Multimodal investigation of the neurocognitive deficits underlying dyslexia in adulthood
Dyslexia is a neurobiological disorder characterised by reading difficulties, yet its underlying causes remain unclear. Neuroimaging and behavioural studies found anomalous responses in tasks requiring phonological processing, motion perception, and implicit learning, and showed gray and white matter abnormalities in several brain regions of dyslexics compared to controls, indicating that dyslexia is a heterogeneous condition and promoting a multifactorial approach. In order to evaluate whether the combination of behavioural and multimodal MRI can have greater sensitivity in identifying neurocognitive traits of dyslexia compared to monocomponential approaches, in 19 dyslexic and 19 control subjects we acquired behavioural cognitive assessments, multiple (phonological, visual motion, rhythmic) mismatch-response functional MRI tasks, structural diffusion-weighted and T1-weighted images. To examine between-group differences in the multimodal neurocognitive measures, we applied univariate and multivariate approaches. Results showed that dyslexics performed worse than controls in behavioural phonological tasks. Neuroimaging analyses revealed that individuals with dyslexia present reduced cerebellar responses to mismatching rhythmic stimuli, as well as structural disorganization in several white matter tracts and cortical regions previously implicated in dyslexia. Most importantly, in line with the view of dyslexia as a multifactorial phenomenon, a machine learning model trained with features from all three MRI modalities (functional, diffusion, and T1-weighted) discriminated between dyslexics and controls with greater accuracy than models including just one modality. The individual classification scores in the multimodal machine learning model correlated with behavioural reading accuracy. These results confirm that dyslexia should be approached as a composite condition characterised by multiple distinctive cognitive and brain features.