[High-resolution distortion-product otoacoustic emissions: method and clinical applications]. / Mit hoher Frequenzauflösung gemessene otoakustische Distorsionsprodukte: Methode und klinische Anwendungen.

Unlike pure tone thresholds that assess both peripheral and central sound processing, distortion-product otoacoustic emissions (DPOAEs) selectively mirror the functioning of the cochlear amplifier. High resolution DPOAEs are missing in the toolbox of routine audiometry due to the fact that high resolution DPOAE measurements are more time-consuming when compared to normal clinical DP grams with rough frequency resolution. Measurements of high resolution DPOAEs allow an early assessment of beginning sensory cell damage due to sound overexposure or administration of ototoxic drugs. When using a rough grid, sensory cell damage would be overlooked as in the early state damage only appears at some distinct cochlear sites. A review is given on the method and application of high resolution DPOAEs.

Influence of artifacts and pass/refer criteria on otoacoustic emission hearing screening.

A screening device for otoacoustic emission (OAE) testing was evaluated via measurements of click evoked OAEs (CEOAEs) and/or distortion product OAEs (DPOAEs) obtained in normally hearing young adults and newborns. Moreover, measurements were performed in a passive cavity and in ears with severe sensorineural hearing loss, in which the occurrence of a pass result was assumed to indicate artifacts. Different parameter settings were investigated. In comparison to normally hearing young adults, overall pass rates in newborns were similar using CEOAEs (93 to 100%), but lower using DPOAEs (65 to 95% vs. 83 to 100%). Pass rates in ears with severe hearing loss were 10% at 2 kHz, 13% at 3 kHz, and lower at other frequencies. Pass rates in the test cavity were generally low. The influence of test frequency on pass rates and calculations using computer simulation indicated the presence of artifacts. The discrimination of such artifacts from biological signals is difficult or impossible, and they may represent a serious problem for efficient OAE screening.

Methods for estimating the sound pressure at the eardrum.

The problem of estimating the sound pressure generated at individual eardrums is systematically investigated. In audiometry, the reference to the pressure at the eardrum is usually realized by using a coupler such as the IEC 711 ear simulator, which is intended to approximate an average ear. The errors caused by individually shaped ear canals are calculated for a typical audiometric earphone (Beyer DT 48) in combination with the IEC 711 ear simulator and with an "ideal" coupler. These errors can reach 15 dB and are clearly more important than deviations of the ear simulator from an average ear. In order to obtain correct estimations, the chain matrices of individual ear canals have to be determined. Best estimates are obtained using the "reflectance phase method," but the "pressure minima method" also provides surprisingly good results, except in narrow frequency ranges. The reflectance phase method is checked using a physical model of the ear canal and the middle ear. The resulting errors of estimation remain within a limit of 3 dB up to more than 10 kHz.

Auditory evoked brain potentials as a function of interstimulus interval in adults with Down's syndrome.

Auditory-evoked potentials (AEP) to repeated binaural tone pips of several interstimulus intervals (ISIs) greater than or equal to 1 s were recorded from the vertex of young adults with Down's syndrome (DS; trisomy 21) and normal young adults. The following results were obtained: (1) AEP amplitudes, as well as one latency measure, of both experimental groups increased with the lengthening of ISI. (2) AEP peak latencies of the DS group were longer than AEP peak latencies of the normal group for all the ISIs employed. (3) AEP amplitudes of the DS group tended to be larger than AEP amplitudes of the normal group. These findings are discussed in relationship to issues of attention.

Effects of rate of repetitive stimulus presentation on the visual evoked brain potentials of young adults with Down's syndrome.

Visual evoked brain potentials (VEP) to repeated stimuli of several interstimulus intervals (ISI) were recorded from young adults with Down's syndrome (DS). The following results were obtained: (i) An ISI effect previously observed in normals: VEP amplitudes increased with increase in ISI; some ISI effect on latency was also observed; (ii) VEP amplitudes of DS subjects were larger than VEP amplitudes of normals; (iii) VEP peak latencies of DS subjects were longer than VEP peak latencies obtained from normals; (iv) ISI had a more pronounced effect on VEP amplitudes of DS than normal subjects. These results are discussed with respect to CNS differences and issues of attention and information processing.