Zhang, Spahr, and Dorman (2010) assessed speech recognition in eight adult, post-lingually deafened, monaurally fitted cochlear implant (CI) patients. Seven of the patients wore hearing aids (HAs) in the contralateral ear. An area of "frequency overlap" between HA and CI perceptions was present between approximately 250 and 750 Hz as all eight patients had residual hearing less than or equal to 65 dB HL at 500 Hz and below, and had residual hearing greater than or equal to 65 dB HL at 1000 Hz and higher.
The authors sought to determine whether reducing the overlap of low frequency information from the CI and HA combined would improve speech recognition. In other words, the authors sought to address whether the combined low-pass acoustic with high-pass electric presentations facilitated improved performance, as compared to the combined simultaneous acoustic plus electric wideband presentations. Historically, results relating to improved versus degraded speech recognition with respect to the overlapped spectral representation have been conflicting.
Monosyllabic words in quiet and sentences in noise were assessed in three listening situations: (1) electric stimulation alone, (2) acoustic stimulation alone, and (3) combined electric-and-acoustic stimulation (EAS). Acoustic stimuli were unfiltered or low-pass filtered at 250, 500 or 750 Hz. Electric stimuli were unfiltered or high-pass filtered at 250, 500 or 750 Hz. In EAS tests, unfiltered acoustic and electric signals were paired, as were 250 Hz low-pass acoustic and 250 high-pass electric, and the same pairing protocol was used with 500 Hz and 750 Hz.
The authors found that reducing the spectral overlap between acoustic and electric signals did not increase speech understanding. Indeed, as total bandwidth increased in both acoustic and electric presentations, performance increased and the highest performance levels were achieved in response to combined and unfiltered acoustic and electric stimulation.
Zhang, Spahr, and Dorman noted that even though low frequency information in isolation allows little or no speech intelligibility, low frequency acoustic information (<250 Hz) accounts for the majority of speech perception benefits and significantly improves performance when patients combine acoustic and electric stimulation. In fact, the authors note that even for patients who only have residual hearing at 125 Hz, they too, may benefit from EAS.
For More Information, References, and Recommendations
Zhang T, Spahr AJ, Dorman MF. (2010) Frequency Overlap Between Electric and Acoustic Stimulation and Speech perception Benefit in Patients with Combined Electric and Acoustic Stimulation. Ear & Hearing 31(2):195-201.