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**Chapter 4** **Physical, Sensory, and Perceptual Development in Infancy** **Physical Changes** The first two years of life, excluding prenatal development, are marked by the most significant physical changes. Babies grow 25 to 30 centimetres and triple their body weight in the first year. By age...
**Chapter 4** **Physical, Sensory, and Perceptual Development in Infancy** **Physical Changes** The first two years of life, excluding prenatal development, are marked by the most significant physical changes. Babies grow 25 to 30 centimetres and triple their body weight in the first year. By age 2 for girls and 2½ for boys, toddlers reach about half their adult height, allowing a reliable prediction of adult height by doubling their current height. However, 2-year-olds have proportionately larger heads than adults to accommodate their nearly full-sized brains. **The Brain and the Nervous System** Different body systems develop and grow at varying rates and times. For example, the reproductive system is fully formed at birth but doesn't significantly change until puberty. On the other hand, the brain and nervous system undergo rapid development in the first two years. At birth, the most developed parts of the brain are the midbrain and the medulla, located in the lower skull and connected to the spinal cord. These regulate vital functions like heartbeat, respiration, attention, sleep, waking, elimination, and head and neck movement. The least developed part at birth is the cortex, the convoluted grey matter around the midbrain, which is involved in perception, body movement, thinking, and language. New research methods are being developed to accurately track the structural and functional development of the infant brain. The Baby Connectome Project aims to map all neural connections, contributing to our understanding of how the human brain, which doubles in size during the first year, develops. Specialized resting-state functional magnetic resonance imaging (rs-fMRI) scans reveal that brain networks become increasingly organized and specialized as connections within and between brain regions strengthen. These networks connect via hubs that facilitate information exchange across neural networks within the developing brain. Over infancy, the brain subdivides into separate regional networks, and the emerging connector hubs improve at exchanging information within and between these regions, leading to advancements in cognitive and motor tasks. **Synaptic Development** Brain structures consist of two basic cell types: neurons and glial cells. Millions of these cells exist at birth, with synapses, or neuron connections, already forming. Synapse development stems from the growth of dendrites and axons. Synaptogenesis, the process of creating synapses, happens rapidly in the cortex in the first few years after birth, leading to a fourfold increase in the brain's overall weight by age 4. However, synaptogenesis doesn't occur smoothly and continuously, but rather in bursts. The brain undergoes a continuous cycle of synaptogenesis (formation of synapses) and synaptic pruning (removal of unnecessary synapses) throughout life, becoming more efficient with each cycle. Interestingly, a 1-year-old has denser dendrites and synapses than an adult, but their network operates less efficiently. This inefficiency, however, has an advantage: infants, with their surplus of unused synapses, can recover more easily from brain insults, such as malnutrition or head injury, compared to adults. This adaptability of the brain is referred to as neuroplasticity. Developmentalists highlight several key insights from the cyclical process of synaptogenesis and pruning in neurological development. Firstly, brain development adheres to the principle of "Use it or lose it." Children exposed to a stimulating and intellectually challenging environment maintain a more complex network of synapses than those with less stimulation. Infants' brains exhibit greater neuroplasticity than older children and adults. However, this period of high neuroplasticity is also when children are most susceptible to significant deficits, similar to a fetus's vulnerability to teratogens during rapid growth. Therefore, young infants require adequate stimulation and a structured environment to optimize their period of rapid growth and neuroplasticity. Insufficient diet or lack of stimulation in early months can subtly but significantly impact a child's future cognitive development. Some argue that excessive television watching in early months could hinder brain development. Recent findings on the ongoing process of synaptogenesis and synaptic pruning throughout life have led developmental psychologists to revise their views on the relationship between brain development and behavior. Previously, it was believed that the brain was nearly fully organized by age 2, implying that subsequent developments were mostly due to experience. However, it is now understood that changes in psychological functioning are associated with physical changes in the brain throughout a person's entire lifespan. **Myelinization** A key aspect of neuronal development is the formation of sheaths around individual axons. These sheaths, made of a substance called myelin, electrically insulate axons from each other and enhance their conductivity. The process of forming this myelin sheath is known as myelinization. The process of myelinization, or the development of myelin sheaths around axons, follows both cephalocaudal (head-to-tail) and proximodistal (near-to-far) patterns. For instance, nerves serving the neck and shoulder muscles are myelinized before those serving the abdomen, allowing babies to control their head movements before they can roll over. Myelinization occurs most rapidly in the first two years after birth and continues at a slower rate throughout childhood and adolescence. By the second birthday, the brain areas involved in vision are mature, while those governing motor movements are not fully myelinized until around age 6. Certain brain structures, such as the reticular formation, take longer to myelinize. The reticular formation, responsible for maintaining attention and sorting information, begins myelinization in infancy but continues intermittently throughout childhood and adolescence, completing only in a person's mid-20s. As a result, infants enhance their task focus during the first two years. A 12-year-old, while better at concentrating than an infant, is still less efficient compared to an adult. **Reflexes and Behavioural States** Changes in the brain result in predictable changes in babies' reflexes, sensory capacities, and patterns of waking and sleeping. In fact, such changes, or the absence of such changes, can be important indicators of nervous system health. **Reflexes** Humans are born with various adaptive reflexes that aid survival. Some reflexes, like the rooting and sucking reflexes, which help in obtaining nourishment, fade away during infancy or childhood. Others, such as withdrawal from pain and pupil dilation or constriction in response to light changes, persist throughout life. Research indicates that stimulating certain reflexes could enhance development. For example, infants encouraged to exercise the stepping reflex were found to walk earlier. However, weak or missing adaptive reflexes in newborns could indicate brain dysfunction, necessitating further evaluation. Primitive reflexes, controlled by the less sophisticated parts of the brain (the medulla and midbrain), have unclear purposes. These include the Moro or startle reflex, where a baby throws her arms out and arches her back when startled, and the Babinski reflex, where stroking the bottom of a baby's foot causes her to splay and then curl her toes. These reflexes typically disappear by 6 to 8 months of age, and their persistence could indicate a neurological issue. **Behavioural States** Researchers have identified five different states of sleep and wakefulness in neonates, known as states of consciousness. Infants typically cycle through these states, moving from deep sleep to lighter sleep, then to alert wakefulness and fussing. After feeding, they become drowsy and return to deep sleep, repeating this cycle every two hours. Neonates sleep about 80% of the time, equally during the day and night. By 8 weeks, the total sleep time decreases slightly, and circadian rhythms, or day/night sleep patterns, start to appear. At this age, babies begin to sleep through two or three consecutive two-hour cycles without fully waking up, often said to be "sleeping through the night." By 6 months, babies still sleep around 13 hours per day, but their sleep becomes more regular and predictable, with clear nighttime sleep patterns and more consistent daytime naps. Babies' sleep patterns vary significantly. In a meta-analysis, 0-2-month-old babies averaged 14.6 hours of sleep per day, ranging from 9.3 to 20 hours. By 6 months, the average sleep time drops to \~13 hours per day, with a range of just under 9 to 17 hours, a pattern that remains relatively stable until 2 years of age. Night wakings are common in infancy, and some babies don't develop a long nighttime sleep period until later in their first year. Cultural beliefs significantly influence parents' responses to infants' sleep patterns. For instance, North American parents often view a newborn's erratic sleep cycle as a problem needing correction, focusing on getting babies to sleep through the night. In contrast, many European parents see newborns' sleep patterns as normal development and expect babies to naturally establish stable sleep patterns during the first two years, without parental intervention. Infants express different needs through distinct cries. A basic cry, often indicating hunger, follows a rhythmic pattern of cry, silence, breath, and may include a whistling sound during inhalation. An anger cry is generally louder and more intense. A pain cry has a sudden onset, contrasting with the gradual build-up of the other two cries, which often start with whimpering or moaning. Cross-cultural studies indicate that an infant's crying frequency increases over the first six weeks and then decreases. Parents across cultures use similar techniques to soothe crying infants, such as picking them up, holding them, and talking or singing to them. Using a pacifier also often helps. Despite concerns that attending to a crying baby might lead to more crying, research suggests that prompt attention in the first three months results in less crying later in infancy. However, 5-19% of infants experience colic, characterized by intense crying for three or more hours a day, three or more times a week, for over three weeks, without an apparent reason and resistant to soothing. Colic typically emerges around 2 or 3 weeks of age and naturally subsides around 3 to 4 months of age. The crying is usually worst in the late afternoon or early evening. The cause of colic remains unclear, with suggestions of multiple and possibly interrelated causes. Factors implicated include gastrointestinal issues like gut inflammation and impaired gut microbiota, neurological factors like migraines, and psychosocial factors such as inadequate parent-child interactions, parental anxiety or depression, and household stress. No single treatment has been universally effective, but some interventions provide relief in certain cases. Key to managing colic is reassuring parents that it's a self-limiting disorder without long-term adverse effects. Additional family support may be needed to reduce risks of parental depression, child abuse, and early breastfeeding cessation. Behavioral interventions, like reducing environmental stimuli and developing routines for feeding, sleeping, and comforting, can be beneficial. Providing a small dose of the probiotic *Lactobacillus reuteri* DSM17 938 has been shown to help exclusively breastfed infants with colic. **Developing Body Systems and Motor Skills** The acquisition of motor skills is dependent on brain development and significant changes in various body systems such as bones, muscles, lungs, and heart. Physical development follows two key patterns: the \*\*cephalocaudal pattern\*\*, which means development progresses from the head downward, and the \*\*proximodistal pattern\*\*, indicating that development moves from the centre of the body outward. These patterns are crucial to understanding the process of physical development. **Bones** During infancy, there are significant changes in the **size, number, and composition of bones**. The growth in the length of long bones in the arms and legs contributes to height increase. The number and density of bones in specific body parts change, enhancing coordinated movement. For instance, a newborn's wrist is a single cartilage mass, which separates into three distinct bones by the age of one. This separation process, continuing into adolescence until the wrist has nine separate bones, is crucial for the development of manipulative skills over the first two years. The process of bone hardening, known as **ossification**, starts in the final weeks of prenatal development and continues until puberty. The hardening of bones in various body parts follows the proximodistal and cephalocaudal patterns. Ossification plays a significant role in motor development. For instance, an infant cannot stand if their leg bones are too soft, regardless of the development of their muscles and nervous system. **Muscles** At birth, the body has all its muscle fibres, but they are small and have a high water-to-muscle ratio. Newborns' muscles also contain a significant amount of fat. By the age of one, the water content in an infant's muscles equals that of an adult, and the fat-to-muscle ratio starts to decrease. These changes in muscle composition result in strength increases, enabling one-year-olds to perform activities like walking, running, jumping, and climbing. **Lungs And Heart** The lungs grow quickly and become more efficient in the first two years. This improvement in lung efficiency, along with the strengthening of heart muscles, provides a two-year-old with greater stamina than a newborn. As a result, by the end of infancy, children can engage in extended periods of continuous motor activity without needing rest, often to the exhaustion of their parents. **Motor Skills** The development of various motor skills in children during the first two years is due to changes in all body systems. These skills are typically categorized into three groups by developmentalists: 1. **Locomotor skills** (or gross motor skills): These include abilities like crawling that allow infants to move around in their environment. 2. **Non-locomotor skills**: These skills, such as controlling head movements, enhance a baby's ability to interact with objects and people using their senses and motor skills. Many of these skills are also used in play. 3. **Manipulative skills** (or fine motor skills): These involve the use of hands, for example, when a 1-year-old stacks one block on top of another. These three areas of development are summarized in Table 4.1 over the first 24 months. Age (in months) Locomotor Skills Non-locomotor Skills Manipulative Skills ----------------- -------------------------------------------------------------------- -------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------ 1 Stepping reflex Lifts head slightly; follows slowly moving objects with eyes Holds object if placed in hand 2--3 Lifts head up to 90-degree angle when lying on stomach Begins to swipe at objects in sight 4--6 Rolls over; sits with support; moves on hands and knees ("creeps") Holds head erect while in sitting position Reaches for and grasps objects 7--9 Sits without support; crawls Transfers objects from one hand to the other 10--12 Pulls self up and walks grasping furniture; then walks alone Squats and stoops; plays patty cake Shows some signs of hand preference; grasps a spoon across palm but has poor aim when moving food to mouth 13--18 Walks backward, sideways; runs (14--20 mos.) Rolls ball to adult; claps Stacks two blocks; puts objects into small container and dumps them out 19--24 Walks up and down stairs, two feet per step Jumps with both feet off the ground Uses spoon to feed self; stacks 4 to 10 blocks Cross-cultural research indicates the need for caution when referring to standardized motor development milestones. Observations in various cultural settings reveal considerable variability in the age's infants reach these milestones. Compared to mainstream Canadian standards, infants from some regions are ahead, while others are delayed. Within Canada, diverse Indigenous parenting practices may lead to varied developmental pathways and timelines among Indigenous children. Specifically, Indigenous children often achieve gross motor skills earlier but language skills slightly later than broader Canadian norms. Therefore, it might be safer to measure an Indigenous child's developmental milestones against their peers within the same Indigenous cultural group. Significant differences could then be seen as potential risk indicators. The University of Manitoba's psychologist Warren Eaton is conducting the ongoing infant Milestone Study to develop Canadian and cross-cultural norms, addressing challenges in determining clear developmental milestone measures. The study has tracked hundreds of babies to observe week-to-week changes as they progress towards motor skills milestones. The timing of when a baby first sits, creeps, crawls, and walks varies, but a seasonal trend has been observed where babies born in spring reach crawling and walking milestones earlier. The project has expanded to include the development of gestures, words, and personality up to 24 months of age. A comprehensive Canadian screening tool, the **Nipissing District Developmental Screen (NDDS)**, is now available. It assists professionals and parents in identifying aspects of a child's development that may need early intervention. The NDDS can be used with children aged 1 month to 6 years to screen for potential issues in critical areas such as gross motor, fine motor, vision, hearing, speech, language, communication, cognitive, social/emotional, and self-help skills. This culturally sensitive tool is being used across Canada and internationally, and is available in English, French, and several other languages. **Gender Differences** During infancy, girls tend to mature physically faster than boys in some respects. For instance, the separate bones in the wrist appear earlier in girls, giving them a slight advantage in developing manipulative skills like self-feeding. On the other hand, boys are generally more physically active, as found by researchers at the University of Manitoba. Both human and primate studies show that male infants prefer rough-and-tumble play from a very early age. Differences in physical aggression between boys and girls become apparent towards the end of the second year, a finding consistent across various cultures. **Explaining Motor Skill Development** Despite variations in the pace of physical development due to gender differences, the sequence of motor skill development remains virtually identical for all children, including those with significant physical or mental anomalies. For instance, children with developmental delays may progress through motor milestones at a slower rate, but they follow the same sequence. Motor skill development adheres to the cephalocaudal and proximodistal patterns. Such consistencies suggest that some form of maturation is likely involved. Research suggests that experience significantly influences motor development in infants. The transition from crawling to walking, despite being unsteady and fraught with falls and bumps, is driven by several benefits that outweigh the immediate costs. These benefits include a broader perspective, the ability to see and interact with distant objects, faster exploration, and the opportunity for new interactions like sharing. These incentives, along with praise and encouragement from caregivers, motivate infants to persist with walking. Even though walking can be goal-driven, such as reaching a desirable object, the mere act of walking around can lead to the discovery of interesting objects, people, or scenes, providing another incentive to continue walking. **Health Promotion and Wellness** Babies depend on adults to help them stay healthy. Specifically, babies need the right foods in the right amounts, and they need regular medical care. **Nutrition** Breastfeeding is considered nutritionally superior to bottle-feeding for most infants, according to decades of research. However, the quality of breast milk, which influences infant gut microbiota, can vary due to factors like maternal lifestyle, breastfeeding practices, environmental factors, and possibly ethnicity. In Canada, health authorities recommend exclusive breastfeeding for the first six months of an infant's life. Yet only half of mothers breastfeed exclusively until four months, and a quarter until six months. There are also regional disparities in breastfeeding trends, with the highest rates in the western provinces and the lowest in Québec and the Maritime provinces. Breastfeeding practices also differ among Canadian subgroups. Older, better-educated women with higher incomes and who are not single parents are more likely to initiate and maintain breastfeeding. Recent immigrant mothers tend to breastfeed exclusively longer than Canadian-born mothers, although this trend diminishes with longer residency in Canada. Breastfeeding offers numerous benefits for both full-term and preterm babies. It promotes faster weight and size gain, and breastfed infants are less likely to experience health issues such as diarrhea, gastroenteritis, bronchitis, ear infections, and colic. They also have a lower risk of infant mortality. Long-term benefits include a reduced risk of chronic diseases like diabetes, heart disease, obesity, certain types of cancers, and neurodevelopmental disorders such as ADHD, autism, and learning disabilities. These benefits may be partly due to the positive impact of breast milk on gut microbiota and immune system function. Given these advantages, physicians strongly recommend breastfeeding whenever possible, even if it's only for a few weeks after birth or needs to be supplemented with formula feedings. There are circumstances where breast milk alone doesn't meet the nutritional needs of babies, particularly preterm infants. These babies' intestinal tracts are less mature than those of full-term infants, requiring special formulas with amino acids and fats that full-term babies can produce themselves. Despite this, preterm babies still need the immunological benefits of breast milk. Therefore, physicians often recommend feeding them expressed breast milk fortified with necessary proteins, fats, vitamins, and minerals. **Development in the Real World** **The Critical Role of Gut Microbiota** Breastfeeding offers key health benefits by providing infants with essential nutrients for healthy growth and functioning, and by strengthening their immune system. The advantage of breast milk over formula lies in its role in enhancing the infant's gut microbiota, the microbial population in the gastrointestinal tract. This emerging evidence highlights the unique health benefits of breastfeeding. Canadian researchers are leading in the field of infant gut microbiota research, identifying several factors that contribute to its development. Gut microbiota colonization begins prenatally and is influenced by various early life exposures. Postnatal exposure levels to gut microbiota can vary based on factors such as delivery method (vaginal vs. C-section), feeding method (breastfeeding vs. formula), maternal antibiotic use, neonatal antibiotic administration, and birth location (home vs. hospital). Studies indicate that compared to babies delivered via scheduled C-section, those born vaginally have healthier gut microbiota with greater diversity and abundance. Exclusive breastfeeding further enhances a newborn's gut microbiota. Conversely, infants who are partially or exclusively bottle-fed exhibit less microbiota diversity and abundance than those who are breastfed. The National Institutes of Health (NIH) launched the Human Microbiome Project in 2008 to study how the microbiome impacts human health and disease. This research has enhanced our understanding of the vital role microbiota play in the gut-brain network, a two-way communication pathway between the brain and the gut. Specifically, the gut microbiota and the brain interact to affect immune and metabolic functions, which are crucial for growth, development, health, and disease prevention. Future research is needed, given evidence from previous studies showing reciprocal interactions among gut microbiota and the immune and nervous systems, which significantly influence our physical and mental health throughout our lives. Breastfeeding is not recommended for all babies in Canada, particularly when mothers are substance abusers or rely on certain medications, as these can negatively impact infant development. In such cases, high-quality infant formula, properly prepared and sterilized, is usually suitable. While breastfeeding can continue beyond 2 years, healthy, full-term infants are ready for solid foods at 6 months to meet their increasing nutritional needs. The first foods should be iron-fortified infant single-grain cereal, followed by puréed vegetables, fruits, and then meat or meat substitutes. This gradual introduction helps identify potential food allergies. By 1 year of age, the baby should be consuming a wide variety of foods as per Canada's Food Guide. **Malnutrition** Malnutrition during infancy can severely affect a baby's brain development, as the nervous system grows most rapidly in the first two years of life. Macronutrient malnutrition, which is caused by a calorie-deficient diet, is the leading cause of death worldwide for children under the age of 5. Severe calorie deficit in infants can lead to a disease called marasmus, characterized by weight less than 60% of the normal for their age, often leading to permanent neurological damage. Many also suffer from parasitic infections causing chronic diarrhea, complicating the treatment of marasmus. However, marasmus can be reversed with a program of dietary supplementation, intravenous feedings, and parasite treatment. On the other hand, some infants' diets may have near adequate calories but lack sufficient protein, leading to a disease called kwashiorkor. This is common in countries where infants are weaned early onto low-protein foods. Kwashiorkor-like symptoms can also occur in chronically ill children due to their bodies' inability to utilize dietary protein. Like marasmus, kwashiorkor can cause various health problems and permanent brain damage. A small percentage of infants in North America have feeding issues, such as an underdeveloped sucking reflex, putting them at risk for macronutrient malnutrition. However, most nutritional problems in developed countries involve micronutrient malnutrition, a deficiency in certain vitamins and minerals. Canada stands out as a global leader in food fortification, having added micronutrients to its food supply for over 60 years. For instance, vitamin D has been added to milk since the 1960s, virtually eradicating childhood rickets. Iodine is added to table salt, eliminating endemic goitre, and vitamin A is added to low-fat milk and margarine substitutes. Canada also leads in promoting and distributing three essential micronutrients---vitamin A, iodine, and iron---in developing countries. As a result, mortality rates can be reduced by 23% by providing vitamin A to young children in at-risk countries, as found by George Beaton, a renowned pioneer in human nutrition requirements. **Health Care and Immunizations** Regular medical checkups are crucial for infants' development. During these routine visits, babies' motor skills are assessed, and any lag in motor development may indicate the need for additional screening for developmental or intellectual anomalies. Vaccination is a key component of infant care. The Canadian Immunization Guide recommends starting routine immunizations at 2 months of age, continuing through childhood and adolescence. It's important for parents to be informed about the benefits, potential adverse reactions, and risks of vaccines. Resources like the Canadian Paediatric Society's "Caring for Kids" website provide information on vaccine effectiveness, safety, and side effects. Parents can also set up a vaccination schedule based on their child's age and location by visiting the Government of Canada website. In the early 20th century, infectious diseases were a major cause of childhood death, but mass vaccination programs have largely eradicated them. In Canada, children typically start receiving vaccinations at 2 months of age for diseases like diphtheria, tetanus, pertussis, poliovirus, haemophilus influenzae type b, pneumococcal, meningococcal, and rotavirus. Influenza vaccinations generally start at 6 months, and measles, mumps, rubella, hepatitis B, and varicella vaccines are administered around the first birthday. However, public complacency about immunizations can lead to outbreaks, as seen in Québec in 2011 with nearly 776 confirmed measles cases due to vaccination failure. Diseases like measles will remain rare only if parents continue to vaccinate their children diligently. While medical checkups and immunizations are essential, some parents opt for medical procedures that aren't medically necessary. **Illnesses In the First Two Years** Over half of infants in Canada experience a respiratory illness in their first year. Research indicates that babies in daycare centres have about double the infections compared to those raised entirely at home, likely due to exposure to a wider range of germs and viruses. Generally, the more people a baby is exposed to, the higher the likelihood of illness. However, overly sanitizing a baby's environment can be counterproductive, as exposure to everyday germs, such as those found in dirt and animals, can help children develop a robust immune system. **Development in the Real World** **Circumcision** Circumcision is a contentious issue in Canada, with consensus unlikely soon. Female circumcision, involving the removal of some or all the labia and clitoris, is illegal in Canada, yet some parents take their daughters abroad for the procedure. Despite this, rates of female circumcision in girls aged 0-14 have significantly decreased in regions where it was historically prevalent. Male circumcision, involving the partial or complete removal of the foreskin, is legal but not medically necessary and thus not recommended for all newborn males in Canada. Still, about one-third of Canadian male neonates undergo the procedure. Both female and male circumcision are deeply rooted practices, often tied to religion, culture, or tradition. However, critics argue that non-medically necessary circumcision raises serious practical, ethical, constitutional, and human rights concerns. Neuropsychologists suggest that the timing of respiratory illnesses leading to ear infections in infants is crucial. Infants with chronic ear infections are more likely to have learning disabilities, attention disorders, and language deficits during school years. This is hypothesized to be due to the temporary hearing impairment caused by ear infections, potentially affecting the development of brain areas vital for language learning in the first two years of life. Therefore, pediatricians stress the importance of good hygiene practices in daycare centres, including regular disinfection of toys and prompt treatment of respiratory infections in infants. **Preterm and Low-Birth-Weight Infants** In Canada, infants born live before 37 weeks of gestation are classified as preterm. The preterm birth rate has remained steady at approximately 8 per 100 live births for the last decade. Even though preterm rates are notably higher for infants from multiple births, singleton births still make up about 80% of all preterm births. Infants born before 32 weeks of gestation often lack fully developed adaptive reflexes, making survival tasks like sucking, and swallowing difficult. As a result, many preterm infants require alternative feeding methods such as intravenous feeding or tube feeding. These infants are at a higher risk for various health complications including neurological impairment, cardiovascular disorders, respiratory difficulties, gastrointestinal complications, immunologic deficiencies leading to increased susceptibility to infections, and neonatal mortality. They are also more likely to experience long-term issues such as motor, cognitive, visual, hearing, behavioural, and growth problems. Most infants with low birth weight (less than 2.5 kilograms at birth) are preterm, but it's possible for a full-term infant (37 weeks or more of gestation) to also be low birth weight. Preterm and low birth weight infants typically reach developmental milestones slower than full-term babies due to their younger maturation age. However, if a correction is made for the baby's gestational age, most of the difference in physical development disappears. For instance, a 12-month-old born two months early would have a corrected age of 10 months. It's important for parents of preterm infants to consider this when comparing their child's progress with full-term babies. By age 2 or 3, a physically normal preterm infant can usually catch up to their peers, but they are typically behind in the early months. Parental responses, particularly from mothers, significantly influence the developmental progress of preterm infants. A recent intervention, known as kangaroo care, encourages parents to increase skin-to-skin contact with their infants. This involves holding the newborns for extended periods. Research indicates that preterm infants who receive kangaroo care develop more rapidly than those given conventional neonatal care. Additionally, kangaroo mother care has been found to alleviate pain response in premature neonates. **Post-Term Infants** Infants born after 42 weeks of gestation are classified as post-term. These pregnancies carry a higher risk for maternal medical complications and fetal and neonatal mortality. However, the rate of post-term deliveries in Canada has significantly decreased from 4.4% in 1991 to 0.3% in 2014, thanks in part to ultrasound dating and the induction of post-term pregnancies. Despite this overall decrease, the rates of post-term deliveries vary considerably across Canada's provinces and territories. **Infant Mortality in Canada** In Canada, about half of infant deaths occur in the neonate stage and the rest between 4 weeks and 1 year of age. Over the past century, the country's infant mortality rate has significantly declined from 134 per 1000 live births in 1901 to approximately 5 in 2007. However, compared to 17 peer countries, Canada has made less progress in reducing infant deaths, tying with the UK for the second highest infant mortality rate, with the US performing worse. Despite a significant reduction in regional disparities in infant mortality rates across Canada, socioeconomic disparity remains a problem. Lower-income families, those with the lowest education levels, and those experiencing high levels of unemployment and material deprivation have higher rates of infant mortality. Additionally, infant mortality is 1.5 times higher in remote areas compared to urban areas. Infant mortality rates are particularly high among Indigenous populations in Canada, with rates being 3.9, 2.3, and 1.9 times higher among Inuit, First Nations, and Métis people respectively, compared to the general population. These elevated rates are linked to complex, overlapping socioeconomic factors and health inequities, many of which are tied to the long-term effects of colonization. **Sudden Infant Death Syndrome** Sudden Infant Death Syndrome (SIDS) refers to the unexpected death of a seemingly healthy infant under one year of age. In Canada, SIDS-related deaths have significantly decreased over recent decades, now accounting for about 5% of all infant deaths. Most of these cases (90%) occur in the postnatal period (28 to 364 days). There is a notable variation in SIDS incidence across Canada, with Québec having the lowest and Nunavut the highest rates. While the exact causes of sudden infant death syndrome (SIDS) remain unknown, certain practices can help reduce its risk. These include: 1. **Sleep Position**: Place the baby on their back to sleep. 2. **Bedding**: Remove quilts, duvets, pillows, soft toys, and crib bumpers that could cover the infant's head. Use a fitted sheet on a crib mattress that meets current Canadian safety regulations. 3. **Sleep Surface**: Avoid letting the baby sleep or nap on soft surfaces or loose bedding, either alone or with someone else. 4. **Sleep Location**: For the first six months, have the baby sleep in a crib or cot near the parent's bed. 5. **Bed Sharing**: Avoid bed sharing or sleeping with the baby on a sofa, especially if the parents smoke, are more tired than usual, or have consumed substances that promote fatigue. 6. **Smoke-Free Environment**: Provide a smoke-free environment during pregnancy and in the home after the infant's birth. These measures can help provide a safe sleep environment for infants and potentially reduce the risk of SIDS. **Sensory Skills** The study of sensory skills involves understanding the information received by sensory organs. This includes questioning whether the structure of an infant's eye allows for color perception, and if the structures of the ear and cortex enable an infant to distinguish between different pitches. A key insight from recent research is that newborns and young infants possess significantly greater sensory capacities than previously believed by physicians and psychologists. **Vision** Until about 50 years ago, it was commonly believed that newborn infants were blind. However, it is now understood that while newborns have less developed visual skills compared to older children, they are certainly not blind. Despite this, an estimated 5 to 10% of babies experience some form of visual problem. Therefore, visual assessments are recommended at birth and during all routine health check-ups, as well as when there are any changes or issues with the eyes or vision. Undetected or untreated visual problems can result in lifelong issues, potentially impacting educational achievement and quality of life. **Visual Acuity** Visual acuity, the clarity of vision, is typically measured as 20/20 in adults, meaning they can see and identify something at 20 feet that an average person can also see at the same distance. A person with 20/100 vision needs to be 20 feet away to see what an average person can see at 100 feet. Thus, the higher the second number, the poorer the visual acuity. At birth, a baby's visual acuity ranges from 20/200 to 20/400, but it improves rapidly during the first year due to synaptogenesis, synaptic pruning, and myelination in the neurons serving the eyes and brain's vision processing centers. In other words, a newborn's visual acuity is about 40 times worse than an adult's, but by 6 months, it improves to being only eight times worse. Most children gradually reach adult levels of visual acuity around the age of 7. While newborns have poor distant vision and may struggle to distinguish between nearby people, this is not as negative as it might initially seem. Newborns have good close-up vision, which is sufficient for interacting with caregivers and nearby objects, such as a breast, a bottle, or a crib mobile. This close-up vision is adequate for their needs at this stage of development. **Tracking Objects in the Visual Field** Tracking is the process of following a moving object with your eyes, a skill used daily in various situations such as driving or watching a moving object. For newborn infants, who can't yet move independently, tracking is crucial for recognizing objects that move towards or away from them. Initially, tracking in infants is fairly inefficient. Infants younger than 2 months can track for brief periods if the object moves very slowly. However, a significant improvement occurs around 6 to 10 weeks, when babies' tracking abilities rapidly become more skillful. **Colour Vision** Contrary to the notion that infants are born blind, research shows that while an infant's visual acuity is initially poor, it rapidly improves, and other visual capacities are well developed early on. However, certain visual skills rely on specific visual stimulation during sensitive developmental periods. For instance, visual deprivation starting at 6 months of age through adolescence can hinder the development of normal peripheral vision. On the other hand, sensitivity to the global direction of motion (e.g., when an observer sees an identifiable group of dots moving together in a particular direction among dots moving randomly) is only affected by visual deprivation occurring near birth. This statement is referring to the impact of visual deprivation, or lack of visual stimulation, on the development of certain visual skills at different stages of growth. This highlights the importance of early and regular eye health check-ups for infants to ensure any visual deprivation is detected and addressed promptly. **Hearing and Other Senses** As you learned in the chapter on Theories of Development, babies can hear long before they are born. However, like vision, hearing improves considerably in the early months of life. The other senses follow a similar course. **Auditory Acuity** Newborns have better auditory acuity than visual acuity, and their hearing is nearly as good as adults' within the general range of human voice pitch and loudness. However, their ability to hear high-pitched sounds is less developed, requiring these sounds to be louder for detection compared to older children and adults. While children's hearing continues to improve until adolescence, newborns already exhibit significant auditory capabilities. **Detecting Locations** Newborns possess the basic auditory skill to determine the general direction of a sound, which improves with age. This ability is due to the slight time difference in sound arrival between the two ears. However, if a sound comes from the midline, equidistant from both ears, this system fails as the sound arrives simultaneously at both ears. While newborns can turn their heads towards the general direction of a sound, their ability to locate sounds with precision is not well developed at birth. Research by Barbara Morrongiello of the University of Guelph shows that a 27-degree shift from the midline is needed for 2-month-old infants to show a changed response. This threshold decreases to a 12-degree shift for 6-month-olds, and by 18 months, infants can discriminate a 4-degree shift, nearly matching the skill level of adults. **Smelling and Tasting** The senses of smell and taste, which are closely linked, are less studied than vision and hearing. If an individual cannot smell (e.g., due to a cold), their taste sensitivity decreases. Taste buds on the tongue detect five basic flavors: sweet, sour, bitter, salty, and umami, while smell is registered in the nose's mucous membranes. Newborns respond differently to all five basic flavors. This was demonstrated in studies by Jacob Steiner, where newborns, who had never been fed, were given flavored water and their reactions were photographed. The babies responded differently to sweet, sour, and bitter flavors. They can also taste umami, a savory flavor found in high-protein foods like meat and cheese, and typically respond positively to it. Some researchers believe that newborns' preference for umami and sweet flavors explains their attraction to breast milk, which is naturally rich in sugars and glutamates. **Senses of Touch and Motion** Infants' senses of touch and motion are highly developed, aiding in feeding through reflexes like the rooting reflex, which responds to touch on the cheek, and the sucking reflex, which is triggered by touch in the mouth. The neonatal brain is sensitive to gentle social touching, contributing to early brain development, particularly in somatosensory processing and tactile sensation perception. Infants show heightened sensitivity to touch on the mouth, face, hands, soles of the feet, and abdomen, with reduced sensitivity elsewhere. They also respond to temperature changes by increasing physical activity in cold environments and show calmness when stroked and discomfort when exposed to irritants like rashes or scratchy clothing. **Perceptual Skills** Perceptual skills studies focus on how individuals interpret and integrate sensory information. Research has shown that even very young infants can make detailed distinctions among sounds, sights, and sensations. These infants don't just react to individual events, but also pay attention to and respond to patterns. This suggests a sophisticated level of perceptual processing from an early age. **Studying Perceptual Development** To understand what babies can perceive, despite their inability to talk or respond to common questions, researchers use three basic methods. One of these is the "preference technique" developed by Robert Fantz in 1956. In this method, a baby is shown two pictures or objects, and researchers record how long the baby looks at each. If many infants consistently look longer at one picture, it indicates that babies can discern a difference between the two. This method also provides insights into the types of objects or images that attract babies' attention. Another method used by researchers involves the processes of habituation and dishabituation. Habituation is when a baby gets used to a stimulus and shows a reduced response to it. Dishabituation, on the other hand, is when a baby responds to a familiar stimulus as if it were new. In this method, researchers repeatedly present a baby with a specific sight, sound, or object until the baby habituates, or loses interest. Then, they introduce a new or slightly different stimulus and observe if the baby shows renewed interest (dishabituation). If the baby does show renewed interest, it indicates that the baby perceives the new stimulus as different from the original one. The third method researchers use is based on the principles of operant conditioning. In this approach, an infant could be trained to turn her head in response to a specific sound, using the sight of an interesting moving toy as reinforcement. Once the learned response is firmly established, the sound can be systematically altered to see if the baby continues to turn her head, indicating that she recognizes the change in the sound. **Looking** A key question in visual perception is whether infants perceive their environment similarly to older children and adults. This includes their ability to see fine details, judge distances, and visually scan objects in an orderly manner. Developmentalists suggest that the way infants look at objects can provide significant insights into what they are learning from visual information. **Early Visual Stimulation** Appropriate visual stimulation during infancy is crucial for the development of normal visual perception later in life. This was supported by a study conducted by psychologists at McMaster University, who studied infants born with cataracts. These infants, who initially could only see light and dark, had their cataracts removed between 2 and 6 months of age and were fitted with corrective lenses. However, when examined years later, these individuals exhibited subtle visual abnormalities, such as an inability to distinguish the relative position of facial features in the same way as those with normal sight. While not all visual processes are affected by early deprivation of visual stimulation, there are critical periods in early infancy and beyond when specific types of visual stimulation are necessary for the development of normal visual perception. For instance, early exposure to certain types of visual input, such as mid to high spatial frequency stimuli (like thin-striped patterns), helps establish the neural foundations for normal visual development. Lack of early experience can lead to abnormal visual development many years later. This "sleeper effect" is particularly evident in sensitivity to mid and high narrow-striped images, face processing, and facial identity based on the spacing of internal facial features. **Depth Perception** Depth perception is a crucial perceptual skill that has been extensively studied. It's necessary for everyday tasks such as reaching for objects or judging the distance to an oncoming car before making a turn. Similarly, infants need depth perception for various simple tasks, like determining the distance to an object for reaching, assessing the distance to the floor when considering crawling off a couch, or aiming a spoon at a bowl of pudding. Depth perception can be judged using three types of information. - **Binocular cues**: These involve both eyes, each receiving a slightly different image of an object. The closer the object, the more these views differ. Information from eye muscles also provides clues about an object's distance. - **Pictorial (monocular) cues**: These require input from only one eye. For instance, if one object partially obscures another, the hidden object is perceived as farther away, a cue known as interposition. Relative sizes of similar objects can also indicate distance, with smaller objects appearing farther away. Linear perspective, such as railroad tracks seeming to converge with distance, is another monocular cue. - **Kinetic cues**: These come from either your own motion or the motion of an object. If you move your head, nearby objects seem to move more than distant ones, a phenomenon called motion parallax. Similarly, moving objects, like a person walking or a train on a track, appear to cover larger distances in a given time if they are closer. Researchers have studied kinetic cues in young babies by observing their reactions to objects appearing to move towards them. If the infant has depth perception, they should react, such as by flinching, moving aside, or blinking, as the object seems to approach closely. Such reactions have been observed in 3-month-olds. The development of an infant's ability to judge depth and the cues they use is still an active research area. Current conclusions suggest that infants first use kinetic information around 3 months of age. Binocular cues are used starting at about 4 months. Lastly, linear perspective and other pictorial (monocular) cues are utilized, possibly between 5 to 7 months of age. **What Babies Look At** In the first two months, a baby's visual attention is driven by the search for meaningful patterns. Babies scan their surroundings until they find a sharp light-dark contrast, typically indicating an object's edge. Once found, they focus on this edge, moving their eyes across and around it. Motion also attracts their attention at this stage. Between 2 and 3 months, as the cortex develops more fully, babies' attention shifts from the location of an object to its identity. They start to scan an entire figure rapidly, rather than focusing on edges, leading to more time spent looking for patterns. In an early study by Albert Caron and Rose Caron (1981), babies were shown a series of pictures with a specific relationship, such as a small figure above a larger one. Once the babies lost interest in these pictures (habituated), they were shown a new figure that either followed the same pattern or a different one. If the babies had truly habituated to the original pattern, they would show little interest in a similar pattern but renewed interest in a different one. The study found that 3- and 4-month-old babies did exactly this, indicating that even at this early age, babies pay attention to patterns, not just individual stimuli. **Faces: An Example of Responding to a Complex Pattern** While infants do not systematically prefer faces over other complex images, they do show a clear preference for certain faces, including attractive ones and their mother's face, from the earliest hours of life. When scanning a face, infants under 2 months old tend to focus on the outer edges, such as the hairline and face shape. This is supported by findings that newborns couldn't distinguish their mother's face from a stranger's if the hairline was covered. However, after 4 months, covering the hairline didn't affect the baby's recognition of their mother. Generally, babies start focusing on a face's internal features, especially the eyes, around 2 to 3 months. By about 6 months, infants can engage in reciprocal eye gaze with their parents, indicating signs of active social communication. **Effects of Visual Deprivation** Canadian researchers, including Daphne Maurer and Sybil Geldart of McMaster University, Richard Le Grand of Kwantlen University College, Catherine Mondloch of Brock University, and Henry Brent of Toronto's Hospital for Sick Children, studied the impact of early visual deprivation due to congenital cataracts on face processing development. They found that early visual input is crucial for developing face-processing expertise in adulthood. Individuals deprived of early visual stimulation didn't develop the ability to recognize faces holistically, i.e., they couldn't automatically recognize faces based on the configuration of features like the eyes, nose, and mouth. Instead, they processed faces as a collection of independent facial features. Another characteristic of adults deprived of early visual stimulation is their inability to distinguish the relative spacing of facial features in the same way as normal-sighted people. They also had difficulty identifying faces when head orientation or facial expressions changed. In research on sensitivity to facial differences, three types of facial stimuli were used: variations in eye and mouth shape (set a), variations in facial contours (set b), and variations in the spacing of the eyes and the eyes and mouth (set c). All test samples were presented close to life-size. It was found that patients who had early visual deprivation due to congenital cataracts (treated in infancy) struggled to discriminate between faces in set c, which involved variations in the spacing of facial features. **Research Report** **Babies' Preferences for Attractive Faces** Research on infant perception suggests that certain perceptual rules are innate, including a preference for attractive faces. In a series of experiments, infants aged 2-3 months and 6-8 months were shown color slides of adult Caucasian women, half rated as attractive and half as unattractive. The infants consistently looked longer at the attractive faces when presented with mixed pairs. This preference was observed regardless of race and even extended to images of other infants. Infants preferred to look at images of other infants rated as attractive by adults over those deemed unattractive. Attractiveness in faces is often associated with average features, as opposed to distinctive ones. Other factors contributing to attractiveness include facial symmetry, femininity in females, and masculinity in males. Experience influences the perception of attractiveness during development, and one's exposure to a variety of faces also affects this perception. However, it's challenging to determine what learning experiences could lead to such preferences in a 2-month-old. This suggests the possibility of an innate template for the "correct" or "most desired" face shape and configuration in our species, and we tend to prefer faces that best match this template. From an evolutionary perspective, attributes signaling attractiveness may also indicate mate quality. For instance, attractiveness is often perceived as a sign of good health. Face recognition improves significantly between the ages of 7 and 11, but it doesn't reach an adult-like level until adolescence. The ability to distinguish differences in the spacing among facial features, such as the distance between the mouth, nose, and eyes, develops more slowly than the ability to distinguish differences in facial contours. Functional magnetic resonance imaging (fMRI) studies have confirmed that facial recognition involves different neural pathways for processing facial features compared to processing the spacing among these features. **Listening** When we turn from looking to listening, we find similarly intriguing indications that very young infants not only make remarkably fine discriminations among individual sounds but also pay attention to patterns. **Discriminating Speech Sounds** Early studies have shown that babies as young as 1 month can differentiate between speech sounds like 'pa' and 'ba'. By around 6 months, they can distinguish between two-syllable words like 'bada' and 'baga', and even respond to a syllable hidden within a string of other syllables. Interestingly, babies are better at discriminating certain types of speech sounds than adults. Up to about 6 months, babies can accurately discriminate all sound contrasts present in any language, including sounds not present in the language spoken to them. However, from about 6 months, they start losing the ability to distinguish vowel pairs not present in their language, and by age 1, this extends to non-heard consonant contrasts. These findings align with our understanding of the rapid growth of synapses in early life, followed by synaptic pruning. Initially, many connections are created, allowing discrimination along all possible sound continua. However, only the pathways used in the child's language are strengthened or retained. Research from the University of British Columbia has shown that accurate speech perception and language development involve not just hearing, but also the movements of the mouth in sound production. By temporarily restricting oral-motor movements (like those of the tongue) using teething toys, thereby inhibiting the accurate production of sounds while listening to new speech sounds, it was found that the speech perception of 6-month-old infants was hindered. This suggests that both sensorimotor experiences and hearing play crucial roles in the development of speech perception and language acquisition. **Discriminating Individual Voices** Newborns appear to have the ability to distinguish between individual voices. Research by DeCasper and Fifer (1980) found that newborns can differentiate their mother's voice from another female voice, though not their father's voice from another male voice, and they show a preference for their mother's voice. There's also a correlation between gestational age and recognition of the mother's voice, with premature infants being less likely to recognize their mother's voice compared to full-term babies. This suggests that in utero learning contributes to newborns' preference for their mother's voice. **Discriminating Other Sound Patterns** Research indicates that infants, from a very early age, pay attention to and differentiate between patterns or sequences of sounds. For instance, studies by Sandra Trehub and her colleagues at the University of Toronto found that babies as young as 6 months listen to melodies and recognize their patterns. In these studies, 6-month-old babies were trained to turn their heads towards a loudspeaker playing a specific six-tone melody. The babies continued to respond to new melodies that had the same contour and pitch range as the original. However, they perceived the melodies as different if the contour changed or if the notes were significantly higher or lower. This suggests that, like visual patterns, babies pay attention to and respond to auditory patterns, not just individual sounds, within the first few months of life. **Combining Information from Several Senses** Intermodal perception refers to the ability to integrate information from different sensory modalities, such as sight and sound. This ability is crucial in infant learning as it helps babies adapt to their environment and synchronize the multisensory information they receive. Research has shown that babies who are exposed to combined auditory-visual stimuli are better at recognizing new stimuli compared to those who are exposed to either auditory or visual stimuli alone. This suggests that the integration of multiple sensory inputs enhances perceptual learning and recognition. For instance, if a baby is exposed to an audiovisual recording of someone singing, they would be able to recognize a change in either the singer (visual stimulus) or the song (auditory stimulus) more quickly than babies who were exposed to only the visual or auditory aspects of the recording. This demonstrates the importance of intermodal perception in early cognitive development. In older infants, the ability to perceive and transfer information between different sensory modalities, such as touch and sight or sound and sight, can be clearly observed. For example, in a study, 6-to 8-month-old babies were exposed to audio recordings of speech or singing. Afterwards, they were shown two silent visual recordings, one of which featured the previously heard speaker or singer. The infants spent significantly more time looking at the silent recording of the person they had heard earlier. This suggests that they understood the connection between the sound pattern and the movement pattern, demonstrating not only intermodal perception but also a sophisticated use of cross-modal cues to match auditory and visual cues to the identity of unfamiliar individuals. **Explaining Perceptual Development** The study of perceptual development has historically been a key area of debate in the nature versus nurture discussion. Nativists believe that most perceptual abilities are innate, while empiricists contend that these skills are learned. Currently, developmentalists are reevaluating the relationship between nature and nurture, and how these two factors interact to influence development. The nativist position on perceptual development argues that many perceptual skills are innate, as evidenced by the range of abilities observed in newborns and very young infants. These abilities include good auditory acuity, adequate visual acuity, excellent tactual and taste perception, some color vision, and a rudimentary ability to locate sound sources. Newborns can also make sophisticated discriminations, such as identifying their mother by sight, smell, or sound. However, research with other species suggests that a minimum level of experience is necessary for the development of perceptual systems. For instance, animals deprived of light experience a deterioration of the visual system and a decrease in perceptual abilities. Similarly, animals deprived of auditory stimuli show delayed or no development of auditory perceptual skills. This suggests that both innate abilities and experience play a role in perceptual development. The development of perceptual skills is best understood as an interaction between innate abilities and experiential factors. A child can make visual discriminations between people or objects within the first few days or weeks of life. However, the specific discriminations a child learns and the number of separate objects they recognize depend on their experiences. For instance, a newborn's ability to distinguish their mother's face from a very similar woman's face must be a result of experience, yet the capacity to make such a distinction must be innate. Therefore, in the nature versus nurture debate, both sides are correct as both innate abilities and experiences contribute to the development of perceptual skills. **List of Key Terms** **adaptive reflexes:** reflexes, such as sucking, that help newborns survive; some adaptive reflexes persist throughout life. **auditory acuity**: how well one can hear. **Canada's Food Guide**: guidelines for a balanced and healthy diet based on three major food groups: fruits and vegetables; whole grains; and proteins---especially plant-based protein foods. **Colic:** an infant behaviour pattern involving intense, inconsolable bouts of crying, totalling three or more hours a day. **depth perception**: ability to judge the relative distances of objects. **developmental milestones**: near-universal, age-related events whose first appearance signals noteworthy change or growth (Eaton, 2015). **Dishabituation**: recurrence of a response to a stimulus that has undergone habituation. **Empiricists**: theorists who argue that perceptual abilities are learned. **gut microbiota**: the population of microbes that colonizes the gastrointestinal tract. **Habituation**: the decline in responding that occurs as a stimulus becomes familiar. **infant mortality**: death within the first year of life. **intermodal perception**: formation of a single perception of a stimulus that is based on information from two or more senses. **Macronutrients**: large amounts of carbohydrates, fats, and proteins that are needed for energy and for body-and brain-building elements. **Microbiome:** all the genetic material within the microbiota---the entire community of microorganisms, such as bacteria, viruses, and fungi, living in or on the body, including the human gut, skin, and body orifices. **Micronutrients:** essential vitamins and minerals that are needed in small amounts to regulate physical and mental processes. **Myelinization:** a process in neuronal development in which sheaths made of a substance called myelin gradually cover individual axons and electrically insulate them from one another to improve the conductivity of the nerve. **Nativists**: theorists who claim that perceptual abilities are inborn. **Neuroplasticity:** the ability of the brain to reorganize its neural structures and functioning in response to experiences. **preference technique**: a research method in which a researcher keeps track of how long a baby looks at each of two objects shown. **primitive reflexes**: reflexes, controlled by "primitive" parts of the brain, that disappear during the first year of life. **reticular formation**: the part of the brain that regulates attention. **states of consciousness**: different states of sleep and wakefulness in infants. **sudden infant death syndrome (SIDS)**: the term used to describe the sudden and unexpected death of an apparently healthy infant. **synaptic pruning:** process by which unused or unnecessary neural pathways and connections are eliminated. **Synaptogenesis:** the process of synapse development. **Tracking**: the smooth movements of the eye used to follow the track of a moving object. **visual acuity**: how well one can see details at a distance