Bilateral Loudness Balancing and Distorted Spatial Perception in Recipients of Bilateral Cochlear Implants

Area of Science:

• Audiology
• Neuroscience
• Biomedical Engineering

Background:

• Bilateral cochlear implants (CIs) aim to restore binaural hearing.
• Effective sound localization relies on fused, centered auditory images.
• Current mapping procedures may not fully optimize spatial hearing in CI users.

Purpose of the Study:

• To evaluate if loudness balancing in bilateral CIs yields fused auditory images.
• To determine if these balanced images facilitate accurate sound lateralization.
• To assess the impact of interaural level differences (ILDs) on auditory perception.

Main Methods:

• Direct stimulation to determine most-comfortable levels (C levels) in bilateral CI recipients.
• Sequential loudness balancing of electrode pairs.
• Subjective fusion experiments and lateralization experiments with imposed ILDs.
• Assessment of auditory image location, number, and compactness.

Main Results:

• Bilateral loudness balancing was achieved in ~80% of cases with small C level adjustments.
• However, ~23% of percepts were not fused, and 45% of zero-ILD stimuli were not perceived as centered.
• Auditory image locations were often offset from physical ILDs, indicating spatial map distortions.

Conclusions:

• Bilateral CI recipients frequently perceive unfused or spatially offset auditory stimuli even after loudness balancing.
• Current mapping may not provide optimal percepts for sound-source localization.
• Development of novel CI signal processing and mapping procedures is suggested to enhance binaural hearing benefits.