The purpose of this longitudinal study was to describe objectcentred interactions between mothers and their 2-4-month-old infants, before and during the emergence of reaching and grasping movements. We hypothesized that when reaching movements emerge at around 3 months, mothers alternate between attention stimulation and reaching stimulation, before joint actions between mother and infant develop around objects. Twelve dyads were recorded when infants were 2 months, 3 months and 4 months. The interactive sessions lasted 5 min. Three ageappropriate toys the infant could handle were available to the mother. A principal component analysis (PCA) was performed on verbal and non-verbal maternal behaviours, motor infant behaviours and co-occurrences of those behaviours. The developmental course of prehension in infants when playing with their mother follows similar pathways, as was described when they are observed alone. Mothers appeared to early scaffold prehension skills by verbal and non-verbal means. Moreover, maternal behaviours change according to the infant's behaviour, and conversely, infant's behaviours influence maternal behaviours: mother plays first an active part in joint action, while later on, the infant achieves joint action when motor skills develop. ProcedureThe dyads were longitudinally videotaped at home (2 months) or in the lab (3 and 4 months) by a single female experimenter. The infants were positioned semi-reclined in a baby-seat, facing their mother. A mirror was placed behind the child, so that the camera filmed the child and the reflection of the mother. An observation lasted 5 min, and the mother was simply told to interact with her infant as she usually did, using the toys. Data CodingThe following behaviours (note 1) were detected.

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