Taken together, the above studies could suggest the presence of an early link between hand and mouth, and it is a pre-functional act of feeding behaviour which is not a reflex or automatic behaviour, but can be differentially elicited using orally-administered substances with different degrees of sweetness. Smell or olfaction is the other chemosense, it enables us to perceive and distinguish the odour of different substances. Both the primary and the accessory olfactory (vomeronasal organ) systems are sufficiently mature by the third trimester of gestation 19, 20 .
A growing body of evidence indicates that newborn infants are able to detect and respond to odour stimulation soon after birth with varied goal-directed behavioural responses such as head turning, crawling and hand movement. Several studies have revealed that newborns shortly after birth will turn their heads in response to olfactory stimuli of a biological nature such as amniotic fluid 21,22 colostrums, and breast-milk. 23 Researchers have found that infants are attracted more to familiar odours like those experienced in uterus.
For instance, it has been found that newborn infants are more likely to turn their head toward their own amniotic fluid AF than to unfamiliar AF 21-23 . Furthermore, breast-fed and bottle-fed infants have been found to turn their head to the odour of the AF when presented simultaneously with either a control or milk-formula stimuli. Babies appear not to distinguish between their own AF and their mother’s colostrum in the first three days of life 24, 25.
This may indicate that babies are unable to detect or distinguish differences in the low intensity odours of such biological substances, or they do not have a preference for one over the other 24 . The inability to distinguish between amniotic fluid and colostrum may occur because the odours are similar, since the odour of both has been found to be affected by the food consumed in the last weeks of pregnancy 26, 27 and gas-chromatographic analysis indicates the existence of common voltaic compounds in both 28 .
To support this conclusion , Marlier et al 25 compared newborn’s response toward amniotic fluid and milk-formula. They found that newborns orient preferentially toward the odour of the amniotic fluid, implying babies are able to identify the odour of amniotic fluid. Furthermore from four days after birth breastfed as well as bottle-fed newborns change their preference and orient to human milk more than to amniotic fluid 23, 24 or milk-formula 29 Preferential responsiveness to particular odours is not limited to the odours they experienced prenatally or odours from the breast region.
Cernoch and Porter 30 have shown that two-week- old breast-fed infants preferentially orient their head toward a pad impregnated with their mothers’ axillary odour relative to the axillary odour of an unfamiliar lactating mother. However, this discrimination in performance has been demonstrated only in breast-fed neonates; bottle-fed newborns showed no preferential head orientation to either pad. The results of the study seen in breast-fed infants were attributed to the continued exposure to axillary odour during feeding times. Maternity cues, such as breast odour, can also be used to elicit goal-direct gross body movement early in life.
Healthy newborn babies when placed in a prone position between their mother’s breasts either immediately 38, 39 or a few days after delivery 40 were able to initiate crawling movements which brought them into contact with the unwashed nipples rather than the washed one. Varendi ; Porter 41 were able to show that this crawling response was due to maternal visual, tactile and auditory stimuli, but rather resulted from olfactory stimulation. In their study 1-3 day-old newborn infants were placed prone on a warm bed , 17 cm away from a pad that was either holding their mother’s breast odour or a control pad.
The infants initiated crawling movement toward the pad that was holding their mother’s breast odour rather than toward the control pad. It has been hypothesized that the attractiveness to the odour of the breast is due to similarity of odorous compounds between amniotic fluid and the secretions produced by the glands in the areola region 28 . Consequently, at birth, the olfactory stimulation from the breast region resembles the familier olfactory stimuli experienced prenatally.
Although infants develop sophisticated motor and manipulative skills in their first year of life, there are not many studies in the literature about motor responsiveness to odour in infants older than 2 weeks of age. Mennella ; Beauchamp 42 reported that 6-13 month- old infants selected and manipulated toys that were impregnated in either vanilla or ethanol scents in preference to unscented toy. Furthermore the degree of infants preference for toys impregnated with odours were found to be related to parental use of these volatile substances during pregnancy and during breast-feeding.
Taken together, the above studies may suggest that newborn babies have the ability to retain, discriminate and respond differentially to the chemical features of odours that are presented in utero and in early postnatal life. Physiological studies have suggested that norepinephrine plays an important role in early olfactory learning 43 . Norepinephrine neurons in the locus coeruleus send projections to the olfactory bulbs and facilitate olfactory learning 43 ,which may then guide the brain to initiate positive motor responses.