A Comparison of CIC and BTE Hearing Aids for Three-Dimensional Localization of Speech
Best, V., Kalluri, S., McLachlan, S., Valentine, S., Edwards, B. & Carlile, S. (2010)
Localization of external sound sources is achieved in a number of ways. In additional to visual cues, listeners use binaural time and intensity differences to localize sounds on a horizontal plane (Woodworth, 1938). Monaural spectral cues provide additional information about vertical location and help differentiate sound sources that are in front of or behind the listener (Blauert, 1997). There is ample evidence that localization of sound sources may be an important first step in the perception of speech in complex listening environments (Freyman et al, 2001; Bregman, 1990; Arbogast et al, 2002). Several studies have shown that hearing aid users demonstrate poorer aided localization than when unaided (Noble & Byrne, 1990; Byrne et al., 1992; Keidser et al, 2006; Vanden Bogaert et al, 2006). This is thought to be due to disruption of binaural time and intensity cues by bilateral hearing aids. Therefore, monaural localization cues are valuable hearing aid wearers and may have particularly important implications for their ability to understand speech in noisy situations.
Two factors known to reduce the availability of monaural spectral cues are of particular relevance to hearing aid users: reduced audible bandwidth (Butler, 1986; Middlebrooks, 1992; Blauert, 1997) and sensorineural hearing loss (Byrne et al, 1992; Noble et al, 1994, 1997; Byrne & Noble, 1998; Rakerd et al, 1998). These factors reduce spectral cue localization because of decreased audibility of high frequencies. Sensorineural hearing loss is accompanied by decreased frequency resolution, which can itself impair spectral cue localization (Jin et al, 2002). Additionally, hearing aid users lose pinna-related spectral cues, particularly with behind-the-ear (BTE) models in which the microphone is placed above the pinna. Completely-in-the-canal (CIC) instruments are thought to preserve pinna-related spectral localization cues because of microphone placement at the ear canal entrance.
The purpose current study was to contrast spatial localization abilities in users with CIC and BTE hearing aids and normal hearing listeners. Two measures of localization were analyzed:
– Lateral localization (horizontal localization – left/right with reference to midline)
– Polar localization (encompassing up/down and front/back dimensions)
The authors recruited eleven subjects with mild to moderate sensorineural hearing loss and four subjects with normal hearing. Hearing-impaired subjects were fitted with CIC and BTE instruments. All hearing instruments had 1.5 mm vents and both CIC and BTE instruments had bandwidth out to approximately 6800Hz. Directional microphones, noise reduction processing and environment classification features were disabled. Hearing aids were programmed to match CAMEQ gain targets (Moore, 1999) and fittings were verified with real-ear measurements. Prior to localization testing, additional probe microphone measurements were conducted to determine aided audibility of the speech stimuli to be used in the test sessions.
Hearing-impaired subjects were tested with both CIC and BTE hearing instruments after a period of “accommodation” or acclimatization to each type of instrument. The experiment was therefore conducted in six phases:
1. Localization testing (both hearing aids)
2. Accommodation period (4-6 weeks, hearing aid A)
3. Localization testing (hearing aid A)
4. Accommodation period (4-6 weeks, hearing aid B)
5. Localization testing (hearing aid B)
6. Localization testing (unaided)
Subjects with normal hearing were tested under two conditions. In one condition, the speech was a broadband stimulus (up to 40,000Hz) and in the other it was low-pass filtered at 6800Hz to approximate the bandwidth of the hearing instruments worn by the hearing-impaired subjects.
Listeners were presented with monosyllabic words at an average level of 65dB for hearing-impaired listeners and 55dB for normal hearing listeners. Subjects were asked to “point their nose” toward the perceived location of the speech. Testing was completed in an anechoic chamber and head orientation was monitored with an electromagnetic tracking system.
The results indicated that for lateral localization errors, there was no difference between CICs and BTEs, no significant difference between aided and unaided results, nor was there a significant effect of accommodation. Performance for normal hearing subjects was more accurate than that of the hearing-impaired subjects. There was a great deal of variability among hearing-impaired subjects; those with poorer low-frequency thresholds had increased lateral localization errors. Previous studies have shown that aided lateral localization is usually worse than unaided and the authors surmised that the performance of the subjects in this study could have been related to their relatively good low-frequency hearing thresholds or the availability of airborne sound through the hearing aid vents.
Analysis of polar angle localization errors yielded similar results. There was no significant effect of hearing aid use, hearing aid style or accommodation. Performance was substantially better for normal hearing subjects, regardless of bandwidth condition, though errors were slightly greater for the limited bandwidth condition. Although vertical localization in particular was expected to be related to the availability of high-frequency cues, no significant correlational was found for unaided individual performance and high-frequency pure-tone thresholds, or aided results and high-frequency aided sensation level.
Performance with CIC instruments yielded significantly fewer front/back reversals than performance with BTEs and results for both hearing aid types showed significant improvement after accommodation periods. Unaided responses were more accurate than either aided condition and subjects with normal hearing did better than hearing-impaired subjects in any condition. The front/back reversal rate was not correlated with high frequency audiometric thresholds or aided sensation levels, nor was the benefit of CICs over BTEs correlated with high Hz sensation level. Previous research shows that front/back localization is primarily related to conchal resonance, which occurs around 4000-5000Hz (Hebrank & Wright, 1974). CIC microphone placement should allow for preservation of these cues, whereas BTE configurations would not. Interestingly, unaided performance in the current study was still better than aided, despite the likelihood that cues in the 4000-5000Hz range would have been inaudible for these subjects without their hearing aids.
The results of this study indicate that hearing-impaired listeners are likely to experience some decreased sound localization ability relative to normal hearing listeners, regardless of hearing aid style. The degree to which localization ability is affected may be related to audiometric thresholds, venting, directionality, compressions settings and other variables. Though lateral and vertical localization was not affected by hearing aid microphone location in this study, CIC instruments afforded better front/back localization than BTE devices. It is possible that new hearing aid technology will allow for enhanced spectral cue availability. For instance, improvements in feedback control allow more stable high-frequency gain and new, deep- fitting CIC instruments may increase the availability of ear canal and pinna-related spectral cues.
The decrease on front/back localization errors following accommodation periods in this study underscores the importance of acclimatization to new hearing aids. Improvement in localization ability over time is not necessarily something that would warrant adjustments to hearing aid settings, but it should be discussed with new hearing aid users with reference to their expectations during the trial period and thereafter.
Though sound source localization is important for speech perception in complex listening environments, it should be noted that the hearing instruments in this study were programmed without directionality. For many hearing aid users, directional microphones will improve the ability to understand primary speech stimuli in front of the listener so binaural and monaural localization cues may be of decreased significance in some circumstances.
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