Hearing Loss and Central Auditory Processing

63 Hidden Hearing Loss

Learning Objectives

Be able to discuss the impact of exposure to loud noises related to this condition.

Be able to describe the impact on behavioral thresholds.

Hidden hearing loss wasn’t reported before 2009 (Kujawa et al, J. Neurosci 19:14077-14085) but it is rapidly garnering broad attention. Even when the hair cells are not damaged and behavioral thresholds recover completely after loud noise exposure, 50% of the synapses are gone! But hearing thresholds as measured clinically and reported in audiograms are still the same. The technical name for hidden hearing loss is cochlear synaptopathy.

To explore this finding of noise-induced primary neuropathy further, and to clarify interpretation, the observations were repeated for an exposure that produced only temporary threshold shift (TTS) in fully adult animals (Kujawa and Liberman, 2009). In this work, mice from the same inbred strain were exposed to a band of noise placed in the region of best threshold sensitivity. The noise was titrated in level and duration to produce a large, acute threshold shift (30–40 dB at 24 h), but one that recovered by 2 weeks, without hair cell loss. Immunostained cochlear whole mounts and plastic-embedded sections (Fig. 6.5.1), imaged by confocal and conventional light microscopy, were assessed to quantify hair cells, cochlear neurons, and synaptic structures providing the communication conduits. Hair cell-based distortion product otoacoustic emissions (DPOAEs) and neural-based auditory brainstem responses (ABRs) or compound action potentials (CAPs) of the auditory nerve were used to assess the peripheral consequences of the noise on function (Fig. 6.5.1).

The left four panels compare imaging from two mice with different exposure to noise. The right two panels show graphs of their hearing loss.
Fig.6.5.1. Noise-induced and age-related loss of synapses and SGNs. Evaluating synaptopathy by triple-staining cochlear whole mounts for a pre-synaptic marker (CtBP2-red), a postsynaptic marker (GluA2-green) and a hair cell marker (Myosin VIIa-blue). Confocal z-stacks in the IHC area from a control (A) and a noise-exposed mouse (B), 2 wks post exposure. Light micrographs of osmium-stained plastic sections from noise-exposed ears, 2 wks (C) or 2 yrs (D) post exposure. Exposure in B and D was 8–16 kHz, 2 h, 100 dB SPL, delivered at 16 wk to CBA/CaJ mice. (E) In aging ears from the same inbred strain, synaptic counts at IHCs decrease steadily from 4 to 144 wks and parallel ganglion cell loss follows whereas, (F) threshold loss begins comparatively later and accelerates beyond 80 wks, mirrored by accelerating loss of OHCs. IHC loss is trivial at any age. Red symbols flag 80 wk data points for all measurements. After Kujawa and Liberman 2006, 2009; Sergeyenko et al. (2013). (Cedit: M. Charles Liberman and Sharon G. Kujawa, Noise-induced and age-related loss of synapses and SGNs, CC BY 4.0)

Presaging the ganglion cell losses, results of these studies revealed an acute loss of synapses between IHCs and the peripheral terminals of the spiral ganglion neurons that contact them (Kujawa and Liberman, 2009). Although thresholds recovered, by design, and no hair cells were lost, IHC synaptic losses were greater than 40% in basal cochlear regions, when assessed 24 h post noise, and were stable 2 and 8 weeks later. Losses were proportional in magnitude and cochlear location to the SGC loss observed in the previous series, suggesting that this interruption of IHC-to-neural communication set the stage for neurodegeneration.


Three graphs are shown. All three show positive trends.
Fig 6.5.2 Response amplitudes and synapse counts. Permanent reductions in ABR, but not DPOAE amplitudes in ears with recovered thresholds after noise. Shown are DPOAE (A) and ABR wave 1 (B) response growth functions in the region of maximum acute TTS 1 d and 2 wk after exposure (as in Fig. 1) to 16 wk CBA/CaJ mice; unexposed controls shown for comparison. Neural response amplitude declines are proportional to synaptic and neural losses in aging CBA/CaJ, where synapses are plotted vs mean wave 1 amplitudes (at 80 dB SPL in 4–128 wk animals (C). Panels A,B from Fernandez et al., 2015; Panel C from Sergeyenko et al., 2013. (credit: M. Charles Liberman and Sharon G. Kujawa, Response amplitudes and synapse counts, CC BY 4.0)

Real world examples:

  • Hidden Hearing Loss:  Sarah, a college student, struggles to follow conversations in noisy environments despite passing routine hearing tests, illustrating hidden hearing loss caused by damage to cochlear synapses from exposure to loud parties and music festivals.
  • Effects of Loud Noises on Hearing Thresholds:   – John, a construction worker, experiences temporary shifts in hearing after exposure to loud machinery, highlighting the long-term impact of noise on cochlear synapses despite intact hearing thresholds.
  • Synaptic Changes in Cochlear Synaptopathy:   – Mary, a retiree and music lover, notices difficulty discerning music despite normal hearing thresholds, demonstrating how synaptic deterioration in the cochlea affects auditory processing and enjoyment of music.
  • These real-life scenarios provide concrete examples of how hidden hearing loss, noise exposure, and synaptic changes manifest in everyday situations, enhancing understanding and relevance of the discussed concepts.

For an additional explanation about hidden hearing loss, take a peak at the video link here and included below.


Cheryl Olman PSY 3031 Detailed Outline
Provided by: University of Minnesota
Download for free at http://vision.psych.umn.edu/users/caolman/courses/PSY3031/
License of original source: CC Attribution 4.0

National Center for Biotechnology Information, “Cochlear synaptopathy in acquired sensorineural hearing loss: Manifestations and mechanisms” by M. Charles Liberman and Sharon G. Kujawa
Provided by: Hearing Research
URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5438769/
License: CC BY 4.0


Kujawa, S. G., & Liberman, M. C. (2009). Adding insult to injury: cochlear nerve degeneration after “temporary” noise-induced hearing loss. Journal of Neuroscience29(45), 14077-14085.

Liberman, M. C., & Kujawa, S. G. (2017). Cochlear synaptopathy in acquired sensorineural hearing loss: Manifestations and mechanisms. Hearing research349, 138-147.


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Introduction to Sensation and Perception Copyright © 2022 by Students of PSY 3031 and Edited by Dr. Cheryl Olman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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