Essay About Auditory Nerve And Electrical Signals

Basis of Processing Sound Strategies
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Basis of Processing Sound Strategies
Introduction to Coding Strategies:
D.J. Allum
Coding strategies define the way in which acoustic sounds in our world are transformed into
electrical signals that we can understand in our brain. The normal-hearing person already has a
way to code acoustic sounds when the inner ear (cochlear) is functioning. The cochlea is the
sensory organ that transforms acoustic signals into electrical signals. However, a deaf person
does not have a functioning cochlea. The cochlear implant takes over its function. Technically,
it is relatively easy to send electrical current through implanted electrodes. The more difficult
part is to make the electrical signals carry the appropriate information about speech and other
sounds. This responsibility is taken over by coding strategies. The more efficient the coding
strategy, the better the possibility that the brain will interpret the information as having
meaning. Without meaning, sound is only unwanted noise.
Some basic vocabulary is useful in understanding coding strategies:
Frequency. Speech is composed of a range of frequencies from high-frequency sounds
(sss, piii) to low-frequency sounds (ah). These frequencies also occur for sounds in our
environment. The speech-frequency range is from about 250 Hz to 6,000 Hertz (Hz).
Amplitude. The amount of amplitude, or intensity, defines how loud a sound is heard.
The usual range from the softest to the loudest sound is about 30 dB. The normal range
for human hearing is around 120 dB.
Tonotopic. A special characteristic of the cochlea and the auditory nerve. It means that
the apical region of the cochlea (and the nerve near this region) is more sensitive to low
frequencies and that the basal region is more sensitive to high-frequencies. The
relationship between the most basal to the most apical region is a progression from
high-to-low frequency sensitivity.
Filters. Filters are used to divide, electronically, acoustic signals into different ranges.
For instance, for a speech-frequency range of 4,000 Hz, we could divide the total range
by 10 and each filter would hold 400 Hz.
Stimulation Rate. The number of times an electrode is turned on and off, i.e., activated
with electrical stimulation.
The normal cochlea is like a series of filters. Sounds that have high-frequencies will fall into
filters at the basal end of the cochlea and those with low-frequencies will fall into filters in the
apical end, i.e., in a tonotopic arrangement. Since the cochlea cannot accomplish this for a
deaf person, the cochlear implant takes its place. It is important to remember that the auditory
nerve is tonotopic even if the cochlea cannot transmit information because of deafness. The
auditory nerve lies in waiting for stimulation to arrive at a certain place in the cochlea. Thus, a
series of electrodes are placed in the cochlea. Each electrode is associated with a place in the
cochlea (base or apical) and with a filter (high-frequency to low-frequency). This is how the
auditory nerve receives information. Because speech is composed of different frequencies and,
therefore, normally analyzed in different parts of the cochlea (tonotopic order), a coding strategy
needs to divide speech electronically into different frequency bands and then send the
information