Lexical Access Changes Based on Listener Needs: Real-Time Word Recognition in Continuous Speech in Cochlear Implant Users
Authors: Smith Francis X. , McMurray Bob 
Published: January 2022

Objectives:
A key challenge in word recognition is the temporary ambiguity created by the fact that speech unfolds over time. In normal hearing (NH) listeners, this temporary ambiguity is resolved through incremental processing and competition among lexical candidates. Post-lingually deafened cochlear implant (CI) users show similar incremental processing and competition but with slight delays. However, even brief delays could lead to drastic changes when compounded across multiple words in a phrase. This study asks whether words presented in non-informative continuous speech (a carrier phrase) are processed differently than in isolation and whether NH listeners and CI users exhibit different effects of a carrier phrase.

Design:
In a Visual World Paradigm experiment, listeners heard words either in isolation or in non-informative carrier phrases (e.g., “click on the…”). Listeners selected the picture corresponding to the target word from among four items including the target word (e.g., mustard), a cohort competitor (e.g., mustache), a rhyme competitor (e.g., custard), and an unrelated item (e.g., penguin). Eye movements were tracked as an index of the relative activation of each lexical candidate as competition unfolds over the course of word recognition. Participants included 21 post-lingually deafened cochlear implant users and 21 NH controls. A replication experiment presented in the Supplemental Digital Content, http://links.lww.com/EANDH/A999 included an additional 22 post-lingually deafened CI users and 18 NH controls.

Results:
Both CI users and the NH controls were accurate at recognizing the words both in continuous speech and in isolation. The time course of lexical activation (indexed by the fixations) differed substantially between groups. CI users were delayed in fixating the target relative to NH controls. Additionally, CI users showed less competition from cohorts than NH controls (even as previous studies have often report increased competition). However, CI users took longer to suppress the cohort and suppressed it less fully than the NH controls. For both CI users and NH controls, embedding words in carrier phrases led to more immediacy in lexical access as observed by increases in cohort competition relative to when words were presented in isolation. However, CI users were not differentially affected by the carriers.

Conclusions:
Unlike prior work, CI users appeared to exhibit “wait-and-see” profile, in which lexical access is delayed minimizing early competition. However, CI users simultaneously sustained competitor activation late in the trial, possibly to preserve flexibility. This hybrid profile has not been observed previously. When target words are heard in continuous speech, both CI users and NH controls more heavily weight early information. However, CI users (but not NH listeners) also commit less fully to the target, potentially keeping options open if they need to recover from a misperception. This mix of patterns reflects a lexical system that is extremely flexible and adapts to fit the needs of a listener.