By Joseph Bochner, Ph.D.
Early childhood deafness generally has a profound and lasting impact on educational and career attainments later in life because it reduces the quality and quantity of language experience, thereby inhibiting the acquisition of spoken language and literacy abilities. Cochlear implants, however, can enhance the quality and quantity of linguistic input and facilitate the development of spoken language and literacy skills in early onset childhood deafness.
We studied the categorization of speech sounds in two groups of adult cochlear implant (CI) users with early-onset deafness and a group of 21 hearing controls. Thirty of the CI users were implanted before age 4 (early CI group), and 21 were implanted after age 7 (late CI group). Specifically, we evaluated listeners’ identification and discrimination of speech sounds distributed along two acoustic continua: a voicing continuum consisting of the syllables /ba/ vs. /pa/; and a place of articulation continuum consisting of the syllables /ba/ vs. /da/.
Our results, published in the Journal of Speech, Language, and Hearing Research in November 2022, showed that, for the /b/–/p/ contrast, when voice onset occurs at about 20 milliseconds (ms) or less after release of air pressure behind the lips, CI users with early onset deafness generally perceive the sound as /b/. But, when voice onset occurs at longer intervals, they generally perceive the sound as /p/.
The accompanying figure illustrates the probability of an average listener in each group identifying the stimuli on the /ba/–/pa/ continuum as /ba/. The /ba/–/pa/ category boundary for each listener group is denoted by a dot on their curve. Stimuli to the left and above the /b/–/p/ category boundary were identified as /ba/, while stimuli to the right and below the boundary were identified as /pa/.
Performance of each listener group on the /b/–/p/ identification task. Category boundary locations for each group are shown with a dot. VOT = voice onset time; CI = cochlear implant. Credit: Bochner et al./Journal of Speech, Language, and Hearing Research
For each group, the /b/–/p/ contrast was quite sharp, as reflected in the steep slope of the curve in the vicinity of the /b/–/p/ category boundary. The figure shows substantial similarity among the three listener groups, with the average /p/–/b/ boundary located at 17.3 ms for the hearing listeners and only about 5-6 ms longer for listeners in the two CI groups.
Distinctive identification and discrimination functions were observed for each group on both the voicing and place of articulation continua, indicating similar performance between CI users and hearing controls and between the early and late CI groups. Compared to hearing participants, however, the CI groups generally displayed longer/higher category boundaries, shallower identification function slopes, reduced identification consistency, and reduced discrimination performance.
The results also showed that earlier implantation was associated with better phoneme categorization (PC) performance within the early CI group, but not the late CI group. Within the early CI group, earlier implantation age but not PC performance was associated with better speech recognition. Conversely, within the late CI group, better PC performance but not earlier implantation age was associated with better speech recognition.
These findings indicate that, within the early CI group, the age at which a child receives a cochlear implant affects their eventual development of phoneme (speech sound) categories in adulthood, and that the linguistic categories within each CI group closely resemble those of hearing controls.
Importantly, individuals implanted prior to their 2nd birthday seem to acquire top-down processing abilities that enable them to become more proficient at speech recognition than those implanted in later years, whose speech recognition generally appears to proceed from the bottom up.
These findings are important because they provide valuable insight into the interaction between the learner’s age (developmental period effect) and their linguistic experience (quality and quantity of linguistic input) in early development. In particular, we interpret these findings as indicating a midsize developmental period effect occurring before age 4, which partly determines phoneme categorization ability.
Moreover, the findings suggest another midsize developmental period effect associated with the acquisition and use of top-down linguistic processing occurring around age 2.
Joseph Bochner, Ph.D., is a professor at the National Technical Institute for the Deaf at Rochester Institute of Technology. His 2017 Emerging Research Grant was generously funded by Royal Arch Research Assistance.
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