Spracherwerb und neurologische Entwicklung; Language acquisition and neurological development; Sprache und Denken (Ontogenese) ; Language and thought (ontogenesis) ; Cognitive development; Language acquisition and cognitive development ; Morbus Down; Mongolismus; Trisomie 21; Down's syndrome ; Pragmatic competence (pathol.) ; Pragmatische Kompetenz (pathol.) ; Communicative competence (pathol.) ; Kommunikative Kompetenz (pathol.) ; Conversational analysis (pathol.)