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Quantitative Analysis of Neural Systems An Analysis of Parameter

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					Quantitative Analysis of Neural
           Systems:
An Analysis of Parameter Variations in
   Cochlear Implant Performance

                   Jorge Roldan
         Dept. of Biomedical Engineering
           Louisiana Tech University
               November 14, 2002
Cochlear Implants
• Medical device designed to electrically
    stimulate the auditory nerve
•   Main features
    – Speech Processors
       • Speech detected by microphones and sent to processors
       • Signals are filtered, analyzed, and digitized
    – Internal components
       • Signals transmitted via FM signals to implant
       • Corresponding current delivered through electrode
    – Speech Coding Strategies
       • Controlled the way signal is processed
       • Emphasize different qualities such as pitch and loudness
       • SPEAK – Spectral Peak Coding Strategy
       • CIS – Continuous Interleaved Sampling
  Physiology
• Auditory Nerve System
   – Comprised of central and peripheral
     systems
      • Peripheral – converts sound into neural
        codes
          – External, middle and inner ear
      • Central – interprets signals as speech
        patterns, music, noise etc.
   – Inner Ear and cochlea
      • Transduces mechanical forces (fluid
        waves) to electrical signals via cilia
   – Frequency Spectrum
      • Fundamental Frequency (vowels)
      • Harmonics vary across frequency
        indicating resonances (formats)
Implant Parameters I
• Number of implanted electrodes
  – Can vary from 1 to 22
  – 7 electrodes sufficient for speech perception
  – Greater number allow for different configurations,
    frequency allocations
  – flexibility to eliminate faulty electrodes or those that
    produce unwanted sensations
• Electrode configuration
  – Refers to location, orientation, and separation
     • Best performance at apical location
     • 3.75 mm distance more closely approximates normal
       frequency delivery
     • Minimizes electrode interaction and interference
     • Monopolar vs. Bipolar
         – Bipolar should be more specific
         – Monopolar offer broad stimulation
Implant Parameters II
• Frequency bandwidth and allocation
  – Signal bandwidth can be no higher than half the
    sampling frequency
  – Normal human tonal frequencies
     • 20 Hz – 20 kHz
     • Only 3kHz needed to detect first 3 formats
     • f >6.7 kHz produces no improved effect on speech recognition
• Rate and duration of stimulus pulses
  – Pulse rate determined by pulse width (vice versa)
  – Faster rates (>800 pps) offer greatest consonant
    recognition
  – Long pulse widths offer greater dynamic range when
    using slow rates
Implant Parameters III

• Amplitude Mapping Functions
  – Transformation of acoustic amplitude to
    electrical amplitude
  – Normal speech sounds range from 40 – 60 dB
  – Implants listeners have dynamic range from
    6-20 dB
  – Power-Law or Logarithmic compression
     • Varying Exponent
Conclusions
• Electrodes sufficient for perception can be low
    but greater numbers offer flexibility
•   High pulse rates with small width provide best
    performance
•   Bandwidth above 6.7 kHz show no significant
    effect
•   Frequency allocation should be optimized for
    given electrode configuration
•   Power-law transformations have no effect on
    speech perception. Logarithmic transformations
    work well
      BEAGLE MEAT
It’s what’s for DINNER !!!

				
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