DEVELOPMENT OF TRANSIENT EVOKED OTOACOUSTIC EMISSIONS IN THE NEONATALRAT

Otoacoustic emissions (OAEs) represent acoustic energy generated by th e cochlear amplifier which contributes to auditory sensitivity and fre quency discrimination. Therefore the OAEs can serve as a noninvasive t ool to study the cochlear amplifier. While transient evoked OAEs (TEOA Es) are generally recorded clinically in man, it has been difficult to record them in animals and instead cubic distortion product OAEs (DPO AEs) 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 r ats. TEOAEs were recorded and compared to the DPOAEs on several postna tal 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, fol lowed by the DPOAEs to lower frequencies. With age, their amplitude al so 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 t ypes of OAEs were similar. This may be evidence that similar mechanism s account for their maturation which probably initially involves a red uction in the air-bone gap with maturation of the outer and middle ear s, and then elevation of the endocochlear potential and additional mic romechanical maturations.