Infancy: Body and Mind

This is Unit Three, part two, on motor skills, the senses, and Piaget's account of infant thinking. Part one closed on newborn reflexes, most of which fade in the first months. Part two opens with the movements that replace them, beginning with walking. Carry one question through this part: in infant development, what is fixed and what varies? The milestones come in a steady order, yet one healthy child walks at nine months and another at seventeen. At the entry on motor milestones across cultures, the part gives the answer: the sequence stays and the timing moves. By the end you will be able to say what is fixed and what varies in motor milestones. To explain dynamic systems theory. To tell sensation from perception, and describe newborn vision. To explain what the visual cliff does and does not show. And to name Piaget's six sensorimotor substages, and say when he placed object permanence.

Two entries: when walking arrives, and a milestone many babies skip on the way.

When infants begin to walk.

HGD 0328 Walking

The average age of walking is twelve months, within a normal range of about eight to seventeen or eighteen months; one infant may walk at nine months and a healthy cousin not until seventeen. The sequence toward walking follows the cephalocaudal and proximodistal patterns. The timing varies widely and is not in itself a warning.

A milestone many babies skip.

HGD 0329 Crawling Is Optional

Crawling is not a required milestone. Some babies never crawl, moving from sitting straight to walking, or scooting instead; others commando-crawl on the belly or crab-crawl. The misconception is that every baby must crawl before walking. Skipping it does not signal a problem.

Six entries on motor skills, large and small, the theory of how they come together, and what culture does to their timing. The opening question is answered here.

Skills that use the large muscles.

HGD 0330 Gross Motor Skills

Gross motor skills use the large muscles, progressing from supporting the head at six to eight weeks to sitting alone at four to eight months, standing alone between seven and seventeen months, and walking alone between eight and seventeen. The ranges are wide because milestones vary with culture, experience and the individual child.

Skills that use the small muscles.

HGD 0331 Fine Motor Skills

Fine motor skills use small muscles for precise tasks, from two-handed reaching at about four months to a thumb-and-forefinger grasp around nine months. The ulnar grasp, the whole hand without the thumb, gives way to a pincer grip, and voluntary grasping replaces the palmar reflex at about six months. Practice and toys at home improve fine motor skill.

A theory of how motor skills come together.

HGD 0332 Dynamic Systems Theory

Dynamic systems theory, developed by Esther Thelen, holds that motor milestones emerge from many interacting systems: the body, the senses, the brain and the environment. Walking needs balance, leg strength, stepping, vision and spatial awareness all at once. No single system causes the milestone.

Genes, twins, and practice.

HGD 0333 Genes, Twins and Practice

Identical twins are more alike than fraternal twins in when they reach motor milestones, which points to genetic influence on timing. Yet in Myrtle McGraw's nineteen thirties twin studies, practice produced very different abilities. Genes and experience both shape motor timing, and the dynamic systems approach now extends to cognition and language.

The payoff: milestones across cultures.

HGD 0334 Motor Milestones Across Cultures

Motor development follows a similar sequence everywhere, but its timing depends on experience and culture, and that answers this part's question. Infants in Tajikistan who spend long hours in gahvora cradles reach milestones later. In northern China, infants kept in sandbags for toileting sit, crawl and walk several months later. In western Kenya, infants given more practice sitting may sit independently earlier. After the Back to Sleep campaigns, back-sleeping infants reached some milestones later, and tummy time is associated with earlier sitting, standing and walking. The sequence stays; the timing moves. On the exam, culture changing when a skill appears, but not its order, is about experience.

Where new skills are practised.

HGD 0335 Play and Motor Skill

Play draws motor and cognitive skills together, as when a spoon becomes a tool for banging, stirring and feeding. A baby may point at a dog by the first birthday, and play patty-cake by eighteen months. Play is an exam keyword under cognitive development, not only a pastime; it is where new skills are practised.

Seven entries on seeing: what perception adds to sensation, and how an infant's vision develops.

Detecting, and interpreting.

HGD 0336 Sensation and Perception

Sensation is detecting a stimulus through the senses; perception is interpreting it, making sense of what has been detected. A baby's eyes register light and shape, and the baby recognises a parent's face. The two develop together but are not the same, and infant research often measures perception by where babies look.

Vision at birth.

HGD 0337 Newborn Vision

Vision is the least developed sense at birth: sight is blurry and best at close range, with low acuity and weak contrast sensitivity. Acuity sharpens to about twenty over two hundred by three months, enough to see a large letter. The fovea keeps developing into the second year, so full acuity comes later.

Colour, from the start.

HGD 0338 Colour Vision

Newborns have basic colour vision for bright blue, green and red, and by four to six months they discriminate colours almost as well as adults, as the retina's cones strengthen. The misconception is that newborns see only black and white. They see colour from the start, less sharply.

How infants look at faces.

HGD 0339 Looking at Faces

Infants prefer faces to other patterns from about two to three months, and the way they scan a face changes with age: at one month, the hairline and chin; at three to four months, the eyes and mouth; at nine months, mostly the eyes. By three months, infants look longer at female faces when the primary caregiver is female.

Seeing depth.

HGD 0340 Depth Perception

Depth perception is absent at birth. It begins around two months with binocular vision, as the eyes start to work together, and cues from pictures are used by about six months. Depth sensitivity grows with experience of moving; infants who crawl or walk are more sensitive to drops.

The classic test of depth perception.

HGD 0341 The Visual Cliff

The visual cliff, designed by Gibson and Walk in nineteen sixty, is a glass-topped table over a checkerboard that seems to drop away halfway across. Few crawling infants aged six to fourteen months would cross onto the deep side. Later work showed that locomotor experience matters: new walkers avoid the drop more than experienced crawlers. The misconception is that the cliff proves babies are born afraid of heights. Karen Adolph's research suggests infants are learning how to move safely, and each new way of moving has to be learned. The cliff also studies social referencing: an infant looks to a caregiver's face before deciding.

What infants prefer to look at.

HGD 0342 Visual Preferences

Measured by looking time, infants prefer curved lines to straight, faces to patterns, patterns to solids, and three-dimensional objects to flat ones. An infant gazes longer at a new toy than at one seen many times, and that preference for novelty is what lets researchers test infant memory.

Five entries on the other senses, several of them at work before birth, and on combining them.

Hearing, at birth and before.

HGD 0343 Newborn Hearing

Hearing develops around the fifth month of prenatal development and works at birth. Newborns prefer speech and the sound of their native language, and they can still tell apart sounds from languages they have never heard. Newborn hearing screening allows early intervention when hearing loss is found.

A classic study of prenatal hearing.

HGD 0344 The Cat in the Hat Study

In DeCasper and Spence's study, mothers read The Cat in the Hat aloud during pregnancy, and their newborns sucked harder on a pacifier wired to play the familiar story. Unexposed newborns showed no preference. The high-amplitude sucking technique measures what infants prefer by how hard they suck.

Touch, and pain.

HGD 0345 Touch and Pain

Touch is the most highly developed sense at birth; touch receptors form around the eighth week of prenatal development. Kangaroo care holds a newborn skin to skin against a parent's chest. Newborns are sensitive to pain during medical procedures, and gentle touch improves mood, sleep and growth in preterm infants.

Taste, and smell.

HGD 0346 Taste and Smell

Taste and smell begin before birth. Newborns distinguish sweet, sour, bitter and salty, with distinct facial expressions within hours, and a six-day-old turns toward a pad scented with its own mother's milk. The fetus swallows amniotic fluid from about week twelve, and newborns favour flavours met there, such as garlic or vanilla.

Combining the senses.

HGD 0347 Intermodal Perception

Intermodal perception is awareness through several senses at once, such as linking what is heard with what is seen. Four-month-olds match a ball's sound to its bounce, and two-month-olds notice when speech and lips do not match. Motor skills help: turning the head and reaching let an infant see and touch the same object.

Four entries to close the part, on Piaget's account of infant thinking and its six substages.

How Piaget said knowledge is organised.

HGD 0348 Schemas, Assimilation and Accommodation

Piaget held that children organise knowledge in schemas. Assimilation fits new information into a schema, as when a child calls a parrot a bird; accommodation changes the schema to fit, as when the child meets a penguin that cannot fly. Piaget saw children as driven toward equilibrium, a balance between what they know and what they meet.

The first three substages.

HGD 0349 The First Three Substages

Piaget's sensorimotor stage begins with simple reflexes, such as sucking adapted to a bottle; then primary circular reactions with the body, such as thumb-sucking repeated; then secondary circular reactions with objects, such as a rattle shaken again and again. Circular means repeated: an action that happens by chance and is interesting enough to repeat.

Knowing that hidden things still exist.

HGD 0350 Object Permanence

Object permanence is understanding that objects continue to exist when they cannot be seen, as when a baby lifts a blanket to find a hidden toy. Piaget placed it at eight to twelve months, arriving with goal-directed behaviour in the fourth substage. Later research found signs of it much earlier, around four months.

All six substages, in order.

HGD 0351 The Six Sensorimotor Substages

Piaget divided the sensorimotor stage into six substages. Simple reflexes, birth to one month. Primary circular reactions, one to four months, repeating actions with the body. Secondary circular reactions, four to eight months, repeating actions on objects. Coordination of secondary circular reactions, eight to twelve months, goal-directed behaviour and object permanence. Tertiary circular reactions, twelve to eighteen months, the child as a little scientist. Beginnings of thought, eighteen months to two years, with symbolic thought and deferred imitation. The substages describe ways of thinking, not deadlines, and Piaget's ages turned out to underestimate infants, as part three shows.

Look at what you can now do. You can say what is fixed and what varies: walking comes anywhere from about eight to eighteen months, crawling is optional, and culture moves the timing of milestones but not their order. You can explain dynamic systems theory: a milestone emerges from body, senses, brain and environment working together, and no single system causes it. You can tell sensation, detecting, from perception, interpreting, and say that newborn vision is the least developed sense, blurry, but in colour from the start. You can explain that the visual cliff shows depth avoidance growing with experience of moving, not an inborn fear of heights. And you can name Piaget's six substages, from simple reflexes to the beginnings of thought, and say that he placed object permanence at eight to twelve months, while later research found signs of it around four. Part three covers what that research changed.