The ability to communicate in challenging situations with high levels of background noise is a fascinating property of the healthy auditory system. Despite normal sensitivity to pure-tones, many listeners complain about having difficulties in such situations. Recent animal studies have shown that noise over-exposure that produces temporary threshold shifts can cause the loss of auditory nerve (AN) fiber synapses. This neuronal degeneration has been termed “hidden hearing loss” or, more accurately, “synaptopathy”, since it is not reflected in the traditional pure-tone threshold. The envelope following response (EFR) has been proposed as a potential objective method to assess synaptopathy in humans. In this study, an AN computational model was used to investigate the effects of off-frequency contributions (i.e. away from the characteristic place of the stimulus) and the differential loss of different AN fiber types on EFR level-growth functions. The extent to which background noise can mask these off-frequency contributions was also examined.
EFRs were measured using a 64-channel EEG system with active electrodes. EFR level-growth functions were recorded in adults for SAM tones with a carrier frequency of 2 kHz. Modulations depths of 25% and 85% were tested at sound pressure levels (SPL) of 40 to 90 dB. Subjects were either normally hearing (NH) or mildly hearing impaired (HI) at frequencies above 2 kHz. A humanized AN model (Zilany et al., 2014) was used to simulate the EFR level-growth functions at the tested modulation depths and levels for several degrees of synapatopathy.
The AN model can account for the general trends obtained from human EFR level-growth functions. The simulations reveal that on- vs off-frequency EFR level-growth functions show completely different shapes. Off-frequency contributions have a large impact in the overall EFR magnitudes. The total EFR responses are strongly dominated by the high-SR fibers, especially at lower intensities. Background noise minimize the effects of off-frequency contributions on the overall EFR.
Off-frequency contributions of high-SR fibers dominate the total model EFR responses, suggesting that the loss of low- and medium-SR fibers has only little impact on measured EFRs. However, the impact of off-frequency high-SR fibers is reduced in broadband noise, placing greater emphasis on low- and medium-SR fiber responses.
This work was supported by CHeSS at the Technical University of Denmark. This work has been done in collaboration with CompNet at Boston University and the Massachusetts Eye and Ear Infirmary, thanks to an ACN Erasmus Mundus scholarship.
|Period||11 Feb 2017|
|Event title||40th MidWinter Meeting of the Association for Research in Otolaryngology|
|Location||Baltimore, United States, Maryland|
|Degree of Recognition||International|
- hidden hearing loss
- envelope following responses (EFR)
- cochlear synaptopathy