Longitudinal Changes in the Structure of Speech Categorization Across School Age Years: Children Become More Gradient and More Consistent

A critical aspect of spoken language development is learning to categorize the sounds of the child's language(s). This process was thought to develop early during infancy to set the stage for the later development of higher-level aspects of language (e.g., vocabulary, syntax). However, many recent studies have shown that speech categorization continues to develop through adolescence. This work, which uses more sophisticated forced-choice tasks and stimuli that span speech continua (e.g., /b/ to /p/ in small steps) suggests that children show increasingly sharp category boundaries over development. However, this contrasts with the consensus view in adult speech perception that speech categories are graded and this may help listeners be more flexible. Here, our longitudinal study revisited these results using a new Visual Analogue Scaling (VAS) task, which employs a continuous rating scale to potentially unpack the underlying cause of a shallower or more gradient boundary. We tested 225 school-aged children (in grades 1-3) over 4 years (through grades 4-6). A Bayesian Hierarchical psychometric model was fit to extract the Slope of the categorization function and Response Consistency (the trial-by-trial variance). Results show that as children age, they show shallower (more gradient) slopes. It also highlighted a new variable, suggesting that older children exhibit greater trial-by-trial categorization consistency. These findings suggest that sound categorization continues to develop through early school age. These findings also suggest that children become increasingly sensitive to fine-grained gradient detail in the signal. The dramatic changes in consistency represent a challenge to current theoretical models that focus primarily on underlying category representation. SUMMARY: Speech categorization continues to develop through the school years, refining children's ability to categorize speech sounds more precisely and flexibly. A longitudinal study tested 225 school-aged children using a Visual Analogue Scaling (VAS) task and Bayesian Hierarchical psychometric modeling. With age, children showed shallower (more gradient) categorization slopes and greater trial-by-trial response consistency. Findings challenge theoretical models, revealing continued development of speech categorization and increasing sensitivity to fine-grained gradient detail in the signal.