Issues in binaural hearing in bilateral cochlear implant users

Abstract

Despite the success of bilateral cochlear implants (CIs) in restoring sound localization abilities in profoundly deaf individuals, their localization accuracy is still poorer than that of normal hearing listeners. One factor could be the behind-the-ear location of the microphones. However, when CI users were tested with stimuli filtered through individualized head-related transfer functions (HRTFs), they showed very little difference in sound localization performance with different microphone locations (behind-the-ear versus in-the-ear). Another factor is the different implantation depths of the electrode arrays at the two ears. Since CIs are typically fitted independently in each ear at the clinic, it is likely that binaural information at a particular frequency can be presented mismatched across the ears. By simulating different amounts of interaural frequency mismatch at single electrode pairs, CI users showed poorer fusion and lower binaural sensitivity with increasing interaural mismatch. Good lateralization and fusion was achieved on or near a pitch-matched pair of electrode. Additionally, results from a separate study showed lateralization performance was typically maintained with simultaneous stimulation of multiple, pitch-matched pairs of electrodes. These results demonstrate methods beyond just changing the microphone position are needed to improve sound localization performance in CI users. [Work supported by NIH-NIDCD (R01-DC003083) and NICHD (P30-HD03352).]