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. Physiological studies confirmed that the human cochlea has normal adult function after the 20th week of gestation 44 .
It has also been found, through the use of high-resolution ultrasound imaging for observing eye-blink responses to vibroacoustic stimulus applied to the maternal abdominal wall, that infants are able to detect and respond to sounds by the beginning of the third trimester 45 . Newborn infants have a well developed sense of hearing 46-51 . Soon after delivery, they show the ability to remember and discriminate the voice of their own mother from the voice of an unfamiliar women 52 .
DeCasper & Fifer 52 placed earphones over the newborns’ ears and a non-nutritive nipple in the mouth. The non-nutritive nipple was connected through a pressure transducer to a recording device that produced either the voice of the infant’s mother or unfamiliar female voice. They reported that newborns increased their sucking rate to activate the playback of the voice of their own mother in preference to the unfamiliar woman’s voice.
A review of the literature on newborns’ behavioural response toward acoustic stimulation reveals that under different experimental conditions46-51, 53 newborns are able to detect and orient their head toward laterally presented sound. Although the overall response to a sound stimulus was positive, it has been found to be effected by different factors. First, the distribution of off-centre to at-centre sound trails: head turns toward off-centre sound source has been found to be improved as the probability of laterally presented sound increased.
47 Second, sound duration: infants turn their heads reliably to sounds lasting 1 second or longer. 54,55 . Third, sound repetition rate: lateral head turns can be elicited using rapidly repeating or continuous brief sounds but not slowly repeating, or brief sounds 55 . Fourth, sound frequency: mid and high frequency and broadband sounds elicit more reliable head turning than low frequency sounds56 . Finally, sound location: due to motor control difficulties, newborns were able to locate and orient their heads to sound sources presented near midline more than ones presented at 90° off-centre. 53