ability able according action activity adaptation adult already appear appropriate aspects attempts attention becomes beginning behaviour bring changes characteristics child communicative consequences considerable context continuity conversation course described developmental direct dyadic early effect emerge encounters environment evidence examined example experience expression face fact findings function further gaze gesture give hand important increase indicate individual infants influence initially instance interaction interest involved labels language largely learning less looking maternal means months mother mutual namely nature object observed occur parent participants particular patterns period person play pointing possible present reference relation relationship remains respect responses result role seen sequences situation skills social social interaction specific speech stage stimulation suggests task thing topic turn utterances various verbal visual vocal young