies. With age, their amplitude also increased, thresholds decreased and a notch appeared in their input-output functions. The TEOAEs were measurable during the continuum of the appearance of the DPOAEs and the developmental sequences of both types of OAEs were similar. This may be evidence that similar mechanisms account for their maturation which probably initially involves a reduction in the air-bone gap with maturation of the outer and middle ears, and then elevation of the endocochlear potential and additional micromechanical maturations.