Frequency overlap between electric and acoustic stimulation and speech-perception benefit in patients with combined electric and acoustic stimulation

Objectives: Our aim was to assess, for patients with a cochlear implant in one ear and low-frequency acoustic hearing in the contralateral ear, whether reducing the overlap in frequencies conveyed in the acoustic signal and those analyzed by the cochlear implant speech processor would improve speech recognition. Design: The recognition of monosyllabic words in quiet and sentences in noise was evaluated in three listening configurations: electric stimulation alone, acoustic stimulation alone, and combined electric and acoustic stimulation. The acoustic stimuli were either unfiltered or low-pass (LP) filtered at 250, 500, or 750 Hz. The electric stimuli were either unfiltered or high-pass (HP) filtered at 250, 500, or 750 Hz. In the combined condition, the unfiltered acoustic signal was paired with the unfiltered electric signal, the 250-Hz LP acoustic signal was paired with the 250-Hz HP electric signal, the 500-Hz LP acoustic signal was paired with the 500-Hz HP electric signal, and the 750-Hz LP acoustic signal was paired with the 750-Hz HP electric signal. Results: For both acoustic and electric signals, performance increased as the bandwidth increased. The highest level of performance in the combined condition was observed in the unfiltered acoustic plus unfiltered electric condition. Conclusions: Reducing the overlap in frequency representation between acoustic and electric stimulation does not increase speech understanding scores for patients who have residual hearing in the ear contralateral to the implant. We find that acoustic information <250 Hz significantly improves performance for patients who combine electric and acoustic stimulation and accounts for the majority of the speech-perception benefit when acoustic stimulation is combined with electric stimulation.