Download FIAT 8 - UCLA Statistics

Cochlear Implants
STATS 19 SEM 2. 263057202.
Talk 8.
A.L. Yuille.
Dept. Statistics.
[email protected]
Nerve Cells and the Brain:
• The brain is made up of two types of cells.
• Neurons and Glial Cells.
• Neurons are the basic elements of the nervous
•
•
system. Information travels along them by
electric impluses.
Neurons transmit to other neurons by synapses.
Glial cells play a supporting role. Maintain brain
structure. Do not conduct electricity.
Nerve Cells and the Brain.
• Neurons and
•
•
Networks of Neurons.
Brain has
100,000,000,000
neurons.
Each communicates
with several 1,000
other neurons.
Nerve Cells and the Brain
• Number of nerve cells
•
•
•
100,000,000,000 is about the
number of trees in the Amazon
rainforest.
Number of nerve cell connections
1,000,000,000,000,000 is about
the number of leaves in
rainforest.
Number of connections in the
world’s telephone system (biggest
machine on the planet).
Brain is the most complicated
system in the Universe.
Magnetic Resonance Imaging (MRI)
• MRI can get 3D pictures of
•
•
•
organs in the brain – by
placing patient in a huge
magnet.
Hydrogen atoms (2/3 body)
absorb and give off magnetic
energy.
Calculate images from
variations in how this magnetic
energy is absorbed and
emitted in the body.
Low energy, non-invasive.
Functional Magentic Resonance
Imaging (fMRI).
• Use fMRI to measure
•
•
blood flow in the brain.
Active areas require more
blood. fMRI can measure
the oxygen content of the
blood.
Molecules in the blood
cells respond differently
to the magnetic field
depending on how much
oxygen they are carrying.
Language and the Brain.
• Moghul Emperor Akbar Khan (17th
century).
• Put 12 babies with dumb nurses in a
castle.
• At age 12, brought them to his palace –
and found they did not speak any
language at all.
Language and Grammar
• Linguistics: rules of syntax (grammar)
operate independent of meaning
(semantics).
• Chomsky’s claim: people have an innate,
universal, system of syntax which makes
laws governing elementary sentences.
• “Colourful green ideas sleep furiously” –
grammatical, but not meaningful.
Language and the Brain
• Left-hemisphere dominant for language.
(strongest for right-handed males).
• Broca’s area. Identified in 1861. Damage
to area impairs speech production.
• Speech production: but also involves some
speech comprehension. (Syntactic?)
• Wernicke’s area. Identified 1873. Damage
to it affects speech comprehension.
• But also production. (Semantic?)
Cochlear Implants.
• The Most Successful Implant Device.
• Provides good artificial input to the
auditory system, but does not solve the
Cortical Part of Language.
• A pre-linguistically deafened child or adult
find it very hard to use them.
• First six years are crucial to language
acquisition and usage.
Cochlear Implants
• Can probably train completely deaf
humans to hear, provided it is done early.
• Experiments on kittens than are born deaf.
• Implants given at age 3-4 months.
• Several months of training, kittens
behaviour showed they “hear” sounds
normally. (Brain activity confirms this).
Cochlear Implants
• 500,000 Americans have severe hearing loss.
• 32,000 people world wide would implants. Cost
•
•
•
$30-50,000.
Most common hearing loss (85%) is caused by
damage to cochlear hair cells in the inner ear.
Damage cn be genetic, or caused by disease
(measles, meningitis, injury, aging, drugs).
For adults, implants can be miraculous. 10
seconds like pinball. In two minutes, could hear
and have conversations.
Cochlear Implants.
• Tiny microphone attached near the ear.
Implants send sounds through a
processor, back to a transmitter, that
delivers the sound through electrodes,
stimulating the auditory nerve.
• 40,000 Americans are deaf-blind. They
seem to benefit from Cochlear implants.
Cochlear.
• Recipients of Cochlear implants suggest
a true hearing percept (i.e. not like having to
guess what the sound means).
• Awareness of sound in the environment.
• Can combine with lip reading, or be used
independently.
• Typically understand 60% speech within two
weeks, 80% within a few months (sufficient for
telephone use).
Cochlear Implant.
• Takes over the operation of the Cochlear –
not just a hearing-aid (amplifier).
• Damaged hair cells can cause damage of
adjacent auditory neurons (very bad).
• But usually auditory neurons remain.
Auditory System.
• Cochlear nerves from ears transmit acoustic
•
signals to cochlear nucleus in lower brainstem.
Then proceed to temporal lobes of the cerebral
cortex.
Cochlear and the Ear
• Outer Ear – Middle Ear – Inner Ear.
• Outer Ear picks up acoustic pressure
waves.
• Converts them into mechanical vibrations
by a series of small bones in middle ear.
• In Inner Ear, the Cochlear (snail shaped
cavity full of fluid) transforms them into
fluid vibrations.
Cochlear and the Ear
• Pressure variations within the fluid of the
•
•
•
•
Cochlear lead to displacement of the basilar
membrane.
The displacements contain information about the
frequency of the acoustic signal.
There are hair cells attached to the basilar
membrane.
Bending of hair cells releases electrochemical
substance which causes auditory neurons to fire.
Signals presence of excitation at a particular site
Auditory Fequencies
• Sound can be decomposed in frequency
•
•
•
components.
Different frequency components cause
maximum vibration at different points along
the basilar membrane.
Low frequencies cause biggest amplitudes at
the apex of basilar membrane (fig 3).
High frequencies near the base (stapes).
Audio.
Audio.
Auditory Frequences.
• Cochlear implant must activate a range of
•
•
•
frequencies.
Signal processor in implant must decompose the
auditory signal into frequencies (Fourier
Analysis).
Activate places in the Cochlear by electrodes to
correspond to input frequencies.
Loudness depends on how many auditory
neurons are activated. Increase strength of
electric current.
Cochlear Implants
• Electrode arrays inserted in the scala
tympani to depths of 22-30 mm.
• Often 22 electrodes are used (frequency
channels).
• First devices used a single electrode, was
not ideal.
Cochlear Implants
• Transmission to implant.
• Transcutaneous. Radio frequency link.
Transmitter and implanted receiver held in
place by a magnet.
• Percutaneous. Transmits to electrodes by
plug connections.
Performance
• Before – sentence recognition 30% or
less.
• After – aim between 70% to 100%.
Patients can often fill in missing words by
context.
Children
• Implant will help child:
• (1) Produce speech (feedback).
• (2) Understand speech.
• Implants give steady improvement for both
•
•
skills. (Up to four years).
The earlier the better.
Fastest improvement for postlingually deafened
children.
Factors affecting performance
• Duration of Deafness – the shorter the
better.
• Age of onset of deafness – better if
postlingual.
• Age at implantation – Best to implant
prelingual children early (2 years min).
Cochlear History.
• Pythagoras – sound is an oscillation of the air.
•
•
•
•
(6th century BC).
Fallopio (1561) discover the Cochlear.
Cochlear Figure.
Corti (1851) found hair cells on the organ of
Corti.
Helmholtz suggested that the organ of Corti
vibrates in response to different sound
frequencies.
Cochlear History
• Von Bekesy (Nobel Prize 1961) showed
that vibrations of the Cochlear did
correspond to sound waves.
• Different parts of the Organ of Corti were
sensitive to different tones.
• Zwicker (1957) showed that the auditory
system organized sounds into 24
channels. 30,000 nerve fibers.
Hearing Loss
• Two types of hearing loss.
• (I) Conductive Hearing Loss: Damage to
the apparatus that transmits sound to the
Cochlea (e.g. eardrum). Can be treated by
hearing aids or surgery to raise volume.
• (II) Sorineural: Destruction of the hair
cells within the organ of Corti.
Hearing Loss
• How many electrodes needed? 30,000 for all the
•
nerve fibers? 24 for Zwicker’s channels? 6 based
on frequency analysis?
Kiang (1965) discovered that an entire
population of nerve fibers must be used to
transmit a single frequency of sound (single
fibers can produce impluses at most 300 times
per second, but speech involves frequencies up
to 4,000 cycles per second).
Newer Discoveries
• New discoveries keep suggesting ways to
•
•
•
improve Cochlear Implants.
Better understanding of how the nerve fibers
encode the speech frequencies.
Understanding the temporal pattern of the nerve
fibers.
Detailed understanding of how the hair cells
change sound into electrical nerve impulses
(2002).
Speech versus Vision
• Speech input is simpler than Vision input.
• Acoustic signals can be decomposed in
terms of frequencies. 30,000 hair cells.
• Images cannot be described so simply
(jpeg encoding). 10,000,000 retinal cells.
• In both cases, fixing the input doesn’t help
if the language/vision area of the cortex
have not developed.
Speech versus Vision
• But for speech, prelingual deaf children can
•
•
•
learn to speak and hear.
Cochlear implants provide sufficient information
to train the language areas of the cortex?
And/or Chomsky may be right, syntax may be
largely innate.
Retinal implants have a very long way to go to
catch up to Cochlear implants.