5. MATERIALES Y METODOS
5.1 Descripción del lugar de investigación
The results of the present study suggest that a hierarchy exists in CI users such that sound localization abilities are strongest in BiEAS users, followed by BiCI users with both CI+HA and EAS+HA users being the weakest. Across head movement conditions, simulated BiEAS users’ performance was better than simulated BiCI users. The
difference between these two groups is that simulated BiEAS users have bilateral access to low-frequency fine-structure information in the acoustic component of the EAS-signal
(Section 1.6.5). Consequently, simulated BiEAS users are able to use on-going ITDs,
which are arguably the most salient cue for localizing wideband noise (Macpherson & Middlebrooks, 2002), whereas simulated BiCI users are not. Therefore, simulated
BiEAS users are able to make frequent comparisons between the two ears by attending to temporal differences in the period of the waveform, whereas simulated BiCI users are only able to analyze the difference in level between the two ears.
Across head movement conditions, simulated BiEAS users’ performance was also better than EAS+HA users. The same result was found when comparing simulated BiCI to CI+HA users which is consistent with real users’ data (Seeber et al., 2004; Ching et al., 2007). In both cases, the bilateral users have fewer differences in the spectral ranges of the signals at each ear relative to CI+HA and EAS+HA users. Therefore, it is likely that the different spectral ranges of the signals account for the reduced sound localization abilities of CI+HA and EAS+HA users. These listeners may not be able to integrate the two different signals when localizing, and may only attend to the CI component (Section
4.1.1), or they may weight high-frequency information in the CI component of the signal
more than other localization cues (EAS+HA; Section 4.1.1). As both simulated CI+HA
and EAS+HA users were equally poor performers and were the worst performers of all simulated CI users, it is likely that the asymmetrical stimulation between the ears made it difficult to attend to sound localization cues.
Findings from sound localization tests involving head movement suggest that BiEAS users derive the most benefit from head movement cues. All simulated listeners’ performance improved with unlimited head movement; however, this is a completely
different paradigm from limited head movement tests. It is expected that all listeners’ would improve with unlimited head movement throughout the duration of the stimulus because they can use constant auditory feedback to accurately orient to the target while the stimulus is still on. In contrast, performance in the controlled movement condition illustrates whether or not listeners can make use of head movement cues to differentiate front from rear sources without the ability to orient to the target while the stimulus is still playing. Only BiEAS users derived benefits from limited head movement because only their front/back error rates were significantly different from what was observed when no head movement was available. Bilateral EAS users obtained as much benefit from limited head movement as they did from free head movement; a result which was not observed in any other device simulation condition. However, the benefits simulated BiEAS users derived from controlled head movement may be greater than the benefits real BiEAS users would derive in the same condition. The BiEAS simulation might overestimate benefits because real hearing impaired listeners with moderate low-frequency loss are impaired in head movement benefits, likely because they lack sensitivity to dynamic ITD due to the nature of their hearing impairment (Macpherson, Cumming, & Quelch, 2012). Future studies assessing head movement benefit in real BiEAS users are needed in order to determine whether the findings of the present study are representative of their
performance or whether real BiEAS users’ benefits are impaired in the same was as bilateral hearing impaired listeners with moderate low-frequency loss.
Together these findings suggest that matched bilateral stimulation predicts good
performance while mismatched bilateral stimulation predicts poor performance on tests of sound localization. In addition, the ability to track low-frequency ITDs as the head moves is only available to BiEAS users. Because simulated CI users’ performance was
predictive of real users’ performance (Section 3.4), one would predict that in real users,
BiEAS user’s localization performance would be better than BiCI users, while CI+HA and EAS+HA users would have the worst performance among binaural listeners. As simulated performance was predictive of real users’ performance for BiCI and CI+HA users, it is likely that device limitations account for the issues that real CI users face when localizing. Device limitations likely account for localization errors in real CI users because the simulated users had similar errors despite having both peripheral and central
auditory systems that were intact. In contrast, if the integrity of the peripheral and central auditory systems account for performance on sound localization tests then real CI users’ performance would have been worse than simulated users. However, this was not the case because both real and simulated CI users’ performance was consistent. Lastly, if the processing used to generate the simulated device conditions degraded the signal more than a real speech processor and if the period of acclimatization were not sufficient, it would be expected that simulated users’ performance would be worse than real users’ performance. For all of these reasons, the device simulation was representative of real users’ performance. Therefore, in future studies of sound localization in real CI users, it is expected that BiEAS users would be the best performers followed by BiCI users, CI+HA and EAS+HA users leaving UCI users as the worst performers across real CI users.