Frequency following responses to simulated bimodal speech: Acoustic bandwidth effects

Abstract

Bimodal hearing, which combines a cochlear implant (CI) with a contralateral hearing aid, provides significant speech recognition benefits in quiet and noise. These benefits may be supported by improvements in fo and F1 neural encoding with the addition of acoustic input in the non-implanted ear. Because the frequency following response (FFR) objectively quantifies subcortical neural phase-locking to fo and F1 features, it may be a useful tool for measuring the optimal amplification outcomes with difficult-to-test bimodal users. The present study aimed to parametrically examine acoustic bandwidth effects on speech-evoked FFRs using simulated bimodal stimuli, which comprised of a vocoded speech in the right ear and a low-pass filtered speech at 125, 250, 500, and 750 Hz in the left ear, respectively. A Fourier analyzer was constructed to measure neural phase locking strength at the fo and subsequent harmonics. Results suggested that FFR amplitudes to lower harmonics increased as the acoustic low-pass bandwidth increased up to 500 Hz, and the minimal acoustic bandwidth needed to derive FFR bimodal benefit was at 250 Hz. The FFR fo strength was likely enhanced by the resolved harmonics in the additional acoustic signal.