Longitudinal Changes in the Structure of Speech Categorization Across School Age Years: Evidence against perceptual narrowing in later developmental periods.

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 other, higher-level aspects of language (e.g., vocabulary, syntax). This development was thought to follow a form of perceptual narrowing in which infants lose access to phonetic detail that is not relevant to their native language categories. More recent research has shown that speech categorization continues to develop through adolescence (e.g., Hazan & Barrett, 2000; Slawinski & Fitzgerald, 1998). This work which uses more sophisticated forced choice tasks and stimuli that span speech continua (e.g., /b/ to /p/ in small steps) is also broadly consistent with narrowing as children show increasingly sharp category boundaries over development. Here, our longitudinal study revisited these results using a new Visual Analogue Scaling (VAS) task which uses a continuous rating scale to potentially unpack the underlying cause of a shallower or more gradient boundary. We tested 282 school-aged children (in grades 1-3) over three years (through grades 4-6). A Bayesian Hierarchical Growth Curve 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 and decreased inconsistency. These findings suggest that sound categorization continues to develop through early school age. These findings challenge perceptual narrowing because they indicate that children are increasingly sensitive to fine-grained gradient detail in the signal. The dramatic changes in consistency represent a challenge to current theoretical models that focus on the nature of the representation.