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A patient with sensorineural hearing loss can have the symptom of hearing "double" in the damaged ear. Not having a time-delayed echo, but hearing as if he (or other people) speak with "two voices" at once.

Is there a theory about the mechanism behind this? Does it happen on the level of the malfunctioning hair cells, or because the brain has trouble remapping an incomplete input of frequencies, or something else?

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    $\begingroup$ Would you happen to have a source to this phenomenon? It sounds interesting. $\endgroup$ – AliceD Jun 15 '15 at 12:24
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    $\begingroup$ @AliceD It's personal experience, sadly :( But the ENT wasn't surprised to hear about it, so I assumed it's common enough in his patients. $\endgroup$ – rumtscho Jun 15 '15 at 13:11
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    $\begingroup$ @aliceD Is the site really strict enough to close a question if it was inspired by a personal situation when I was careful to write the main text on a theoretical level? I think I'd best ask the mods, or on Meta. $\endgroup$ – rumtscho Jun 15 '15 at 13:17
  • $\begingroup$ Meta and mods are your best chance for sure. While we're on it: What is the hearing function like? Any hearing loss? Unilateral or bilateral? In case of hearing loss - what is the audiogram like (high-frequency hearing loss, or a noise-dip at 4 kHz, or other)? Is the double-voice phenomenon temporary (say a few minutes at a time, or only after a loud concert, or is it chronic and persistent)? $\endgroup$ – AliceD Jun 15 '15 at 13:19
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – rumtscho Jun 15 '15 at 13:37
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Short answer
Damage to the inner ear can result in an asymmetric distortion in pitch perception between the two ears. This phenomenon can result in the same tone being perceived as a different pitch by the two ears.

Background
Damage to the inner ear (the cochlea) can lead to hearing loss. Hearing loss can sometimes lead to changes in perceived pitch. For people with unilateral cochlear damage, or asymmetrical hearing losses, the same tone presented alternately to the two ears may be perceived as having different pitches in the two ears (diplacusis). Sometimes different pitches are perceived even when the hearing loss is the same in the two ears (Moore, 1996). Diplacusis is associated with a higher perceived pitch in the relatively more impaired ear (Burns & Turner, 1986).

The precise origin of these pitch shifts is unclear, but are related to hearing loss (Moore, 1996). What is known, however, is that damage to the cochlea can lead to (Moore & Carlyon, 2005):

  1. Reduced frequency selectivity in the ear, i.e., a reduced pitch discrimination. This may alter the perception of pitch in complex tones;
  2. Altered velocity of the travelling wave on the basilar membrane. When the outer hair cells (the acoustic amplifiers in the cochlea) are damaged, the speed in which the acoustic wave travels through the cochlea may be altered, possibly leading to an altered perceived pitch;
  3. Dead regions in the cochlea, where the inner hair cells (the mechanoreceptors of the inner ear) are absent. In this case a tone is perceived by adjacent regions in the cochlea. Because the cochlea is a frequency-to-place converter (basically a Fourier transformer), it may result in altered pitch perception;
  4. Reduced phase locking (Koppl, 1997) in the auditory nerve. Phase locking refers to auditory nerve fibers following the waveform of the tonal stimulus, which may provide pitch information (in humans this process is mainly confined to low frequencies, ~1 kHz and below). Reduction of this property may alter pitch perception.

References
- Burns & Turner, JASA (1986), 79(5): 1530-40
- Koppl, J Neurosci (1997), 17(9): 3312–21
- Moore, Ear Hear (1996); 17: 133-60
- Moore & Carlyon, Springer Handbook of Auditory Research (2005); 24: 234-77

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    $\begingroup$ Top-notch answer as usual! $\endgroup$ – Krysta Jun 17 '15 at 14:28
  • $\begingroup$ How large can the pitch be (in Hz)? $\endgroup$ – draks ... Jun 23 '15 at 6:10
  • $\begingroup$ @draks... you mean the pitch or pitch difference? $\endgroup$ – AliceD Jun 23 '15 at 11:53
  • $\begingroup$ Sorry, the pitch difference, please... $\endgroup$ – draks ... Jun 23 '15 at 12:50

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