Hearing
Alexander Liss
The mechanism of hearing is well studied.
Sound processing involves Middle and
Inner Ears.
The Middle Ear is an air filled cavity
separated from external environment with elastic tympanic membrane
(eardrum).
The Inner Ear is a liquid filled cavity, which has two openings into the Middle Ear both covered with an elastic membrane: oval window and round window.
There
is a complex mechanism of small bones suspended with ligaments and muscles in
the Middle Ear:
Working
of two muscles of the Middle Ear is understood only partially. The function of stapedius as a guardian against an overload of the Internal
Ear, especially from internal sounds (yelling), is well studied. Some forms of
vibration of stapedius, possibly caused by its
"spasm", cause tinnitus (ringing in the ear).
The cavity of the Inner Ear has complex configuration, which is often
described as a "vestibule" adjacent to the oval window, which
"leads" to two distinctive areas: one consists of three semicircular
channels and the other is cochlea housed in a spiral dead-end
cavity.
Semicircular channels contain small bony
particles suspended in the liquid and hairs-receptors. The entire system reacts
to heads and tails of pressure waves in the liquid of the Inner Ear, because
these fronts of waves move particles, which in turn bend hairs-receptors.
Such
waves could be caused by the body acceleration (tree semicircles allow
determination of the direction of such acceleration).
The
cochlea has a special membrane of varying thickness along its spiral cavity
with hairs-receptors embedded in it.
Sound
in the ear is generally described as a sound wave propagating through a chain
of transmitters:
1. tympanic membrane, which
passively reflect a wave of external environment; it is silently assumed that
variations of movement of different points of this membrane are unimportant;
2. ossicles chain with its muscles, which collects
energy of the sound on a large area of the tympanic membrane and passes it
through a small area of the oval window; muscles are assumed to be static;
3. oval window - cochlea - round window.
It
is silently assumed that after the wave exits the Inner Ear, its energy
dissipates.
It
is assumed also, that the cochlea works as a specter analyzer, because
different parts of its membrane react (resonate) to different frequencies of
sound. However, it is not explained how hairs-receptors in the cochlea
translate high frequency vibration into signals sent along nerves. Note that
high-pitched tone causes 20,000 cycles of displacement per second. It should be
explained in this resonance model, how hairs-receptors of cochlea, which
react to a single displacement, translate this into slow waves of neural
signals, but there is no such explanation.
There
is one additional cause for doubt.
The idea of specter analyzer comes from technology and mathematical presentation of sound with its spectrum - a sum of sinusoidal waves. However, sinusoidal wave does not exist in nature - it is a mathematical abstraction. When mathematicians tried to present a complex wave as a sum of simple waves of different amplitude, they found that formulae are simplest, when simple waves have sinusoidal form. This does not mean that this presentation of a wave is suited in description of the workings of cochlea. Even when one wants to stick to the idea of resonance of cochlea's membrane, one better choose forms of waves specific for the cochlea and present sound as a sum of waves of this type.
In
sum, these assumptions are highly unrealistic.
Let
us introduce a different set of assumptions and arrive to a different more
realistic model.
First,
we revisit role of muscles.
Maintaining
static level of muscles tension is an expensive task for a body. Muscle fibers
contract and relax randomly and coordinating their actions that an average
tension stays in a narrow range is difficult. It is much less expensive to
organize periodic contractions and relaxations of a muscle. Hence, most likely,
tensor timpani and stapedius are in the process of periodic
contraction and relaxation. Periods and strength of contraction of these
two muscles should be independent, because they are innervated from different
nerve pathways.
When
stapedius contracts, it tilts stepes,
reduces area of contact between bony stepes and the
membrane of the oval window. Consequently, this reduces level of energy of
sound in the Inner Ear (bones transmit sound better than air). In addition, it
changes the form of sound wave entering the Inner Ear and distribution of its
energy in the Inner Ear.
Hence,
in addition to protecting the ear from energy overload, stapedius
actively participates in sound analysis.
When
tendon tympani contracts, it increases tension in the tympanic membrane and
this changes its sound qualities. Hence, tendon tympani also actively participates in sound analysis.
If
this assumption is true, then weakening of muscles with age should lead to
worsening of hearing, which is a known fact. Exercising of these weakened
muscles should improve hearing. There is anecdotal evidence that it is true
also, and there is a cottage industry dedicated to such exercises, but there is
no scientific evidence in this area yet.
Second,
we revisit role of the tympanic membrane.
The
tympanic membrane is under tension; hence, it has to have particular forms of
vibration, where it resonates. The set of these forms depends on degree of its
tension exerted by the tendon tympani. When the membrane’s tension
changes, the set of this forms changes.
Hence,
the reaction of tympanic membrane depends on the form of the sound wave in
external environment. Obviously, the form of a sound wave is modified by the
external ear. This is not important for a sound analyzer, it is important that
different forms of sound waves are still different after transformation by the
external ear.
That
one ear can discriminate between different forms of waves and even could help
to determine location of the sound this way without help of another ear is an
easily verifiable fact.
When
the tympanic membrane has high tension, it resonates at higher frequencies,
hence when tensor timpani changes its tension it changes sound in the ear.
Third,
we assert that different characteristics of sound are analyzed in different
places of the Internal Ear.
It
is known that different ears have different capacity to differentiate tones and
clicks and often one ear is used to analyze tones and the other to analyze
clicks. This means that there are separate mechanisms in the ear to do these
two forms of analysis.
Natural
sounds have a head (an interval of growing strength) and a tail (an interval of
diminishing strength).
Most
likely heads and tails of sounds are analyzed in semicircular channels, which
are attuned to detection of displacements caused by heads and tails of sounds.
The body of the sound with relatively stable intensity of the sound most likely is analyzed by cochlea.
Fourth,
we dispel the notion of the running sound wave in the ear and replace it with
an assumption of the standing wave.
There
is no dampening device in the ear; hence, one cannot assume that energy of the
sound wave dissipates at returning into the Middle Ear; hence, a realistic
model has to deal with sound environment in the entire assembly of the Middle
and Inner Ears under changing external conditions.
Functioning
of hairs-receptors in cochlea could be logically explained, when there is a
standing wave in it, which causes the membrane in the cochlea to assume a
relatively static form in response to a sound with fixed spectrum.
This
standing wave is defined by all elements of the ear, not only by
characteristics of the cochlea. For example, the Middle Ear creates two
pathways of sound, one through bones to the oval window and the other through
air to the round window with differential of speed of sound over these
pathways. The stapedius controls the ratio of sound
energy coming via these pathways, hence it controls
the standing wave.
With
this assumption, it is easy to understand the pattern of innervations of the
cochlea's hairs-receptors. The signals from a few distinctive bands of
receptors along the length of cochlea's membrane are collected into a single
nerve fiber. The pattern of these bands corresponds to classes of standing
waves and it means that cochlea is hardwired for such initial classification of
sounds before information comes to the brain.
The
usefulness of such preliminary classification is obvious, because in the sea of
possible sounds there is a subset of sounds, which are generated in nature and
which are of interest.
Ears
of some people produce a fine tuned classification, which allow fine
differentiation of harmonic tone combination. Harmonic tone combination
something present in nature - sounds produced by vibration of natural objects
have strong overtones.
This
wired in ear classification explains why sounds produced by musical instruments
are perceived as more pleasant than electronically generated sounds - they are
produced by vibrating natural bodies, and ear is
attuned to recognition of such sounds.
Now
we summarize an alternative model of sound analysis by the ear, based on above
assumptions:
1) Muscles of the Middle Ear
(tensor tympani and stapedius) are in the process of
periodic contraction and relaxation and their actions are independent. The
brain controls parameters of these actions and receives back information about
current state of muscles (degree of their contraction).
2) Sound environment in the Middle Ear and in the cochlea is a slowly changing standing wave; this standing wave is changing even for constant external sound, because of actions of muscles.
3) The tensor tympani affects the standing wave trough changing properties of
tympanic membrane.
4) The stapedius
affects the standing wave trough changing the level of sound energy passing
through the oval window (back and forth).
5) The tympanic membrane affects
the standing wave for two reasons:
a) there are particular forms of
vibration, where it resonates,
b) it is affected by the external sound
environment of a particular form of wave.
6) Receptors in the cochlea detect
properties of this standing wave.
7) Fixed innervations of the
cochlea receptors define initial classification of sound provided by ear
itself.
8) Rapidly changing external sound
environment (heads and tails of natural sounds) is detected and analyzed in
semicircular channels of the Internal Ear.
9) Brain receives information from
muscles, cochlea and semicircular channels and extracts patterns, which it
interprets as characteristics of external sound environment.
This
model could be used as a basis for classification of hearing disorders.
Muscles
related disorders could be related to one of the muscles of the Middle Ear or
to both of them.
Inability
to execute cycles of contraction and relaxation of short period, leads to
inability to differentiate short sounds, where a period between a sound head
and a sound tail is short, and hence inability to understand fast speech.
Disruption
of brain control over muscles activity leads to diminished ability to
differentiate tones of sounds.
Unusually
strong contractions of tensor tympani present to the brain unheard before high
pitch sounds of external and internal environment; this could be confusing.
Inability
of tensor tympani to make strong contractions (weakening of muscles) leads to
removal from analysis of standing waves, which correspond to the highly tense
state of the tympanic membrane, when it resonates on high frequency component
of external sound. This leads to loss of ability to detect a high frequency
component of sound and inability to hear sounds, which are completely high
frequency.
Inability
of stapedius to relax well leads to diminished
ability to form standing waves and hence diminished ability to differentiate
tones; however differentiation of clicks (heads and tails of sounds) should be
relatively intact.
Inability
of stapedius to make strong contractions (weakening
of muscles) leads not only to diminished protection from strong sounds but also
to a lower range of potential contractions of stapedius
and thus to limited range of standing waves ear could produce for a given
external environment and this means diminished ability to differentiate of
sounds of different spectrum. This could cause problems with speech
recognition.
Too
short period or too strong contraction in the cycles of contraction and
relaxation could lead to generation of sound by the mechanism of the Middle Ear
(ringing, clicks in the ear, tinnitus). This mechanism has a range of normal
operations and this range narrows with age because of loss of original flexibility.
Hence, with age, it could produce sounds on its own more often.
Disorders
related to the tympanic membrane could be caused by its damage or diminished
elasticity.
Damaged
membrane messes up entire picture of correspondence of external sound environment
and internal standing waves and this leads to consistent misinterpretation of
sound by the brain.
Membrane
of diminished elasticity could resonate on high frequencies at some level of
its tension, but there is no level of tension, which could make it resonate on
low frequencies. This leads to loss of ability to detect a low frequency
component of sound and inability to hear sounds, which are completely low
frequency.
Presence
of control circuitry in the system (contractions of muscles controlled by the
brain) introduces possibility of a special kind of disorders, where the system
flips in an undesirable pattern of operation (pattern of cycles of contraction
and relaxation, similar to muscles twitching). This could manifest itself in
distorted sound perception, in noises produced by the mechanism of the Middle
Ear, and even in pain.
Problems
with ear usually cause anxiety and stress and this actually could reinforce
such bad state of the body.
Note
that hearing aide does not help with any of these disorders. However, there are
a few things, which could be helpful.
One
group of measures includes exercises of muscles of the Middle Ear. They should
become a regular routine of aging people, because muscles weaken with age.
The
other group of measures includes techniques of flipping the body out of bad
patterns of cycles of muscles contraction and relaxation. In some cases, it is
possible to flip body back into an acceptable state with change of activity or
with a special exercise. For example, sometimes, it is sufficient to change
sound environment and engage in physical activity to get rid of an incident of
tinnitus. The very understanding that a healthy body could accidentally fall in
a bad state of operations and the only thing, which is needed in this case is
to kick it back into normal state, could help solving this kind of problems.