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Causes/Kinds of Hearing Loss

Diplacusis—The Strange World of People with Double Hearing

© September 2011 by Neil Bauman, Ph.D.

Question: A musician explained, "I suddenly began experiencing a strange phenomenon with my hearing. I now hear music through my right ear at the correct pitch, while, at the same time, I hear the same music a semitone higher in my left ear. This is frustrating and scary. I can no longer perform my music. A major part of my life has suddenly been snatched from me. Have you ever heard of this before? Am I going crazy? What can I do to correct this condition?"

Another person related, "I've suddenly begun to experience a rather disturbing auditory phenomenon. Sounds as heard by my right ear are pitched a bit lower than the same sounds as heard by my left ear. This gives music a very frightening and eerie chorus effect that is becoming more and more disconcerting. What causes it? Does it ever go away?"

Answer: I have written about people who hear music off-pitch—either certain notes, or all notes. (See "When You Hear Music in the Wrong Key" including all the comments.) In most cases, these people heard the same music off-pitch with both ears. This alone was disconcerting and destroyed their enjoyment of music.

However, it is even more frustrating when you hear the same notes at different pitches in each ear and you don't know which ear to believe. For example, your left ear may hear a note as F while your right ear may hear the same note as F# (F sharp).

Rest assured, when this happens you are not going crazy, but something definitely has messed up the pitch perception between your ears. This condition is known as diplacusis (dip-lah-KOO-sis).

What is Diplacusis?

Diplacusis is a disconcerting condition, especially for musicians, because you hear the same note at two different pitches—often at the correct pitch in one ear and either higher (sharp) or lower (flat) in the other ear. This makes playing, singing or listening to music sound sour (sharp or flat depending on the direction of the frequency-shift). This can be devastating to a musician who has previously had perfect pitch.

The dictionary defines diplacusis as "abnormal perception of sound either in time or in pitch, such that one sound is heard as two. This fancy name comes from two Greek words "diplous"—double, and "akousis"—hearing. Thus, diplacusis is really double hearing or hearing double. (1)

Diplacusis occurs when your ears have a significant difference in frequency selectivity. This results in clashing interpretations (dissonance) of the tones you hear.

Fortunately, although many people hear tones at different pitches in each of their ears, this difference is normally slight. In fact, when the difference in pitch is less than about one semitone (halftone), the average person typically does not notice it. This difference in pitch normally escapes our notice because the slightly different pitches of sound from our two ears merge in our conscious perception such that we only hear one pitch of sound. (2)

Musicians, however, because of their musical training, may be considerably more sensitive to these slight pitch differences. As a result, they may be aware of, and bothered by, smaller pitch differences than even a semi-tone.

Kinds of Diplacusis

Diplacusis or "double hearing" comes in various "flavors".

Diplacusis binauralis (by-nar-RAL-is) is where you hear the same sound differently in each of your ears. For example, you may hear a different pitch of sound in each ear, or the timing may be different in each ear.

A subset of diplacusis binauralis is diplacusis dysharmonica (dis-har-MON-ih-ka) where only the pitch is different in each ear. Some authorities use the term "Interaural Pitch Difference" (IPD) rather than diplacusis, but they both refer to the same condition. (2)

Diplacusis echoica (eh-KOE-ih-ka), as it's name implies, is where you hear the same sound repeated in the affected ear—thus you hear the original sound followed by an "echo" of the original sound.

Finally, there is diplacusis monauralis (moh-nar-RAL-is). This is where you hear a single sound as two different sounds in the same ear. (1)

In my experience, by far the most common "flavor" of diplacusis is diplacusis dysharmonica. This is the annoying condition that numbers of musicians experience and the "kind" of diplacusis we will discuss here.

What Causes Diplacusis?

Diplacusis involves a shift of pitch perception. This can happen when the hearing in one ear is damaged (unilateral hearing loss), or the hearing in one ear is damaged more than it is in the other ear (asymmetrical hearing loss). However, the degree of pitch distortion does not appear to bear any simple relationship to the degree of hearing loss. (3)

Incidentally, diplacusis was first observed way back in the 1880s in people with unilateral hearing loss. (2)

If one ear has normal hearing, and the other one has sensorineural hearing loss, you can have a lot of diplacusis. Bilateral sensorineural hearing loss results in less diplacusis but there are probably pitch distortions because both ears are likely messed up in the same way. (4)

In fact, there is a high degree of correlation between the occurrence of diplacusis and damage to the inner ear. (2) Diplacusis is typically experienced as a result of sensorineural hearing loss. Onset is usually spontaneous and can occur at the time of an acoustic trauma or in the midst of an ear infection. Sufferers may experience the effect permanently, or it may go away on its own. (4)

Fig. 1: The top row of inner hair cells shows their slightly rounded stereocilia bundles. The bottom three rows of outer hair cells show their deep
V-shaped stereocilia bundles. (5)

For people who have some degree of sensorineural (inner ear) hearing loss, here is a detailed account of how diplacusis may develop. First, we need a bit of background.

The cochlea in your inner ears contains four rows of hair cells. There is one row of inner hair cells and three rows of outer hair cells. On the top of each hair cell are bundles of minute hair-like projections called stereocilia (Fig. 1.). These stereocilia sway (dance) in unison to received sound signals.

The job of the inner row of hair cells is to transmit the sound signals from the cochlea to the auditory nerve. As they sway, they open "ion-gates" that allow the sound signals to be transmitted to the auditory nerve.

If the row of inner hair cells die, you will not hear anything as there is no other way to transfer the sound signals from the cochlea to the auditory nerve. If sections of inner hair cells die or are severely damaged, you no long will hear the frequencies of sound to which those hair cells were tuned.

Fig. 2: Close-up of a healthy stereocilia bundle perched on top of an outer hair cell. (5)

The outer three rows of hair cells (Fig. 1.) have a different function. Their job is to filter and amplify the sounds we want to hear. If all of them were to die, we could still hear (that's the inner hair cells job), but sounds would be unregulated as to volume and all sounds would appear to run together.

Like the inner hair cells, "each set of outer hair cells and associated stereocilia  (Fig. 2.) have a favorite frequency and 'dance' energetically when they receive it. This 'dance' amplifies the signal to the inner stereocilia, which react passively and signal the brain. An array of outer and inner stereocilia operate together as a narrowband amplifier." (5)

Fig. 3: The black (bottom) line shows the normal operation of the sharply-tuned cochlear amplifier. When some inner hair cells die or are damaged only the volume is reduced (red [top] line) but the signal is still sharply defined. (5)

Thus, when all is working properly, there is a sharply defined peak for a given sound (The black line in Fig 3.).

However, damage or death to some of the inner row of hair cells reduces sound sensitivity so you don't hear as well (the red line in Fig. 3.), but if there is no corresponding damage to the outer rows of hair cells, the signal peaks are still sharply defined.

In real life this seldom happens. What typically happens is that loud sounds first damage the outer hair cells rather than the inner hair cells. In fact, you could have 50% of the stereocilia on a region of the outer hair cells damaged before you would see any change in hearing on an audiogram. A person could suffer damage to 75% of the outer hair cells before he would notice he had a hearing loss, but by that time you would expect some inner hair cells to be severely damaged too. Thus, the first consequence of noise damage is not hearing loss itself as you might expect, but fuzzy or blurred hearing.

Fig. 4: Close-up of a damaged stereocilia bundle perched on top of an outer hair cell. Notice that many of the stereocilia are missing or flopped over and deformed. (5) (Compare with Fig. 2 above.)

The stereocilia on top of a hair cell may be damaged (Fig. 4) and thus will not be able to do their "dance" properly. Since they do not "dance" effectively, there is less amplification of their favorite frequencies.

The result is that both the amplitude and the sharpness of the resulting signal is drastically changed. Compare the black line (healthy hair cells) in Fig. 5 with the red line (damaged hair cells).

Not only is there less volume, and fuzzy hearing, something surprising also occurs. The frequency is shifted a bit. Notice that the lowest part of the red line and the peak of the black line no longer coincide.

Fig. 5: The black (bottom) line shows the normal operation of the sharply-tuned cochlear amplifier at one particular frequency. When some outer hair cells die or are severely damaged sound signals become softer and fuzzier (red [top] line) resulting in a shallow broad curve with no clear peak. Notice that the central frequency is also shifted slightly away from where it should be, thus giving a slightly different frequency of signal in the damaged ear resulting in diplacusis. (5)

Since the frequencies are slightly different, the result is that you now hear the same sound at two different frequencies. The fuzzy, shifted frequency in the your more damaged year and the normal or near normal sound in your undamaged or lesser-damaged ear. This is one possible explanation for diplacusis.

Another interesting phenomenon is that when your brain receives information from damaged sections of hair cells it struggles to determine whether it is intensity (volume) or frequency that has changed. Thus, for instance, it may seem that the music you are listening to has gone sharp during a crescendo. (5)

Loud sounds do not only damage or kill a single hair cell along with its stereocilia, but loud sounds also typically damage or kill neighboring hair cells as well.

Incidentally, if a cell is badly damaged, it programs itself to die through a process known as apoptosis. Since hair cells are a specialized kind of nerve cell, when they die your body does not replace them, thus leaving you with a permanent hearing loss.

Fig. 6 shows the results of severe noise damage. Notice that most of the hair cells are missing  and have been replaced by scar tissue. When this happens hearing becomes very faint and fuzzy. The result is that you will no longer hear much at all, and whatever you do hear will be so fuzzy that you will be not be able to distinguish small differences in pitch between your two ears and therefore you will never notice diplacusis.

Fig. 6: Missing and badly-damaged hair cells. The inner row of hair cells is at the top and the 3 rows of outer hair cells are in the middle. Notice how most are dead and thus missing. Compare with Fig. 1. (5)

Diplacusis can occur whether you have a temporary hearing loss or a permanent hearing loss. If you have a permanent hearing loss, your diplacusis may prove to be permanent as well (2) such as is the case with my wife.

Diplacusis does not just occur in people with hearing loss, however. It is also found in some people with normal hearing. The main difference between diplacusis in people with hearing loss and in people with normal hearing is that in people with hearing loss there is a pronounced intra-ear frequency shift that typically exists in a fairly broad frequency region, while in people with normal hearing the intra-ear frequency shift rarely exceeds +/-2% and is distributed in a quasi-random manner such that the mean across frequencies essentially is zero. (2)

Apart from having a sensorineural hearing loss, a number of people seem to get diplacusis after they have had a cold or ear infection. This may be because of a viral attack on the inner ear, or because of a middle ear infection and/or blocked Eustachian tube causing a conductive loss.

For example, one man explained, "I have had a cold for the past week or so. Last night I suddenly noticed that the pitch I hear in my right ear is a semitone higher than the same pitch in my left ear."

Another person explained, "I have had this happen to me several times over the years, always in conjunction with an ear infection. Fortunately, it never affected me longer than a few days."

Still another person explained, "I have had this happen twice in the past 15 years or so, and both times it was related to a bad cold and probable ear Infection. In my case the effects diminished gradually over several weeks, and finally disappeared."

Finally, a singer explained, "I woke up to an ear ache in the middle of the night a week ago and couldn't hear music properly the next morning. I went to my doctor, who said my eardrum was inflamed and that I had a sinus infection. He put me on Azithromycin and steroids and now my cold/sinus infection seems to have mostly subsided, although I am still hearing about a quarter tone higher pitch in my left ear than my right."

Ménière's disease also seems to result in diplacusis in some people. (6) The amount of diplacusis seems to vary in degree with the amount of hearing loss, becoming more significant when the hearing decreases and less noticeable as hearing improves. (7)

How Common is Diplacusis?

Since hearing loss (inner ear damage) affects as many as 1 in 4 people, and since hearing loss is seldom identical in both ears, diplacusis is not as rare a phenomenon as you might think. (5) However, diplacusis appears to be more common—or at least more noticeable—among musicians since they have trained their ears to discern smaller differences in pitch than people in the general public.

One study of musicians revealed that 24% experienced tinnitus, 25% experienced hyperacusis, 12% experienced distortion in their hearing, and 5% experienced diplacusis in one of its forms. Of course, many of these musicians experienced two or more of the above symptoms at the same time. (5)

Cases of diplacusis on the order of 1% to 2% can be found in many people with normal hearing, especially those that are fatigued or have been exposed to loud noise. (6)

What Can I Do About My Diplacusis?

Since diplacusis is most often associated with some degree of hearing loss, if you have some condition that reduces your hearing such as wax in your ears, middle ear infections or clogged sinuses/clogged Eustachian tubes, the obvious thing to do is to treat these conditions. As the "gunk" dissipates and your ears/sinuses clear, your hearing will improve and your diplacusis will likely fade away.

For some people, being properly fitted with hearing aids helps to reduce their diplacusis.

Unfortunately, for others, nothing seems to work. You may be left to cope with your diplacusis as best you can. In any case, the good news is that you now understand what is likely going on with your ears, and that makes it easier to deal with.

_____________

(1) Stedman's Medical Dictionary, 27th Edition. 2000. Lippincott Williams & Wilkins.

(2) Terhardt, Ernst. 2000. Diplacusis binauralis (IPD). http://www.mmk.ei.tum.de/persons/ter/top/diplacusis.html.

(3) Larkin, W.D. 1983. Pitch Vulnerability in Sensorineural Hearing Impairment. http://www.ncbi.nlm.nih.gov/pubmed/6651625.

(4) Diplacusis. 2011. Wikipedia. http://en.wikipedia.org/wiki/Diplacusis.

(5) A Sound Ear II. 2008. The Association of British Orchestras.  http://www.abo.org.uk/user_files/ABO Publication Downloads/ASoundEarII.pdf.

(6) A Dictionary of Hallucinations. 2009. Jan Dirk Blom. p. 144.

(7) Karino, Shotaro, et. al. 2002. Application of Binaural Beat Phenomenon to Evaluation of Diplacusis Binauralis Dysharmonica. http://www.aro.org/archives/2002/2002210.html.