Down Syndrome from a Neurodevelopmental Perspective
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Psychology. Please check back later for the full article.
Down syndrome (DS; trisomy 21) is the most common genetic disorder associated with intellectual disability. It occurs in one out of every 700 to 1,000 live births. DS is caused by trisomy of human chromosome 21, which results in the altered expression of over 300 genes. This neurodevelopmental syndrome is characterized by distinctive facial dysmorphology and an uneven cognitive phenotype including relative strengths and weaknesses. Relative strengths include visual processing, receptive vocabulary, and social-emotional functioning (though performance in these domains generally falls below the level expected for typically developing individuals). Relative weaknesses include verbal working memory, expressive language, and motor ability. However, the phenotype of individuals with DS is far from homogeneous, and a wide range of individual differences is present at every level of description. On the genetic level, the trisomy can occur through different mechanisms at distinct developmental time points, and the expression of trisomy 21 may be modulated by different genes across individuals. On the level of the brain, individual differences in brain structure and/or function correlate with variation in cognition and behavior, including communication skills. Large individual differences can also be observed on the cognitive level. For example, while some toddlers with DS are nonverbal, others reach expressive vocabulary levels close to those of typically developing children. A wide range of individual differences has also been reported in other areas, including the motor domain, sleep, parent-child interaction, and medical and psychiatric comorbidities.
In order to understand a neurodevelopmental syndrome such as DS, it is crucial to consider individual variations at multiple levels of description and the interactions between them over developmental time. A more complex, dynamic view that goes beyond a description of DS as a homogenous group is thus required.