Estimates of peripheral compression using auditory steady-state responses (ASSR) - and some perspectives
The healthy auditory system shows a compressive input/output (I/O) function as a result of healthy outer-hair cell function. Hearing impairment often leads to a decrease in sensitivity and a reduction of compression, mainly caused by loss of inner and/or outer hair cells. Compression is commonly estimated based on behavioral procedures (e.g., Plack et al., 2004), which are time consuming and rely on assumptions regarding the ability to selectively investigate frequency-specific cochlear processing; or on objective recordings such as otoacoustic emissions (OAEs) (e.g., Neely et al., 2003), which allow to selectively study cochlear processing but the interpretation of results for individual data is challenging because of its large variability.
Auditory steady-state responses (ASSR) are another objective method which allows fast, reliable and frequency-specific measurements of hearing function. It is investigated here whether ASSR can be used to estimate compression in the peripheral auditory system. A multi-frequency ASSR method was used to asses compression at four frequencies simultaneously (Lins and Picton, 1995). It is hypothesized that compressive behavior is observed in normal-hearing (NH) listeners, while in hearing-impaired (HI) listeners sensitivity and compression are reduced. Compression estimated via ASSR recordings are later compared to estimates of cochlear compression from distortion-product otoacoustic emissions (DPOAEs) recordings.
Results show clear, solid and repeatable compressive ASSR I/O functions in all frequencies for all NH subjects. For HI subjects, ASSR reveal the loss of sensitivity at low stimulus levels, which hinders the interpretation at the impaired frequencies in HI where reduction of compression may occur. Slopes of the I/O functions at supra-threshold levels are smaller (more compressive) in ASSR than in DPOAE I/O functions.
It will be discussed how ASSR I/O functions can be used for the objective evaluation of supra-threshold coding of sounds and how this measure could be used for audiological applications