Quantitative Analysis of Neural Systems An Analysis of Parameter by MikeJenny


									Quantitative Analysis of Neural
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
• Auditory Nerve System
   – Comprised of central and peripheral
      • Peripheral – converts sound into neural
          – 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
  – 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
• Electrodes sufficient for perception can be low
    but greater numbers offer flexibility
•   High pulse rates with small width provide best
•   Bandwidth above 6.7 kHz show no significant
•   Frequency allocation should be optimized for
    given electrode configuration
•   Power-law transformations have no effect on
    speech perception. Logarithmic transformations
    work well
It’s what’s for DINNER !!!

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