Coronal underspecification as an emerging property in the development of speech processing

Is the developing lexicon phonologically detailed or are representations underspecified? Experimental results from toddlers suggest phonological specificity. By contrast, the Featurally Underspecified Lexicon (FUL) theory (Lahiri & Reetz, 2010; Lahiri 2018), motivated by evidence such as the cross-linguistic prevalence of phenomena such as coronal assimilation (rainbow → rai[m]bow), proposes that coronal sounds are unspecified for place of articulation even in the adult lexicon. FUL therefore predicts that asymmetries in mispronunciation sensitivity are also present in the developing lexicon. Recent research (Ren et al., 2019) has rejected this, reporting similar sensitivity to mispronunciation of coronals and non-coronals at 19 months. Using a more sensitive experimental paradigm, we provide new evidence demonstrating a lack of asymmetries at 18 months, but mispronunciation sensitivity for coronals disappears by 24 months. In an intermodal preferential looking study, growth curve analysis shows that 18-month-olds are sensitive to misprounciations of words with a coronal (e.g., duck vs. *buck) and non-coronal (e.g., bird vs. *dird) onset. At 24 months, mispronunciations of coronal-onset words were treated just like the accurate pronunciations. We conclude that coronals are underspecified in the developing lexicon at 24 months. We propose a model under which initial representations are phonetic in nature and require exact acoustic input whereas phonological coronal underspecification at the lexical level emerges gradually as a result of exposure to variation in the input such as coronal assimilations that only become detectable patterns with growing lexical and segmentation skills.