Understanding Developmental Dyslexia: Linking Perceptual and Cognitive Deficits to Reading Processes
Peter F. de Jong
Understanding the mechanisms responsible for developmental dyslexia (DD) is a key challenge for researchers. A large literature, mostly concerned with learning to read in opaque orthographies, emphasizes phono-logical interpretations of the disturbance. Other approaches focused on the visual-per-ceptual aspects of orthographic coding. Recently, this perspective was supported by imaging data showing that individuals with DD have hypo-activation in occipito-temporal areas (a finding common to both transpar-ent and opaque orthographies). Nevertheless, it is difficult to infer causal relationships from activation data. Accommodating these findings within the cognitive architecture of reading processes is still an open issue. This is a general problem, which is present in much of the literature. For example, several studies investigating the perceptual and cognitive abilities that distinguish groups of children with and without DD failed to provide explicit links with the reading process. Thus, several areas of investigation (e.g., acoustic deficits or magnocellular deficiencies) have been plagued by replication failures. Furthermore, much research has neglected the possible contribution of comorbid symptoms. By contrast, it is now well established that developmental disorders present a large spectrum of homotopic and heterotopic co-morbidities that make causal interpretations problematic. This has led to the idea that the etiology of learning difficulties is multifactorial, thus challenging the traditional models of DD. Recent genetic studies provide information on the multiple risk factors that contribute to the genesis of the disturbance. Another critical issue in DD is that much of the research has been conducted in English-speaking individuals. However, English is a highly irregular orthography and doubts have been raised on the appropriateness of automatically extending interpretations based on English to other more regular orthographies. By contrast, important information can be gotten from systematic comparisons across languages. Thus, the distinction between regular and irregular orthographies is another potentially fruitful area of investigation. Overall, in spite of much research current interpretations seem unable to integrate all available findings. Some proposals focus on the cognitive description of the reading profile and explicitly ignore the distal causes of the disturbance. Others propose visual, acoustic or phonological mech-anisms but fail to link them to the pattern of reading impairment present in different children. The present Research Topic brings together studies based on different methodological approaches (i.e., behavioural studies examining cognitive and psycholinguistic factors, eye movement inves-tigations, biological markers, neuroimaging and genetic studies), involving dyslexic groups with and without comorbid symptoms, and in different orthographies (transparent and opaque) to identify the mechanisms underlying DD. The RT does not focus on a single model or theory of dyslexia but rather brings together different approaches and ideas which we feel are fruitful for a deeper understanding developmental dyslexia.