Mothers and infants participated in three conditions: distal joint watching, proximal joint watching and free play. In the joint watching conditions, the infant was seated in a highchair (distal joint watching) or in the mother's lap (proximal joint watching) and watched a calm aquarium video for 90 s. The order of distal and proximal joint watching conditions was counterbalanced. In the free play condition, mother and infant were instructed to freely play face-to-face, as they would at home, but without toys and without singing for five minutes. The free play condition always followed the two watching conditions. Only interpersonal synchrony data from the free play condition are reported here (see [36 (link)] for a comparison of experimental conditions). Mothers' and infants’ brain activity was measured using fNIRS. Their behaviour was micro-coded using ELAN (v. 5.9; Max Planck Institute for Psycholinguistics, The Language Archive, Nijmegen). The dyads' electrocardiography was also measured but analysed in separate papers [34 (link),35 (link)]. Mothers were sent questionnaires on affect, postnatal depression, attachment style, and infant temperament (not reported here) before the first visit to the laboratory.
At 12 months of age, we visited families at their homes, observed the mother–infant dyad for at least 90 min, and rated them on the Attachment Q-Sort [37 ]. Parents filled out a questionnaire on infants’ expressive language skills (ELFRA-2 [38 ]) when infants were 24 months old.
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