Evaluation of a Bio-inspired Coding Strategy for Cochlear Implant Users
Condition: Hearing Loss, Complete
MeSH terms: Hearing Loss; Deafness
Study type: Interventional

Summary: 
      This study will determine the facilitation, refractoriness and spatial spread effects of
      auditory nerve fiber responses to electrical stimulation via a cochlear implant.

      The performance of CI users in melody contour and speech recognition in noise tests with
      their own clinical sound processor and a MATLAB implementation of their coding strategy will
      be compared and a bioinspired coding strategy will be evaluated in comparison with the
      conventional ACE coding strategy.
    

Detailed description: 
      The advanced combination encoder (ACE) strategy, one of the most widely used clinical speech
      processing strategies available to cochlear implant (CI) users, attempts to optimize
      transmission of input acoustic signals, but does not explicitly consider the auditory nerve
      fibers' (ANFs) capacity for conveying this information. The ACE strategy decomposes temporal
      frames of the incoming sound signal into frequency bands with a bank of band-pass Fast
      Fourier Transform (FFT) filters. The temporal envelopes in each band are then extracted and
      typically eight to ten bands with the highest energy content are selected to amplitude
      modulate the biphasic pulses. This selection is performed irrespective of the ANFs responses
      to the electrical stimulation. However, some studies have shown that temporal response
      properties of ANFs impose limitations for electrical stimulation. Apart from that, the degree
      of spread of neural excitation which is typically larger than in the acoustic case, may
      result in responses which can diminish information provided on the individual channels. Thus,
      a coding strategy taking into account temporal properties of ANFs as well as spatial spread
      of the electric field could be beneficial for CI users.

      One of the prominent temporal characteristics of ANFs in response to the electrical
      stimulation is refractoriness. This phenomenon can be defined as a reduction in the
      excitability of ANFs immediately following an action potential and has been observed in CI
      recipients via measuring the electrically evoked compound action potential (ECAP). The
      refractory period can be divided into an absolute refractory period (ARP) during which the
      auditory nerve is incapable of responding to the following pulse and a relative refractory
      period (RRP) during which a response from the neuron is possible under specific
      circumstances. Refractoriness can impose limitations on the maximum stimulation rate of CIs
      since ANFs cannot respond to a stimulus presented during the ARP.

      Apart from refractoriness, an ongoing high rate pulse train produces spike rate adaptation
      (SRA) in ANFs in which the neurons progressively lose their ability to respond to every
      pulse. This decrement in neural excitability is even larger than can be explained by
      refractoriness. Animal studies of ANFs at high rate pulse trains have shown SRA and
      adaptation has also been observed in the ECAP amplitude of human CI users in which the
      amplitude decreased as the stimulation rate increased. In parallel to spike rate adaptation,
      accommodation can contribute to the spike rate decrement. Accommodation reduces excitability
      for the second pulse (probe) response when there is a subthreshold response to the first
      pulse (masker) and the masker-probe interval (MPI) is large enough to allow the membrane
      potential to decay back near or below the resting potential. Accommodation in addition to SRA
      is considered as reduction in the excitability of ANFs and its effect accumulates over
      sequential non-spiking responses.

      Electrical stimulation of ANFs in animals with pairs of pulses has also shown facilitation
      which is defined as an increase in nerve excitability caused by sub-threshold stimulation in
      short intervals. Apart from animal studies, the facilitation effect has also been observed in
      human CI recipients. Facilitation happens when the neuron does not respond to the first
      pulse, but if the membrane potential remains near the threshold long enough, the second pulse
      can produce a response. It was shown that facilitation is more evident at a low MPI and for a
      masker with an intensity equal or less than that of a probe. Although the facilitation effect
      was observed in human CI users, systematic ECAP measurement to quantify this effect were not
      yet performed. Thus, neural response telemetry (NRT) measurements with negative masker offset
      and short MPIs need to be done to determine a non-monotonic behaviour of facilitation and
      confirm the predictions by Cohen in his model simulations.

      Apart from ANFs temporal considerations, electrical current spreads out widely along the
      cochlea and excites a wide range of populations of ANFs which leads to a decrease in the
      selectivity and the number of effective channels. Thus, spatial spread of the electric field
      has a major impact on the spectral resolution of CI users and decreases the excitability of
      the affected ANF population.

      In the planned study refractoriness, spatial spread and facilitation effects will first be
      determined from NRT measurements with CI participants. Then, all the aforementioned phenomena
      are integrated in a bio-inspired coding strategy for a better selection of channels with
      highest energy content. This new strategy will be compared to the conventional ACE coding
      strategy.
    

Intervention name(s): Coding strategy for cochlear implants
Intervention type(s): Device
Intervention description(s): Comparison of two variations of a coding strategy based on electrophysiological objective measures with standard reference coding strategy
Other intervention name(s): Nucleus Cochlear Implant System
Intervention model: Single Group Assignment
Intervention model description: Single center, single-armed, parallel design with four laboratory sessions for CI subjects
Masking: None (Open Label)
Primary purpose: Basic Science
Keywords: cochlear implant; coding strategy; objective measures