The effects of frequency overlap on functional listening outcomes in postlingually deafened bimodal cochlear implant users with aidable residual low-to-mid frequency hearing

OBJECTIVES: To assess the effects of frequency overlap across ears on functional listening outcomes in postlingually deafened experienced bimodal users with aidable low-to-mid frequency acoustic hearing. METHODS: Two experimental conditions were compared: (1) frequency overlap (full cochlear implant [CI] bandwidth) and (2) frequency non-overlap (CI stimulation restricted to frequencies beyond the 65 dB HL threshold). Hearing aid (HA) amplification remained constant, with gain applied up to and including the 65 dB HL threshold. Fourteen participants were assessed on speech perception in quiet, co-located, and spatially separated noise, localisation, and subjective evaluation of sound quality. RESULTS: Frequency overlap resulted in significantly better outcomes on all measures except localisation, which showed no difference. Bimodal listening resulted in better speech perception in quiet and co-located noise compared to CI or HA alone. When noise was presented to the HA side, speech perception improved relative to noise on the CI side. Participants preferred the frequency overlap condition. DISCUSSION: Findings suggested that providing full CI frequency bandwidth may offer perceptual benefits for some experienced bimodal users under specific programming conditions. No evidence of signal interference across ears was observed. The consistent absence of interference may reflect, in part, the restriction of HA amplification to below cochlear dead region thresholds; however, further studies are required to investigate this relationship. CONCLUSION: While this study provides preliminary evidence supporting the potential benefits of frequency overlap in bimodal fittings, further research involving larger and more diverse samples is necessary to confirm these effects and to assess longer-term outcomes.