One benefit of listening with two ears versus one ear for individuals with normal hearing is the ability to localize sound sources on the horizontal plane with high accuracy - i.e. 6-7° of error [e.g. Grantham et al., 2007; Yost et al., 2013]. Localization ability is contingent on access to interaural level difference (ILD) cues in high frequencies (above 1.5 kHz) and/or interaural time difference (ITD) cues in low frequencies (under 1.0 kHz) [Blauert, 1997]. In this report we describe the sound source localization abilities of patients who have undergone hearing preservation cochlear implant (CI) surgery and who have two ears with low-frequency (LF) acoustic hearing. At issue with these patients is the level of sound source localization performance that is allowed by access to the ITDs available in the bilateral areas of LF acoustic hearing.
Individuals with relatively good LF hearing and precipitously sloping high-frequency hearing loss can benefit from a surgical technique for cochlear implantation that preserves the LF hearing in the implanted ear and therefore have bilateral LF acoustic hearing. Gifford et al. [2013] reported that hearing preservation patients are able to resolve ITDs although not as well as normal-hearing (NH) listeners. Six listeners with preserved hearing had ITD thresholds that ranged from 131 to 1,271 µs compared to NH listeners with a range of 30-60 µs for signals at 250 Hz. Given these data, it is reasonable to suppose that some hearing preservation patients would be able to localize sound sources on the horizontal plane - but with less accuracy than NH listeners.
A study by Dunn et al. [2010] suggests this is the case. Patients using a short electrode array of 10 mm and bilateral hearing aids were tested on localization. Dunn et al. [2010] reported that hearing preservation listeners could localize with a root mean square (RMS) error of about 25°.
The first aim of this project was to attempt to replicate the results of the Dunn et al. [2010] study. The second aim was to filter stimuli to better constrain the availability of ILD and ITD cues. The third aim was to extend our knowledge of sound source localization by hearing preservation patients by (i) testing patients with deeper electrode insertions than those used by the patients in Dunn et al. [2010], (ii) testing patients with and without symmetrical LF acoustic hearing and (iii) determining whether hearing aids have a significant effect on sound source localization accuracy.
Methods
Subjects
Twelve adult CI users with hearing preservation and a minimum of 1 year of CI use were tested following approval by the IRB at Arizona State University. All but 2 of the participants had been, or were enrolled in, the clinical trials for either the MED-EL EAS or the Cochlear Nucleus Hybrid device. Two of the MED-EL participants did not participate in the EAS clinical trial but had preserved hearing in the implanted ear. Eight subjects had symmetrical LF acoustic hearing, i.e., differences no greater than 15 dB between ears at 250 Hz (fig. 1a).
Fig. 1
a Mean audiometric thresholds for the hearing preservation patients with symmetrical LF hearing (n = 8). Squares indicate thresholds for the implanted ear. Error bars indicate ±1 SEM. b Mean audiometric thresholds for the patients with asymmetrical LF hearing (n = 4).
Four subjects lost a significant level of hearing resulting in asymmetrical LF hearing with differences of 45-60 dB at 250 Hz between ears (fig. 1b). Three of the subjects lost hearing following surgery and prior to activation. One subject lost hearing in the implanted ear 7 years postoperatively due to an autoimmune disorder. This subject was tested approximately 3 months following the loss of hearing in the implanted ear. Typically, the poorest ear is chosen for this type of surgery, and as such, preimplant audiometric thresholds would not be better than the unimplanted ear. Listeners used their preferred program on their own processors. Demographics for hearing preservation listeners are provided in table 1.
Hearing Aids
All hearing preservation subjects used their own behind-the-ear hearing aid on the contralateral ear. Participants used their everyday settings. However, hearing aids were evaluated using real ear measurements to assess whether their settings met NAL-NL1 prescriptive targets [Dillon et al., 1998] in the LF region. For the symmetrical listeners, the prescriptive target was met for both hearing aids. For 3 of the participants with asymmetrical hearing, the processor-integrated hearing aid was unable to meet target due to the degree of hearing loss, even with gain settings set to maximum. All subjects with asymmetrical hearing reported that the addition of the hearing aid reduced listening effort. Critically, every subject with asymmetrical hearing showed improved performance on at least one measure of speech understanding in the ipsilateral hearing aid plus CI condition compared to the CI-alone condition.
Test Stimuli
Three 200-ms filtered (48 dB/octave) Gaussian noise stimuli of different spectral content were presented in random order. The stimuli were (i) low-pass (LP) noise-filtered from 125 to 500 Hz, (ii) high-pass (HP) noise-filtered from 1,500 to 6,000 Hz and (iii) wideband (WB) noise-filtered from 125 to 6,000 Hz.
Test Environment
Testing was conducted in an 11 × 15 foot sound-deadened room. The stimuli were presented from a 13-loudspeaker array with an arc of 180° in the frontal horizontal plane. There was 15° of separation between loudspeakers. To reduce edge effects, stimuli were not presented from loudspeakers 1 (far left) and 13 (far right). Loudspeakers were placed 1.67 m from the listener's head and were at the level of the listener's pinnae.
Test Conditions
Presentation of the stimuli was controlled by Matlab. Four blocks of 33 trials each were presented at 65 dBA. Each stimulus (LP, HP, WB) was presented 4 times per loudspeaker resulting in 132 presentations. Overall level was roved ±2 dB to ensure that small differences between the output of the loudspeakers were not a cue.
Listeners were evaluated in the following four conditions, which were counterbalanced among subjects: (i) unaided, no CI, (ii) unaided plus CI, (iii) bilaterally aided, no CI, and (iv) bilaterally aided plus CI. None of the hearing preservation listeners were able to hear the HP stimuli without the CI due to the severity of their high-frequency hearing loss. Therefore, the HP condition was eliminated for the unaided and aided conditions without the CI but was administered in the unaided and aided conditions using the CI.
Subjects were instructed to look at a red dot on the center speaker (speaker 7) at midline until a stimulus was presented. Each subject identified the speaker of the sound source by pushing a button on a numbered keypad corresponding to the number of the loudspeaker.
Results
RMS error in degrees was calculated after Rakerd and Hartman [1986] using the D statistic. Chance performance was calculated using a Monte Carlo method of 100 runs of 1,000 Monte Carlo trials. Mean chance performance was 73.5° with a standard deviation of 3.2° for the three noise stimuli.
To provide a reference level of sound source localization accuracy, i.e. for NH listeners, we have used data from Yost et al. [2013] using the same room and with the same stimuli.
Because hearing asymmetry is known to affect sound source localization [Moore, 1996; Simon, 2005], the hearing preservation group was divided into two groups for all statistical analyses - patients with symmetrical LF hearing at 250 Hz and those with large asymmetries at 250 Hz.
The results for the NH listeners and the two groups of hearing preservation patients are shown in figure 2.
Fig. 2
Localization error as a function of spectral content for NH listeners and for hearing preservation listeners with symmetrical (symm.) and asymmetrical (asymm.) LF hearing in the combined condition (bilateral hearing aid plus CI). The gray bar represents ±1 standard deviation for chance performance. The dotted horizontal lines indicate mean scores. The vertical dotted lines are included to facilitate visual segregation of the data from the three listener groups. * p < 0.01.
Patients with Symmetrical Hearing Loss
For the 8 patients in this group, the mean sound source localization accuracy in the combined condition (CI plus bilateral hearing aids) for the LP, HP and WB stimuli were 23, 58 and 33° of error, respectively. A repeated-measures ANOVA revealed a main effect for conditions (F2, 23 = 19.6, p = 0.0006). Post tests (Holm-Šídák) indicated that (i) the scores in the LP condition differed from those in the HP condition, (ii) the scores in the HP condition differed from those in the WB condition and (iii) that the scores in the LP condition did not differ from those in the WB condition - although the mean scores suggest poorer performance in the WB condition. We return to this issue in the section on the effect of hearing aids on performance.
Patients with Asymmetrical Hearing Loss
For the 4 patients in this group, the mean sound source localization accuracy for the LP, HP and WB stimuli in the combined condition was 76, 60 and 50° of error, respectively. Both aided and unaided results for the low-passed condition were at chance levels of performance for all 4 listeners. The small number of listeners precluded a useful statistical evaluation of the differences in mean scores. However, inspection of figure 2 reveals that none of the patients in the asymmetrical hearing loss group performed as well as the patients in the symmetrical hearing loss group when the stimulus was an LP noise signal, i.e., the signal that maximized the availability of ITD cues.
Effect of Hearing Aids and CIs on Sound Source Localization
This analysis compares the performance of patients who showed symmetrical LF hearing loss (i) with and without amplification for their acoustic hearing and (ii) with and without the CI. The signals were the LP and WB noise signals. The results are shown in table 2. Inspection of table 2 indicates that, for the LP signal, neither amplification nor the use of a unilateral CI significantly altered sound source localization performance, i.e. all mean error scores were between 19 and 23°.
Table 2
RMS errors for LP and WB stimuli for hearing preservation listeners with symmetrical LF hearing in the unaided and aided conditions with and without the CI
A similar inspection of table 2 for the WB stimulus suggests a different outcome. For this stimulus, the presence of a unilateral CI did not alter the mean error scores, but amplification did. In figure 3, the scores from the WB unaided condition (without and with CI) and for the WB aided condition (without and with CI) are plotted. Performance in the aided and unaided conditions differed significantly: aided = 33° of error, unaided = 22° of error (t15 = 3.562, p = 0.0038).
Fig. 3
Localization error for the wideband condition for hearing preservation patients with symmetrical LF hearing loss in the unaided and aided test conditions. Each patient in each group contributed two scores. The unaided condition consists of responses when tested with a unilateral CI and one without a unilateral CI. The bilaterally aided condition consists of listening with and without the CI for each listener. * p < 0.01.
Discussion
The present study has replicated and extended the work of Dunn et al. [2010]. These authors reported for patients with shallow (10-mm) electrode arrays that (i) the mean sound source localization error, to spectrally and temporally complex signals, was about 25° and (ii) that the presence of a unilateral CI used in conjunction with bilateral LF acoustic hearing did not detrimentally alter sound source localization accuracy. We have obtained similar outcomes for patients with deeper electrode insertions (nominally 16-24 mm). We find a mean sound source localization error of 33° for a WB stimulus and no deleterious effect of a unilateral CI combined with bilateral LF acoustic hearing.
ITDs and Sound Source Localization to the LP Stimulus
In the work reported here, the LP stimulus served to reduce the possibility that ILD cues were used for sound source localization. That is, over the range 200-500 Hz, maximum ILDs are small - from 3 to 6 dB [e.g. Shaw, 1974]. It is reasonable to suppose that the performance of the patients with symmetrical LF hearing loss in the combined condition (bilateral hearing aids plus one CI), e.g. 23° of error, reflects use of ITD cues.
LF Symmetry and Sound Source Localization Ability
Our research extends the work of Dunn et al. [2010] by documenting that large asymmetries in LF hearing between ears have a detrimental effect on sound source localization accuracy. Listeners with asymmetrical LF hearing showed sound source localization to the LP stimulus in the combined condition that was near the level of chance performance and the mean level in response to the WB stimulus was 50° of error. One practical consequence of these outcomes is that, before surgery, patients should be told that they will localize reasonably well, following surgery, only if there is minimal additional hearing loss in the operated ear.
Hearing Aids Impair Sound Source Localization Performance for WB Stimuli but Not LP Stimuli
We have found that sound source localization errors in response to the WB stimulus were larger by about 10° in conditions where amplification was provided for acoustic hearing than in conditions in which amplification was not provided. In contrast, errors in response to LP stimuli were not affected by the presence of amplification.
Our data do not speak to the mechanisms underlying the poorer performance using the WB stimulus in the amplified test conditions [see and contrast localization results by Boymans et al., 2008; Kobler and Rosenhall, 2002; Van den Bogaert et al., 2006 for patients with and without conventional hearing aids). However, given the steeply sloping hearing losses and poor thresholds above 500 Hz, it is likely that dead regions were present [Zhang et al., 2014] and amplification into dead regions could distort relevant information for localization [e.g. Moore, 2004]. Finally, we note that outside of the laboratory, the patients were accustomed to listening to WB stimuli with amplification and with a single CI. This, however, was not the condition that allowed the best sound source localization performance. Because sound source localization was best in test conditions that were relatively ‘unpracticed', i.e. those without amplification, we suspect that amplification was indeed detrimental to sound source localization ability.
All of our subjects used different hearing aids on each ear - that is, they used a hearing aid coupled to the processor on their CI ear and used a conventional behind-the-ear hearing aid on their contralateral ear. More research needs to be conducted to determine whether other schemes for amplification would produce different results.
Summary
Hearing preservation patients with symmetrical LF acoustic hearing coupled with a single CI are able to locate sound sources on the horizontal plane, in the most favorable test conditions, with approximately 20° of error. Test performance, in response to LP stimuli, suggests that the patients were using ITD cues for sound source localization. The presence of a unilateral CI combined with bilateral LF acoustic hearing does not impair sound source localization accuracy, but amplification for acoustic hearing can be detrimental to sound source localization accuracy. Finally, patients with asymmetrical LF hearing loss show much poorer results than patients with symmetrical LF hearing.
Acknowledgments
Author L.H.L. was supported by NIDCD F31DC011684 and MED-EL Corp.; M.F.D. by NIH R01 DC 010821; R.H.G. by NIH R01 DC009404, and W.A.Y. by AFOSR FA9550-12-1-0312. Portions of the data in this article were presented at The World Congress of Audiology 2014 in Brisbane, Australia and at CI 2014 in Munich, Germany.
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