Which cross-over frequency is best for electro-acoustic stimulation?
Reviewed by Kerri Millward and Morag Lockley

Advances in technology and improved soft surgical techniques have led to individuals with better hearing thresholds, especially at the low frequencies, becoming candidates for cochlear implants (CI). Surgeons are more confident that residual hearing can be preserved thus making those with sloping losses candidates for electro-acoustic stimulation. However, clinicians then face the dilemma of how to aid these losses, in particular where the acoustic stimulation should stop, and the electric stimulation begin. Currently each company has set defaults in their software where they believe this cross-over should be based on the gain of the hearing aid. However, would these patients benefit from having electric stimulation taking over at better hearing thresholds? The National Acoustic Laboratories in Sydney investigated this by looking at an audiometric-based criterion and set cross-over frequencies based on where residual hearing thresholds were 60, 75 and 90dBHL. Each participant had a unilateral CI and bilateral in-the-ear hearing aids fitted. Binaural skills were investigated using speech in noise and localisation tests as well as listening questionnaires. No one particular cross-over frequency was significantly better than the others. However, an interesting finding was the patients wore their devices more when wearing their preferred cross-over frequency. They also tended to perform better on the speech in noise test on their preferred setting. This led the authors to conclude that a variety of cross-over frequencies should be trialled with patients to establish which cross-over frequency is preferred and where performance is optimal. However, more work is needed in this area to look at whether there should be an overlap of electric and acoustic stimulation and on the impact of any dead regions. 

The effect of cross-over frequency on binaural hearing performance of adults using electric-acoustic stimulation.
Incerti PV, Ching TYC, Cowan R.
COCHLEAR IMPLANTS INTERNATIONAL
2019;20(4):190-206.