Audiological and Demographic Factors that Impact the Precision of Speech Categorization in Cochlear Implant Users

The ability to adapt to subtle variations in acoustic input is a necessary skill for successful speech perception. Cochlear implant (CI) users tend to show speech perception benefits from the maintenance of their residual acoustic hearing. However, previous studies often compare CI users in different listening conditions within-subjects (i.e., in their typical Acoustic + Electric configuration compared with Acoustic-only or Electric-only configurations) and comparisons among different groups of CI users do not always reflect an Acoustic + Electric benefit. Existing work suggests that CI users with residual acoustic hearing perform similarly to Electric-only listeners on phonetic voicing contrasts and unexpectedly poorer with fricative contrasts which have little energy in the range of the Acoustic + Electric listeners' acoustic hearing. To further investigate how residual acoustic hearing impacts sensitivity to phonetic ambiguity, we examined whether device configuration, age, and device experience influenced phonetic categorization in a large individual differences study.

CI users with various device configurations (Electric-only N = 41; Acoustic + Electric N = 95) categorized tokens from five /b-p/ and five /s-ʃ/ minimal pair continua (e.g., bet-pet; sock-shock). We investigated age, device experience, and when applicable, residual acoustic hearing (pure tone hearing thresholds) as predictors of categorization. We also examined the relationship between phonetic categorization and clinical outcomes (CNC, AzBio) in a subset of our sample.

Acoustic + Electric CI users were better able to categorize along the voicing contrast (steeper categorization slope) compared with Electric-only users, but there was no group-level difference for fricatives. There were differences within the subgroups for fricatives: bilateral users showed better categorization than unilateral users and bimodal users had better categorization than hybrid users. Age was a significant factor for voicing, while device experience was significant for fricatives. Critically, within the Acoustic + Electric group, hybrid CI users had shallower slopes than bimodal CI users.

Our findings suggest residual acoustic hearing is beneficial for categorizing stop voicing, but not frication. Age impacts the categorization of voicing, while device experience matters for fricatives. For CI users with ipsilateral residual acoustic hearing, those with better hearing thresholds may be over-relying on their acoustic hearing rather than extracting as much information as possible from their CI, and thus have shallower fricative categorization.