When a newborns cheek touched by something it turns its head toward the object and open its mouth this is known as the reflex?

Primitive Reflexes

Normal newborns exhibit a large number of easily elicited primitive reflexes that are often altered or absent in the infant with neurologic impairment. These reflexes may be transiently depressed in the infant who has experienced difficulty in achieving the transition between intrauterine and extrauterine existence. The persistent absence or asymmetry of one or more of these reflexes may be a clue to the potential presence of neuromuscular abnormalities requiring further investigation (seeChapter 3).

The rooting reflex may be elicited by lightly stimulating the infant's cheek and observing the reflexive attempts to bring the stimulating object to the mouth. The sucking reflex is activated by placing an object in the infant's mouth and observing the sucking movements. In the grasp reflex (Fig. 2.15), transverse stimulation of the midpalm (without touching the back of the hand) or midsole leads to flexion of the digits or toes around the examiner's fingers.

The Moro reflex (Fig. 2.16) evaluates vestibular maturation and the relationship between flexor and extensor tone. Elicitation of the reflex involves a short (10 cm), sudden drop of the head when the infant is supine. The full response involves extension of the arms, “fanning” of the fingers, and then upper extremity flexion followed by a cry. An incomplete but identifiable reflex becomes apparent at approximately 32 weeks' gestation, and by 38 weeks it is essentially complete. Very immature infants demonstrate extension of the arms and fingers but do not show true flexion or make a sustained cry. Marked asymmetry of response may be associated with focal neurologic impairment.

Galant's infantile reflex is a truncal incurvation reflex. It can be elicited by holding the infant in ventral suspension and stroking from shoulder to hip along one side of the spine. The infant will contract the abdominal musculature and laterally flex toward the stimulated side. Lack of the Galant reflex may indicate a spinal cord lesion.

These reflexes and a host of other less commonly used reflexes are termedprimitive, because they are present at or shortly after birth and normally disappear after the first few months of life. Just as their absence may indicate neurologic impairment at birth, their abnormal persistence may also be a cause for concern and further evaluation.

Sensory System

The newborn is able to recognize touch and pain, which is evident from observing the rooting reflex and the infant's withdrawal from painful stimuli. Tactile stimuli can cause the infant to become more alert with either the initiation or the cessation of the associated motor activity, and a painful stimulus causes withdrawal from that stimulus and is often associated with crying. Saint-Anne Dargassies demonstrated that a preterm infant of 28 weeks can differentiate touch from pain.

As it has been demonstrated that infants can experience pain, attempts to alleviate that pain have proven to be beneficial. One can discern the severity of discomfort by observing subtle findings, such as the intensity of cry and the extent of facial grimacing. The reliable assessment of proprioception is not possible until later childhood.

Perception of visual stimuli occurs early in human development. By the age of 26 gestational weeks, the infant will blink to a light stimulus, and a pupillary response to light can be demonstrated at approximately 29 weeks. From 32 to 36 weeks, the infant will turn toward a diffuse light, and at 32 weeks exposure to light will provoke lid closure as long as the light source is present. By 32 weeks of gestation, there is some visual fixation that improves thereafter, and at 34 weeks most infants will track in a small arc or a red object such as a 4 in. ring. At term, fixation and following a visual stimulus are well developed.

Optokinetic nystagmus (OKN) is present in some infants at approximately 36 gestational weeks and is consistently present in term babies. By using OKN, it is believed that the acuity of the newborn infant is approximately 20/150. Perception of color sensitivity can be shown at approximately 2 months, and binocular vision and depth perception are present at approximately 3–4 months. At the age of 2–3 weeks, the infant can imitate facial gestures, and from 12 to 20 weeks some attention is directed to the hand(s). At approximately 4–5 months, the infant can be excited when observing that food is being prepared, and at 6 months the infant can adjust his or her head and body position to gaze at an object. Normal visual function continues to improve throughout infancy and early childhood.

Perception of auditory stimuli also occurs in early human development. Vibroacoustic stimulation has been monitored by ultrasound in utero and responses could be elicited as early as 24 or 25 weeks of gestation. A preterm infant of 28 weeks of gestation startles or blinks to a loud clap or noise and a normal term infant may respond more quickly to a similar stimulus. By the age of 3 or 4 months, the normal infant will turn toward the source of the sound.

Prelanguage communication begins soon after birth when the infant watches the mother's face, and by 6 weeks the infant will smile responsively to the mother. Communication is carried out by crying, cuddling, or resisting being held. Later, the infant communicates by laughing, screaming, or having a temper tantrum. Soon after the infant begins to smile, he or she begins to vocalize and by 3 or 4 months can make some consistent sounds, sometimes squealing with delight. By the age of 7 or 8 months, the infant can make a sound to attract attention and by 10 months may know one meaningful sound or word and respond to ‘no.’ Between the ages of 15 and 18 months, the infant can utter some meaningful words, and by 21–24 months he or she begins to put several words together. One study showed that out of 1824 boys and 1747 girls, 3% said their first word at approximately 9 months, 10% did so by 10 months, and 90% did so by 18 months of age. With maturation, the infant learns what effect his or her sounds or words have on other people. The sounds become more meaningful and the infant has an understanding of many words before he or she can articulate them.

Innumerable developmental processes occur during the early postnatal period, some of which occur simultaneously and others at different times. There is an inexorable progression and an elegant integration of development that continues until maturation.

Developmental Reflexes

The most frequently elicited primitive or developmental reflexes include the rooting, sucking, palmar grasp, and Moro responses. These reflexes are fully developed and strong in the healthy, full-term newborn and typically disappear in the months to follow. Their persistence beyond the anticipated age of disappearance is an indicator of underlying central nervous system dysfunction. Other primitive reflexes include the crossed extension (where a limb withdraws by contracting of the flexor muscles and relaxation of the extensors and the opposite occurs in the other limb), the placing reflex (when there is flexion followed by extension when the infant is held upright and the dorsum of a foot is moved along the under edge of a table), and stepping reactions (when the soles of the feet of an infant held upright touch a flat surface she will take steps by placing one foot in front of the other), all of which make their appearance by 36-38 weeks’ postmenstrual age.

8.1.1 The Evolution of Inhibitory Control

A common way in which infantile patterns are eliminated from the adult repertoire is by the inhibition of their expression (Peiper, 1963). The reality of this inhibition is illustrated by a common phenomenon, the release from inhibition that can occur with age-related dementia. For example, if the cheek of a 1-month old infant is gently stroked with one's finger, it will exhibit a rooting reflex, whereby it orients its mouth to the stimulus; the same thing can be elicited in an elderly person with dementia (Teitelbaum, 1967). Thus, the neural circuit that regulates the behavior pattern is not lost, but rather is simply inhibited by a new control circuit, usually situated in the anterior portions of the cortex. Then, as these late forming circuits are the first to be disorganized by age-related disruptions, the circuits deeper in the brain are released from the inhibition and so are once again expressed (Kolb & Whishaw, 2009). There is good reason to suspect that this is similar to that occurring over evolutionary time. As most neural circuits have multiple functions, it would be difficult to eliminate a particular neural circuit so as to eliminate an unwanted behavior pattern, as this would likely disrupt other useful behaviors that are controlled by that neural circuit. Therefore, a simple way in which to eliminate unwanted behaviors is to inhibit the expression of the neural circuit in the appropriate context (Kavanau, 1990).

If scratching with either the foot or the hands is under inhibitory control, this has several implications for the distribution of the relative balance of scratching done by the hand and foot in species that use both. First, it would imply that the more infrequent mode of scratching is being used at a lower frequency than it could be. Head scratching was observed in two troops of Allen's swamp monkeys (Allenopithecus nigroviridis), with one troop consisting of five juvenile and adolescent monkeys, and the other, five adult monkeys (Table 4.2). In the troop of older monkeys, due to complications arising from diabetes, one of the animals had had its right hind limb amputated at the level of its hip. Based on the scores derived from all the intact individuals, the frequency with which they scratched their head with their hands was ≤ 5%. For the monkey with the amputated leg, on the side with the missing leg, 100% of the head scratching involved the ipsilateral hand, thus showing that, as predicted, the hand is capable of scratching at a higher frequency than is normally the case. This particular example is instructive in another way as well.

When the monkey with the missing leg scratched itself on the side of the body with the intact hind leg, it scratched its head with its ipsilateral hand in 20% of scratches. Again, this illustrates that these monkeys are physically able to scratch with their hands more often than the usual low frequency. However, to scratch on the intact side with the foot required major postural adjustments. The monkey had to lean on its right side and support its body by placing its right hand on the ground, which freed its left foot to scratch. Indeed, in the absence of the right leg, all this monkey's body weight had to be supported by its right arm, a bigger postural shift than would be the case in monkeys with both hind legs intact. That such a postural shift was sometimes exceedingly difficult for this monkey was suggested by the increase in its use of its hand to scratch its head on its intact side to above the level on the body seen in intact monkeys. Thus, even though it required exaggerated postural adjustments, wherever possible, the monkey with the amputated leg maintained the dominant bias in using its foot to scratch its head on its intact side. These observations further mitigate the claim that the choice of scratching tactic is determined by biomechanical factors that make that particular form of scratching more efficient (see above).

The presence of both types of scratching also implies that the inhibition of one pattern is not complete, raising the possibility that the two strategies could compete with one another. We have seen several such examples of apparent competition. For example, in a troop of Guinea baboons (Papio papio) containing over 60 adults (Table 4.2), it was estimated that they scratched their heads with their hands about 33% of the time. On several occasions, as a baboon was completing scratching with its hand, it thrust up its ipsilateral foot and scratched the same location with its foot. Sometimes, this occurred so quickly that the baboon lost its balance and would start to fall toward the side being scratched; this led to the baboon hurriedly placing its ipsilateral hand on the ground for support while it continued to scratch its head with its foot.

The third piece of evidence is developmental. The adult, white-handed spider monkey (Ateles geoffroyi) scratches its head 100% of the time with its hands, whereas the similarly sized, adult black howler monkey (Alouatta caraya) scratches its head about 45% of the time with its hands (Table 4.2). Observations on infant spider monkeys, around 6 months or younger, revealed that, on occasion, they also scratched their heads with their feet (Pellis, 2010). Comparing a male howler monkey with a male spider monkey both 1.5-months old showed that the young howler monkey scratched its head around 95% of the time with its feet, while the young spider monkey did the same about 30% of the time. That is, the infants exhibited a reduced inhibition of scratching with the feet as compared to the adults, just as would be expected if, during later development, inhibitory mechanisms were established (see above).

Together, these data support the hypothesis that switching from the fully foot-typical scratch to the fully hand-typical scratch involves the evolution of an inhibitory mechanism that suppresses the older pattern.

Neurobehavioral Testing

It is difficult to detect subtle neurobehavioral differences among neonates during the assignment of Apgar scores or the performance of the initial neurologic examination; therefore, investigators have developed and studied methods of documenting neonatal neurobehavioral status (Table 9.6). In the past, the neonate was considered incapable of exhibiting higher cortical function. However, investigators have noted that the term neonate is able to sense and respond to a variety of stimuli in a well-organized fashion.231–233

In 1973, Brazelton234 described the Neonatal Behavioral Assessment Scale (NBAS) with the following four variables as key determinants of neonatal neurobehavior: (1) various prenatal influences (e.g., infection); (2) the maturity of the infant, especially its CNS; (3) the effects of analgesics and anesthetics administered to the mother before and during delivery; and (4) the effects of difficulties encountered during delivery (e.g., trauma). The NBAS was developed as a tool to detect neurobehavioral abnormalities that resulted from any of these four variables.

This scale consists of 47 individual tests with 27 evaluating behavior and 20 evaluating elicited or provoked responses. The 47 tests can be completed in approximately 45 minutes. The NBAS evaluates the ability of the neonate to perform complex motor behaviors, to alter the state of arousal, and to suppress meaningless stimuli. The goal is to provide an extensive evaluation of neonatal cortical function and to detect subtle differences among groups of infants. Habituation (i.e., the ability to suppress the response to meaningless, repetitive stimuli) is considered an excellent indicator of normal early cortical function.231

In 1974, Scanlon et al.235 described the Early Neonatal Behavioral Scale (ENNS), which consisted of tests that were easy to perform and score quantitatively during the neonatal period. The ENNS was developed primarily for the evaluation of the effects of maternal medications (e.g., analgesic and anesthetic agents) on neonatal neurobehavior. The ENNS consists of (1) 15 observations of muscle tone and power, reflexes (e.g., rooting, sucking, Moro), and response to stimuli (e.g., light, sound, pinprick); (2) 11 observations of the infant's state of wakefulness; (3) an assessment of the ability of the neonate to habituate to repetitive stimuli; and (4) an overall general assessment of neurobehavioral status. This test can be performed in 6 to 10 minutes.

In 1982, Amiel-Tison et al.236 described the Neurologic and Adaptive Capacity Score (NACS) to differentiate neonatal depression secondary to maternally administered drugs from depression caused by asphyxia, birth trauma, or neurologic disease. Whereas the ENNS concentrates on the infant's habituation ability, the NACS emphasizes motor tone as a key indicator of drug-induced abnormal neurobehavior. The basis for this emphasis on neonatal motor tone is explained as follows: unilateral or upper body hypotonus may occur as a result of either birth trauma or anoxia, but global motor depression is more likely a result of anesthetic- or analgesic-induced depression. A total of 20 criteria are tested in the areas of adaptive capacity, passive tone (e.g., scarf sign), active tone (e.g., assessment of the flexor and extensor muscles of the neck), primary reflexes (e.g., Moro), and alertness. The total possible score is 40, and a score of 35 to 40 is considered normal. The NACS can be performed in 3 to 4 minutes.

The Functional Organization of Behavior

Early Work on Learning

Much of the earliest research on infant learning was methodological in nature. Driving questions were whether human infants were even capable of learning and if so, what types of learning they would exhibit. Researchers were also interested in determining when learning might begin to take place in development. Before these questions could be answered, fundamental methodological details had to be worked out. For instance, it was important to rule out the possibility that a change in behavior reflected acclimatization to a stimulus or a response to a particularly appealing reward, behaviors that mimic but do not reflect learning. Additionally, a number of early studies failed because researchers did not know how much or how little exposure to a stimulus would promote learning or which types of rewards were needed.

The earliest studies investigated whether infants could be classically conditioned to associate a conditioned stimulus with an unconditioned response by pairing a CS with an UCS. In 1920, John B Watson and Rosalie Raynor exposed an 11-month-old child named Albert to an UCS (a loud sound) each time he touched a CS (a white rat). The loud sound caused Albert to cry and withdraw his hand, so that subsequently, merely seeing the rat led to the same behavior, demonstrating memory of a learned stimulus–response pairing.

Infants just a few hours old can be classically conditioned also. In 1984, E M Blass and colleagues followed a stroke on the forehead (the CS) by immediate oral delivery of sucrose to infants through a glass pipette (the UCS). A control group was exposed to the same number of CS–UCS pairings, but the time interval between the pairings varied across trials. Another control group received only the UCS in order to rule out the possibility that repeated deliveries of sucrose itself would result in a change in behavior. During an extinction phase where infants received the CS but not the sucrose delivery, the experimental group cried when the sucrose was not delivered whereas infants in the two control groups did not. All three groups experienced withdrawal of sucrose, so this in itself could not explain the experimental group’s behavior. Their behavior could only be explained in terms of their having learned the predictive CS–UCS relationship. This study, and others in this general vein, established that infants could be classically conditioned from birth.

Researchers were also interested in determining whether infants were capable of learning using operant principles. Operant conditioning involves reinforcing a naturally occurring response to increase or decrease the rate of that response. For instance, positively reinforcing a response leads to an increased rate of that behavior. This is in contrast to classical conditioning, which involves a learned association between a CS and UCS. The first study to obtain evidence for operant conditioning in newborns was conducted in 1966 by Siqueland and Lipsett. They paired differential auditory stimuli with an unconditioned tactile stimulus (stroking the infant’s cheek). The stroking produces an unconditioned rooting reflex to that side and by necessity a head turn. In their study, when the pairing of the positive auditory stimulus (a buzzer) and the tactile stimulus produced a head turn, this was always followed by a positive reinforcer (administration of sugar water). However, a head-turn in response to the pairing of the tactile stimulus and the negative auditory stimulus (a tone) was never reinforced. Responding to the presence of the positive auditory stimulus increased over time whereas responding to the negative auditory stimulus decreased suggesting that infants were discriminating between the two types of auditory stimuli based on operant conditioning.

An operant paradigm that has been used extensively to study learning and memory in infants since then, is the mobile conjugate reinforcement procedure developed by Carolyn Rovee-Collier in 1969. In this procedure, an infant is placed on his back in a crib beneath a mobile. A ribbon runs from a suspension hook on the mobile to the infant’s ankle (in the reinforcing condition) or from a hook that will not move the mobile (the nonreinforcing condition). First, a measure of baseline kicking is obtained by observing the number of kicks in the nonreinforcing condition (a pretest). After obtaining a baseline measure, the ribbon is tied to the suspension hook so that when the infant kicks he moves the mobile, resulting in a high rate of kicking and attention to the mobile. Memory of the learned experience can then be assessed after various delays by positioning the infant beneath the mobile, attaching his ankle to the nonreinforcing hook (to prevent additional learning), and measuring the number of kicks he produces. Learning is evidenced by a greater rate of kicking relative to baseline when infants are tested with the same mobile as compared to a different one. This procedure was used to show that 2-month-old infants could remember a learning experience occurring 24 h earlier.

Infants and young children do not necessarily need reinforcement to learn. In a seminal study of observational learning reported in 1961, Bandura showed that 4-year-olds who simply observed an adult beating up a Bobo doll were more likely to direct similar behaviors toward the doll in subsequent play than were children in a control condition who did not see an adult exhibiting such behavior. This type of learning is referred to as observational because learners imitate an observed behavior with no stimulus–response pairing or any kind of reinforcement. Observational learning does not appear to have a lower age limit. In deferred imitation, another type of observational learning, an experimenter models a sequence of actions and the infant is later tested on the ability to reproduce the behavior. In 1992, Patricia Bauer and Jean Mandler showed that infants as young as 11.5 months of age can learn and later imitate novel actions in an event sequence like making a rattle. Events were simple, consisting of two or three actions, but infants readily reproduced the actions in their correct order. Interestingly, the type of sequence matters, such that arbitrary sequences involving a series of events that are not causally related (like banging, turning, and stacking a ring on a dowel) are much more difficult to learn than causally related actions that require actions to be performed in a certain order (such as making a rattle by putting a ball in a paper cup, joining the mouth of the cup with the mouth of another paper cup, and shaking). In a different experimental paradigm demonstrating observational learning, Rachel Barr and colleagues in 1996 showed that infants as young as 6 months of age can remember and imitate portions of a sequence they have observed being modeled. The sequence, consisting of removing a mitten from a puppet’s hand, shaking the mitten (causing a bell inside the mitten to sound), and placing the mitten back on the puppet, was imitated by the infants after a 24 h delay. In most cases, exposure was very brief. The event sequences were modeled just twice with 11.5-month-olds and six times with the 6-month-olds. Additionally, infants as young as 2 months of age can engage in learning without feedback as shown by Naomi Wentworth and Marshall Haith in a study reported in 1992. Infants were exposed to an alternating left–right pattern of visually presented pictures. On one side (left or right) the picture was always the same and on the other side the picture varied. Infants showed learning of the visual content of the picture as evidenced by their tendency to anticipate the location of the stable picture and to respond to it more quickly as compared to the location of the unstable picture.

Given these findings it becomes important to ask just how early learning occurs in human development. Given the results detailed above, it is reasonable to think that learning may occur as soon as infants are able to process sensory information, indeed that they might even begin learning in utero.

One of the earliest indications of fetal learning was the finding that newborns prefer their mother’s voice to that of another female speaker. They also prefer sentences from their native language to sentences from another language. Passages read in French produced higher sucking rates in French newborns than passages read in Russian. Other studies have shown that these preferences are not specific to French. Therefore such preferences must be shaped by prenatal experience with maternal speech. What might infants be learning? We know from intrauterine recordings that low-frequency components of maternal speech, including its rhythmic qualities are audible in utero and infants born prematurely at 24 weeks are able to react to sounds, raising the possibility that learning may begin this early.

In 1986, Anthony DeCasper and Melanie Spence showed that newborns, whose mothers read a passage aloud each day during the last 6 weeks of pregnancy, were able to discriminate the passage from an unfamiliar one at birth. Two-day-olds were tested, using a high-amplitude operant sucking procedure, to see whether the familiar passage would be more reinforcing than an unfamiliar one, even when read in another woman’s voice. It was, suggesting that infants had learned features from their training passage involving its prosodic (or rhythmic) qualities over and above features specific to their mother’s voice. The passages were not read aloud before the newborns were tested, and thus learning must have occurred in utero. A later study, by DeCasper and colleagues, used heart rate as a dependent measure to test learning in 37-week-old fetuses. Mothers recited one of two rhymes out loud, once a day, over 4 weeks. At 37 weeks’ gestational age the fetuses were stimulated with recordings of the familiar and unfamiliar rhymes. The familiar rhyme elicited a decrease in fetal heart rate, whereas the unfamiliar one did not, suggesting discrimination of the two passages, and hence learning.

6.1 Development of oral motor behavior involved in food intake

For many years, infant oral motor behavior had been described in term of reflexes and responses, e.g., the rooting reflex (head turn in the direction of the stimulated perioral skin accompanied by mouth opening and ‘labial grasping’) and the sucking response (rhythmical sucking induced by the insertion of a nipple or finger into the infant’s mouth; Prechtl, 1977; Sheppard and Mysak, 1984). Gradually it became clear, however, that most oral motor behavior, especially sucking, chewing and swallowing, is organized with the help of CPG-networks located in the brainstem. These networks are modulated by supraspinal activity and exhibit experience-dependent plasticity (Barlow, 2009; Delaney and Arvedson, 2008).

Fetal sucking and swallowing movements have been observed from 12 weeks PMA onwards; they may include thumb sucking (De Vries et al., 1982). Sucking movements emerge at the same age as the rooting reflex. The latter was reported by Minkowski (1938), who studied motor behavior of fetuses in a bath of physiological saline, immediately after they had been delivered to safe maternal life. The incidence of sucking and swallowing movements in the first half of gestation is low (De Vries et al., 1985). Moreover, the first jaw, lip, tongue and pharynx movements are relatively simple (Miller et al., 2003). The movements become gradually more complex: simple jaw and lip opening movements develop into repetitive mouth opening and closing movements similar to those present in neonatal sucking, and tongue movements develop from simple forward thrusts and cupping movements to the anterior-posterior movements needed for the successful sucking of the neonate. The latter behavior is consistently present from 28 weeks PMA onwards and is used during fetal sucking and swallowing. Fetal sucking and swallowing is clearly associated with hand-face contact (Miller et al., 2003).

After birth, nutritive sucking and swallowing have to be combined with respiration. This is a challenging task, which is not well mastered at 32–33 weeks PMA. At that age sucking, swallowing and respiration is characterized by exploration of the possible combinations to coordinate the three activities. The infants do not only show the swallow-expiration sequences that are typical for later life (Kelly et al., 2007), they also exhibit, for instance, breathing during swallowing, and alternating blocks of suck-swallow (without respiration, lasting 5–7 seconds) and respiration (without swallowing, lasting 10-16 s; Vice and Gewolb, 2008). From about 34 weeks PMA, total oral feeding may be achieved in low risk infants (Delaney and Arvedson, 2008). At that age and in the few following weeks, sucking and swallowing is characterized by a large variation in tongue movements, and by a suck-swallow ratio that is higher than the typical 1:1 ratio of the full-term newborn (Bulock et al., 1990). With increasing age, in particular after 36 weeks PMA, the frequency of typical and efficient tongue movements increases, the sucking rhythm stabilizes with a dominant 1:1 suck-swallow ratio, and sucking is less often interrupted by breathing bursts (Bulock et al., 1990; Gewolb et al., 2001; Gewolb and Vice, 2006; Vice and Gewolb, 2008). The phase of secondary variability emerges.

Craig and Lee (1999) demonstrated that the sucking of term newborns is characterized by well adapted pressure changes, that have kinematic characteristics similar to those exhibited during adult sucking. This finding supports the idea that term newborns are with sucking behavior in the phase of secondary variability. Like the preterm infants, also term born neonates face the challenge to combine sucking and swallowing with respiration. During the first postnatal days the infants explore various combinations, be it without breathing during swallowing, and the alternating blocks of suck-swallow and respiration shown at early preterm age (Bamford et al., 1992; Kelly et al., 2007; Weber et al., 1986). At this early age about half of the swallows occur mid-expiratory (Kelly et al., 2007). However, already at one week postnatally this pattern ceases to be the most prevalent one; the adult timing of swallowing emerges, i.e., swallowing at the cusp of inspiration and expiration. This coordination pattern occurs at the age of one week in 30% of swallows; its prevalence increases with increasing age to 37% at 6 months, and 75% at 12 months (Kelly et al., 2007). During the first postnatal months, sucking efficiency increases: the number of sucks per minute increases, the length of sucking bursts increases, and more milk per unit time is transferred (Qureshi et al., 2002; Sakalidis et al., 2013). The latter is associated with a doubling of milk intake in the first postnatal month (volume of milk per suck; Qureshi et al., 2002).

During the first post-term months infants are fed human milk or infant formula. From the age of 4 to 6 months also other types of food are introduced, delivered on a spoon instead of by breast or bottle (Wilson et al., 2012). The infants initially get semisolid foods, e.g., pureed food, which is orally explored and handled by sucking and munching (Gisel, 1991). Soon thereafter, usually from 6 months onwards, infants can also handle solid food; the chewing movements emerge. At 7 months of age, the chewing rate and the number of chewing cycles are already adapted to the texture of the food (puree, semisolid, solid; Wilson et al., 2012). The chewing rate does not change with increasing age, but chewing efficiency improves between 6 and 24 months: less chewing cycles and less time is needed to grind food (Gisel, 1991). This is accompanied by an increasingly better lip control, increased efficiency of tongue movements, and a decreased involvement of the perioral structures in the act of swallowing (Stolovitz and Gisel, 1991). Steeve et al. (2008) showed that at 9 months the coordination of the activity of the masseter and temporal muscles (bilaterally) and their antagonist, the anterior belly of the digastric muscle, was characterized by the basic coordination of adults, but expressed with large variation in the exact timing and degree of contraction of the muscles. With increasing age – at least until the age of 4 years - the synchrony of agonistic activity and reciprocal antagonistic activity increases (Green et al., 1997; Steeve et al., 2008), suggesting a better selection of the adult pattern of efficient muscle coordination (Fig. 6).

Reflex-like Behaviors Give Way to Volitional Actions

The kicking-to-mobile-movement contingency is by no means the only case in which learned, volitional behaviors replace, emerge from, and/or depend upon behaviors that were initially spontaneous or reflexive. A host of such transitions occur in a variety of different developmental domains in an infant’s first months (Figure 3).

For instance, the side-to-side head turning reflex, a variant of the rooting reflex, is thought to play an important role in feeding by providing an opportunity for the neonate’s mouth to come into contact with the mother’s nipple [9]. As neonates gain increasing experience with feeding, this initially spontaneous action transitions to a directed head turning reflex at approximately 2–3 weeks of age [4,9], and then transitions again to volitional directed head turning at approximately 3 months [11]. The hand grasping reflex, present at birth, begins to disappear at approximately 4–5 months of age, coinciding with the emergence of voluntary reaching and grasping [38]. Similarly, the toe grasping reflex, also present at birth, begins to fade as infants gain voluntary control over feet and legs and begin to crawl [10].

These transitions from reflexive behaviors to volitional actions do not appear to progress according to lockstep chronological timers; instead, they depend more heavily on individualized experiences that are necessary to facilitate later transitions. For instance, the directed head turning reflex persists longer in infants who are breast-fed compared with infants who are bottle-fed [4]; voluntary reaching and grasping emerge earlier in infants who are afforded the experience of picking up objects [77]; and the decline of the toe grasping reflex is more closely related to the acquisition of voluntary control of the feet than to chronological age [10]. Each of these examples can be thought of as an individualized timescale of development: initial behaviors provide pivotal opportunities that enable subsequent learning, in highly individualized fashion. (For more on individualized timescales of development, and recent statistical advances in the measurement thereof, see Figure 4.)