Traditional clinical hearing tests often fails to diagnose patients with a common form of inner ear damage that might otherwise be detected by challenging behavioral tests. These findings of a University at Buffalo were published in the Journal Frontiers in Neuroscience.
This “hidden hearing loss” presents itself as essentially normal hearing in the clinic, where audiograms-the gold-standard for measuring hearing thresholds are typically conducted in a silent room.
The reason that some forms of hearing loss go unnoticed in the clinic is that hearing involves a complex partnership between ear and the brain. The central auditory system compensates for significant damage to the inner ear by raising up its volume control, partially overcoming the deficiency.
The hearing pathway:
About 95% of sound input to the brain comes from the ear’s inner hair cells. Ear damage reduces the signal that goes to the brain. This results in trouble with hearing, however the brain has a central gain control that can turn up its volume control to better hear a distant sound. Sound is converted to neural activity by the inner hair cells in the auditory part of the ear-cochlea. Sound evoked neural activity then travels from the cochlea to the auditory nerve and into the central auditory pathway of the brain. In its pathway the information is relayed into a structure known as the inferior colliculus, before arriving finally at the auditory cortex in the brain. Interpretation of speech takes place there.
In people with hair loss, sound is less faithfully converted to neural activity in the cochlea. However, this weakened sound-evoked activity is progressively amplified as it travels along the central auditory pathway to the inferior colliculus and onward. The time it reaches the auditory cortex, things are hyperactive because the brain has recognized a problem.
When the signal gets high enough to activate a few neurons it is like the brain has a hearing aid that turns up the volume.
However, it is still unclear how many people have this kind of hearing loss, but there is a common complaint that people have hearing difficulties in noisy environments as they grow older. Adding background noise for testing hearing capabilities could help in recognizing deficits.
Researchers also suggest that this compensation might be causing to other auditory disorders such as tinnitus and hyperacusis.
When a person has excessive gain in the central auditory system, it could eventually result in over-amplification of sound or even make silence sound like noise.