Research Overview

I am interested in understanding atypical development. What causes it? How do these causes change brain development? How do these changes in brain development alter the development of psychological functions? Which altered psychological functions underlie the patterns of abnormal behaviors that distinguish different developmental disorders? What does atypical development tell us about normal development?

Answering these questions about a disorder will provide an integrated scientific explanation for that disorder. Achieving such explanations will have profound consequences for both basic developmental science and clinical practice. For instance, understanding the different ways that language development goes wrong in dyslexia, autism, Down syndrome, and fragile X syndrome will help us understand how it goes right in typical language development. The same is true for other domains like executive control, social cognition, and emotion regulation. As for clinical practice, having an integrated scientific explanation of a disorder almost inevitably leads to revolutionary changes in diagnosis, early identification, and preventive treatment.

For over 25 years, my colleagues and I have been pursuing such integrated explanations of several developmental disorders: dyslexia, ADHD, autism, and mental retardation. Our lab includes a Child Neuropsychology Clinic, and we have had clinical experience with all these disorders. To understand the developmental pathways that lead to the different neuropsychological phenotypes found in these disorders, my colleagues and I have pursued two strategies: working forward from genotype to phenotype, and backward from phenotype to genotype.

If the genetic etiology of a syndrome is known, one can work forward from genotype to phenotype. This is the approach we have taken with phenylketonuria, fragile X syndrome, and Down syndrome. In some of these disorders, we have been able to demonstrate a dose-response relation between an aspect of the disorder's biology and the severity of the neurocognitive phenotype. In the Down syndrome work, we have collaborated with other scientists who have developed mouse models of Down syndrome. Yet, even with a known mutation, there is variability in the neuropsychological phenotype.

But most developmental disorders are behaviorally-defined and etiologically complex: They are caused by the interaction of multiple genetic and environmental risk factors. Because they are behaviorally-defined, one must work backward from phenotype to etiology. This is the approach we have taken to understand the development of several related, complex behavioral disorders: dyslexia, other speech and language disorders, and ADHD. In addition, since these disorders are comorbid with each other, we can use genetic and neuropsychological methods to test hypotheses to explain their comorbidity. We now know that each of these disorders is familial and heritable and that some pairs of them are co-familial and co-heritable, which means that comorbidity is due in part to shared genes. Because molecular work to identify genes influencing these disorders is proceeding rapidly, we can perform molecular tests of shared genetic influence. For instance, we have recently found that the dyslexia locus on chromosome 6p also influences comorbid ADHD. We are also testing the hypothesis that the comorbidity between dyslexia and some speech and language disorders is due to shared genes, in this case, ones that alter the development of implicit phonological representations. Since
our collaborators have identified several risk loci for dyslexia (on chromosomes 1p, 2p, 3p-q, 6p, 15q, and 18p), we are testing whether some of these loci also influence speech delay. We have found that the dyslexia loci on 6p and 15q are also linked to speech delay. We are also testing which cognitive risk factors are shared among these comorbid disorders and which are specific to a given disorder

In sum, it now appears that comorbidity is often due to partly shared etiological and cognitive risk factors. Working out which risk factors are shared and not shared across these developmental disorders is an important goal for our future research.

Our clinical experience with all of these disorders has helped focus our research, and our research results have led to improvements in their diagnosis and treatment. My research and clinical work informs the courses I regularly teach on developmental psychopathology and developmental neuropsychology.

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