Tech Topic: Localization | July 2015 Hearing Review
While the Contralateral Routing of Signals (CROS) hearing aids or Bilateral Contralateral Routing of Signals (BiCROS) hearing aids may achieve audibility of sounds originating from the side of the unaidable ear for people with this hearing loss configuration, localization can still be challenging. This paper discusses how we may enable CROS/BiCROS wearers to improve their localization abilities.
We have previously examined how different hearing aid factors (Localization 101, September 2014 Hearing Review)1 and the use of home-based localization training programs (Localization 102, January 2015 HR)2 helped improve front/back sound localization for hearing aid users who have a symmetrical hearing loss. For this group of individuals, left/right localization is typically not the major difficulty they encounter.
This may not be the case for individuals with an asymmetrical or unilateral hearing loss.3-5 While the Contralateral Routing of Signals (CROS) hearing aids or Bilateral Contralateral Routing of Signals (BiCROS) hearing aids may achieve audibility of sounds originating from the side of the unaidable ear for people with this hearing loss configuration, localization can still be challenging. This paper discusses how we may enable CROS/ BiCROS wearers to improve their left/right localization.
Why Is Localization Difficult for People with Unilateral Hearing Loss?
Individuals with an asymmetric or unilateral hearing loss, where one ear is aidable (or normal) and the other ear is not, can only receive auditory input from one ear. The loss of auditory input from one ear removes any potential for the brain to take advantage of binaural inputs and enjoy binaural benefits.
For example, the intensity difference between ears for a sound presented on one side results in an inter-aural timing difference (ITD) below 1500 Hz and an inter-aural level difference (ILD) above 1500 Hz. These differences provide vital information to the brain on the location of a sound in the left/right domain. A unilateral hearing loss means the absence of the ITD and ILD cues, and a lack of sufficient input to make the left/right discrimination.
Fitting individuals with one unaidable ear and one aidable ear with a unilateral hearing aid on the aidable ear may improve audibility for the aidable ear; however, not for sounds arriving at the unaidable ear because of the head shadow.6 Aiding the unaidable ear is not possible either because of the extent of the loss or because of the potential distortions from the unaidable side. In this case, no inter-aural cues are available because only one ear is aided. Thus, left-right localization remains challenging even in the unilaterally aided condition.
CROS and BiCROS hearing aids
A CROS or BiCROS device can improve audibility for sounds arriving at both the unaidable and aidable ears. However, this solution does not re-establish inter-aural cues that are essential for localization.
In a CROS or BiCROS system, a transmitter is worn on the unaidable ear and a receiver is worn on the aidable ear. A CROS hearing aid transmits sounds from the unaidable side to the ear with normal hearing. The receiver does not include an active microphone for sound pick up. In contrast, a BiCROS system has a microphone on the aidable side, which also picks up sounds and amplifies them along with the transmitted sounds from the side of the unaidable ear. Thus, sounds from both sides (aidable and unaidable) are delivered to the aidable ear for amplification.7
A CROS/BiCROS system alleviates the effects of the head shadow6 and provides audibility of sounds from the side of the unaidable ear. It offers improvements in speech recognition ability in quiet and in some background noise conditions when compared to the unaided and unilaterally aided conditions.8-12 Real-world subjective benefits were also supportive of the CROS/ BiCROS hearing aid.11,13,14
However, neither system restores true binaural functions in that sounds from both the aidable and unaidable sides are heard in the same aidable ear at the same or very similar levels. No inter-aural differences (time or intensity) exist between sounds from both ears. This makes localization of sounds a challenge with a CROS/BiCROS hearing aid.10
Optimizing the CROS/BiCROS hearing aid for localization
The direct transmission of sounds in a CROS/BiCROS system effectively eliminates any inter-aural differences and is perhaps the main reason for confusion in left/right localization. If one can recreate in a CROS/BiCROS system some kind of acoustic differences between sounds originating between the aidable and unaidable ears while keeping audibility, its wearers may be able to perceive this acoustic difference and use it in left/right localization.
The On/Off switch as a means to create acoustic difference. In an unaided condition, a sound that is presented on the side of the unaidable ear is typically not heard by the aidable ear, or heard at a lower volume (than it is if presented on the aidable side) because of the head shadow effect. With the CROS transmission, the sound from the unaidable ear will be heard at its full volume. If a CROS/ BiCROS wearer can learn the difference in sound quality/level between having the CROS and not having the CROS, such difference could serve as a cue for left/right localization.
The difference in sound level/quality can be created in the aided CROS condition by wearer deactivation of the CROS transmission. The wearer can be instructed to pay attention to the loudness of a sound while the CROS microphone is activated. Then he/ she is instructed to listen for the same sound while deactivating the CROS microphone. If the sound originates from the side of the unaidable ear, deactivating the CROS transmitter should eliminate that transmission path. The sound has to overcome the head shadow and be picked up at the aidable ear. This decreases the loudness of the sound. Because the magnitude of the head shadow can be as large as 15 dB SPL in the high frequencies,15 this magnitude of intensity difference can be used as a cue for the CROS/ BiCROS wearers to determine the location of a sound. A sound that originates on the aidable ear side or the front or back should not be affected by deactivating the CROS transmitter.
A CROS/BiCROS device that allows its wearers to create level differences is a step towards optimizing the CROS hearing aid for localization. This can be achieved with a convenient On/Off switch on the CROS transmitter of the CROS/BiCROS system. In such a case, the user can press the On/Off switch at any time and experience any potential loudness change as a means for left/right localization. A convenient On/Off switch also avoids the perils of removing the device or battery entirely. Such an on-board control was shown to be effective not only in improving localization, but also speech understanding in noise and subjective comfort.16
Localization Training as an Important Element for CROS/BiCROS Wearers
Having a convenient On/Off switch on the CROS transmitter is a first step in achieving left/right localization. An equally important, if not more important element, is convincing the wearer that such manipulation of the On/Off switch is beneficial, albeit seemingly tedious at first. This cannot be done simply by alerting the wearers to the presence of such a switch on the device, or instructing them how to press the On/Off switch only. Rather, personal experience with the effect of adjusting the On/ Off switch is needed for effective compliance.
This can be achieved through demonstra- tion and training. The CROS/BiCROS wearers should be instructed to use the device (CROS microphone activated) at all times. They should also be instructed to deactivate the CROS microphone when speech under- standing is difficult (too loud, too noisy) or when they desire to know the direction of a sound source (ie, localization). They should be provided the rationale for deactivating the On/ Off button and how it may aid localization.
A protocol on demonstration/training on the On/Off switch. The in-office demonstration/training on the use of the On/ Off switch should be part of the hearing aid orientation session. This may be on the same day as the hearing aid fitting (if the wearer is experienced), or on a follow-up visit if the wearer is a first-time user who already has a long list of things to remember relative to the proper use of the hearing aids.
The demonstration/training can be carried out easily in most clinics with sound- field audiometry setup. Facilities where such a set-up is not available can use a laptop or desktop computer connected to two loud-speakers, and they can request a copy of the program from the authors.
The loudspeakers should be placed to the left and to the right sides of the CROS wearer at equal distance. A continuous stimulus, such as any type of noise or sinusoids from an audiometer, a recorded speech list used during speech audiometry, or even music played through a computer or CD player, may be used for demonstration. These stimuli are presented from one loudspeaker at a time.
The wearers are instructed to deactivate and activate the On/Off button and listen for any differences in loudness. After listening to the stimuli presented from one loudspeaker for 2-3 times, the same stimuli can be presented from the opposite loudspeaker, and the wearers are instructed again to listen for any difference in loudness with On/Off switch adjustment. When the wearers report that the sound becomes softer when the CROS microphone is turned off, the clinician should inform the wearers that the sound is coming from the side of the unaidable ear or CROS transmitter.
A brief explanation may be valuable at this time. When the wearers report that no change in loudness is noted, the clinician should inform the wearers that the sound comes from the side of the aidable ear or the receiver side. Again, an explanation reinforces the wearer’s perception. Although in real-life the same sound may come from the front or the back, explaining such a possibility should wait until the wearers are familiarized with the task.
After the wearers have demonstrated some familiarity with the loudness changes associated with adjusting the On/Off switch, the clinician should reinforce the wearers’ learning by randomly presenting the sound stimuli from either loudspeaker and ask the wearers to identify the location of the sound source with the help of adjusting the On/Off switch.
To help wearers remember their tasks, a cue card is provided as they perform the reinforcement task (Figure 1). This training should continue until the wearers are correct in their identification for at least four consecutive trials. The entire demonstration/training exercise typically takes less than 10-15 minutes.
Assessing the Effectiveness of the Training Protocol
A study was conducted to evaluate the effectiveness on the appropriate use of the On/Off switch on left-right localization. The study included 9 participants, all of whom were BiCROS candidates with one unaidable ear and one aidable ear with a mild-to-moderately severe degree of hearing loss.
The Widex CROS-Fashion was used as the transmitter (Figure 2) and the Widex Dream ?440 m-CB was used as the receiving hearing aid. The appropriate instant fit ear-tip was used to couple the receiving hearing aid.
This CROS system uses Widexlink near-field magnetic induction (NFMI) wireless technology17 to transmit sounds efficiently from the unaidable ear to the receiving hearing aid. It features a 25 kHz sampling frequency (which allows transmission band-width of 12 kHz) and True Input Technology (which allows for linear input up to 113 dB SPL without clipping or compression limiting). In addition, the CROS transmitter microphone was configured to have an option of omnidirectional or broadband fully adaptive directional microphone.
The effectiveness of the CROS system, and of the directional microphone on the transmitter was reported in a previous article.12 Of special importance in this study is the availability of an On/Off switch and a volume toggle on the CROS transmitter; it allows the wearer easy access for controlling the availability and level of the transmitted sounds.
The Widex CROS-Fashion transmitter can be paired to any Dream hearing aid model at all technology levels. The receiving hearing aid used for the study was the Dream 440 m-CB, a 15 channel wide dynamic range compression (WDRC) device. The Dream hearing aid also includes an extended input limit of 113 dB SPL, a multi-channel, fully-adaptive directional microphone with a pinna compensation algorithm,18 a speech intelligibility index (SII) based noise reduction algorithm, active feedback cancellation system, and an impulse noise reduction algorithm.
Evaluation conditions. Four hearing aid conditions were evaluated:
2) Aided (ie, use of the study hearing aid on the aidable ear only);
3) Fixed BiCROS (the use of the CROS transmitter on the unaidable ear and the Dream hearing aid on the aidable ear); and
4) Adjusted BiCROS (the use of the CROS transmitter on the unaidable ear and the study hearing aid on the aidable ear).
The default settings on the hearing aids were used, and participants were trained on the use of the On/Off switch while following the demonstration/training protocol described above. They were allowed to use the On/ Off switch during the localization test in the Adjusted BiCROS condition.
Stimuli and test setup. Study participants were seated in the middle of a sound booth, with loudspeakers placed at 0°, 90°, 180°, and 270° azimuth. Each loudspeaker was placed 1 meter from the center of the participant’s head. Stimuli were presented at 30 dB sensation level (SL, re: the highest threshold mea- sured for each of the three test stimuli).
The localization test program used three stimuli (alarm, female speech, and white noise) each for a maximum duration of 1 minute. The participants were asked to determine where (front, back, left, right) they heard the test stimulus and respond using a touch screen. Each stimulus was presented through each loudspeaker 3 times in a random order (3 stimuli x 3 presentations x 4 loudspeakers = 36 presentations). A second adjusted BiCROS condition was also completed using only the right and left loudspeakers (3 stimuli x 3 presentations x 2 loudspeakers = 18 presentations). No feedback regarding accuracy of localization attempts was provided.
Localization Results. The results on the accuracy of localization for the different hearing aid test conditions are displayed in Figure 3. Performance for stimuli originating from the front (blue), the back (red), the side of the aidable ear (green), and the side of the unaidable ear (purple) is displayed for the different conditions.
Localization accuracy for sounds originating from the side of the unaidable ear showed a significant shift in performance from 49% in the unaided to 35% for the aided and 13% for the fixed CROS condition. The performance rose to 51% in the adjusted CROS condition. This highlights the possible degradation in localization performance with a fixed CROS system and the potential benefit that On/Off adjustment may make.
Performance for the side of the aidable ear revealed an unaided and aided accuracy of 72% and 77%, respectively. Performance decreased to 48% with the fixed BiCROS and further decreased to 37% in the adjusted BiCROS condition. This uncertainty may reflect the improved hearing on the side of the unaidable ear.
When only two directions were examined—left and right—high accuracy levels were achieved. Sounds originating from the side of the unaidable ear were correctly identified 93% of the time (purple bar), while sounds originating from the side of the aidable ear were correctly identified 77% of the time (green bar).
Localization accuracy for sounds originating from the front showed some small differences across the different hearing aid conditions. It is of interest to note that a 12% improvement in localization accuracy over the unaided condition was realized with the adjusted CROS condition. On the other hand, localization of sounds from the back remained poor even with the use of the On/ Off switch. This again reinforces the observations that hearing-impaired people have difficulties localizing sounds from the back.12 In addition, it supports the contention that candidates for the CROS/BiCROS configuration have additional problems in localization from other directions.
CROS On/Off switch usage. An important factor to consider when evaluating the utility of a feature, such as the CROS On/Off switch, is whether or not it is actually used outside the clinic. Study participants were given the opportunity to use the CROS aids in their daily lives for over 1 month. Upon return, they reported on how the CROS volume control and On/Off switch were used in their daily lives.
Table 1 includes the responses of 9 study participants regarding the frequency of On/Off switch use. All but one participant?????????????????????????????????????? reported using the On/Off switch at least once or twice during the week. Just under half, or 4 of the 9 participants, reported using the On/Off switch 3 or more times each day. The single participant who reportedly never used the On/Off switch preferred to leave the CROS on so that he could feel comfortable hearing sounds from all directions. However, he uses the volume control on the transmitter to regulate loudness of sounds.
How often do you turn off the CROS?
- Three or more times each day = 4
- Once or twice each day = 1
- Three or more times during the week, but changes are not made every day = 1
- Once or twice during the week = 2
- Never = 1
Table 1. Responses to one question examining study participants’ use of the CROS On/Off switch in their daily lives (n=9).
The current study demonstrates:
- CROS/BiCROS candidates have poor localization ability for sounds originating from any directions. All sounds are identified to originate from the side of the aidable ear. This is true for the unaided and aided (aidable ear only) conditions.
- A fixed BiCROS does not improve localization performance for sounds originating from the front or the back, and could degrade performance for sounds presented from the side of the unaidable ear.
- A BiCROS system with an easy On/ Off switch could provide a loudness cue for sounds originating from the left and the right. Thus, a convenient and discreet On/Off mechanism should be considered a critical part of a CROS/ BiCROS system.
- Training BiCROS candidates on the proper use of the On/Off switch can improve left-right localization in situations where sounds are restricted to the left and right sides only.
- The current in-clinic training resulted in subjects using the On-Off switch to optimize their real-world communication on a regular basis.
- A dedicated demonstration/training on the effective use of the On/Off switch on a CROS/BiCROS system should be part of the hearing aid orientation session for these candidates.
- The current in-clinic training (of 10-15 minutes) may not be sufficient to improve real-world localization where sounds can originate from any directions. More directed laboratory and home training may be necessary to bring about such changes.
Kuk F, Korhonen P. Localization 101: Hearing aid factors in localization. Hearing Review. 2014;21(9):26-33. Available at: https://hearingreview.com/2014/08/ localization-101-hearing-aid-factors-localization
Kuk F, Crose B. Localization 102: Essentials of a Home-based Front-back Localization Training Program. Hearing Review. 2015;21(1):20-25. Available at: https://hearingreview.com/2014/12/ localization-102-essentials-home-based-front-back- localization-training-program
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Eric Seper, AuD, is a research audiologist and Francis Kuk, PhD, is the director of the Widex Office of Research in Clinical Amplification (ORCA) in Lisle, Ill.?
Correspondence can be addressed to Dr Kuk at: [email protected]
Original citation for this article: Seper, E. Kuk, F. Localization 103: Training BiCROS/CROS Wearers for Left-Right Localization. Hearing Review. 2015;22(7):16.?