Phonological encoding and lexical processing of spoken words in dyslexia: linking behaviour and neural responses

Developmental dyslexia is associated with phonological processing deficits. Despite phonological deficits, auditory processing of everyday speech is unaffected, raising the question: How do individuals with a phonological deficit process spoken words to achieve normal comprehension? Our study investigates the relationship between phonological skills and neural responses to spoken words at the phonological and at the lexico-semantic level. Fourteen right-handed participants with dyslexia diagnosis and 14 without a history of reading difficulties heard single Spanish words and occasionally had to indicate whether the word was animate while magnetoencephalographic (MEG) signals were recorded. The effects of phonological neighbourhood and word frequency were extracted from the MEG responses using event-related regression coefficients. Participants with dyslexia diagnosis had significantly lower phonological scores than typical readers. Importantly, individuals with lower phonological skills not only showed weaker neural responses to phonological neighbours 200-500 ms after word onset, but also reduced sensitivity to word frequency between 200-650 ms. Source space analysis localised phonological and lexico-semantic effect peaks to the left superior temporal and left inferior frontal gyri, key areas for phonological and combinatorial processes that have been linked to dyslexia. The results provide evidence that phonological deficits impact not only sublexical stages of spoken word processing, but also lexico-semantic processing. These deficits cannot be compensated through neural re-organization of lexical-distributional information at the single word level. Remediation strategies which strengthen bottom-up processing may thus be even more important for dyslexia than previously assumed because phonological training likely benefits word processing at multiple linguistic levels beyond phoneme representations.