2.5 Data analysis
\(MAE=\frac{\sum_{i=1}^{n}\left|\alpha\par \begin{matrix}i\\ \text{RESP}\\ \end{matrix}-\alpha\par \begin{matrix}i\\ \text{TARG}\\ \end{matrix}\right|}{n}\) (1)
\(\alpha_{\text{RESP}}=g\bullet\alpha_{\text{TARG}}+b\) (2)
The mean absolute error (MAE) was calculated using Equation 1 to assess the sound localisation accuracy under different conditions. Theα RESP and α TARG referred to the response azimuth (in degrees) and target azimuth (in degrees), respectively. Additionally, the best linear fit of the target–response relationship for each participant was also computed using Equation 2, where g is the response gain (slope, dimensionless), and bis the response bias (offset in degrees). In this study, the right side was defined as the impaired side; therefore, azimuth coordinates for patients with left ear impairment and controls with NH with left ears plugged were inverted.
Paired and independent t- tests were conducted to evaluate differences under different test conditions. The p-values of <0.05 and <0.001 were considered statistically significant. SPSS version 26.0 and GraphPad Prism version 8.0 were used to analyse the data and draw diagrams.
RESULTS
3.1 Hearing thresholds
For patients with UMA, the mean ± SD of the unaided hearing threshold was 51.36 ± 5.02 dB HL, which significantly improved to 27.64±2.38 dB HL with a mean FHG of 23.73 ± 3.47 dB HL (p < 0.001). For the NH comparison group, the mean hearing threshold was 15.36 ± 3.88 dB HL. The aided mean hearing threshold of the UMA group was still higher (worse) than the mean hearing threshold of the NH group (p < 0.001). Detailed outcomes across 0.5–4 kHz are depicted in Fig. 1a.
3.2 Speech perception abilities
In Fig.1b-c, the average unaided WRS and SNR for patients with UMA were 18.27 ± 14.63 % and -5 ± 1.18 dB SPL, respectively, while the average aided WRS and SNR conspicuously changed to 85.45±7.38 % and -7.73 ± 1.42 dB SPL, respectively (WRS: p < 0.001; SNR: p < 0.05). The mean WRS and SNR of the comparison group were 99.27 ± 1.35 % and -10.55 ± 2.77 dB SPL, respectively. Fig. 1b-c also indicates significant differences in the speech levels between the patients aided with BCDs and their peers with NH (WRS: p < 0.001; SNR: p < 0.05).
3.3 Sound localisation in patients with UMA andstimulated UHL
Fig. 2 shows the individual sound localisation target-response plots for two children with UMA and one control (P6, P7, and N2) under monaural (unaided and P, left column) and binaural (aided and UP, right column) listening conditions. Under the unaided condition, P6 showed a poor localisation ability and perceived most stimuli from the healthy ear side. However, P7 exhibited relatively better sound localisation accuracy than P6. When aided with the BCD, the sound localisation accuracy improved in P6 (delta gain = 0.422, delta MAE = -14.28°) and worsened in P7 (delta gain = -0.167, delta MAE = 18.58°). Under the P condition, all data points of N2 fell along the diagonal dotted line, indicating a sharply deteriorated localisation performance (gain = 0.12, MAE = 65°).
Individual data on gender, age, MAE, response gain, bias, and r2for all participants are presented in Table 2. The MAE and response gain under monaural listening conditions (unaided and P) are plotted against those under binaural listening conditions (aided and UP) in Fig. 3a–b. Varying sound localisation performance was observed in the 11 children with UMA under the unaided condition. When the mean gain and MAE of all children with UMA were compared between the unaided and aided conditions, no significant differences were identified (gain: p = 0.104, MAE: p = 0.436). Control listeners showed good sound localisation performance in the UP condition. All of them exhibited considerable deterioration after being plugged (gain: p < 0.001; MAE: p < 0.001), with most of them unable to localize the stimuli presented from the plugged side. Although no significant difference was observed between the unaided and P conditions (gain: p = 0.073; MAE: p = 0.073), the results indicated that children with UMA showed better sound localisation performance (smaller MAE and gain) than the stimulated UHL listeners. This phenomenon might be related to the adaptation to congenital unilateral asymmetric hearing loss; however, the benefit of this adaptation was insufficient for children with UMA to localize sound as accurately as the normal controls did.
3.4 Influence of a BCD on sound localisation accuracy
Fig. 3c–d shows the localisation accuracy of the patients with UMA and the NH controls on the impaired (the atretic and plugged) side and the contralateral (the healthy and unplugged) side, respectively. A better sound localisation accuracy was observed in children with UMA (43.18 ± 30.58° vs. 83.18 ± 37.82°, p < 0.05) on the impaired side than in controls in the P condition (Fig. 3c). When aided with a BCD, there was no difference in the MAE between the unaided and aided conditions on the impaired side (43.18 ± 30.58° vs. 34.14 ± 17.9°, p = 0.303) or the contralateral side (26.97 ± 24.68° vs. 27.42 ± 14.52°, p = 0.79), indicating that the BCD use was not detrimental to the original sound localisation ability of the patients with UMA.
According to the criterion in a previous study, 11 children with UMA were divided into two subgroups: good performers (n = 5; gain > 0.75) and poor performers (n = 6; gain ≤ 0.75) 14. When the MAE outcomes were separately compared bilaterally, a significantly better sound localisation accuracy on the atretic side was observed in good performers (15.67° ± 10.71° vs. 66.11° ± 19.77°, p < 0.05, Fig. 4a). Linear regression was further conducted to explore the predictive effect of gain on the benefits of sound localisation accuracy by fitting BCDs (delta MAE, MAEaided − MAEunaided). The results revealed an evident relationship between response gain and delta MAE (r2 = 0.553, p < 0.05), indicating that children with UMA who have poor sound localisation performance (lower gain) show more improvement after being fitted with BCDs (Fig. 4b).