Malleability of the Lexical Bias Effect for Acoustically Degraded Speech.

Objectives: Lexical bias is a phenomenon wherein impoverished speech signals tend to be perceived in line with the word context in which they are heard. Previous research demonstrated that lexical bias may guide processing when the acoustic signal is degraded, as in the case of cochlear implant (CI) users. The goal of the present study was twofold: (1) replicate previous lab-based work demonstrating a lexical bias for acoustically degraded speech using online research methods, and (2) characterize the malleability of the lexical bias effect following a period of auditory training. We hypothesized that structured experience via auditory training would minimize reliance on lexical context during phonetic categorization for degraded speech, resulting in a reduced lexical bias.

Design: In experiment 1, CI users and normal hearing (NH) listeners categorized along 2 /b/-/g/ continua (BAP-GAP; BACK-GACK). NH listeners heard each continuum in a clear and eight-channel noise-vocoded format, while CI users categorized for clear speech. In experiment 2, a separate group of NH listeners completed a same/different auditory discrimination training task with feedback and then completed phonetic categorization for eight-channel noise-vocoded /b/-/g/ continua.

Results: In experiment 1, we observed a lexical bias effect in both CI users and NH listeners such that listeners more consistently categorized speech continua in line with the lexical context. In NH listeners, an enhanced lexical bias effect was observed for the eight-channel noise-vocoded speech condition, while both CI users and the clear speech condition showed a relatively weaker lexical bias. In experiment 2, structured training altered phonetic categorization and reliance on lexical context. Namely, the magnitude of the lexical bias effect decreased following a short period of auditory training relative to untrained listeners.

Conclusions: Findings from experiment 1 replicate and extend previous work, suggesting that web-based methods may provide alternative routes for testing phonetic categorization in NH and hearing-impaired listeners. Moreover, findings from experiment 2 suggest that lexical bias is not a static phenomenon; rather, experience via auditory training can dynamically alter reliance on lexical context for speech categorization. These findings extend theoretical models of speech processing in terms of how top-down information is weighted for listeners adapting to acoustically degraded speech. Finally, these findings hold clinical implications for tracking changes in phonetic categorization and reliance on lexical context throughout the CI adaptation process.