by Francis Kuk, PhD, and Petri Korhonen, MS for HearingReview.com
Localization is the ability to tell the direction of a sound source in a 3-D space. The ability to localize sounds provides a more natural and comfortable listening experience. It is also important for safety reasons such as to avoid oncoming traffic, an approaching cyclist on a running path, or a falling object. Being able to localize also allows the listener to turn toward the sound source and use the additional visual cues to enhance communication in adverse listening conditions.
Hearing loss results in a reduction of localization ability1; however, the use of amplification may not always restore localization to a normal level. Fortunately, the application of digital signal processing and wireless transmission techniques in hearing aids has helped preserved the cues for localization.
A sound source has different temporal, spectral, and intensity characteristics depending on its location in the 3-D space. A listener studies these characteristics (which are described as cues) originating from all directions on the horizontal and the vertical planes to identify the origin of the sound.
Horizontal plane. Localization on the horizontal plane involves comparison of the same sound received at the two ears (ie, binaural comparison for left/right) or between two surfaces of the same ear (ie, front/back).
Vertical plane. Byrne and Noble1 demonstrated that sound sources striking at the pinna from different elevations result in a change in spectra of the sound, especially above 4000 Hz. This change in spectral content may provide a cue on the elevation of a sound source.
Monaural versus binaural cues. Because localization typically involves the use of two ears, people with hearing in only one ear (either aided monaurally or single-sided deafness) should have extreme difficulties on localization tasks. However, some people in this category still report some ability to localize. Frequently, a person moves their head/body to estimate the direction of a sound. The overall loudness of the sound and the spectral change of the sound as it diffracts the pinna from different orientations act as cues. This spectral cue relies on a stored image of the sound, so cognition is involved with monaural cues.3
Physiological structures in the brainstem related to localization. The superior olivary complex (SOC) in the pons is the most caudal structure to receive binaural inputs. This suggests that the low brainstem is the most critical to binaural interaction.
Any lesion that affects input to the SOC could disrupt the cues for localization. Because cognitive factors (including memory, expectations, and experience) also could affect localization, it is possible that localization performance is not easily described by lesions just to one structure, but to many structures within the brain. It also makes it difficult to predict localization performance through the audiogram and any one set of information.4
Prevalence and ramifications. One conclusion from almost every study is that hearing-impaired people have poorer localization ability than normal-hearing people. The use of amplification may improve localization in that it provides the needed audibility. However, it is common to find that aided localization is poorer than unaided localization when tested at the wearer’s most comfortable listening level (ie, audibility is not involved).
In general, left/right discrimination is only mildly impaired in hearing-impaired listeners; front/back discrimination is consistently impaired1 to the point that it is poorer than the unaided condition.5
These observations were replicated in our internal study at the ORCA-USA facility. Figure 1 shows the accuracy of localization for sounds presented from 12 azimuths in the unaided condition at a 30 dB SL (sensation level) in 15 experienced hearing aid wearers. For ease of presentation, the results across the 12 loudspeakers were collapsed to only front, back, left, and right quadrant scores. The results of using two scoring criteria were reported: “0 degree” criterion requires absolute identification and a “30 degree” criterion suggests that the subjects’ response may be ±1 loudspeaker from the target sound source.
There are two clear observations. First, localization accuracy for sounds is impaired in this subject population. Even with the 30 degree criterion, subjects were only 60-70% accurate in localizing sounds from the front, left, and right. With a 0 degree criterion, it is only between 30% and 45%. Second, localization of sounds from the back was extremely poor. A score of 30% was seen with the 30 degree criterion; the percentage dropped to 20% when a 0 degree criterion was used.
Several authors have suggested that being able to focus one’s attention to a sound source in an adverse listening environment (eg, cocktail party) may help improve speech understanding and communication.6,7 Furthermore, Bellis4 indicated similar processing mechanisms are used in the brainstem for speech in noise task and localization task. There have been bimodal studies8 (eg, one cochlear implant and one hearing aid) that demonstrated a significant correlation between localization ability and speech recognition in bimodal and bilateral cochlear implant subjects. If this can be generalized to other hearing-impaired patients, it would suggest that an improvement in localization could result in an improvement in speech understanding in noise or communication in adverse listening conditions. It is unclear if improving speech-in-noise performance may lead to an improved localization performance. Currently, neither one of these hypotheses has been validated.
Despite the use of amplification, aided localization is still poorer than that of normal hearing listeners.9 In some cases, aided localization ability may be even poorer than unaided localization when tested at the same sensation level.
This has two important implications. First, audibility is not the only issue that governs localization ability. Otherwise, ensuring audibility by testing at the same sensation level should restore localization to a normal level for all wearers. Additional means to restore localization are also needed. Second, the manner in which amplification is applied could affect the aided localization performance. This is seen in the poorer aided performance than the unaided performance.
From the device manufacturer’s perspective, it is important to understand the reasons for the poorer performance, and design better algorithms so the aided performance can be as “normal” as possible. The following are some reasons for the poorer aided performance.
When a pair of hearing aids that provide linear processing is worn, the same gain is applied to both ears. This preserves the inter-aural level difference. A WDRC hearing aid provides less gain to the louder sound but more gain to the softer sound (the opposite ear). This will reduce the interaural level difference between ears and could affect the accuracy of sound localization to the sides of the listener. For example, Figure 3 shows that the interaural level difference may decrease from 15 dB to 10 dB from the use of fast-acting WDRC hearing aids.
The components of a hearing aid (including the DSP chip) could introduce delay to the processed signal. The impact of the delay is minimal if the extent of the delay is identical between ears. However, if the delay is different between the two ears, the time of arrival (or phase) will be different, and that may result in a disruption of the localization cues.
Drennan et al12 determined the ability of hearing-impaired listeners to localize and to identify speech in noise using phase-preserving and non-phase-preserving amplification. At the end of 16 weeks of use, speech understanding in noise and localization was demonstrated to be slightly better with the phase-preserving processing than non-phase- preserving processing.
Hearing aids that destroy this pinna cue could result in front/back localization problems. Because the microphones of a BTE/RIC are typically over the top of the ear and are not shadowed by the pinna, wearers of BTE/RICs are more likely to be deprived of this pinna cue. Because the microphones of custom products (CIC/ITE/ITC) are at the entrance of the ear canal and are shadowed by the pinna, it is likely that wearers of these hearing aid styles will still retain the pinna cues. Indeed, Best et al13 compared the localization performance of 11 listeners between BTE and CIC fittings and found fewer front/back reversal errors with the CIC than the BTE. The loss of pinna cues could affect a significant number of hearing aid wearers because over 70% of hearing aids sold in the United States are BTE/RIC styles.14
One must provide the necessary acoustic cues so the impaired auditory system has as much accurate information to process as possible. Appropriate training may be necessary to retrain the altered brain to interpret the available information.
In summary, the use of bilateral wireless hearing aids that exchange compression parameters between ears of the custom style (CIC, ITE/ITC) with a large vent and a broad bandwidth should provide the best potential to preserve any localization cues. The availability of a directional microphone may further enhance front/back localization. If BTE/RICs are desired, broadband bilateral wireless devices with inter-ear features and pinna compensation algorithm fitted in an open mode (or as much venting as possible) are appropriate. The addition of a directional microphone also enhances the effectiveness.
What else can be done to improve localization? Despite having the optimal acoustic cues, the brain may have been accustomed to the old pattern of neural activity so that the new cues may decrease performance. What needs to be available is rehabilitation programs that can instruct these wearers how to understand and utilize the new cues.
Wright and Zhang23 provided a review of the literature that suggests localization can be learned or relearned. The reviewed data indicated human adults can recalibrate, as well as refine, the use of sound-localization cues. Training regimens can be developed to enhance sound-localization performance in individuals with impaired localization abilities. This will be further discussed in a subsequent paper on the topic.
Read this article on HearingReview.com.
Figure 1. Unaided localization accuracy of experienced hearing aid wearers (N=15) for sounds originating from 4 quadrants at a 30 dB SL input level and scored using two criteria (0 degree and 30 degree).
Figure 2. Inter-aural level difference in the unaided condition.
Figure 3. Inter-aural level difference in a fast acting WDRC hearing aid.
Figure 4. In-situ directivity index (DI) of an omnidirectional microphone on a BTE, of the natural ear with pinna, and of the pinna compensation algorithm microphone system used in the Dream.
Original citation for this article: Kuk F, Korhonen P. Localization 101: Hearing aid factors in localization. Hearing Review. 2014;21(9):26-33.
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