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 Although newborns can detect and orient toward laterally presented sound soon after birth, motor behaviour eliciting procedures should take into consideration the immaturity of the cortical structures of the auditory system, which is believed to be involved in complex temporal processing.
For instance, Clifton et al 46 tested the “precedence effect” phenomenon on newborns, in which a sound is presented from two loudspeakers located in opposite sides of the infant and one speaker is leading the other by a few milliseconds, normally listener locate the sound of the leading speaker. Clifton et al 46 proved that newborn babies were not responsive to the “precedence effect” as to single source stimuli. This inability seems to improve as the infants get older, so by 24 weeks of age infants were found to orient reliably to both precedence effect and a single source stimuli.
In addition to the lack of ability to orient to precedence effect, newborns also showed irresponsiveness to brief sounds i. e. 500 msec and below. 55 Using acoustic stimulation to promote movements has also been documented in the literature. Presenting auditory stimulus in the form of either bell sound or speech by female voice was found to increase the general body movement of newborn babies of 1 to 5 days compared to the effect of pure tone 69.
Four to 12 months old infants were found to be more interested in manipulating objects with sound when compared to the same objects with no sound68. Older infants have been found to use auditory and proprioceptive information to guide their reaching behaviour in the dark and they manage to do that fairly well 64-67 . Although the reaches were found to be successful in 70% 66 to 77% 64 of the trials, Perris and Clifton 64 found that the chances of eliciting or promoting reaching movement increase if there was prior motor engagement i. e. manipulation of the sounding object, and /or prior visual experience.
Vision At the time of birth the sense of vision is the least developed sense compared to other senses. Anatomical data shows that newborn’s peripheral vision is more mature than the central vision 70 . Using preferential looking to evaluate the human monocular visual acuity in the first three months of life, Courage et al 71 found that central as well as peripheral visual acuity are poor in the first month of life.
At birth visual acuity was found to be around 20% of adult visual acuity 72 . A retinoscopic study estimated the focal distance of newborns when fixating an object is around 9 inches 73 . Finally, some behavioural studies found that newborns are able to detect stimuli presented in their peripheral visual field as far as 30-35° 74, 75, 76 from midline. Their ability to discriminate objects in the peripheral visual fields is not developed until around the 4 month after birth 77.
Although newborns have not developed mature vision, several behavioural studies showed that newborn babies have the ability to process some visual information and use it to initiate motor behaviours. For instance, from the first days of their lives, newborn infants are capable of imitating simple motor actions such as opening and closing the mouth, tongue protrusion, opening and closing of the hand, and index finger movement 78-82 . Imitation of facial gestures has also been observed in babies as young as 45 minutes old 83-84.
Recently Nagy et al raised the possibility of the presence of cortical mirror neuron system that may contribute to the emergence of early imitation in newborns 78. Several behavioural studies found that newborn infants are not just able to perform simple motor acts but also able to initiate motor behaviours that are voluntary, controlled and resemble reaching movement. Bower et al 89 and McDonnel 90 found that when newborn infants were presented with an object in 5 different positions, infants changed the direction of their reach to match the direction of the presented object.
Bower et al 89 found that 70% of their reaches were within 5° and approximately 1. 5 cm of the object. Their reaches were not just considered oriented but also intentional because when they were presented with a virtual rather than a real object they became frustrated. Furthermore, van der Meer, et al 91-92 have found that newborn infants can deliberately adjust their arm movement to correct for a force applied to it, but only if they can see their arm either directly or through a monitor 91-92 .
In a further study, van der Meer 93 also showed that neonates have the ability to change direction and control the velocity and deceleration of their arm to put the hand within a 7 cm cross light beam 93 . Interestingly, they noticed that approximately 74% of newborns decelerated the movement of the arm before entering the light, which provided an indication of expectation of light and thus further evidence of an ability to control the arm movement.