Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) periventricular heterotopia ?
    ### Response:
    Periventricular heterotopia is a condition in which nerve cells (neurons) do not migrate properly during the early development of the fetal brain, from about the 6th week to the 24th week of pregnancy. Heterotopia means "out of place." In normal brain development, neurons form in the periventricular region, located around fluid-filled cavities (ventricles) near the center of the brain.
    