Discussion
hearing threshold, head movements, head movements’ behavior, head-movement behaviors, hearing asymmetry, hearing in monaural listening conditions [
UNILATERAL HEARING LOSS
We observed that UCI users can improve their sound localization abilities, despite the substantial impoverishment of the available auditory cues. Thus, acoustic space perception improvement is possible also for people using a single CI, at least in the experimental context we have examined. Specifically, we showed that sound localization of UCI users can improve across trials while engaged in a Spatial VR training and that error reduction extended beyond the trained task. Localization errors decreased after training, as compared to before training, and this decrement (about 13°) was greater after the Spatial compared to the Non-Spatial VR training. Further analyses revealed that hearing asymmetry (as described by PTA at the non-implanted ear) modulated training benefit. Generalization effect of Spatial training was more pronounced for participants with higher hearing asymmetry (i.e., higher hearing threshold at the contralateral ear). Finally, the Spatial VR training had no impact on the audio-visual orienting task, which involves the ability to localize sounds sources indirectly. A possible explanation for the lack of this effect is that active listening was prevented during this task as, for experimental reasons, it was performed using a chin-rest. Although this test represents a firm attempt to test the generalization effect, further studies are needed to investigate the transferability of training effects to tasks in which the ability to localize sounds is implicitly involved.A previous study by Luntz and colleagues already suggested that training UCI spatial hearing skill is possible, but this early report suffered from methodological limitations. They tested only few participants (Interestingly, our short training produced effects also in a sound localization task entailing different sound positions, a different response method (i.e., use of the head as pointer instead of reaching sound sources using the hand), and less visual cues available (i.e., potential sounds sources invisible during the test). The difference between the trained task and the test task (i.e., head-pointing to sound) is clearly evident also in the different performance achieved by the participants in the two sound localization procedures. While in the trained task, the absolute error for participants in the Spatial training group was 24° on average, in the test task, they started from an average absolute error of 52.6° to achieve a performance of 39.3° at the end of training. This difference is likely the consequence of the different priors about sound position in the two procedures: during training, all possible sound positions were visually identified, whereas during test, no visual cue helps participants to locate the sound sources.This result highlights the importance of assessing generalization effect when testing the efficacy of training protocols (see also [Given the large interindividual variability in terms of hearing experience, we investigated if hearing asymmetry influenced sound localization performance and training effects. We observed an increase in the effectiveness of the Spatial training for UCI users with higher levels of hearing asymmetry. This finding supports the idea that it is possible to improve localization of sounds even when auditory cues available are primarily monaural intensity cues, and opens the possibility to offer a similar training to people with unilateral hearing loss. Since the larger training effectiveness was documented in individuals who primarily listen monaurally using their CI, rather than individuals with bimodal experience providing binaural cues, this finding suggest that our training primarily changed the way in which participants exploited the available intensity monaural cues (but see [A further contribution of the present work concerns the study of head movements’ behavior. First, during training, participants requiring progressively fewer head movements to perform the task and reduced the extent of their head rotation when responding to central targets. This corroborates the observation of a trial-by-trial improvement, that we described above in terms of progressive reduction of performance errors. Second, after training, head movements changed between the pre- and post-training measurements. When we focused on the first head-movement onset during the head-pointing to sounds test, we observed that the correct direction of the sound was identified faster after the Spatial compared to the Non-Spatial VR training. Third, participants also started to spontaneously implement novel head-movement behaviors after the training. Specifically, they increased the number of movements and explored a larger portion of space with the head. This was particularly evident after the Spatial VR training, hinting at the possibility that they moved the head strategically to bring their CI toward the sounds. This strategy might have favored the extraction and use of monaural intensity variation at the CI—pointing again to an advantage of the Spatial training mostly related to the use of monaural cues available at the unilateral CI. This strategy has been already documented in previous studies testing people with normal hearing in monaural listening conditions [