As sound travels through the inner ear, it causes vibrations along the basilar membrane within the cochlea. The basilar membrane is organized by frequency or pitch. In other words, different frequencies will vibrate maximally at specific places along the membrane. Along the length of the basilar membrane are rows of outer hair cells (OHC) which play a part in transmitting these patterns of vibration along the basilar membrane. After being detected by the OHCs, sound vibrations are detected by the inner hair cells (IHC) which form a single row along the length of the basilar membrane. The IHCs are responsible for releasing a neurotransmitter which sends sounds to the auditory nerve.
With cochlear hearing loss, the OHCs, IHCs, or both can be damaged. In hearing loss above 55 dB HL, there is most often IHC involvement. In this case, there may be sections of IHCs that are completely non-functional. These sections are often referred to as cochlear dead regions
In other words, a cochlear dead region is a specific area within the inner ear that has lost sensitivity to sound. If IHCs are not functioning at all, they are unable to release the neurotransmitter needed to send the sound to the auditory nerve. When a dead region exists in the cochlea, sounds in that frequency range cannot be detected, even if the sound is presented at a high volume. Dead regions can occur in any part of the cochlea, but they are most commonly found in the high-frequency regions. Individuals with dead regions may have trouble understanding speech, especially in noisy environments. They may also find it difficult to hear certain sounds, such as consonants or high-pitched tones.
Cochlear dead regions are difficult, if not impossible, to detect on traditional pure tone audiometry. In fact, an individual with cochlear dead regions may respond to sound presented at a high volume within that frequency range. This is because if sound is loud enough, even with non-functional IHCs in that specific frequency regions, sections of IHCs nearby to that dead region may respond. This is referred to as “off-place listening”.