The Nature of Phonological Impairments in Dyslexia and Stuttering: Evidence from Phonemic, Semantic, and Design Fluency Tasks

Objective: This study aimed to dissociate the cognitive mechanisms underlying verbal fluency in adults who stutter (AWS) and dyslexic adults. While both groups show impaired phonemic fluency, the degree to which these impairments stem from difficulties relating to efficient access to phonological representation versus degraded phonological representations remains poorly understood.Method: An online sample of 186 adults, comprising 62 AWS, 62 dyslexic adults and 62 neurotypical adults completed a suite of individual difference measures and typed semantic and phonemic fluency tasks. Design fluency served as a nonverbal control to differentiate general executive impairment from domain-specific linguistic challenges and was assessed with a mouse-drawn task. Results: After controlling for nonverbal IQ and a composite measure of phonological and orthographic precision, hierarchical regression analyses showed that dyslexia and stuttering significantly predicted reduced phonemic output. Notably, semantic and design fluency remained unaffected. Critically, a finer-grained analysis showed that while both neurodivergences predicted a reduced number of switches, a proxy for phonological access, this effect was more pronounced in the dyslexia group. Conversely, only the AWS group significantly predicted reduced cluster size, a proxy for the integrity of phonological representations.Conclusion: These findings suggest a functional dissociation: phonological impairment in dyslexia appears localized to the access of the phonological representation mechanisms, while impairments in AWS seem to involve the robustness of the phonological representations themselves. These results support a taxonomic distinction between the two neurodivergent profiles, suggesting that clinical interventions should target retrieval strategies for dyslexia and representational strengthening for persistent stuttering.