Audiometer Implementation Of The Ten(Hl) Test For Diagnosing Cochlear Dead Regions; Appendix 2 - Interacoustics Affinity 2.0 Additional Information

2.0
Affinity Additional Information

1.17 Appendix 2

1.17.1 Audiometer Implementation of the TEN(HL) Test for Diagnosing Cochlear Dead Regions

Until recently, the TEN(HL) test for diagnosing dead regions in the cochlea could only be conducted by use
of a compact disc player connected to an audiometer. Now, the test has been implemented within the
2.0 2.0
Affinity and Equinox
describes: (1) What is meant by a dead region in the cochlear; (2) The basis of the TEN(HL) test for
diagnosing dead regions in the cochlea; (3) The implementation of the TEN(HL) test in the Interacoustics
audiometers; (4) The clinical value of diagnosing dead regions.
What is a Dead Region?
Sounds entering the ear give rise to vibration patterns on the basilar membrane within the cochlea. Each
place on the basilar membrane is tuned to respond best to a specific small range of frequencies; high-
frequency sounds produce maximum vibration towards the base and low-frequency sounds produce
maximal vibration towards the apex. The frequency that leads to a maximal vibration at a given place on the
basilar membrane is called the characteristic frequency (CF) for that place. In an ear with normal hearing,
the patterns of vibration on the basilar membrane are strongly influenced by the activity of the outer hair cells
(OHCs), which are minute sensory cells forming rows along the length of the basilar membrane. The OHCs
play a role in what is called the "active mechanism" in the cochlea.
and length in response to the vibrations on the basilar membrane. This activity of the OHCs enhances the
response to weak sounds (increasing the amplitude of vibration) and sharpens the tuning on the basilar
membrane. This sharpening increases the frequency selectivity of the auditory system, i.e., its ability to
separate out the different frequencies that are present in complex sounds such as speech and music. The
amplified vibrations are then detected by the inner hair cells (IHCs), which form a single row running along
the length of the basilar membrane. In response to vibrations on the basilar membrane, the IHCs release
neurotransmitter, and this leads to neural activity in the auditory nerve.
Cochlear hearing loss is often associated with damage to the hair cells within the cochlea.
can give rise to raised hearing thresholds (i.e., hearing loss as measured by the audiogram) in two main
ways. Firstly, damage to the OHCs impairs the active mechanism in the cochlea, resulting in reduced
basilar-membrane vibration for a given low sound level.
to give a just-detectable amount of vibration. Secondly, IHC damage can result in less efficient stimulation of
the auditory nerve. As a result, the amount of basilar membrane vibration needed to reach the hearing
threshold is larger than normal.
OHCs alone. A hearing loss greater than 55 dB nearly always involves some loss of function of IHCs as well
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as OHCs. From measurement of the audiogram alone, it is not possible to determine what proportion of the
hearing loss is due to OHC damage and what proportion to IHC damage.
In some cases, the IHCs at certain places along the basilar membrane may be completely non-functioning.
In addition, the auditory neurons making contact with those places may be non-functioning. Places with non-
functioning IHCs and/or neurons have been referred to as "lacunae"
used the blunt phrase "dead regions"
"dead zones" is also quite common. Figure 1 shows the dissected cochlea of a person who had been
exposed to intense impact sounds (gunshots) before dying in an incident unrelated to gunshots! The dark
lines are the neurons that would eventually get together and form the auditory nerve. There are essentially
no neurons coming from the basal part of the cochlea, indicating a high-frequency dead region.
Basilar-membrane vibration that occurs within a dead region cannot be detected by the neurons connected
to that region (if there are any). Say, for example, that the IHCs at the basal (high-frequency) end of the
cochlea are non-functioning. Neurons connected to the basal end, that would normally have high CFs, will
not respond. However, if a high-frequency pure tone is presented, it may be detected if it produces sufficient
basilar-membrane vibration at a region closer to the low-frequency, apical end. In other words, a high-
frequency sound may be detected via neurons that are tuned to lower frequencies. This is sometimes called
"off-place listening" or "off-frequency listening". Similarly, if there are no functioning IHCs in an apical region
of the cochlea, a low-frequency tone may be detected via neurons that are tuned to higher frequencies.
By Brian C.J. Moore, Ph.D.
PC-based audiometers (version 2.0.4) made by Interacoustics. This article
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A cochlear hearing loss up to about 55 dB may be caused by damage to
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and that phrase seems to have caught on, although the phrase
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They do this by changing their stiffness
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Hence, the sound level must be larger than normal
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and "holes in hearing"
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