Effectiveness of Directional Microphones in Bilateral/Bimodal Cochlear Implant Users—Impact of Spatial and Temporal Noise Characteristics

Objective: To measure speech reception thresholds (SRTs) in co-located (S0N0) and diffuse noise conditions (multi-source noise field, MSNF) and to assess the impact of beamforming algorithms in MSNF in cochlear implant (CI) users. Study Design: Non-randomized, open, prospective study. Setting: Tertiary referral cochlear implantation center. Patients: Participants included 14 CI users (7 bimodal, 7 bilateral) and 14 normal hearing young adults. Interventions: Cochlear implantation. Main Outcome Measures: SRTs were assessed by means of a German matrix sentence test in either continuous or modulated noise. Loudspeakers were configured in two different conditions: S0N0 and MSNF (speech source in front, four speakers distributed at ±28.6 and ±151.4 degrees). In MSNF, the CI speech processor microphone was set in different directional sensitivity settings: standard (sub-cardioid), fixed (super-cardioid), and adaptive. Results: In continuous noise, SRTs of both CI groups were comparable. In modulated noise, bimodal CI users showed lower SRTs than bilateral CI group, but significant benefit from glimpsing was only demonstrated in normal hearing participants. All subject groups showed significant spatial release from masking (i.e., SRT improvement in MSNF compared with S0N0 condition) in continuous noise. A tendency of improved SRT (1 dB bimodal, 2 dB bilateral) with fixed and adaptive directional sensitivity was found which could not be statistically confirmed due to large between-subject variations. Conclusions: The absence of the glimpsing effect in CI users was reaffirmed in the present study. Although very effective in single noise source conditions, the beneficial impact of beamforming algorithms in multiple noise source conditions is poor.

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