Interaural Place-of-Stimulation Mismatch Estimates Using CT Scans and Binaural Perception, But Not Pitch, Are Consistent in Cochlear-Implant Users

Bilateral cochlear implants (BI-CIs) or a CI for single-sided deafness (SSD-CI; one normally functioning acoustic ear) can partially restore spatial-hearing abilities, including sound localization and speech understanding in noise. For these populations, however, interaural place-of-stimulation mismatch can occur and thus diminish binaural sensi-tivity that relies on interaurally frequency-matched neurons. This study examined whether plasticity-reorganizati on of central neural pathways over time-can compensate for peripheral interaural place mismatch. We hypothesized differential plasticity across two systems: none for binaural processing but adaptation for pitch perception toward frequencies delivered by the specific electrodes. Interaural place mismatch was evaluated in 19 BI-CI and 23 SSD-CI human subjects (both sexes) using binaural processing (interaural-time-difference discrimination with simultaneous bilateral stimulation), pitch perception (pitch ranking for single electrodes or acoustic tones with sequential bilateral stimulation), and physical electrode-location estimates from computed-tomography (CT) scans. On average, CT scans revealed relatively little BI-CI interaural place mismatch (26 degrees insertion-angle mismatch) but a relatively large SSD-CI mismatch, particularly at low frequencies (166 degrees for an electrode tuned to 300 Hz, decreasing to 14 degrees at 7000 Hz). For BI-CI subjects, the three metrics were in agreement because there was little mismatch. For SSD-CI sub-jects, binaural and CT measurements were in agreement, suggesting little binaural-system plasticity induced by mismatch. The pitch measurements disagreed with binaural and CT measurements, suggesting place-pitch plasticity or a procedural bias. These results suggest that reducing interaural place mismatch and potentially improving binau-ral processing by reprogramming the CI frequency allocation would be better done using CT-scan than pitch information.

Automated summary

The study investigates whether plasticity-reorganization of central neural pathways can compensate for peripheral interaural place mismatch in individuals with bilateral cochlear implants and single-sided deafness. While binaural processing did not show adaptation to mismatch, there was evidence of adaptation in pitch perception towards specific electrode frequencies. Results suggest that reducing interaural place mismatch and potentially improving binaural processing by reprogramming the cochlear implant frequency allocation would be better done using CT scan than pitch information.