Explaining the Comprehension-Production Vocabulary Gap Through Neural Networks and Cross-Syndrome Evidence: Insights From Williams Syndrome.

The comprehension-production vocabulary gap is a well-documented hallmark of language development; however, anecdotal evidence suggests that this asymmetry may be reduced in children with Williams syndrome (WS). Here, we use empirical data to characterise the comprehension-production gap and computational modelling to investigate potential mechanisms underlying this distinctive linguistic profile, focusing on children aged 7 months to 6 years. Using parental reports (Communicative Development Inventories), we measured the receptive and expressive vocabularies of children with WS (n = 67) and compared them to typically developing children (n = 1210) and cross-syndrome groups with Down syndrome (n = 27), and fragile X syndrome (n = 15). Results confirm that children with WS show a unique trajectory: alongside general delay, they exhibit a significantly reduced comprehension-production asymmetry not observed in other groups. To elucidate the potential origins of this phenomenon, we implemented a biologically inspired neural network-self-organising map (SOM)-to model early word learning and evaluate visual and auditory map representations. Our findings reveal that WS-like vocabulary patterns can emerge from selective difficulties in visual processing, leading to exemplar-based rather than prototype-based object representations. The model suggests that these visual processing challenges, consistent with known visuospatial difficulties in WS, may contribute to the atypical comprehension-production relationship, while broader processing constraints may underlie general delays. This study provides a mechanistic account of vocabulary development in WS, highlighting the role of visual constraints in shaping lexical outcomes. More broadly, it underscores the need to conceptualise language development as an interaction between sensory input and cognitive subsystems, explaining why the comprehension-production gap is not a uniform feature of language acquisition.