Download The human eye and the sense of sight. Structure Anatomy and

The human eye and the sense of sight.
Structure
Conjunctiva
Cornea
Sclera
Choroid
Retina
Iris
Lens
Aqueous humour
Vitreous humour
Ciliary body
Optic nerve
Anatomy and Function
Continuation of the epidermis of the skin. It protects the cornea
at the front of the eyeball from friction.
Transparent to light. It refracts light to help form an image on
the retina.
The white of the eye: a tough coat of fibres. It protects the
eyeball against mechanical damage and helps to maintain the
shape of the eyeball.
A membrane containing pigment and blood vessels. It nourishes
the retina and prevents internal reflection as it is black and
absorbs light.
Contains light-sensitive receptor cells connected to sensory
neurons. The retina detects light.
A pigmented muscular structure that contracts and dilates to
adjust the amount of light entering the eye.
A flexible, transparent structure which allows light to enter the
back of the eye. It refracts light to allow fine focusing of an
image onto the retina.
A watery fluid that helps to maintain the shape of the eye.
A jelly-like fluid that helps to maintain the shape of the eye.
Contains muscles. It supports the lens and alters the shape of the
lens.
Consists of bundles of sensory neurons. It transmits impulses
generated in the retina to the brain.
Humans use the sense of sight to interpret much of the world around them. What we see is
called “light”. However, humans only see a small part of the entire “electromagnetic
spectrum.” Humans can see only the wavelengths of electromagnetic radiation between
about 380 and 760 nanometres because our eyes do not have detectors for wavelengths of
energy less than 380 or greater than 760 nanometres. Thus we cannot “see” other types of
energy such as gamma or radio waves. Rattlesnakes, however, can detect electromagnetic
radiation in the infrared range and use this ability to find prey.
Type of
animal
Vertebrate
Name of
Part of electromagnetic
Wavelengths detected
animal
spectrum detected
Human
Visible
700-400 nm
Rattlesnake
Infra-red and visible
850-480 nm
Japanese
Ultraviolet and visible
As low as 360 nm
dace fish
Invertebrate
Honeybee
Ultraviolet and visible
700-300 nm
Mantis
Ultraviolet and visible
640-400 nm
shrimp
Humans can detect visible light wavelengths in the range from 380-760 nm. Invertebrate
animals have visual sensitivities close to humans but some insects can detect light in near
ultra-violet.
Nocturnal animals such as the rattlesnake are better hunters during the night because their
prey cannot see them; however the snake is able to detect the prey’s body heat as infrared.
Similarly, deep-sea angler fish have no available light and use bioluminescence to attract
prey.
Humans see only a limited range of the electromagnetic spectrum because it is all that is
necessary to their survival. Other animals have different needs to humans and thus they have
adapted to suit these requirements. Snakes hunt at night for food, thus they are able to detect
the infrared body heat of their prey even though they cannot see the prey visibly.
Ocean-dwelling organisms such as species of coral reef fish and some crustaceans are able to
detect UV light. This can enhance the image that the organisms see, creating more contrast
and thus allowing the organism to see more detail as necessary. As humans do not hunt for
prey at night nor live underwater, we do not have the need to see UV or infrared light.
In order to be seen an object reflects light, generates its own light or transmits light to
our eyes. When light moves from one substance or medium to another, it is bent, or refracted.
The speed at which the light is travelling also changes. The movement of light through a
denser medium is slower and is thus refracted to a greater degree. When light is passed
through a convex lens the rays are refracted toward a central point known as the focal point.
The rays then cross over and diverge from that point. If a screen is placed in the pathway of
the diverging rays, the resulting image is upside down or inverted.
The density of the cornea, aqueous humor, lens and vitreous humor are similar to each other
and all refract light that passes through the eye. The refractive power of air through which
light travels to reach the eyes of terrestrial mammals is lower than the refractive power of
parts of the eye. Therefore, the greatest degree of refraction in the human eye occurs when
light moves into the cornea, since the change in refractive power is at its greatest point the
greater the difference in the refractive power of two media, the more the light is refracted
when it passes from one medium to the other.
The process of the eye being able to focus on objects at different distances is known as
accommodation. Accommodation is achieved by muscles that change the shape of the lens.
Ciliary muscles surrounding the lens contract and the ligaments linking the muscles to the
lens slacken. As a result the lens thickens and causes the light to be focused so we can see
objects that are far away. If the ciliary muscles relax the ring increases in diameter and the
ligaments tighten, stretching and thinning the lens. As a result light is focused on the retina
from objects that are close to us. As the lens changes shape the incoming light rays are
focused onto the retina. The process of accommodation is important because it allows us to
see things clearly at different distances.
DISTANT VISION: when the eye is looking at distant objects, light travels in straight lines.
This light is focused on the retina by the lens in its resting state. The lens is quite flat and at
its lowest strength or refractive power. This means that there is very little refraction or
bending of light as it passes through the lens. The ciliary muscles are relaxed and tension in
the attachments from the lens to the ciliary body keep the lens thin.
Above: The object is far away and the biconvex lens is elongated to slightly converge rays.
The light rays are almost parallel.
NEAR VISION: when the eye is looking at close objects, the light rays tend to diverge as
they reach the eye. This means that the refractive power of the lens must be increased,
achieved by the lens becoming more convex: bulging outwards. The contracting of the ciliary
muscles causes the bulging of the lens; hence the image is focused on the retina.
Above: the light rays diverge from the close object. Highly rounded biconvex lens converge
light rays – the focal length is longer.
Accommodation is important to allow clear vision. If the lens could not change curvature, the
image would not be focused properly, resulting in a blurred image and hampering visual
communication.
DISTANT VISION: the curvature of the lens must be relatively flat. When the ciliary
muscles are relaxed they hold the suspensory ligaments, pulling on the lens and keeping it
relatively flat (elongated lens) and allowing the image of distant objects to be focused on the
retina, as light rays from distant objects tend to be parallel. Light rays are not greatly
refracted when the lens is elongated, or slightly convex.
NEAR VISION: the curvature of the lens must be increased; a thicker lens has greater
refractive power and a shorter focal length. The ciliary muscles thus contract, causing the
suspensory ligaments to slacken. As a result, the lens becomes rounder (its curvature
increases), known as a highly convex lens, refracting the light to a greater degree and
allowing a focused image to fall on the retina.
Therefore, the refractive power of the lens changes from low (flatter lens) when at rest, to
high (rounder lens) at maximum accommodation.
The eyes vary in shape and size from person to person, and these are often hereditary. If the
cornea or lens is not the right shape, or the eyeball is too elongated or too round, the ability of
the eye to refract light and focus it accurately onto the retina is affected. If light is not
accurately transmitted it can result in the weakening of clarity of sight. Difficulties in seeing
are called visual defects and include myopia (short-sightedness) and hyperopia (longsightedness). They are not usually due to disease but as a result of how the body grows.
Myopia is when a person can see near objects clearly but distant objects appear
blurred. Light
from distant objects is brought
into focus at a
point in front of the retina
surface as a result
of an elongated eyeball.
Myopia can be corrected with concave lenses in spectacles or contact lenses ‘Myopia is best
treated with eyeglasses’ (MED networking for health). The concave lens diverge the light
before it reaches the eyes so that the objects in the light path are brought into focus on the
retina.
Hyperopia results from a short eyeball or poor accommodation ability in the lens. It is
the condition in which a person can see distant objects clearly but closer objects appear
blurred. Close objects are focused behind the retina and thus are not clear.
Hyperopia can be corrected with spectacles or contact lenses with convergent lenses, so the
light is converged more strongly for close vision.
Other technologies to correct these visual defects include:
-
-
RADIAL KERATECTOMY: Fine surgical instruments shave small amounts off the
corneal surface, thus refractive power is altered
PHOTO-REFRACTIVE KERATECTOMY: involves the removal of the epithelium
(outer membrane) and the surface of the cornea. The laser is used to shape the
uppermost surface of the cornea.
LASER SURGERY: lasers are used to shave the corneal surface, thus refractive
power is altered.
Depth perception is the sense of depth that occurs when objects are viewed with binocular
vision, dependent on the fact that a person has stereoscopic vision
Depth perception occurs due to the angling of the eyes. The angle of the eyes is signalled to
the brain and is used to judge distance. In the brain the two images of each eye are combined
to give stereoscopic vision. When the eyes face forward each eye sees an image of an object
in the light path. The two images are fused into one image in the cerebral cortex of the brain.
The brain uses slight differences in the images to interpret distances. The person’s eyes are
separated and thus have slightly different views of objects located different distances away.
When an object is a slightly different distance from each eye, it is imaged by each eye at a
different distance from each fovea. This gives the perception of depth as this image is fused
and seen to be a different distance from the eye to another object that is closer to the eye. The
two objects are focused in different places on the retina and thus are seen as two images in
their respective positions so there is depth to the picture that is perceived.
The innermost coat of the eyeball, the retina, is a thin sheet that consists of several
layers of nerve cells, one of which is the layer of visual receptors the rods and cones. Of all
nerve cells in the retina, only the rods and cones respond directly to light, hence the name
photoreceptors.
The rods and cones are the last layer of cells in the retina that light reaches. The
photoreceptors generate impulses which travel along the various neurone layers of the retina
to the optic nerve, which carries signals to the brain.
There are five main layers of nerve cells (neurones) that are directly involved in the
transmission of impulses in the retina:
PHOTORECEPTOR CELL LAYER: the rods and cones that, when stimulated by light,
perform 3 main functions – 1) absorb light energy (involving visual pigments)
2) convert light energy into electrochemical energy, generating
a nerve impulse
3) transmit the impulse towards the bipolar layer.
HORIZONTAL CELL LAYER: occurs at the junction between photoreceptors and bipolar
cells. They connect one group of rod and cone cells with another and then link them to
bipolar cells.
BIPOLAR CELL LAYER: these sensory neurones receive electrochemical signals from the
rods and cones and transmit the signal to the next layer.
AMACRINE CELL LAYER: occurs at the junction between bipolar and ganglion cells.
GANGLIAN CELL LAYER: these neurones receive electrochemical signals from the bipolar
cells. The distal end of ganglion cells is extended into long processes that go on to form the
fibres of the optic nerve. These neurones are responsible for carrying signals from the retina
to the brain.
Studies suggest that horizontal and amacrine cells are involved in processing, or
“summarising” incoming visual information. Most of the interpretation of visual stimuli
occurs in the brain, based on variables such as: strength of light, depth perception and the
number of rods/cones
The retina’s two types of photoreceptor cell - the rods and cones each contain different
pigments that allow them to absorb different wavelengths of lights. Both rods and cones are
elongated cells that contain an outer segment joined to an inner segment that leads to the
conducting part of the cell.
Rhodopsin is the only pigment present in rods which allows rods to only detect black and
white light.
Cones contain iodopsins, of which there are 3 types, each sensitive to different wavelengths,
and thus cones are responsible for colour vision.
The role of visual pigments is to absorb light energy, which the rod or cone cell then converts
to an electrochemical signal the brain can interpret. Rods are scattered around the perimeter
of the retina, but are absent from the fovea. As a result, rods are responsible for most
peripheral vision, including the detection of movement. They are extremely sensitive to light,
responding best to low light intensities. They are used for night vision and to detect light and
shadow contrasts.
Cones are distributed in groups throughout the retina, mostly being concentrated in the
macula, an area of the retina that gives the central 10° of vision. The fovea is a small pit in
the middle of the macula that contains densely packed cones only. They are less sensitive to
light than rods, functioning best in high intensity light, giving daytime vision.
When light enters a rod cell, it splits rhodopsin molecules into its two components. This
reaction results in an impulse in the neurone attached to the rod or cone. The two products
slowly recombine, ready to be split again by more light. This is known as the visual cycle.
The main function of the photochemical rhodopsin is to absorb light in order to set off a
series of biochemical steps to carry a signal to the brain.
Each cone contains one of three types of iodopsin pigments and is therefore most sensitive to
light in one of three wavelengths. These pigments result in cone cells being sensitive to:
-The short wavelengths of blue light. Peak sensitivity approx 455nm
-The medium wavelengths of green light. Peak sensitivity at approx 530nm
- The long wavelengths or red light. Peak sensitivity being at approx 625nm
By comparing the rate at which various receptors respond, as well as the overlap in colours
detected, the brain is able to interpret these signals as intermediate colours. Because cones
detect colour, any defect or damage to the cones will affect the ability or the eye to perceive
colour. A mutation in a gene that codes for a cone pigment leads to the inability of this
pigment to function correctly. As a result, the person is unable to perceive colour in the
normal manner and is said to be either colour deficient or colour blind, depending on how the
mutation affects the pigment.
A person that is deemed to be ‘colour blind’ is not truly colour blind but is usually able to see
only two of the three primary colours of light. As they are unable to detect one of the colours
that normal person that sees three primary colours can, they perceive colour differently and
interpret all colours based on combinations of the two primary colours that they are able to
see.
The human ear and the sense of sound
Structure
Pinna
Anatomy
Large, fleshy external part of the
ear
Function
Collects sounds and
channels it to the ear. It
does so by acting as a
funnel amplifying the sound
and leading it to the ear
canal.
Tympanic membrane
The eardrum – a membrane that
Vibrates when sound waves
stretches across the ear canal
reach it and transfers
mechanical energy into the
middle ear.
Ear ossicles
Three linked movable tiny bones: Amplify the vibrations from
hammer, stirrup and anvil
the tympanic membrane.
Convert the sound waves
striking the eardrum into
mechanical vibrations.
Oval window
Region that links the ossicles of the Picks up the vibrations from
middle ear to the cochlea in the
the ossicles and passes them
inner ear. One of the membraneon to the fluid in the
covered outlets into the air-filled
cochlea
middle ear
Round window
Membrane between cochlea and
Bulges outwards to allow
middle ear. The other of the
pressure differences in the
membrane-covered outlets into the
cochlea
air-filled middle ear
Cochlea
Circular spiral shaped chamber
When vibrated causes a
filled with fluid
nerve impulse to form
resulting in the changes of
mechanical energy into
electrochemical energy
Organ of Corti
A structure within the cochlea. A
Location of the hair cells
sensitive element in the inner ear.
that transfer vibrations into
Situated in one of the three
compartments of the cochlea.
Auditory nerve
The nerve that travels from the ear
to the brain. A bundle of nerve
fibres.
Eustachian tube
Connects the middle ear with the
throat. It is usually kept closed but
opens when we swallow or yawn.
electromagnetic signals.
Can be seen as the bodies
microphone.
Transmits electrochemical
signals to the brain. Carries
hearing information through
the cochlea to the brain.
As air can pass through the
opening, the pressure
between the middle ear and
the atmosphere can be
equalised.
http://www.medindia.net/animation/ear_anatomy.asp
Sound bends around objects and travels around corners. It can travel through substances,
solids, liquids and gases. Whatever the habitat, an animal is always surrounded by a soundtransmitting medium ‘Sound is also useful both day and night. It travels over long distances
and can go around corners. The sender does not have to be visible to the receiver’. (HSC
biology online).
Sound enables animals to communicate without being in visual or direct contact.
When visual, tactile and olfactory senses are impaired or absent, sound can be used as the
primary method of communication. A variety of sounds may be produced by varying the
pitch, loudness and tone. A complete message can be conveyed quickly. Sound, particularly
low-frequency sounds, will also travel long distances.
Sound is a useful form of communication because it is readily produced by vibrating objects
and can effectively travel through material to a receptor. The structure of the ear is such that
the vibrations can be received and the differences in frequency detected by the hair cells in
the organ of Corti. This occurs to give versatility in the sending and receiving of messages
using sound.
Toothed whales and bats use a form of sound communication called echolocation, whereby
the animal emits sounds and listens for the echo to come back to them. This type of SONAR
(Sound Navigation Ranging) works well even in complete darkness. By this process, killer
whales are able to judge distance, direction, size, shape and speed of objects in water.
‘Millions of years before humans invented sonar, bats and toothed whales had mastered the
biological version of the same trick – echolocation’ (discover magazine)
Sound originates when something vibrates rapidly enough to organise the movement of
molecules, so as to send a compression wave through a medium. The wave can only travel
through media which contain particles that can be compressed (this process is known as
compression according to the Success One HSC Biology guide) and spread (The spread of
these particles is known as rarefaction according to Success One HSC Biology guide). The
particles move backwards and forwards in the same direction as the flow of energy. It is
energy that is transferred, not molecules. The frequency of the vibration of the medium
molecules is the same as the frequency of the vibrating object. The frequency of vibrations is
the number of waves which pass a given point in one second, expressed in cycles per second
(one cycle being called a hertz, Hz). Low-frequency sounds have long wavelengths while
high-frequency sounds have short wavelengths. The amplitude of a sound wave is the
maximum distance that a particle moves from its original position. The amplitude determines
the volume of a sound.
The larynx, or voice box, is located in the throat
between the fourth and sixth vertebrae. The function
of the larynx is to produce sound. Inside the larynx
are the vocal cords, which consist of muscles that can
adjust pitch by altering their position and tension.
The larynx forms part of the trachea, the passageway
that leads from the mouth to the bronchi and lungs.
When air passes over the vocal cords in the larynx,
they produce sounds that can be altered by the
tongue, as well as with the hard and soft palate, teeth, and lips.The larynx contains an
arrangement of nine different cartilage that connect the vocal cords that extend across the
trachea opening. The vocal chords together with the entire larynx surround a narrow opening
in the trachea named glottis. The sounds are then formed when air is pushed through the
glottis which causes the elastic fibres to vibrate. The sound changes pitch depending on the
length and tension that is created in the vocal cords. The tighter the vocal chord tension, the
faster they will vibrate resulting in the higher pitch of sound produced. The glottis requires a
narrow slit to form for the sound to be high pitched. Deeper sounds are the opposite, resulting
from less tense and longer vocal chords with a wide glottis opening. The volume of the sound
is a different feature of sound than pitch. Loudness of sound is determined by the force which
the air is pushed past the cords from the lungs.
‘Hearing is the ability to detect sound waves by changing the vibrations of sound to electrical
energy of nerves. Hearing is only found in vertebrates by arthropods’ (EXCEL HSC
Biology).
INSECTS: The tactile bristles on an insect’s exoskeleton and on its antennae respond to low
frequency vibrations, though many insects possess more specialised structures for hearing.
Orthopterans (such as crickets) have a tympanum or ear on each leg just below the knee. The
tympanum is a cavity containing no fluid, enclosed by an eardrum on the outer side and a
pressure release valve on the other. Nerve fibres are connected to the eardrum and pick up
vibrations directly. Female crickets are deaf to some frequencies and sometimes rely on
smells given off by males.
Cicadas possess a pair of large tympana connected to an auditory organ at the base of their
abdomen. When a male cicada sings (as females don’t), he crinkles his tympana to prevent
deafening himself.
FISH: The hearing abilities of fish vary between species. All fish have a lateral line, a pair of
sensory canals, which run the length of each side of the animal. Pressure waves in the
surrounding ocean distort the sensory cells in the canals, sending a message to the nerves.
Some fish actually perceive sound waves by possessing an inner ear containing an otolith (ear
stone) which is lined with hair cells. Auditory nerves detect the differences in vibrations
between the hair cells and the otolith and send a message to the brain.
Fish also have an air-filled swim bladder, located in the abdomen, which vibrates in response
to sound or vibrations.
MAMMALS: Killer whales have an acute sense of hearing. Sound is received by the lower
jawbone, which contains a fat-filled cavity extending back to the auditory bulla.
Sound waves are received and conducted through the lower jaw, middle ear, inner ear and the
auditory nerve to the auditory cortex of the brain.
Dolphins close their ear canals when diving. They detect vibrations through special organs in
the head and some low frequency sounds through the stomach.
Structures used to
detect vibrations
Receptor cells
Insects
Tympanic
membranes, sensory
hairs
Mechanoreceptor
cells
Fish
Lateral line, inner
ear, swim bladder
Mammals
Cochlea
Hair cells in the
inner ear,
neuromasts in
lateral line
Hair cells in Organ
of Corti
The Eustachian tube connects the middle ear to the pharynx which is the chamber of the back
of the mouth and nose. Usually this opening is kept closed, but it opens when we swallow or
yawn.By permitting air to leave or enter the middle ear, the tube equalizes air pressure on
either side of the eardrum.
This diagram outlines the path of a
soundwave through the external,
middle and inner ear and identifies the
energy transformations that occur.
Passing along the length of the cochlea is a ribbon-like structure, the organ of Corti. This has
three main components: the basilar membrane, hair cells and the tectorial membrane. The
basilar membrane is composed of transverse fibers of varying lengths. Vibrations received at
the oval window are transmitted through the fluids of the cochlea causing the transverse
fibers of the membrane to vibrate at certain places according to the frequency. High
frequency sounds cause the short fibers of the front part of the membrane to vibrate and low
frequency sounds stimulate the longer fibers towards the far end. As the basilar membrane
vibrates, the hairs of the hair cells are pushed against the tectorial membrane. This causes the
hair cells to send an electrochemical impulse along the auditory nerve to the brain. The region
of the basilar membrane vibrating the most at any instant sends the most impulses along the
auditory nerve.
The actual perception of pitch depends on the mapping of the brain. Nerves from particular
parts of the organ of Corti stimulate specific auditory regions of the cerebral cortex of the
brain. When a particular part of the cortex is stimulated, we perceive a sound of a particular
pitch.
Some organisms use their two ears to judge the position from which a sound comes. They can
move each ear independently until each ear receives the maximum sound. Humans cannot
move their ears, but can locate the direction of a sound nevertheless. This is because the
sound is heard more loudly by the ear nearest to it and also fractionally earlier.
The pinna is mostly skin and cartilage with some muscles attached to the back, which is what
allows some animals to "wiggle" their ears. The brain uses reflections from the twists and
folds of the pinna to determine the direction of sounds. Sounds coming from the front and
sides become enhanced as they are directed into the auditory canal while sounds from behind
are reduced. This helps an animal to focus on what they want to hear while reducing noise in
the background.
When sound waves are coming from directly in front, behind or above the head, both ears
receive the sound waves equally and the sound will be the same for both ears. When sound is
coming from one side, the receptors in the ear closest to the sound will be stimulated slightly
earlier and also more intensely (because the sound energy is less dissipated). The brain then
locates the sound as coming from one side of the body. The head is said to cast a sonic
shadow on the sound coming into an ear from the opposite side of the body.
Technology involved in improving hearing
Hearing aids and cochlear implants are both devices designed to improve deafness.
A hearing aid is an electronic, battery-operated device that amplifies and changes sound to
allow for improved communication. A hearing aid will not assist hearing if the nerve endings
are damages within the ear. Hearing aids receive sound through a microphone, which then
converts the sound energy to electrical energy. The amplifier increases the loudness of the
signals and then converts the electrical energy back to sound. This sound leaves the hearing
aid through a speaker which directs the sound down the auditory canal. Most hearing aids are
placed in or near the external auditory canal.
Hearing Aids are particularly beneficial in patients who have damage to their middle
or outer ear. They are often used in people
where the auditory nerve or hair cells are
damaged due to ageing, noise, injury,
infection or an inherited condition.
Hearing Aids can only be used on patients
who still have a certain degree of residual
hearing. Hearing aids will not restore
normal hearing or eliminate background
noise. Hearing Aids are used in those
whose lack of hearing is affecting their
daily lives. They are relatively cheap
compared to cochlear implants, they are
also invisible, unobtrusive, removable,
have no physical side effects and do not
require programming or surgery.
Above: is an example of a hearing aid
A cochlear implant is a small, complex electronic
device that can help to provide a sense of sound to a
person who is profoundly deaf or severely hard of
hearing. It bypasses damaged parts of the inner ear
and electronically stimulates the auditory nerve. Part
of the device is surgically implanted in the skull
behind the ear and tiny electrode wires are inserted
into the cochlea. The other part of the device is
external and has a microphone, a speech processor
which is used to convert sound into electrical
impulses, and connecting cables.
‘When used with a microphone and speech
processor it electrically stimulates the auditory nerve
and results in the person being able to hear sound’
(power house museum).
The diagram below demonstrates the structure and
function of the parts of a cochlear implant.
Processor
•sounds are picked
up and sent to the
speech processor
Microphone
•codes the sounds into
an electrical signal
which is sent via a
cable to the coil
•passes the signal
through the skin
cia radio waves
to the implant
Transmitting
coil
Implant
•transforms the signal
into electrical impulses
that then stimulate
cochlea nerves and are
then detected by the
brain to be sound
An implant does not restore or create normal hearing. Instead, it can give a deaf person a
useful auditory understanding of the environment and help him or her to understand speech.
Unlike a hearing aid which amplifies sound, cochlear implants compensate for damaged or
non-working parts of the inner ear. It electronically finds useful sounds and then sends them
to the brain. The person may also have to use the implant in conjunction with lip reading.
Cochlear implants are used where permanent damage has occurred to the inner ear or a
patient has a congenital problem which results in severe to profound, hearing loss. Implants
are only recommended once hearing aids have been tried because they are permanently
implanted into the inner ear. Also cochlear implant technology has seen the greatest
improvement of hearing in those who have the implant installed before the age of 5 or in
those that have lost their hearing after learning to speak.
Position
Type of energy
transfer occurring
Conditions under
which it assists
hearing
Advantages
Hearing Aid
Worn close to the external ear
with speaker directed down into
the auditory canal.
Sound to electrical energy then
to amplified sound energy
Damaged outer or middle ear.
When hair cells have been
damaged by aging, noise, illness,
injury, chemicals or an inherited
condition
- Relatively inexpensive
- No surgery required
Cochlear Implant
Implanted in the skull just
behind the ear. Wires attach to
the cochlear. Also has external
microphone and speech
processor.
Sound to electrical signal
Damaged inner ear or auditory
nerve. Used when a person has
profound deafness.
- Provides some hearing to
profoundly deaf people
- Restores hearing after injury
Limitations of the
- Does not restore normal
- Surgery is expensive
technology
hearing
- Possible post-operative side
- Also amplifies background
effects (eg. facial nerve
noise
damage,
- Need to adjust sound level
infection)
- May be uncomfortable or cause - Need to learn to interpret
pain
sounds
Based on the information here, it can be seen that a person with hearing loss associated with
age or injury would find it more beneficial to use a hearing aid as it is more effective, the cost
would be less and there is no need for surgery. However, a person who is unable to hear due
to damage to the inner ear or part of the auditory nerve would require a cochlear implant as a
hearing aid would have no effect.
http://blogs.discovermagazine.com/notrocketscience/2010/01/25/echolocation-inbats-and-whales-based-on-same-changes-to-same-gene/
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(J. Bailey) 2008(accessed 22/6/10)
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(D. Amoas) 2006(accessed (4/7/10)
http://home.comcast.net/~wnor/lesson11.htm
(unknown) 2005 (26/6/10)
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