Variación del Índice de Sustentabilidad para las fincas de la ARAC

Findings from the present study and evidence in the literature suggest that there are several features of CI devices that limit the user’s ability to localize sound sources

accurately. All implant users (i.e. BiCI, CI+HA, UCI, BiEAS and EAS+HA users) might gain access to spectral cues important for differentiating front from rear sources if the typical microphone placement on the implant devices were changed. Positioning of the microphone BTE does not capture sound that has been filtered by the pinnae; therefore, the acoustic input for the CI does not contain important high-frequency spectral cues. Placement of the microphone at the entrance of the external auditory canal or near its entrance might allow for spectral cues to be captured by the microphone. However, in some HA users, other microphone placement options are not possible, and BTE fitting is necessary. With BTE microphone placement on HA users, it has been shown that use of a cardioid directional microphone can improve front/back sound localization performance (Keidser et al., 2006). Therefore, changing the microphone placement or adding a

directional microphone to the CI device might benefit the listener in terms of front/rear discrimination.

In addition to changing the microphone placement and/or adding directional microphone settings, the overall bandwidth of the speech processor needs to be increased; otherwise spectral cues would be removed when the speech processor filters the acoustic signal into channels. Specifically, the overall bandwidth of speech processors should be increased such that frequency information up to 12 kHz or more would be included in order to

maintain important spectral info (Hebrank & Wright, 1974; for review: Section 1.1.3).

Listeners would also benefit from increased access to spectral cues if more channels with smaller bandwidths were present in the electric output of the speech processor (Friesen et al., 2011; Goupell et al., 2010). Increasing the listeners’ access to spectral cues should improve their ability to localize sound sources throughout the horizontal plane.

Interaural level difference sensitivity for all binaural listeners might be improved if changes were made to the AGC circuitry for all device configurations. First, BiCI users’ sensitivity to ILD is reduced when AGC circuitry is inactivated (Grantham et al., 2008). Inactivation of the AGC circuitry has been shown to improve ILD sensitivity in BiCI users; presumably its inactivation would also be beneficial for ILD sensitivity for other CI users. However, inactivation of this circuitry did not lead to improved localization performance (Grantham et al., 2008) which suggests that acclimatization to the electric input might have accounted for the improved localization performance when the AGC circuitry was active. Therefore, activation of the AGC circuitry might improve ILD sensitivity but does not necessarily improve localization performance in binaural listeners. Inactivation of this circuitry might also degrade speech perception because low-level speech would be difficult to hear and high-level speech might be too loud and might become distorted. As a result, it is possible that improvements in ILD sensitivity gained through inactivation of the AGC circuitry would diminish performance in other aspects of daily life. Second, all users’ sensitivity to ILD might be improved if the AGC circuitry were coordinated between devices. For example, CI+HA users’ sensitivity to ILD may be reduced because of distortions in ILD cues resulting from the AGC circuitry in the HA amplifying sounds that are low intensity at the aided ear while the AGC circuitry in the CI neither amplifies nor compresses the same sounds. In this example, the ILD cue would be reduced thus making it more likely that the CI+HA user would have greater ILD-JNDs.

Interaural level difference sensitivity might also be improved in users with matched bilateral device stimulation if care was taken to match the place of stimulation within the cochlea. It has been shown that BiCI users’ ILD sensitivity is best when stimulated electrode pairs are matched for place of stimulation along the electrode array (Laback et

al., 2004; van Hoesel et al., 1993; Section 1.5.3). Therefore, ILD sensitivity might be

improved if, during surgery, placement of the electrode array was matched between ears and if fitting involved testing to identify which electrode pairs elicited similar pitch percepts. Bilateral fitting procedures could then attempt to optimize stimulation of pitch- matched electrodes. Listeners with mismatched bilateral stimulation might also benefit from the same principles because sensitivity to ILD would improve. Benefits might be gained if frequencies encoded by electrodes in the apical region of the basilar membrane spanned the same frequencies as those in the acoustic signal of the aided ear ILD. Lastly, ILD sensitivity might also be improved if speech processing strategies maintained all of the filtered channels in the electric output. As speech processing strategies, like

ACE, n-of-m and SPEAK (Section 1.4.1), remove spectral channels based on their

relative amplitudes, channels containing ILD information may be removed in one or both ears for BiCI and BiEAS users. Therefore, maintaining all channels in the electric signal might improve sound localization by increasing ILD sensitivity and in turn improve sound localization abilities. However, this might also degrade speech perception because speech processing strategies like ACE (etc.) eliminate filtered channels in the electric output in order to reduce masking of the electric signal by adjacent electrodes.

Interaural time difference sensitivity might be improved in all listeners if bilateral devices were coordinated to optimize stimulation of the auditory nerves In the case of CI+HA and EAS+HA users, imposing a delay on the electric signal relative to the acoustic signal might improve ITD sensitivity. Francart, Brokx and Wouters, (2008a) sought to improve ITD sensitivity in CI+HA users by manipulating when the electric signal stimulated the implanted ear relative to the aided ear. They found that mean delays of 1.5-ms in the electric signal resulted in the perception of a centered sound image in the individuals studied. In principle, delaying the CI output in both BiEAS and EAS+HA users so that the bilateral HAs were coordinated in time with the electric signal might also improve

these listeners’ ITD sensitivity. Interaural time difference sensitivity might be improved in BiCI and BiEAS users if coordinated bilateral speech processors were used. The use of coordinated bilateral processors would reduce, if not remove, the activation delay between the two ears (van Hoesel et al. 2002). Coordinated bilateral processors would also ensure that the jitter in the pulse rates between ears would be the same (van Hoesel et al. 2002). Lastly, speech processing strategies that maintain all of the filtered channels in the electric output might improve ITD sensitivity for the same reasons as described above. Therefore, maintaining all the channels in the electric signal might improve sound localization by increasing ITD sensitivity and in turn improve sound localization abilities. If effective, these changes might also benefit real CI users’ in their ability to understand speech perception in noise if maintaining all the information did not degrade speech perception (see previous paragraph).