A disorder of neuronal migration that is characterized by abnormal cortex morphology, with pathological features including a variably decreased brain size, enlarged ventricles representing a stage of fetal development rather than hydrocephalus, heterotopia of the inferior olivary bodies that lie between the corpus pontobulbare and their normal location, aberrant or absent corticospinal tract, heterotopia of cerebellar granules and abnormally shaped dentate nuclei.