INTRODUCTION

Our purpose was to investigate binaural integration for spectrally degraded speech in normal-hearing (NH) subjects, single-sided deafness (SSD) cochlear implant (CI) recipients, and bilateral deaf bilateral CI recipients.

METHODS

We tested ten adult subjects in each group with a modified version of the binaural fusion test according to Matzker. Speech recognition was assessed for monotic listening with the better-hearing ear or CI, monotic listening with the poorer-hearing ear or CI, and dichotic listening. We employed two presentation modes: (1) low pass (LP)-filtered speech to the better ear or CI and high pass (HP)-filtered speech to the poorer ear or CI, and (2) LP-filtered speech to the poorer ear or CI and HP-filtered speech to the better ear or CI. Five magnitudes of LP and HP filtering, i.e., spectral degradation, for each presentation mode were applied yielding two spectrally overlapping and three nonoverlapping presentation conditions. Sentences from the Oldenburg Children's sentence test were applied to assess speech recognition.

RESULTS

NH subjects, SSD CI recipients, and bilateral CI recipients were able to understand spectrally degraded speech under both monotic and dichotic listening conditions for both the presentation modes. Speech performance decreased with the increasing loss of spectral information in all the three subject groups. In the NH subjects, speech recognition scores significantly improved for dichotic compared with each monotic listening type under two spectrally nonoverlapping conditions. The SSD CI subjects showed a significant improvement in speech scores for dichotic listening compared with monotic listening with the NH ear under one nonoverlapping condition, i.e., a dichotic benefit in speech recognition with CI. We saw a dichotic benefit in the bilateral CI recipients who achieved significantly better speech scores for dichotic compared with monotic listening with the better CI in three nonoverlapping conditions.

CONCLUSIONS

All the three groups (NH subjects, SSD CI recipients, and BiCI recipients) showed binaural integration for the side-separated presentation of spectrally degraded speech. Use of strictly side-separated dichotic stimulus presentation avoided the possible occurrence of physical effects such as the head shadow effect and therefore, confirmed the binaural benefit attributable to central binaural processing.