Abnormal neuronal migrations in the developing human brain are generally thought of in the context of early gestational events induced by genetic factors, teratogens, or infections. Although the major neuronal migrations that form the cortical plate occur by the 16th week of gestation, late migrations from the germinal matrix into the cerebral cortex continue until five months postnatally. The external granular layer of the cerebellar cortex continues to migrate until 1 year of age. Ample opportunity thus exists for disturbances of these migratory processes in the postnatal period.
Focal zones of imperfect cortical lamination, small hamartomas, and microscopic subcortical heterotopia are often encountered during postmortem examination of infant brains. These focal structural anomalies are usually dismissed as "incidental findings." In survivors, however, such "minor" lesions perhaps contribute to the later seizure disorders, developmental delays, learning disabilities, perceptual disorders, and motor incoordination that are so common following premature birth or neonatal