Exploring Critical Periods in Early Language Development: Phoneme Discrimination and Word Order Processing

The timing and underlying mechanisms of language acquisition have been central to debates on critical periods for decades. While developmental trajectories in phoneme discrimination abilities are well established, far less is known about whether grammatical structures, such as word order, are subject to similar developmental constraints. This thesis investigates how learning mechanisms and developmental timing interact across different levels of linguistic structure, with a particular focus on frequency-based bootstrapping as a cue for word order acquisition. Across four studies, the current thesis explores when and how linguistic learning mechanisms operate and how their developmental trajectories unfold. Studies 1 and 2 demonstrate that both adults and infants use distributional frequency cues to infer word order, extending prior findings to Turkish, a novel, typologically distinct, and underrepresented functor-final language. These results provide cross- linguistic evidence that frequency-based bootstrapping operates across development in both infants and adults, generalizes beyond well-studied Indo-European languages, and supports the validity of online experimental methods for more inclusive psycholinguistic research. Studies 3 and 4 address questions of developmental timing by examining whether short-term exposure modulates linguistic representations during infancy. Study 3 investigates the flexibility of word-order preferences in monolingual Italian-learning infants aged 4–12 months following exposure to training with contrasting word orders. Study 4, using the same age range and a similar experimental design, examines phoneme discrimination as a comparison domain with a well-documented trajectory of perceptual narrowing. Although the preliminary data did not reveal robust learning-related effects, descriptive age-related differences point to potential methodological constraints. Taken together, these findings suggest that frequency-based mechanisms support early sensitivity to word order, but that their flexibility may vary across representational levels.