Evaluation of cochlear processing and auditory nerve fiber intensity coding using Auditory Steady-State Responses (ASSR)

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Description

Objectives:
The compressive nonlinearity of the peripheral auditory system is commonly assumed to be a result of healthy outer-hair cell function, and to be a good indicator of the system’s integrity. It has recently been shown that auditory steady-state responses (ASSR) elicited by sinusoidally amplitude modulated (SAM) tones, with modulation frequencies around 80 – 100 Hz, show compressive growth as a function of stimulus intensity for medium levels. These responses are thought to reflect coding of the acoustical stimulus envelope at the level of the brainstem, hence after cochlear processing. Recent research in laboratory animals shows that noise exposure producing temporary threshold shifts can cause auditory nerve fiber (ANF) deafferentation, predominantly affecting low-spontaneous rate (SR) fibers. In the present study, it is hypothesized that deafferentation of low SR fibers leads to a reduction of ASSR amplitude at levels coded by this class of fibers.

Methods:
Multi-channel ASSR input/output (I/O) functions were measured in two groups of audiometrically normal-hearing adults. For the first group, ASSR I/O functions were obtained at stimulation sound pressure levels (SPL) ranging from 20 to 80 dB in steps of 5 dB using multi-frequency stimulation with four octave-spaced SAM tones and a fixed modulation depth of 85%. For the second group, ASSR I/O functions were obtained using a single SAM tone presented at 3 levels below 60 dB SPL and at 9 levels in the range from 60 to 90 dB SPL. ASSR growth functions were recorded at four modulation depths (65, 75, 85 and 100%).

Results:
Results showed that ASSR growth functions can be measured in NH listeners for input levels between 20 and 80-90 dB SPL. Significant compression was found for input levels between 30 to 60 dB SPL, and a saturation of the growth functions was observed at levels above 60 dB SPL. The lower-level part of the ASSR I/O functions showed compression of about ~0.25 dB/dB, which is similar to compression recorded directly in animal cochleae. The slope for levels above 60 dB SPL showed larger variability across subjects.

Conclusions:
The slope of ASSR I/O functions could be used to estimate peripheral compression simultaneously at four frequencies. First results suggest, that the slope of ASSR I/O functions above 60 dB SPL might be used to evaluate the integrity of intensity coding of low-SR fibers. Hence, ASSR might prove to be useful for evaluation of both cochlear and ANF integrity.

Funding
This work was funded by the Oticon Center of Excellence for Hearing and Speech Sciences at the Technical University of Denmark.
Period11 May 2015
Event titleXXIV Biennial Symposium of the International Evoked Response Audiometry Study Group: null
Event typeConference
LocationBusan, Korea, Republic of

Keywords

  • hearing loss
  • auditory steady-state responses (ASSR)
  • peripheral compression
  • hidden hearing loss
  • deafferentation