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Interaural time and level difference thresholds for acoustically presented signals in post-lingually deafened adults

D Wesley Grantham1, Daniel H Ashmead, Todd A Ricketts

  • 1Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA. d.wesley.grantham@vanderbilt.edu

Ear and Hearing
|December 20, 2007
PubMed
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Adults with bilateral cochlear implants (CIs) show poor interaural time difference (ITD) thresholds but good interaural level difference (ILD) thresholds for acoustic noise. Localization relies on ILDs, not ITDs, due to CI processing limitations.

Area of Science:

  • Auditory Neuroscience
  • Cochlear Implant Technology
  • Signal Processing in Hearing

Background:

  • Bilateral cochlear implants (CIs) aim to restore binaural hearing.
  • Understanding sound localization cues (ITDs and ILDs) is crucial for CI users.
  • Previous research often used direct electrical stimulation, differing from acoustic presentation.

Purpose of the Study:

  • To measure interaural time difference (ITD) and interaural level difference (ILD) thresholds for acoustic noise in bilateral CI users.
  • To assess the correlation between ITD/ILD thresholds and horizontal-plane sound localization accuracy.
  • To test if ILD cues primarily mediate localization in bilateral CI users.

Main Methods:

  • Eleven postlingually deafened adults with bilateral MED-EL COMBI 40+ CIs participated.

Related Experiment Videos

  • Acoustic noise stimuli (200ms, 100-4000 Hz) were presented via headphones.
  • ITD and ILD discrimination thresholds were measured using a two-interval forced-choice adaptive procedure.

    • Main Results:

    • Interaural time difference (ITD) thresholds were poor (best ≈400 µs; 5/11 subjects <1000 µs).
    • Interaural level difference (ILD) thresholds were relatively good (mean 3.8 dB with compression, 1.9 dB without).
    • ILD thresholds correlated significantly with localization errors (r=0.87, p<0.01), particularly for sources near midline.

    Conclusions:

    • Acoustic signal processing in bilateral CIs elevates ITD and ILD thresholds compared to electrical stimulation.
    • CI processing limits ITD information, making it unsuitable for localization tasks.
    • Sound localization in noise for bilateral CI users relies predominantly on interaural level difference (ILD) cues.