[1] Northwestern Univ, Dept Phys Therapy & Human Movement Sci, Feinberg Sch Med, Chicago, IL 60611 USA
[2] Northwestern Univ, Dept Pediat, Feinberg Sch Med, Chicago, IL 60611 USA
[3] Kennedy Res Ctr Intellectual & Neurodev Disabil, Chicago, IL USA
[4] Univ Chicago, Sect Dev & Behav Pediat, Comer Childrens Hosp, Chicago, IL 60637 USA
[5] Univ Chicago, Dept Publ Hlth Serv, Chicago, IL 60637 USA
[6] Benioff Childrens Hosp, Dept Pediat, 675 Nelson Rising Lane,Box 0663, San Francisco, CA 94143 USA
[7] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA USA
[8] Univ Calif San Francisco, Dept Neurol, San Francisco, CA USA
[9] Univ Calif San Francisco, Weill Inst Neurosci, San Francisco, CA 94143 USA