title: Endogenous and exogenous smiling systems in early infancy.
mesh_terms: D001766:Blindness / Q000151:congenital; D002652:Child Behavior; D002657:Child Development; D005149:Facial Expression; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D012895:Sleep, REM; D012919:Social Behavior