HIV/AIDS Treatment

A cochlear implant consists of several components.  All cochlear implants have microphones, external speech processors, signal-transfer hardware, transmitters, receivers, and electrodes. Each plays an important part in converting sound to an electrical stimulus.  The microphone simply receives and transduces sound into an electrical representation.  This is done in an analog (continuous) fashion.  The external speech processor and signal-transfer hardware shapes the electrical signal.  This requires amplifying, compressing, filtering, and shaping.  Amplification is necessary to increase some signal levels to the point that they can be used in the electrical circuits.  Compression is a necessary second step of signal modulation.  The normal human ear can hear gradations of sound intensity in a range of 120 dB.  Persons with severe to profound hearing loss do not have this same range.  In the high frequencies their dynamic range (the difference between their absolute threshold and painful sound) can be only 5 dB!  The range in the lower frequencies is often 10-25dB.  This means that significant compression of the sound energy must take place in order to render it useful.  Thus, all cochlear implants employ gain control of one kind or another.  These systems monitor the output voltage and adjust the ratio of compression to keep the output in a range where it provides useful, but not painful stimuli.

Filtering of the input signal is the next step.  Frequencies between 100 Hz and 4000 Hz are generally those most important for understanding speech.  Sound energy is analyzed using several different types of filters.  This allows the unimportant frequencies to be removed and the frequencies of interest to be separately modified.  Useful sound information is filtered into frequency bands.  This information can then be analyzed for speech patterns and channeled to the appropriate portion of the electrode array.   The cochlear implant surgery needs this information as well as us.