Asymmetrical hearing and unilateral hearing loss require solutions that do not seem to follow typical hearing aid fittings for bilateral sensorineural hearing loss. Last week’s post on this subject showed the results of “loudness catch-up” of the poorer ear that indicated a pattern somewhat similar to the steep growth of loudness associated with classic sensorineural hearing loss. This post shows the measurement results from a second of twenty-nine cases, and then describes the measurement method and suggests how the results might be used.
Wayne Staab, Section Editor
Asymmetrical Hearing loss, Loudness Balance, and Hearing Aid Fittings
Christopher Schweitzer, Ph.D., and Don Kim, Au.D.
A second example, with results of a 52-year-old male with normal hearing on the left ear and sudden severe loss on the right subsequent to a vaccine reaction two years prior is shown in Figure 5.
Initially troubled with distortion experiences with amplification with even very mild gain values, he eventually reported localization improvements and hearing in quiet with amplification levels far less than standard prescription gain proposals{{1}}[[1]]Schweitzer, C. & Wakefield, E. (2009). Gentle amplification treatment for severe unilateral cochlear injury. Paper presented at ASHA convention. New Orleans[[1]]. Balance tasks were more difficult for this subject due to the substantial distortion of the injured ear. He was, however, able to achieve repeatable results shown in the Figure 5 bars, which are also plotted for speech and music. (Description of the bars is available in Part I of this series). As with the Figure 1 subject, a significantly smaller amount of difference in presentation levels was required to achieve a sense of “center of the head” lateralization experience for tones, speech, and music at Preferred Listening Levels (PLL) than implied by the threshold audiogram. An experience of binaural auditory perception was clearly achieved, both surprising and, curiously, amusing the subject.
Figure 5. Threshold and plotted examples of levels to achieve balances for selected signals for a male subject with adventitiously acquired RE asymmetrical hearing loss.
These findings speak to the high value that Nature assigns to the principle of Balance, (with regard to audition, rather than to vestibular function), even when injury and medical mishaps conspire to disrupt it. Differences of 50 and 60 dB at threshold were in some cases condensed to 5 dB or less at comfortable sensation levels. In several cases, subjects reported that the auditory “image” jumped rapidly from right to left, requiring a few moments to engage the adjustments to locate the sound within the head. Another interesting report from several subjects with long-term severe asymmetries was that a “phantom” image to the bad side was perceived briefly when all stimulation was moved back to the better side. The cortical activity involved with cognitive tasks such as with these judgments is presumably complex when stimuli are moved and mixed in the manner described. Generally, however, there was a systematic reduction of the difference to achieve balance as a function of sensation level.
Although most studies of binaural loudness summation make use of symmetrical hearing loss subjects, this finding of an interaction with sensation level is consistent with studies of binaural loudness summation patterns as a function of level{{2}}[[2]]Hawkins, D., Prosek, R., Walden, B., & Montgomery, A. (1987). Binaural loudness summation in the hearing impaired. J Speech Hear Res. 30, 3743[[2]]. It further emphasizes the uncertain relation between threshold audiometry and PLL, especially when inter-ear differences are present.
Asymmetrical Hearing Loss Balancing for Music
To further investigate the related question of whether binaural perception of signals for unilateral or asymmetrical sensorineural impairments can still produce “center of the head” lateralization, or stereophonic listening experiences, a balance control (Able Planet IBVT) was used to gain insight. Subjects were asked to adjust the balance of pink noise and music to a comfortable, center-of-the-head condition. For the music, noise-canceling headphones (Able Planet NC-200) were used, and music was delivered via a digital player. The experimental balance control was adjusted by the subject while music was simultaneously routed to an identical set of headphones for SPL measures of the relative dB difference between the L/R headphones as shown in Figure 6. Values were recorded when the subject reported identical balanced levels on three consecutive adjustment trials.
Figure 6. Measurement scenario for determining the relative SPL differences between headphones when Ss adjusted the music to a Center of Head position.
Due to the additional set up and time requirements for this portion of the studies, only three subjects were recruited to adjust the basic volume for a passage of Bonnie Raitt’s “Something to Talk About” played at a comfortable listening level to the better ear. The balance control was then adjusted to reposition the sound until a center-of-head (Figure 2 – No. 15) position was achieved. This was usually accomplished in a few seconds after over-adjusting briefly (bracketing) to the worse ear. The IBVT control was then “locked” into position with a secure toggle switch. By splitting the signal to a matched set of headphones the SPL difference between the individual outputs were obtained on a standard sound level meter in A-weighted slow mode secured into a coupler. A recorded pink noise signal was played through the MP3 device to achieve a stable reading of the levels.
Measured output differences in dB SPL for the two earphones were recorded as inter-ear level differences. For the subject in Fig. 1 (previous post), those differences ranged from 2.5 dB to 7.5 dB, depending on the level at the better ear as shown in Figure 7. Similar results were observed for the other two subjects.
Figure 7. Illustration of the use of the Able Planet In Balance control to achieve a “Center of Head” experience for the music passage. Differences were measured using a steady pink noise as shown in Figure 6. For subject, depending on the loudness listening level, the differences ranged from 2.5 to 7.5 dB. Smallest differences were observed at loudest sensation levels.
Closing Comments
Both unilateral and asymmetrical sensorineural hearing loss are unique to each subject due to his or her particular auditory history and inter-ear dissimilarities. Nevertheless, the robust and fundamental binaural processing of signals, even from ears of unequal sensitivities and stimulation histories could be readily observed in these subjects.
These findings are instructive at several levels of discourse. Results obviously varied among the members of this sample, but the histories and patterns appeared to pertain to cochlear damage of varying durations. While the numerous differences among the 29 subjects argue against averaging these findings, it was tempting to simply compare the Average Threshold difference between “good” and “bad” ears with the Average Balanced level difference. For what it’s worth, those numbers are: 49.2-dB Threshold versus 7.4 dB for Suprathreshold Balanced (at PLL) difference values across all the various signal types and sensation levels. Clearly, something similar to the speculative pattern of Figure 2 (previous post) was at work for these listeners.
Potential Hearing Preservation Benefit
The adjustments with the balance control unit for individuals with asymmetrical hearing patterns were accomplished by reduction in the levels delivered to the better ear, since the control is a passive adjustment that effectively attenuates levels to the ear of better sensitivity. This might suggest hearing conservation implications for extended use. Reductions in accumulated level/hours of listening are always presumed to be advantageous for hearing health. Further investigation is warranted.
Implications for “Prescriptive Gain” Proposals
It is unlikely that any of the commonly favored hearing aid gain prescriptions make significant allowance for asymmetrical patterns, such as have been discussed here. Clearly, there is need to integrate an appreciation of binaural integration in such proposals and to understand that output properties must be organized around Preferred Listening Levels, however unpredictable they may be. It begs the question of hearing aid prescriptions for everyday listening based on unnatural conditions, namely minimal peripheral neurons from individual ears{{3}}[[3]] Schweitzer, C. & Donnelly, R. (2013). Why it’s time to retire the audiogram (for hearing aid fittings). https://hearinghealthmatters.org/hearingviews/2013/why-its-time-to-retire-the-audiogram-for-hearing-aid-fittings[[3]]. We do not suggest these findings represent hard science or should be generalized, but perhaps they will serve to generate conversations on the relation of audiometry to everyday listening experiences.
