Psychophysical properties of low-frequency hearing: implications for perceiving speech and music via electric and acoustic stimulation

Abstract

We have investigated the psychophysical properties of low-frequency hearing, both before and after implantation, to see if we can account for the benefit to speech understanding and melody recognition of adding acoustic stimulation to electric stimulation. In this paper, we review our work and the work of others and describe preliminary results not previously published. We show (a) that it is possible to preserve normal or near-normal nonlinear cochlear processing in the implanted ear following electric and acoustic stimulation surgery - though this is not the typical outcome; (b) that although low-frequency frequency selectivity is generally disrupted following implantation, some degree of frequency selectivity can be preserved, and (c) that neither nonlinear cochlear processing nor frequency selectivity in the acoustic hearing ear is correlated with the gain in speech understanding afforded by combined electric and acoustic stimulation. In another set of experiments, we show that the value of preserving hearing in the implanted ear is best seen in complex listening environments in which binaural cues can play a role in perception.

Fig. 1

AF shapes for the 5 individual EAS patients (EAS1–EAS5) and the average for the normal-hearing listeners (NH AVG). AF shapes for the pre- and postimplant conditions are represented by dashed and solid lines, respectively. The mean AF shape for the normal-hearing listeners is represented by the bold line in each panel. Subject EAS3 demonstrated no frequency selectivity either before or after implantation and thus her filter shapes are represented by superimposed horizontal lines.

Fig. 2

SRT data for unilateral co-chlear implant (CI) recipients (n = 25), bilateral cochlear implant recipients (n = 10), bimodal listeners (n = 24), EAS listeners in the bimodal condition (n = 5, implanted ear occluded), and EAS listeners in the ‘combined’ condition (n = 5, electric plus binaural acoustic hearing). Error bars represent ± one standard error.