Although predictability and symmetry are generally appreciated, audiologists often are presented with the more challenging configuration of asymmetrical hearing loss patterns. These may manifest in a unilateral loss configuration, with one ear severely damaged and the other entirely normal, or with both ears distressed but one significantly more than the other. Besides the extra care required in the diagnostics to apply appropriate masking and reasonably identify the causal factors, there is the issue of supra-threshold inter-ear balance.

Most daily hearing experiences for communication and entertainment obviously occur at levels considerably above threshold, at sensation levels which are most comfortable for the individual listener. At these levels, inter-ear relations are not easily predicted for patients with significant threshold asymmetry. Hence, a case can be made for including inquiries into inter-ear sensation level difference patterns in a comprehensive rehabilitation plan.

There has been relatively little systematic study or methods developed for the clinical study of supra-threshold inter-ear properties, especially with asymmetrical threshold patterns. A fundamental premise of normal binaural processing is the assumption that the two auditory inputs are well matched or balanced.^1,2

Similar to the growth of loudness patterns in cochlear loss, it seems that the inter-ear differences at threshold often will not remain consistent throughout a typical supra-threshold listening range. While nonlinearity of loudness growth in cochlear etiologies is well studied,^3,4,5there seems to be little commentary about how inter-ear performance varies in asymmetrical sensorineural configurations. There is some recent evidence that a kind of "catching up" may occur in the SPL to SL relation in asymmetrical listeners.⁶

A general example of how this property may manifest is shown in Figure 1. The reader may notice a familiar pattern similar to that found in the loudness growth pattern of cochlear-impaired listeners. It is perhaps less well understood whether such level-dependent irregularities with cochlear injuries manifest within an individual's hearing experience when only one ear is damaged or one is more severely damaged than the other. In other words, do large threshold differences resolve into much smaller differences at comfortable listening levels for entertainment and/or communication?

A related question is whether binaural perception of signals for unilateral or asymmetrical sensorineural impairments still can produce "center of the head" lateralization or rich stereophonic listening experiences. This relates to the usefulness of a balance control in headphones.

Challenges and Alternatives

Typical treatment paths for severe unilateral loss or asymmetrical sensorineural loss patterns distill down to such limited options as: abandonment, as in the use of a CROS device, or (unintentional) over stimulation, as in attempts at trans-cranial power approaches, including those implemented via bone anchored devices. Recently, another approach, designated gentle stimulation, was reported.⁷This approach exploits the inter-ear loudness convergence properties described above. Clinical observations of patients with single severely damaged ears, who typically report mostly distortion and discomfort at standard amplification prescription levels, may gradually adjust to very low sensation levels of amplification with closely monitored intervention strategies. The authors have observed that several dozen such patients, who were either told by professionals not to bother with amplification on the 'dead ear' or who had abandoned hearing aid use due to problems of over-stimulation, often have reported reasonable benefit with this approach.

Headphone Enhancement

A central aspect of everyday hearing is the enjoyment of entertainment, such as music, television or drama. Without some allowance for inter-ear loudness differences, the experience is usually less than vivid. By use of a left-right adjustable balance control, a much richer experience can be experienced, often at surprisingly less of a level "off-set" than the threshold difference would suggest. The benefits of a balance control for improving the stereo listening enjoyment of unilaterally impaired or asymmetrical listeners were investigated recently.⁶Subjects of varying asymmetrical sensorineural hearing loss pattern were equipped with a prototype control. After clinical balancing data were obtained, they were asked to adjust the control until a standardized music passage was heard as a "center of head" experience. The music was simultaneously routed via a splitter

to an identical set of headphones so that sound pressure levels could be recorded, as shown in Figure 2.

The findings to date on a limited number of listeners with severe unilateral loss or asymmetrical degree of sensorineural loss are generally consistent with the loudness convergence behavior described above. An example is a 49-year-old female with a congenital, severe sensorineural impairment of the left ear and normal right ear thresholds. The difference at threshold for her left/right hearing levels averages about 60 dB for frequencies 250, 500, 750, 1k, and 1.5k Hz.

In Figure 3, hearing levels are plotted for selected signals at which balance sensations were achieved. The results were striking examples of the previously mentioned "catching up" behavior for the several frequencies tested. For this subject, it was clear that a significantly smaller amount of difference for higher presentation levels to the two ears was required to achieve a sense of "center of the head" lateralization experience for a variety of signals than implied by the threshold audiogram. Balanced binaural auditory perception was clearly achieved, often amusing and surprising this subject. Generally, there was a systematic reduction of the difference to achieve balance as a function of sensation level. In other words, as presentation levels were increased to the better ear, the amount of difference to the injured ear was further reduced, as suggested in Figure 1. Interactions of sensation level and inter-ear difference emphasize the clinical argument for including such investigations, especially when inter-ear differences are present.

Figure 4 illustrates how listeners manipulated the position of a music passage to a center (balanced) position with a finger-operated control attached to the headphone line. The range of measured level difference (depending on the sensation level) to the better ear is for one subject shown at the bottom of the figure.

Practical Applications

The clinical tradition for hearing assessment

is to first separate the two naturally communicating acoustical sensors (ears) and then to measure them independently. Perhaps it is characteristic of the discovery process that sometimes a great deal can be learned about complex systems, such as hearing, from modest changes in protocol and the serendipitous presentation of a few individuals with non-standard hearing patterns.

With regard to the experimental balance control device, subjects with cochlear asymmetries expressed strong value and interest, regardless of their individual auditory details. The following comment from one subject is representative: "It's kind of a weird but enormously satisfying sensation to have the music moved over to the middle of my head."

Hence, one small enabling tool for listeners with unequal hearing sensitivity seems to provide genuine perceptual benefit. Being able to reposition the listening experience toward a "center of the head" position was an unambiguously desirable feature. In several cases, it produced an unprecedented and enjoyable auditory experience. Focusing on threshold level measures will often obscure such phenomena as loudness convergence and the implications for everyday auditory experience.

Christopher Schweitzer, PhD, is chief audiology officer and Kevin Semcken is chairman and CEO of Able Planet. For more information: www.ableplanet.com.

References

1. Yost, W, Gourevitch, G. (1987). Directional Hearing. Springer-Verlag, New York.

2. Schweitzer, C. (1997). Binaural Processes: Implications for assessment and amplification. Seminars in Hearing, 18:4. New York. Thieme.

3. Marozeau, J, Florentine, M. (2007). Loudness growth in individual listeners with hearing losses: A review. JAcousSocAm Express Letters DOI 10.112.

4. Buus, S, Florentine, M. (2001). Growth of Loudness in Listeners with Cochlear Hearing Losses: Recruitment Reconsidered. J Assoc Res Otolaryng, 05, 120-139.

5. Buus S, Mu¨sch H, Florentine, M. (1998). On loudness at threshold. J Acoust Soc Am,04:399-410.

6. Schweitzer, C, Smith, DA. (2009). Meeting in the middle with unequal ears. HHear Rev,(in press).

7. Schweitzer, C, Wakefield, E. (2009). Gentle amplification treatment for severe cochlear injury. Paper at ASHA convention. New Orleans.