Contribution of low- and high-frequency bands to binaural unmasking in hearing-impaired listeners

This study investigated the contribution of interaural timing differences (ITDs) in different frequency regions to binaural unmasking (BU) of speech. Speech reception thresholds (SRTs) and binaural intelligibility level differences (BILDs) were measured in two-talker babble in 6 young normal-hearing (NH) and 9 elderly hearing-impaired (HI) listeners with normal or closeto- normal hearing at and below 1.5 kHz. Target sentences were presented diotically, embedded in a stream of diotic or dichotic maskers. Both target and masker sentences were split into frequency regions above and below 1.25 kHz. In the dichotic listening conditions, the maskers were lateralized to the left side by introducing 0.68-ms ITDs in either the low-frequency band, the high-frequency band, or both bands simultaneously. BILDs were found to be similar in both listener groups when the ITDs were imposed on the lowfrequency band only. ITDs in the high-frequency band alone did not produce any BILD in any of the groups. However, when ITDs were imposed in both frequency bands, the NH listeners yielded significantly greater BILDs than the HI listeners. The results suggest that, on a group level, HI listeners relied solely on ITDs in the low-frequency band while NH listeners were able to utilize envelope ITDs above 1.25 kHz to facilitate the BU of speech.