Effect of long-term noise exposure on the developing and developed ear in the Rat.

There have been reports that the developing ear is more sensitive than the adult ear to noise-induced hearing loss. This was investigated by testing auditory function in rats, both electrophysiologically and histologically, following exposure to broad-band noise (12 h/day for 15 days) at different stages of auditory development (neonates and adults), and also in age-matched controls. An exposure of 90 dB SPL broad-band noise caused no long-term change in auditory function in either age group. A higher exposure (102 dB SPL) caused greater long-term changes in hearing in the adult compared to the young noise-exposed rats, although histology showed greater damage to hair cells in the younger animals. Therefore, functionally, the developing ear does not seem more vulnerable than the developed ear to acoustic trauma.

Transient evoked otoacoustic emissions in laboratory animals.

Transient evoked otoacoustic emissions (TEOAEs) are much used clinically. However, it has been difficult to record them in small laboratory animals, and experimental manipulations designed to determine the generation mechanisms of this type of emission could not be performed. After refining the technique, based on the use of short clicks and a short amplifier gain suppression period, TEOAEs were recorded using the same instrumentation and techniques in rabbits, Psammomys obesus (fat sand rats), mice, rats and guinea pigs. Distortion product emissions were also recorded. The responses in each species differed with respect to threshold, magnitude, frequency spectrum and duration (endpoint). The ability to record TEOAEs routinely in laboratory animals should now allow for further experimentation on the mechanisms of their generation, on the cochlear amplifier in general and on the comparison of TEOAEs with distortion product emissions in individual species and animals.

Effect of temperature on the transient evoked and distortion product otoacoustic emissions in rats.

In order to study the energy dependence of the cochlear amplifier, transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) were recorded in rats during gradual cooling to 27 degrees C and heating to 40 degrees C. In the range 33-39 degrees C, the TEOAEs and DPOAEs were maximal in amplitude and almost insensitive to temperature. However, they were significantly depressed (reversibly) at higher and lower temperatures. Intensity functions were plotted at 37, 27 and 40 degrees C for both types of oto-acoustic emissions. At 37 degrees C intensity functions were nonlinear, with a notch at mid-intensity regions. At 27 degrees C, the magnitudes were depressed more at the lower intensities and threshold elevations were observed. As a result, the intensity functions were more linear and the notch was no longer seen. This result provides further evidence for a more active, energy-dependent component of the otoacoustic emissions at lower intensities for both TEOAEs and DPOAEs. The cooling probably affects the lower intensity otoacoustic emissions by inducing a depression in the endocochlear potential, by reducing the motility of the outer hair cells and by introducing a small conductive hearing loss.

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.

Cooling induces a decrease in middle ear compliance.

The effects of cooling rats from 37 degrees C to 27 degrees C and rewarming to 37 degrees C on the conductive mechanism of the middle ear was studied by means of acoustic impedance measurements. Cooling reduced middle ear compliance reversibly, without an effect on external canal volume and middle ear pressure. These results provide evidence for an increase in the stiffness of the tympanic membrane and/or of the ossicular chain and/or a decrease in stapes mobility. Thus a small part of the decrease in the magnitude of otoacoustic emissions during cooling is due to an effect on the conductive mechanism of the middle ear.

Transient evoked otoacoustic emissions can be recorded in the rat.

Transient (click) evoked (TEOAE) and distortion product (DPOAE) otoacoustic emissions can be recorded in most normal human ears. Even though DPOAEs have been recorded in many laboratory animals, there has not been much success in recording TEOAEs in non-primate mammals except for guinea pigs. In this study, TEOAEs were unequivocally recorded in every rat (and guinea pig) ear studied by using short pulses (40 microseconds) to generate the clicks and a short (1.1 ms) amplifier gain suppression period. The responses were reproducible in the same rat, above the noise floor and disappeared post-mortem. They were shorter in duration in rats than in guinea pigs and were made up of a broadband frequency spectrum between 2 and 4 kHz. Post-mortem, the TEOAEs to 65 dB SPL clicks disappeared at about the same time as DPOAEs to low stimulus intensities and before the DPOAEs to high stimulus intensities. The ability to record TEOAEs in rats and other animals should permit further experimentation into the basic mechanisms of generation of otoacoustic emissions in general and TEOAEs in particular.

Comparison of the efficiency of various criteria for artifact rejection in the recording of auditory brain-stem responses (ABR).

Artifact rejection using various criteria has not yet been investigated comparatively in the determination of auditory brain-stem responses (ABR). The aim of this paper is to test the efficiency of three different criteria of artifact rejection: the 'level' criterion, the 'amplitude histogram' criterion, and the 'power' criterion, and to compare their practical applicability. With regard to the identification of the single waves of ABR (percentage frequency of their appearance) and their amplitudes without and with artifact rejection, it can be shown that the frequency of appearance increases severalfold when one of the artifact rejection criteria is used. The difference between amplitude means with artifact rejection (all three criteria) and those without artifact rejection is significant on the 5% level for waves I, II, and III and on the 1% level for wave V. When the three criteria are compared with regard to the amplitude means of the individual waves, it can be shown that the best results are obtained, when the 'level' criterion is used, then follows the 'power' criterion and third is the criterion of 'amplitude histogram'. The differences, however, are not significant. With reference to the 'level' criterion used, our investigation proved that the more complicated 'amplitude histogram' and 'power' criteria are not more efficient.