RECAP: The microphone on the earpiece picks up sound
waves, and via the analog-digital-converter, translates them into a stream of
1s and 0s. This stream is first sent to the Automatic Gain Control, which
increases volume of quiet sounds and decreases volume of loud sounds, and then
to the Bandpass filters, which separates the signal into frequency groups. The
output from these filters get processed by Allison’s MAP, which adjusts
frequency levels according her individual needs and preferences.
The processor’s work is now complete and the signal is
ready to be sent to the internal computer. Recall that the earpiece/processor
is connected via a wire to the external headpiece, which is held in place – on
Allison’s head, resting atop the internal piece – magnetically. So the
processed signal is sent from the earpiece along this wire to the headpiece, at
which point it needs to pass through the skin.
According to Michael Chorost, “[t] he obvious way to send
information through unbroken skin is by radio waves, so the headpiece is a
miniature radio transmitter, broadcasting the data to the implant using AM
waves.” I mean – duh! – Obvious! Everyone knows this is possible because
electricity and radio are manifestations of the same underlying force.
Okay. So the external headpiece turns signal into AM
radio waves and transmits them through the skin to the internal computer. This internal
computer picks up the AM radio waves and converts them back into an electrical
impulse carrying digital data. (At the same time, the internal computer is
sending a message, via FM radio waves, to the external headpiece saying Everything is working on this end! Which
is why, when Allison takes off the headpiece, a little light starts blinking,
indicating a disruption in communication.)
Wait a minute. The earpiece is run off rechargeable
batteries, but how is the internal computer powered? It is powered by the radio
waves. (I know all this information isn’t necessary to understanding the basics
of a CI, but I just thought it was cool.) When radio waves come into contact
with a wire, a flow of electricity is generated. So within the internal casing,
there’s a small coil that picks up the incoming radio signal, and in doing so,
creates enough electricity to run the internal computer and the electrode
array.
Which brings us back to the next step in the process. The
internal computer is wired to the electrode array inside of Allison’s cochlea. The
electrode array it is a strip of silicone with an array of 24 tiny platinum
plates, or electrodes. (An electrode is just something that conducts
electricity.) The silicone strip is about 1 ½ inches long, less than a
millimeter wide, and is inserted into Allison’s cochlea.
Side note 1: The electrode array is
handmade by an electrician looking through a microscope. The skills involved in
making them require at least three months of training and finishing two
electrodes a day is considered a good pace.
Side note 2: When the array was first
developed, it was too stiff and would often damage the cochlea when it was
inserted. In trying to fix this problem, it was made softer, but then it wasn’t
stiff enough to feed into the spiraling cochlea, and the surgeons couldn’t get
it to go in. So the story goes that one day, one of the scientists involved in
developing the implant was on vacation at the beach with his family. He had
found a shell that was similar to the cochlea in its spiral structure, and was
feeding a blade of grass into it when the solution occurred to him: the
electrode array needed to be stiff at the end, like the blade of grass, but
soft at the other end so as to not damage the inner ear!
So the incoming electrical impulse (having been converted
from the AM radio waves) tells the internal computer to ‘fire’ the electrode in
the cochlea, which stimulates the auditory nerves. The auditory nerves send the
message to the brain, but this is outside the realm of the CI. (There’s nothing
wrong with Allison’s auditory nerve – never has been.)
So – done and done. And done quickly – this whole process,
from microphone to auditory nerves, takes ¼ of a millisecond.
Wow. Really super cool awesome shit.
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