On the Use of Envelope Following Responses to Estimate Peripheral Level Compression in the Auditory System
Authors: Encina-Llamas Gerard , Dau Torsten , Epp Bastian 
Published: February 2020

Hearing impairment is a common and increasingly frequent problem among elderly people. The success of restoration or compensation therapies is strongly dependent on the development of reliable diagnostic methods for individual patients. The ability to compress the large level range of incoming sounds into a smaller range of vibration amplitudes on the basilar membrane (BM) is an important property of the healthy auditory system. Sensorineural hearing impairment typically leads to a decrease in sensitivity to sound and a reduction of the amount of compression observed in BM input-output functions in the cochlea. While sensitivity loss can be measured efficiently via audiometry, no measure has yet been provided that represents fast and reliable compression estimates in the individual listener. This would be useful to disentangle outer hair cells (OHC) from inner hair cells (IHC) damage. In the present study, magnitude-level functions obtained from envelope following response (EFR) to four simultaneously presented amplitude modulated tones were measured in normal hearing (NH) and sensorineural hearing impaired (HI) listeners. The slope of part of the EFR magnitude-level function was used to estimate level compression as a proxy of peripheral compression. The median values of the compression estimates in the group of NH listeners were found to be consistent with previously reported group-averaged compression estimates based on psychoacoustical measures and group-averaged distortion-product otoacoustic emission magnitude-level functions in human listeners. They were also similar to BM compression values measured invasively in non-human mammals. The EFR magnitude-level functions for the HI listeners were less compressive than those for the NH listeners. This is consistent with a reduction of BM compression. Given the numerical concordance between EFR-based compression estimates and group-averaged estimates from other methods, the frequency-specific (on-characteristic frequency (CF)) nature of BM compression was analysed through computer modelling. A computer model of the auditory nerve (AN) was used to simulate EFR magnitude-level functions at the level of the AN. The recorded EFRs were considered to represent neural activity originating mainly from the auditory brainstem-midbrain rather than a direct measure of AN activity. Nonetheless, the AN model simulations could account for the recorded data. The model simulations revealed that the growth of the EFR magnitude-level function might be highly influenced by contributions from off-CF neural populations. This compromises the possibility to estimate on-CF (i.e., frequency-specific or “local”) level compression with EFRs. Furthermore, the model showed that, while the slope of the EFR magnitude-level function is sensitive to a loss of BM compression observed in HI listeners due to OHC dysfunction, it is also sensitive to IHC dysfunction. Overall, it is concluded that EFR magnitude-level functions may not represent frequency-specific level compression in the auditory system.