Frequency mismatch and spectral integration in acoustic and electric hearing

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SUMMARY: Bilateral cochlear implant (CI) patients and CI patients with single-sided deafness (SSD) must integrate spectral patterns that might be quite different across ears. Due to interactions between the acoustic- to-electric frequency allocation and the limited extent/insertion depth of the electrode array, CI patients may experience an intra-aural mismatch between the acoustic input frequency and the electrode place within an implanted ear. Bilateral and SSD CI patients may also experience inter-aural mismatch between the frequency-place allocation in each, which may limit binaural benefits for speech and localization. Radiological imaging can estimate electrode positions within the cochlea, but cannot characterize the electrode-neural interface (the proximity of healthy neurons to the electrode), which is the ultimate arbiter of frequency mismatch. Single-channel inter-aural pitch-matching may not fully characterize the effects of mismatch for multi-channel speech perception. Given its additive properties, using band-limited, non-redundant speech (rather than broadband speech) may provide greater insight regarding the effects of frequency mismatch on spectral integration. Optimal bandwidths and frequency ranges for non-redundant speech may be estimated from frequency importance functions. The long-term goal of this proposal is to better understand how frequency mismatch affects spectral integration within and across ears. It is hypothesized that frequency importance functions may be quite different between CI and normal-hearing (NH) listeners, and may be affected by frequency mismatch. It is also hypothesized that the effects of frequency mismatch on spectral integration can be better estimated in noise using band-limited, non-redundant speech. Finally, it is hypothesized that the degree of inter-aural mismatch in CI patients can be accurately and efficiently estimated using complementary, non-redundant speech information presented to each ear. Three aims are proposed to better explore how spectral integration is affected by frequency mismatch. Aim 1 will explore how frequency mismatch affects the frequency importance function for speech intelligibility. Aim 2 will explore how frequency mismatch affects spectral integration within and across ears in NH subjects listening to unilateral, bilateral, and SSD CI simulations, and further evaluate a novel technique to estimate inter-aural mismatch by delivering, complementary, band-limited, non-redundant speech cues to each ear. Aim 3 will use the above novel technique to estimate inter-aural mismatch in real CI patients and further optimize the frequency allocation to reduce inter-aural mismatch. The proposed research is of great theoretical interest, as it will provide greater insights into factors that limit binaural integration in bilateral and SSD CI patients. The proposed research is also of great clinical value, as the results may provide useful clinical tools to accurately and efficiently optimize CI frequency allocations to maximize binaural benefits for bilateral and SSD CI users.