How exactly does a cochlear implant work?
Primarily the CI is implanted into the skull and, using an external microphone and voice processor, transmits information onto the primary auditory nerve and you hear sound. But that is just a brief overview. Below are questions you might have:
How does the Implant Stimulate the Auditory Nerve?
How does an electrode stimulate a neuron?
Wouldn’t More Electrodes be Better?
What is a Channel and How does the Implant translate Pitch?
Types of Individual Electrode Stimulation
Multichannel vs. Single channel Cochlear Implants
How is the Cochlear Implant different from a Hearing Aid or an ABI?
So can someone with an implant understand speech?
Each stimulated spot on the cochlea is a channel.
Each channel is responsible for a frequency of sound.
The cochlea is tonotopically organized, so that the hair cells deep in
the cochlea translate the lower frequencies and the hair cells closer to the
opening of the cochlea interpret the higher frequencies.
http://www.macalester.edu/psychology/whathap/UBNRP/Audition/site/ear.html
The above is a website created by another group of students who were involved in the Behavioral Neuroscience Research Project. This website will explain the cochlea and the function of hair cells in greater depth.
How does the Implant Stimulate the Auditory Nerve?
The basic idea behind the cochlear implant is that there is a series of microelectrodes inserted into the cochlea, which directly electrically stimulates the auditory nerves (a type of neuron) to create “sound.”
How does an electrode stimulate a neuron?
The inside of a neuron normally has a negative electrical
charge, but in order to send a message on to the next neuron it must become
positively charged. Relaying a
message from one neuron to the next is called “synapsing.” The neuron sending the message releases neurotransmitters.
A neurotransmitter is a chemical that will either excite the next neuron
or inhibit it. Exciting the neuron
means that the neurotransmitter opens a chemical gate so that positive calcium
ions can enter and positively charge the neuron.
This sets off a chain reaction that produces a firing of a neuron.
An electrode is like a small, fine needle that conducts an electrical impulse. A microelectrode is a very small electrode, perhaps a thousandth of a millimeter in diameter. By touching the firing microelectrode against the neuron, the inside of the neuron becomes positive for a moment, starting an action potential that carries the message to the next neuron.
Wouldn’t More Electrodes be Better?
Theoretically it would make sense to have as many electrodes as possible. If each electrode stimulated an individual auditory nerve and each auditory nerve is responsible for carrying the message of a specific frequency to the brain, then stimulating more auditory nerves should allow a larger number of frequencies to be interpreted as sound by the brain, right? In fact, there is little reason to believe that more than eight electrodes is necessary or in fact creating a higher quality of sound, and in some respects a larger number of electrodes may be detrimental.
· A larger number of electrodes may just mean that there is bipolar stimulation (e.g., 16 electrodes create 8 channels), not that there are more channels (i.e. more frequencies being “heard”)
· The number of intact auditory nerves limits the number of effective electrodes. Cochlear implants are appropriate for those who have had hair cell damage or death in the cochlea, but whose auditory nerves are still intact. Over time however, the lack of stimulation can lead the auditory nerves to die (link to section on why it is better to treat ppl soon as possible).
· Interference from adjacent electrodes is more likely if there are a larger number of electrodes.
· Not all electrodes will necessarily be used at once, meaning that a larger number of electrodes will be inactive at any given time.
What is a Channel and How does the Implant translate Pitch?
Each localized stimulated spot on the cochlea is referred to as a channel, and the stimulation of a particular channel will result in the neural signal to interpret a particular pitch, or frequency. The human ear distinguishes sounds when they differ in frequency by between 10-17%, despite the fact that the healthy ear is capable of detecting those frequencies in between two recognizably distinct sounds: although a more specific range of frequencies certainly can add more nuance (or color) to what is being heard, this type of distinction is not necessary for comprehension purposes. A smaller number of electrodes are sufficient for conveying the basic tonality of sound without the interference and potential problems of a larger number of electrodes. Cochlear implants with fewer channels are regarded as at least as good as those with more channels, generally speaking.
Think of the channels like you would think of a color spectrum. Each individual point on the color spectrum is unique. A painting using every possible color will be very vibrant, but the viewer would still be able to perceive and understand what the subject of the painting was if the artist only used the red, orange, yellow, green, blue, indigo and violet paint.
Temporally, there are three types of stimulation for
multi-channel cochlear implants.
Types of Individual Electrode Stimulation
The adjacent firing of electrodes creates “channel interaction.” Channel interaction occurs when the stimulation from one electrode interferes with the stimulation of a nearby adjacent electrode, making it less distinct as to which auditory nerve is being activated. There are some methods of minimizing channel interaction.
· Use pulsatile stimulation so that nearby electrodes will not fire at the same time. Physical separation of the channels so that the electrical currents are farther apart. (This involves a tradeoff of fewer channels so that there is not as much channel interaction. This does not intrinsically mean that the quality of the implant is lower.)
· Newer electrode arrays that are designed to limit this type of interaction.
· Combinations of the above
Multichannel vs. Single channel Cochlear Implants
A multichannel cochlear implant stimulates different areas
of the cochlea through an electrode array that covers nearly the whole length of
the cochlea: this type of electrode array is an attempt to simulate the
functioning tonotopic cochlea by stimulating the part of the cochlea that
corresponds to a particular frequency in the environment.
The 22-30mm electrode array winds through most of the spiral of the
cochlea. Ideally all of the
auditory neurons of an implant wearer would be intact and functional, so that
electrodes can lie along the whole electrode array and stimulate different areas
of the cochlea. This allows for a
wider variety of channels. However,
if the surviving auditory nerves are clustered in one area, then a few
electrodes stimulating that area will be as good as a large number of electrodes
stimulating the length of the cochlea.
Advantages
Because the electrode array is spread out, there is likely to be less electrical current clustered in one area (a higher current density could cause channel interaction).
The multichannel implant lets you perceive a wider range of sound in the environment.
Tonotopic stimulation is easier: the cochlea is arranged tonotopically so a wider range of pitch can be “heard.” This can be extraordinarily helpful with speech recognition as well as with developing linguistic capabilities.
Disadvantages
A single-channel cochlear implant transmits only one
channel, and can use only one electrode.
Single channel implants were the original design, and are much simpler
and lower cost in comparison to multichannel.
A single-channel electrode is inserted into the cochlea, but does not
extend beyond the first bend of this snail-shaped organ.
Some frequency information can be transmitted, but only minimally.
Advantages
Disadvantages
How is the Cochlear Implant different from a Hearing Aid or
an ABI?
The cochlear implant is not the same thing as either a
hearing aid or an auditory brainstem implant (ABI). The type of aid which a deaf person is eligible for depends
on what causes that individual’s deafness.
A hearing aid may help a person who has conductive hearing loss (caused by damage to the outer or middle ear) by amplifying the sound. If the damage is to the inner ear (the hair cells of the cochlea are not functional) but the auditory nerves are still intact, then the person may be eligible for a cochlear implant. Those persons whose deafness is caused by a problem from the auditory nerve on may be eligible for an ABI.
So can someone with an implant understand speech?
The level of speech that is understood depends on several factors:
· If a person was old enough to be able to understand speech before becoming deaf then it is more likely that it would take less time to learn to speak and to recognize spoken speech.
· The implant’s inability to process very low sounds is not generally a problem while understanding speech because of a harmonic quality of very low pitches. It’s as though there is a higher frequency that is heard at the same time as the lower pitch, so even if the low pitch cannot be interpreted by the implant the higher associated frequency is still there.
· When the sound perceptions produced by the implant are accompanied by lip reading, then the speech can be understood more readily. In fact, both hearing and deaf people tend use lip-reading to aid understanding.