Effects of F1-F2 Frequency Spacing on Spectral Integration in Combined Electric and Acoustic Stimulation.

Purpose: The aim of this study was to measure the effects of frequency spacing (i.e., F2 minus F1) on spectral integration for vowel perception in simulated bilateral electric-acoustic stimulation (BiEAS), electric-acoustic stimulation (EAS), and bimodal hearing.

Method: Twenty listeners with typical hearing participated in synthetic vowel recognition. Four vowels were used with varying frequency spacings (/ͻ/: 270 Hz, /ʊ/: 653 Hz, /æ/: 1040 Hz, and /I/: 1607 Hz). F1 was acoustically simulated with a band-pass filtering, while F2 was electrically simulated using an eight-channel sine wave vocoder with matched input and output frequency range. Vowel recognition was measured in five listening conditions: BiEAS (F1 and F2 in both ears), EAS (F1 and F2 in the left ear), bimodal (F1 and F2 in opposite ears), cochlear implant alone (F2 alone in the left ear), and hearing aid alone (F1 alone in the left ear).

Results: In EAS, spectral integration was significantly better at a 270-Hz spacing, while in bimodal hearing, spectral integration was significantly poorer at a 1607-Hz frequency compared to other frequency spacings. BiEAS conditions offered the best spectral integration, regardless of frequency spacing. Vowel confusion remained consistent and below chance level across the first three listening conditions. Bimodal interference occurred for the /I/ vowel when the cochlear implant ear perceives the dominant cue and the hearing aid ear perceives the nondominant cue. The F2 place cue is transmitted significantly better than the F1 height cue in BiEAS, EAS, and bimodal conditions.

Conclusions: EAS and bimodal hearing integrates narrower frequency ranges better than wider spacings. EAS hearing provided greater outcomes over bimodal hearing, suggesting that within-ear (EAS) integration is more effective than across-ear (bimodal) integration. Bimodal interference may be a factor for variability in bimodal performance. Cautious interpretation and further research with real EAS and bimodal users are suggested to validate and extend these findings.

Supplemental Material: https://doi.org/10.23641/asha.28127249.