Multidimensional etiology and individual differences in developmental dyslexia

Although reading seems straightforward, a significant percentage of the population struggles persistently to develop fluent reading skills. In the quest for the underlying cause(s) of dyslexia, most studies still examine isolated factors and have stringent inclusion criteria, overestimating the homogeneity within individuals who experience reading difficulties and leaving it unclear how factors within and across diagnostic boundaries interact. This doctoral thesis expands our understanding of the complexity of factors involved in reading acquisition and development by combining behavioral, neural, and intervention data. We examined the role of top-down control in acquiring new letter-speech sound correspondences, revealing substantial inter-individual learning differences and outcomes. Examining the underlying mechanisms of change during a dyslexia intervention showed that especially children with poor letter-speech sound integration at the start of the intervention improved the most after a phonics-based treatment. In line with the multiple deficit model, we argue that while some deficits are present in some, they are absent in others. Future research should step away from searching the ‘core’ deficit underlying each neurodevelopmental disorder and aim to capture complex interrelationships that move beyond diagnostic boundaries. We have shown how complex analysis techniques can be used to study these multivariate interrelationships. Understanding the complex nature of typical and atypical reading development is particularly relevant to educational and clinical practice, which are recently benefiting from new developments such as game-based or online interventions. Better identification of individual differences and understanding why and for whom these interventions work is of great importance for literacy policy and timely, effective interventions, as these can be better matched to children’s needs.