gms | German Medical Science

To benefit from binaural hearing, for example sound localisation and better speech perception in noise, bilateral cochlear implants (CI) are often provided if neither ear provides sufficient residual hearing. Most bilateral CI users show improvements compared to unilaterally implanted CI users. However, binaural benefits fall short compared to normal hearing (NH) listeners. One of several factors that limit the benefits is the interaural tonotopic mismatch. That means that in some subjects corresponding left and right auditory nerve fibers are stimulated with different acoustic frequency bands, effectively comparing apples with oranges. It is assumed that in upstream binaural neurons only place-corresponding fibers interact, at least in postlingually deaf subjects. Reasons for this mismatch lie in different insertion depths, different implant lengths, or locally different electrode-nerve interfaces. If the mismatch would be known, the frequency allocation tables can be adjusted.

This study addresses only the first part of the problem: Measuring which electrode on the left CI compares or interacts best with which electrode in the right CI. Three pair identification methods are employed: (1) find electrode pairs that allow for maximum sensitivity to interaural time differences, (2) find electrode pairs with the most similar pitch sensation, and (3) compare the electrode insertion angles from post-operative x-rays.

With data collection with experienced post-lingually deafened MED-EL subjects ongoing, the following patterns emerge: (1) interaural electrode mismatch is common. (2) ITD sensitivity declines with increasing insertion angle difference. (3) Pitch-matched electrode pairs correspond better with the pair that is stimulated with the same acoustic frequency bands – not with insertion angle pairs. A subject with very large (> 10 mm) mismatch is an exception.

So far, the data indicate that the common problem of mismatch cannot be properly addressed with pitch matching. Strategies for re-mapping the frequency allocation table for a given mismatch need to be developed.