Hearing
Alexander Liss
Morphological description of hearing mechanism often
gets in the way of understanding of its functioning. Following provides
description of this mechanism from functional point of view.
Middle Ear and Inner Ear comprise one Resonating
System and reactions of this System to vibrations of bones and air are detected
and converted into a stream of electrical signals of various intervals passed
in parallel through a set of neural pathways.
Following are morphological Elements of the
Resonating System, which resonate not separately but in concert, while each
element has its own set of free vibrations:
·
Air
Cavity of the Middle Ear
·
A
Three Bones Assembly in the Middle Ear
·
Liquid
Filled Channel (spiral) of the Inner Ear
·
Sensing
Sealed Tube inside this Channel
·
Outer
Membrane separating the Middle Ear and Outer Ear.
Outer Membrane is connected to the Air Cavity and
the Three Bones Assembly and its vibrations induced by vibrations of air
outside the Outer Ear affect vibrations of entire Resonating System, not only
vibrations of the Air Cavity or the Three Bones Assembly.
The Liquid Filled Channel has:
·
one
end connected to the three bones assembly of the Middle Ear via an Oval Window,
with the end of one if these bones serving as a cover of this Window with three
degree of freedom of motion
·
Another
end connected to the Air Cavity of the Middle War via a Round Window with a
membrane separating them; this membrane has a large set of forms of free vibrations
·
Sensing
Sealed Tube is separated from the Liquid Filled Channel with flexible
membranes, which have a large set of forms free vibrations.
Two opposing walls of the Sensing Sealed Tube are
membranes separating it from the Liquid Filled Channel; two other opposing
walls are bones. Membranes are connected to walls and have a form of (twisted)
trapezoid.
External vibrations could reach the Sensing Sealed
Tube via air through the Liquid Filled Channel and via bones.
The Liquid Filled Channel is bent around
the Sensing Sealed Tube and has two compartments: one adjacent to one membrane
of the Sensing Sealed Tube and to the Oval Window, and the other adjacent to
the other membrane and to the Round Window.
The Liquid Filled Channel has a large set
of asymmetric forms of free vibrations, which vary more near the Round Window
and less near the Oval Window.
The membrane separating the Sensing Sealed
Tube and the Liquid Filled Channel at the Round Window is a Basilar Membrane;
it has sensors attached inside the Sensing Sealed Tube.
There are two muscles inside the Middle
Ear, which control the tension the Outer Membrane and the state of the Three
Bones Assembly. One muscle is large and attached close to the Outer Membrane
the other is small and attached to the middle of the Three Bones assembly.
Actions of these muscles affect forms of
free vibrations of the Outer Membrane. This alone affects the forms of free
vibrations of the Resonating System. In addition, they change the form of the Air
Chamber and hence the set of its free vibrations and the tension and the form
of the Three Bones Assembly and hence it’s set of free vibrations.
There are many reasons why the body would
like to modify the stream of sensory information:
·
Throttling
the intensity of the signal
·
Throttling
the information stream
·
Focusing
the sensory system on a particular type signals
·
Introducing
variability in a monotonous signal to keep focus on it.
Close analogy could be found in the way visual
signal is controlled. There is:
It is reasonable to assume that the large
ear muscle provides action similar to large scale eye movement and the small
ear muscle provides “scanning” to assure variability of the perceived signal.
Creatures, which do not have a second small
ear muscle, should have difficulties perceiving monotonous sounds and should
compensate with ear or head movement.
A resonating system with control could have
cases of Auto Vibrations when the control system produces a positive feedback.
It seems that tinnitus (nose in the ear) is the case of Auto Vibration, when
the control system focuses on the vibration not induced by the external
vibrations, and perpetuates this vibration.
The Sensing Sealed Tube is filled with
liquid and it houses a Sensory Mechanism.
It consists of the Basilar Membrane on which sits
one row of Inner Hair Cells and at some distance from this row a three cell
wide belt of Outer Hair Cells.
Hair Cells are sensors, Inner and Outer Hair Cells
are different types of sensors.
In the middle of the Sensing Sealed Tube is
a Ridge protruding from the bonny wall of the Tube. This Ridge is not cellular
tissue and Hair Cells have their other end “touching” it with various degree of
attachment.
The Basilar
Membrane is a (twisted) trapezoid with its broad end facing the Round Window.
Its edge is rigidly attached to the bone. There is one row of Inner Hair Cells
and a belt of tree rows of Outer Hair Cells. They provide information about
vibration of the membrane.
Its broad
end is more rigid than its narrow end, this causes the amplitude of vibrations
be of comparable size over the surface if the membrane. The consequence of this
is higher response of the broad end to higher frequencies of vibrations and
higher response of narrow end to low frequencies.
Amplitude
of vibration near the edge of the membrane is the smallest. This could be a
pattern reinforcing existence of two types of Hair Cells and their placement:
Inner at the edge of the membrane and Outer closer to the membrane’s middle.
Other
creatures have Hair Cells of different types and some have these sensors placed
over entire surface of the membrane.
Some of
types of Hair Cells have a bunch of hairs (stereocilia) on top; they do not
exhibit directionality in motion detection. However human Hair Cells have hairs
neatly placed in rows and this should cause clear directionality of motion
detection.
Hairs of
Inner Hair Cell are in the row parallel to the edge of the membrane; hence it
is most sensitive to displacement perpendicular to the edge:
Hairs of
Outer Hair Cell are in two perpendicular rows, which should cause more
unidirectional sensitivity:
Signals
from one row are added to signals from perpendicular row of hairs.
The Basilar
Membrane vibrates with high frequency (human ear easily detects 20,000 cycles
per second). Neurons could fire with frequency not more than 1,000 per second.
Hence, the assembly of Hair Cell and an attached neuron should work on the
principle of accumulation: a few cycles of stimulation of a Hair Cell cause its
firing and a few firing of a Hair Cell cause firing of the attached neuron.
Number of
cycles causing firing of the Hair Cell should vary depending on the amplitude
of stimulation: higher amplitude of stimulation should cause larger deformation
of “hairs” in the Hair Cell and quicker accumulation of ions in the cell.
Hence, higher amplitude should lead to more frequent firing of the Hair Cell.
In response, the attached nerve cell should fire more frequently.
Frequency
of the attached nerve firing characterizes the vibration in the given place on
a Basilar Membrane. The set of such data characterizes the shape of two
dimensional vibration of the membrane.
In
biological systems the increase of response is proportional to the increase of
the stimulus, i.e. amplitude of the Basilar Membrane vibrations.
If
r - a Hair Cell response,
a – amplitude of membrane vibration and
a0 - a threshold of sensitivity of the Hair Cell
then
r = log(a – a0), when a > a0
Outer Hair
Cells have unique ability to contract at command from the attached nerve cells
and on mechanical stimulation - motility.
Outer Hair
Cells move in space and have to adjust their height that their hairs are not
crashed by the Ridge. This could be the reason of their mechanical motility –
automatic height adjustment as a response to a mechanical stimulus.
Most
likely, the same mechanism used for dampening the monotonous signal, especially
when such contraction is caused by the nerve signals.
When Outer
Hair Cells contract, they become wider, this should make the Basilar membrane
stiffer and shift the resonance pattern.
Hairs have
to bend to generate signal, they cannot be “squashed” or “stretched” and they
are placed upright on a “flat” top of a Hair Cell.
Hence they
essentially detect an equivalent of two dimensional movement of a Hair Cell,
even when it moves in three dimensions.
For the
purpose of signal analysis, only projection of the movement of the top of the
Hair Cell on the surface of the calm Basilar Membrane matters.
Had the
Basilar Membrane been flat, one would say that only the projection on the plane
of Membrane matters, i.e. all variety of sounds is present in the family of
vibrations, where displacement of the Membrane is a transformation of the
Membrane in its plane. This set of vibrations is easier to analyze than one,
which allow movements of the Membrane outside its plane.
The sensor
detects vibrations of the Basilar Membrane and the Ridge. Different forms of the
Basilar Membrane vibrations cause different patterns of signals from the Hair
Cells.
To probe reactions
of the hearing mechanism, simple air pressure vibrations are used, which have a
front (a crest of the wave) of a simple form, usually a sphere, and changes in
time as a sine function.
This is
definitely a “monotonous” vibration, which perception could be quickly
suppressed by the system, unless it has a compensating mechanism as one described
above.
Properly
used, these probes are useful.
They cause
the Basilar Membrane resonate with a clear maximum of the amplitude, which depends on the frequency
of forcing vibration – the higher frequency the closer the maximum to a broad
end of the Basilar Membrane.
A major
property of hearing system is ability to differentiate vibrations with
different shapes of the front, to probe this ability one needs more
sophisticated set of probes.
Traditional
presentation of sound as one dimensional vibration is too simple to describe
this phenomenon. It reflects an important part of this presentation, and this
is a cause of its usefulness, but it is not sufficient.
Had it
been only one row of sensors – Inner Hair Cells, one-dimensional model would be
adequate. However, there are other sensors on the membrane and they provide
additional characterization of the vibration. There are theories claiming that
Outer Hair Cells serve only as sound amplifiers, but this is highly unlikely.
If one looks at the Basilar Membrane covered with sensors in Inner Ear of some
creatures, it is obvious that they are used to detect vibrations of a membrane,
a two dimensional object.
Hence,
traditional presentation of sound is a model, which describes the way the brain
perceives vibrations only partially. A new, more precise presentation is
needed.
For
description of the human perception of vibrations one need to add
differentiation provided by Outer Hair Cells.
For the
purpose of signal processing we assume:
The brain
receives two sets of signals – from Inner and Outer Hair cells.
Every
Inner Hair Cell responds to all sine probe vibrations, however the degree of
response depends on the frequency of vibration and the function plotting the
degree of response from frequency of vibration is unimodal and has a clear
maximum. This is similar to reaction of light sensors to light of different
wavelength.
An Inner Hair
Cell is determined by its place in a row. Degree of response of Inner Hair
Cells is a function of the place in the row. Hence, from Inner Hair Cells the
brain receives a set of signals similar to a function of one variable. This is
a continuous function with relatively small change in its value, when change in
its argument is small.
This
function is changing in time and this change is perceived as change of sound.
Similarly,
an Outer Hair Cell responds to all probe vibrations with varying degree.
One could
define a line along the belt of Outer Hair Cells and for a point in this line
find three corresponding cells. Degrees of their responses are a
vector-function of the place along this line.
This is a
continuous three-dimensional vector-function with relatively small change in
its vector-value, when change in its argument is small.
This
function is changing in time also and this change is perceived as change of
sound. This component of presentation should be sensitive to changes in the
shape of the front of vibration.
Creatures,
which have different types of Hair Cells or different placement of them on the
Basilar Membrane, have different perception of sound. When one tries to analyze
their reactions to sound or their sound based communication, one has to take
this in consideration – they might operate on a kind of vibrations, which is substantially different from one, which
humans used to analyze the surroundings and to communicate, and the difference
could be not in the frequency of vibration but in the change of the vibration
front, etc.
One should
be careful and not assume much from probing hearing of a creature with limited
set of sine vibrations with a fixed shape front.
Definitely,
when a creature has entire Basilar Membrane covered with Hair Cells, it
perceives vibrations differently.
Combining
Probe Sounds should cause the Basal Membrane to have a set of free vibrations,
which is a combination of sets of free vibrations corresponding to all Probes.
Amplitude
of vibrations of mechanical system (cavities, channels, membranes, etc.) should
be a sum of amplitudes of vibrations caused by individual Probes. However, the values
of functions comprising the Sound Presentation is not a sum of corresponding
values for Probes separately, because the increase of Hair Cells response is
proportional to the increase of the amplitude of the Basilar Membrane
vibrations – dependency is not linear.
Still, the
amplitude of vibration of the ear’s mechanical system is a good description of
the sound even when it does not reflect properly perception of sound loudness.
Hence, we could:
Even more,
we could allow this vector of amplitudes to change in time, because when change
is slow enough, the vibrations of the membrane should follow this changing in
time vector of amplitudes roughly proportionally and with only imperceptible
delay.
Hence,
traditional presentation of sound with its (slowly changing in time) spectrum,
allows analysis of sound close to the way it is perceived by the ear, if one
ignores perception of loudness.
Obviously,
the requirement of slow change is crucial. When the spectrum of sound is
changing fast (as it is with some calls of birds), the membrane does not have
time to adjust its pattern of resonance and the match between the spectrum and
perception of the sound dissipates.
Understanding
functioning of the hearing system allows immediate health improvement
recommendations.
If
Tinnitus is an Auto-Vibration caused by a vicious cycle in a perception
mechanism in the brain and in the mechanism of the Ear, then it has to be
interrupted simultaneously in both areas.
The
solution should be a quick relaxation of the perception system (psychological
relaxation), combined with listening to a pleasant sounds (music).
With age,
muscles of the Middle Ear become less efficient and this could lead to
difficulty of hearing monotonous sounds or compensating for low or high level
of the sound.
The
solution should be in regular exercise of these muscles, for example regular
listening to music.
Cochlea
Implant is an implant into the Sensing Sealed Tube, which provides direct
electrical stimulation of the nerves and possibly Hair Cells according to
externally detected sound.
From
presented above theory of cumulative signals, it is obvious that stimulation
should be in form of electrical pulses, which frequency should be proportional
to the intensity of revenant sound “component”, it should not be any continuous
periodic signal.
Two simple
modifications should substantially improve the quality of Hearing Aid Devices.
A Hearing
Aid Device is a sound amplifier.
First,
instead of collecting sound in one point it should collect it in a few points
and emit amplified sound in a set of corresponding points.
Second, it
has to have added functionality (which could be configured for each patient
separately), which allow periodic shift of sound frequencies back and force,
similar to one caused by muscles of the Middle Ear and Outer Hair Cells of the
Inner Ear.