Auditory System
Assist. Prof. A.A. Maharramov
Auditory System
Auditory system
Auditory system
Auditory system
Auditory system
Auditory system
Vestibular System
Human Auditory System
The Auditory System
Outer Ear
• The outer ear is comprised of the pinna
(given the Latin origins, the plural is
pinnae) and external auditory canal (also
called the external auditory meatus). The
eardrum (also called the tympanic
membrane) marks the transition
between the outer ear and the middle
ear.
Anatomy of the Auditory System
The hearing structures of the ear are divided into three
parts:
The outer ear consists of the external ear structure,
called the pinna, and the external ear canal.
The middle ear contains the tympanic membrane, or
eardrum, and three small bones known as the ossicles.
The inner ear contains the cochlea and connected bony
structures, called the "semicircular canals."
These structures work together to transmit sound from
outside the ear to the auditory nerve, which send the
information as an electrical impulse to the brain, where it
can be processed and assigned meaning.
Outer ear
Main article: Outer ear
The folds of cartilage surrounding the ear canal
are called the pinna. Sound waves are
reflected and attenuated when they hit the
pinna, and these changes provide additional
information that will help the brain determine
the direction from which the sounds came. A
single malfunction of an organ in the ear can
cause a person to become deaf.
The sound waves enter the auditory canal, a
deceptively simple tube. The ear canal
amplifies sounds that are between 3 and 12
kHz. At the far end of the ear canal is the
eardrum (or tympanic membrane), which
marks the beginning of the middle ear.
Middle ear
Main article: Middle ear
Sound waves traveling through the ear canal
will hit the tympanic membrane, or eardrum.
This wave information travels across the airfilled middle ear cavity via a series of delicate
bones: the malleus (hammer), incus (anvil) and
stapes (stirrup). These ossicles act as a lever
and a teletype, converting the lower-pressure
eardrum sound vibrations into higher-pressure
sound vibrations at another, smaller
membrane called the oval (or elliptical)
window. The malleus articulates with the
tympanic membrane via the manubrium,
where the stapes articulates with the oval
window via its footplate. Higher pressure is
necessary because the inner ear beyond the
oval window contains liquid rather than air.
The sound is not amplified uniformly across
the ossicular chain. The stapedius reflex of the
middle ear muscles helps protect the inner ear
from damage. The middle ear still contains the
sound information in wave form; it is
converted to nerve impulses in the cochlea.
Main article: Inner ear
The inner ear consists of the cochlea and
several non-auditory structures. The cochlea
has three fluid-filled sections, and supports a
fluid wave driven by pressure across the basilar
membrane separating two of the sections.
Strikingly, one section, called the cochlear duct
or scala media, contains endolymph, a fluid
similar in composition to the intracellular fluid
found inside cells. The organ of Corti is located
in this duct on the basilar membrane, and
transforms mechanical waves to electric
signals in neurons. The other two sections are
known as the scala tympani and the scala
vestibuli; these are located within the bony
labyrinth, which is filled with fluid called
perilymph, similar in composition to
cerebrospinal fluid. The chemical difference
between the two fluids (endolymph &
perilymph) is important for the function of the
inner ear due to electrical potential differences
between potassium and calcium ions.
Human factors involving the auditory system
Virtual reality attempts to offer realistic stimuli
for all human senses. Given the current state
of technology, the details of realism are often
trade-offs for real-time management of the
system components. The auditory system is no
exception. While the human ear is capable of
hearing a multitude of distinct sounds, the ear
can only concentrate on listening to one
particular sound at a given time.
This physiological constraint has lead to studies
in "auditory cognition". Auditory cognition
analyzes such issues as attending to auditory
events, remembering and recognizing sound
sources and events, and perceptions of
acoustic sequences. The theories behind
auditory cognition attempt to explain how the
brain processes and/or filters out certain
sounds.
These studies are useful in Virtual Reality systems, because they allow the
developers to mimic selective sounds in the background while the user
attends to a specific important sound. For example, if the virtual environment
imitates the city of New York, the sound generators need not orchestrate the
sounds of taxi horns, subway noise, the rustling of people, and other typical
background noise. These may be merged as one sound that is played in the
background, since the user may not want to attend to any individual sound in
this sound cluster. However, the user may be interested in hearing his name
being called among the noise. The layering of this sound over the background
noise is a sound to which the user would selectively attend. In this example,
the sound of interest (i.e. the name being called) would require the use of 3D
sound. However, the background noises need not take advantage of the 3D
sound capabilities, since these sounds seem to surround the user, and the
user does not consciously attempt to locate the sound source of each
individual noise.
This section provides some sample theories on
how sounds of interest can be made to be
attention-holding in the virtual environment.
Broadbent's Theory
Broadbent's Filter Theory of Attention has
been used as the basis for most selective
attention models. In his theory, sound
information is passed through a number of
sensory channels. These "channels" are
vaguely defined as having some distinct neural
representation in the brain. Their
representations may be based on a number of
sound attributes, such as pitch, loudness or
spatial position characteristics. Broadbent
postulates that the sound channels lead into
the short-term memory portion of the brain,
where a particular channel may then be
filtered based on the desired sound attributes.
This filter allows only one of the channels to
lead to the long-term memory store and any
output mechanisms necessary to respond to
the input channel.
This theory arose from the cocktail party
situation in which a guest must filter out all
distracting sounds to concentrate on one
conversation. Several studies have suggested
different conditions under which the filter
switches to listen to a different channel. For
instance, when your name is heard in the
middle of a current conversation shows some
priority over which channel is filtered. Some
experiments have suggested that the switching
time of the filter between channels is to be of
the order of 0.25 seconds [Moray, 1970].
Treisman's Theory
Treisman's theory modifies Broadbent's theory,
by proposing that the input selections bypass
the short-term memory area of the brain to
arrive immediately to a filter which is sensitive
to a sound's physical characteristics. This filter
eliminates most unattended sound channels,
but allows a subset of channels to enter a
series of nodes in the brain, known as
"dictionary units". This network of dictionary
units becomes a pattern matcher, where
similar-sounding stimuli trigger signals to the
listener's output activity mechanism in the
brain.
Deutsch and Deutsch's Theory
Deutsch and Deutsch's Response Selection
Theory of Selective Attention alters Treisman's
theory by omitting the initial filter for physical
characteristics. Within the dictionary network,
each signal is analyzed and recognized for its
importance. The importance of the signal fires
a proportional signal to the brain's output
activity mechanism. Hence, the sound that
captures the listener's attention is the sound
that bears the heaviest signal. Applying this
theory to hearing your name during a
conversation shows that tuning into your name
when it is called has importance.
INTENSITY OF SOUND
• I = 10 log(I/I0)
• I0 = 3x10-5W/m2