Assessment of Binaural Benefits in Hearing and Hearing Impaired Listeners
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The primary goal of this study was to investigate which speech material is most appropriate as stimulus in head shadow effect (HSE) and binaural squelch (SQ) tests. The most appropriate speech material was then used to obtain normative values of both tests. The second goal was to explore the results of the HSE, SQ, azimuth localization (LOC), and the Speech, Spatial, and Qualities of Hearing (SSQ) scale in bilateral cochlear implant (CI) users. Study participants consisted of 30 normal-hearing (NH) persons and 34 bilateral CI users. In the first phase of the study, six NHs and 11 CI users underwent both HSE and SQ tests. Tests were done twice with three different speech materials: monosyllabic words (NVA), disyllabic words (BLU), and sentences (LiCoS). In both groups (NH and CI) and for both tests (HSE and SQ), the results for the different speech materials were calculated in terms of (a) effect size; (b) test-retest reliability and (c) inter-individual variability. In the second phase, the speech material selected in the first phase was used to test a further 24 NH participants to obtain normative values for both tests. In the third phase, both tests were administered to a further 23 bilateral CI users who had at least six months of binaural listening experience. In addition to the HSE and SQ tests, the LOC test and the SSQ scale were used. The results of the first phase indicated that BLU and LiCoS were better test materials for HSE and SQ tests compared to NVA. Although BLU and LiCoS revealed similar results in terms of effect size and inter-individual variability, LiCoS was preferred over BLU for the subsequent phases of the study due to its higher test-retest reliability, especially in CI users. In the NH group the mean (± standard deviation) HSE and SQ were 58±14% and 22±11%, respectively. In the CI group, the mean HSE was 49±13% and the mean SQ was 13±14%, and both were significantly lower than those of the NH group (p<0.05). There were no statistically significant correlations between the HSE, SQ, LOC, and SSQ results. Further analysis did also not reveal any significant correlations between the test results and demographic variables (p>0.05). Sentence tests are preferred as stimulus material in the binaural HSE and SQ tests. Normative data are given for HSE and SQ with the LiCoS sentence test. HSE benefits are positive for all bilateral CI users, while SQ benefits are positive in approximately seven out of ten cases. Because of the high test-retest variability, these tests do not seem suitable for individual evaluations, but should only be used for group comparisons.