Hearing Aids Improve Elderly Listening Abilities

Bilateral cochlear implant users experienced a small but measurable reduction in speech intelligibility when their device’s stimulation levels were lowered to account for binaural loudness summation. According to a 2005 study published in *Ear and Hearing*, this performance decrease was on the order of 1 to 2 decibels. However, the study’s lead author Richard van Hoesel and his team also found that the inherent benefit of listening with two ears effectively cancels out this loss, leaving overall performance at least as good as with the better ear alone.

### The Challenge of Fitting Two Ears: Binaural Loudness Summation

When a person uses a hearing device in one ear, clinicians program it with an amplitude-mapping function, or “map.” This map converts acoustic sound into electrical signals that the implant delivers to the auditory nerve. A key consideration is setting the maximum comfortable levels to ensure sounds are not uncomfortably loud.

The challenge increases with two implants. Due to a phenomenon called binaural loudness summation, a sound presented to both ears is perceived as louder than the same sound presented to just one ear. If a standard, single-ear map is used for both implants without adjustment, everyday sounds could become intolerably loud for the user. Clinicians therefore often consider lowering the stimulation levels in each implant to compensate. The 2005 study aimed to test what happens to speech understanding when they do this.

### How the Study Tested Different Amplitude Maps

Richard van Hoesel and colleagues at the Cooperative Research Centre for Cochlear Implant and Hearing Aid Innovation recruited eight adults who used bilateral Nucleus 24 cochlear implants. They tested four different maps, each used at home for one week before formal assessment.

One map was the standard clinical map, fitted as if for a single ear. The three experimental maps all reduced stimulation levels, but in different ways: one reduced levels more near the maximum comfort level, another reduced them more near the threshold, and a third applied a uniform reduction across the entire dynamic range.

After each week of use, researchers tested performance in two ways. First, they used an adaptive speech-in-noise test where both speech and noise came from a speaker directly in front of the listener. This S0N0 setup minimizes the head-shadow and binaural squelch benefits of two ears, isolating the effect of level changes. Second, they tested speech understanding for very soft speech in a quiet room. All tests were conducted with the maps used in both unilateral (better ear only) and bilateral conditions.

### Higher Stimulation Levels Led to Better Speech Understanding

The results were clear. Reducing stimulation levels, regardless of the specific method, led to a modest but statistically significant decrease in performance. In the speech-in-noise test, the drop in performance was about 1 to 2 dB for level reductions designed to mirror binaural loudness summation effects. Performance for soft speech in quiet also suffered with lower levels.

The way the levels were reduced—whether more at the top, bottom, or evenly—had little impact. The primary factor was the overall level of stimulation. Higher levels consistently led to better speech intelligibility. The researchers suggest this may be because the relationship between electrical stimulation and perceived loudness accelerates at higher levels, giving a performance boost even when the signal-to-noise ratio is technically unchanged.

A critical secondary finding came from comparing unilateral and bilateral use with the *same, unadjusted map*. Even in the challenging S0N0 condition, using both ears provided an average benefit of 1.4 dB over using the better ear alone. This demonstrated a persistent binaural advantage, even when spatial cues were absent.

### Clinical Implications: The Bilateral Advantage Offsets Adjustment Loss

This study provides practical guidance for clinicians fitting bilateral cochlear implants. It confirms a trade-off: lowering levels to maintain comfortable loudness can reduce clarity. However, it also shows that the natural benefit of hearing with two ears counterbalances this loss.

The authors concluded that after appropriate level adjustments to account for binaural summation, a user’s speech understanding with both implants will be at least as good as their performance with their better ear alone using an unadjusted, higher-level map. This means patients do not have to choose between comfort and performance; effective bilateral fitting can provide both.

For individuals with hearing disorders, understanding these precise engineering trade-offs can be empowering. It highlights how device programming is a balance of many factors, much like research into how hearing aids can improve specific listening skills in the elderly. Furthermore, the study underscores the brain’s complex role in integrating signals from two ears, a process relevant to conditions like tinnitus and hyperacusis where central auditory processing is altered. Research on the dorsal cochlear nucleus’s role, for example, explores similar central integration mechanisms in different hearing pathologies.

**Source:** van Hoesel R, Böhm M, Battmer RD, Beckschebe J, Lenarz T. Amplitude-mapping effects on speech intelligibility with unilateral and bilateral cochlear implants. *Ear Hear*. 2005;26(4):381-8. doi:[10.1097/00003446-200508000-00002](https://doi.org/10.1097/00003446-200508000-00002). PMID: [16079633](https://pubmed.ncbi.nlm.nih.gov/16079633/).