A thread from amnion to milk to food: flavour continuities over birth and weaning transitions
Résumé
Olfaction and taste engage oral activity from birth. Odorants and tastants are thus in a good position to modulate the first milk intake responses and the selective acceptance of non-milk foods around weaning. In the perinatal period, the biological fluids – amniotic fluid and milk – that contact the nasal/oral chemosensors carry stimuli which are salient to newborns. Amniotic fluid and milk are at first treated by infants as sensorily/motivationally equivalent, giving credit to the double notion of a chemosensory overlap between perinatal ecologies and of neonatal preparation by foetal experience to attend certain stimuli more than others. Tests of several predictions derived from this hypothesis will be presented, leading to the evidence that the foetal brain registers odour stimuli and mediates positive orientation when these odorants are re-encountered in the newborn's environment. Later on, infants continue to extract unprecedented odour cues during nursing interactions, and to expand their odour "lexicon". They encode in particular odorants and tastes associated with the breast and milk, either maternal or artificial. These breast- and/or milk-borne chemosensory cues are retained in the long run, up to 5 to 20 months, and can thus influence acceptance of post-lacteal foods. This acceptance of novelty in foods during the period of diversification is further influenced by the variability of early chemosensory experience. In sum, multiple, redundant processes promote the early learning of odours, some operating in anticipation (fetal learning) while others work in the immediacy of the postnatal constraints. Thus, human foetuses are born with a draft of the version of the odour environment they will have to face later on, as newborns or weanlings.