Page 23 - Volume 12, Issue 2 - Spring 2012

Fig. 3. Schematic diagram of the nine possible conditions (3 masker configurations x 3 listening modes). Binaural listening conditions (1,2,3) are in the top row, Left-ear conditions (4,5,6) in the middle row, and Right-ear listening conditions (7,8,9) in the bottom row. Target (T)-Masker (M) in front occurs in the three conditions in the left column (1,4,7). Masker on the left is shown in the conditions in the middle column (2,5,8) and masker on the right is shown in the three conditions in the right column (3,6,9).
monaural “better ear” or “monaural head shadow” cue, which arises when the monaural condition has an ear with a poor SNR, and a contralateral ear with a better SNR is added (activated) to create the bilateral listening mode. This benefit would occur by comparing speech intelligibility in conditions 5 and 2 or 9 and 3 (see Fig. 3). A smaller effect is also seen when the monaural condition has the ear with the better SNR and the ear with the poorer SNR added to create the bilater- al listening condition (known as the squelch effect). This benefit would occur by comparing speech intelligibility in conditions 8 and 2 and 3 and 6. A third effect known as bin- aural “summation” is also observed, when both target and interferer are in front, and unilateral vs. bilateral listening conditions are compared; the addition of the second ear improves speech reception thresholds. This benefit would occur by comparing speech intelligibility in conditions 1 and 4 or 1 and 7.
When performance is measured at fixed SNR (Agrawal, 2008; Litovsky et al., 2012) and competitors are placed in the left-right configuration shown in Figure 2C, rendering the “head shadow” weak or absent, bilateral CI users show marked decline in performance compared with normal hear- ing subjects. Effectively, bilateral CI users need the SNR to be significantly more favorable than do normal hearing people to understand the target speech. This difference between the two populations suggests that, while the use of bilateral CIs provides a benefit over monaural CIs, as described above, in a listening situation with maskers presented from both the right and left, the ability of bilateral CI users to hear speech in noise is markedly worse than that of normal-hearing lis- teners. Finally, in terms of SRM measures per se, a recent study in bilateral CI users (Loizou et al., 2009) controlled for monaural and binaural cues while still preserving spatial cues that occur in the free field. In this study, target and maskers were convolved through head related transfer functions that had been measured through the ear canals of a human-like manikin. Stimuli were provided to listeners via direct con- nect input to the auxiliary port of the CI in each ear. Results
showed that SRM (quantified as SRTfront-SRTside) due to bin- aural interaction was about 0 dB, in contrast with 6 dB in normal-hearing listeners. SRM due to monaural cues was about 4 dB in both groups of listeners, suggesting that when the CI microphones are bypassed at least the monaural head shadow cue observed in normal-hearing listeners is retained.
A small number of studies in children who receive bilat- eral CIs have also been conducted on this subject. Results suggest that the primary spatial cue used by these children is the monaural head shadow cue; either they do not have access to, or do not utilize binaural cues. The most illustrative example comes from a recent study by Misurelli and Litovsky (2012), in which maskers were placed in the three configura- tions shown in Figure 2 (A, B, and C). SRM was measured for both the front vs. side conditions and front vs. right-left con- ditions. First, it is noteworthy that children perform better when both CIs are used compare with monaural listening conditions (Litovsky et al., 2006b, 2012). Second, children aged 4-9 years showed SRM with the latter conditions, but not the former. That is, when monaural head shadow was present, these children displayed SRM, albeit not as large as that seen in age-matched normal-hearing children. However, when maskers were placed towards both the right and left, greatly reducing or eliminating the head shadow cue, SRM was eliminated. Thus, children who are bilaterally implanted can use spatial cues to segregate target from maskers, howev- er, this effect is dominated by their ability to benefit from having a good SNR in one ear, rather than from being able to integrate inputs arriving at the two ears. It is possible that the binaural integration ability is acquired with listening experi- ence. However, it is important to review the fact that bilater- al CIs are not very effective at preserving binaural cues. While the former consist of cues that are presented to the two ears, but not necessarily in a manner that preserves the syn- chronization of inputs to the two ears, the latter specifically refers to stimulation consisting of well-preserved and syn- chronized inputs in the right and left ears.
Outcomes in patients who use bilateral CIs are curtailed
22 Acoustics Today, April 2012