brain development

1. The human brain: adapted for learning meanings from other people
2. What brain science can and cannot explain
3. Discovery of how neurons link up to make mental activity
4. Reflexes, motives, and consciousness: what develops?
5. The embryo brain (conception to 8 weeks)
6. The fetal brain (8 to 40 weeks)
7. The newborn infant's brain
8. Age-related changes in functions of cortex in different lobes of the hemispheres
9. How language and other meanings fit in: a brain with personality and for all cultures

1. The human brain: adapted for learning meanings from other people

Compared to brains of other mammals, the brain of a newborn human infant has two remarkable features. First, it already has uniquely complex anatomy. All major systems are present, in various states of immaturity. It has core mechanisms of motor coordination that are adapted for peculiarly human behaviours: walking with an agile bipedal body; clever manipulation of objects within many senses; and intimate communication with other minds by facial, vocal, and gestural expression of emotions, interests, and purposes. Second, in line with the principle that the longer a mammal species lives and learns from experience the larger is its forebrain cerebral cortex, the human cortex is extremely large, even in its half-developed stage at birth. Moreover, it matures very slowly. For a few months after birth its networks are being transformed, and some of its tissues and axon pathways develop over decades, responding to exercise and education. The baby's cerebellum (hind brain), which contains more neurons than all the rest of the brain, is also exceedingly complex, large, very immature at birth, and slow developing. Its intricate circuits set the timing for sensory control of rapid and skilled movement sequences of an agile body that walks, talks, and has two clever hands. It grows as the body grows in size and power.

The complexity of parts of the baby's brain underneath the cerebral cortex includes neurochemical 'activating', or modulating, systems of the brain stem that have already guided the multiplication, migration, and growth of cortical neurons in fetal stages. These same neural systems generate emotional expressions after birth. The subcortical emotional brain and the limbic cortex most closely connected with it motivate a child's learning, much of which will depend on communicating with other people and with the interests and feelings in their brains. Throughout an infant's cortex, new synaptically connected neuron systems that are essential for planning action and for coordinating the senses are being assembled. These developments both refine perception of objects and increase the precision of movements to manipulate them.

In ways that science is just beginning to comprehend, the human brain is born prepared for awareness of people and for sharing their actions and consciousness. Recently, imitative or sympathetic 'mirror neuron' systems have been discovered in the infant's cortex soon after birth. For example, territories known from neuropsychology of adults and physiological research on monkeys to be critical for recognizing other individuals and perceiving their actions and expressions, and for sympathizing with their motives and emotions, become active in a 2-month-old's brain when the baby is looking at a woman's face. This 'mother/teacher-perceiving system' of a human baby is essential for the education of speech and all other cultural skills, including arts and sciences, technologies, social manners, beliefs, and philosophical explanations, too. Our brains have to learn a distributed consciousness or community wisdom. Our efforts to share this knowledge are driven by powerful emotions of pride and shame, emotions that are expressed to their companions and teachers by infants long before they can speak.

2. What brain science can and cannot explain

Up to a point, functional imaging techniques — tracking local blood flow or electroencephalic activity in brains — enable responses to stimuli, and even the presence though not details of spontaneous thoughts, intentions, or memories, to be detected and mapped in living, thinking persons. How different lobes of the brain and the two cerebral hemispheres become activated for different kinds of mental activity can be observed. But only part of widely distributed and fast-changing brain effects can be imaged, and it is rarely possible to sample what goes on in a freely moving subject. Research in brain genetics, neurochemistry, physiological activity, and plastic developmental change is filling in details. In spite of these advances, however, we still depend on descriptions of brain anatomy and growth built up over more than 100 years. Some who have been trying to find evidence on natural processes of, say, interpersonal recognition or language conclude that neuroscientists must take a wider view of human mental life, incorporating insights from social sciences, anthropology, and philosophy, as well as comparisons with the neuropsychology of other species. And claims that psychologically and culturally important abilities, such as 'music' or 'language', have been 'located' in one or other structure by functional imaging of brain activity are not to be trusted. They depend on artificial testing conditions and selective sampling. They need relating to 'whole person' experience in a mobile body, where consciously intended actions, intuitions, and emotional states, especially those sensed between people, condition awareness, communication, and learning. These are the factors that determine developments in the brain of a child.

Changes in the distribution of cortical activity following regimes of training or exercise, or exposure to highly stressful situations that interfere with normal rest and recuperation, prove that the intercellular connections of cortical tissue can be changed and functions can be increased, relocated, or weakened. People who rely on different senses for communicating, or who have practised different skills, have different functional brain maps. The brains of those who have suffered abuse or extreme lack of sympathetic company may also be irreversibly changed. The diversity of brains due to experience is clear, but internal factors of intention, attention, and emotion that direct changes in the brain in different persons, and that determine temperament and personality differences, are not well understood. Age-related events of brain growth and behaviour in early childhood reflect developments in subcortical mechanisms that have evolved to generate adaptive body–brain relations in this period of accelerating growth and adaptive response to the environment. Advances in our understanding of the psychological abilities of infants and how a child's mind grows help build a natural foundation for explanations from neuroscience about how different adult brains work (see infancy, mind in).

3. Discovery of how neurons link up to make mental activity

The composition of nerve tissues has been discovered only within the last 150 years. In Charles Darwin's day, neurons and their delicate connections were almost unknown, and Darwin wisely said little of the brain, though he assumed it to be the evolved organ of the mind.

About 1870 an Italian psychiatrist, Camillo Golgi, discovered that single nerve cells could be stained black with silver or gold particles, so that their fine receptor branches (dendrites) and output fibres (axons) could be seen in brain slices under the microscope. In Spain, Santiago Ramón y Cajal used silver staining to lay the foundations of modern neuroanatomy. Cajal pioneered microscope studies of how the brain develops. He imagined the growing tip of each new nerve fibre probing its way with delicate filaments, 'a sort of battering ram, possessing an exquisite chemical sensitivity, rapid amoeboid movements and a certain driving force that permits it to push aside, or cross, objects in its way'. He concluded that formation of the right patterns of connections depends upon how this 'growth cone' chooses a path through the densely packed tissues of brain and body. He also saw that developing nerve cells could move in groups to change their location in the brain. He drew remarkably accurate conclusions about mature brain anatomy by studying the forms and connections of neurons in the relatively uncluttered tissues of fetal mice and birds.

In the 19th century, surgical experiments with animals located regions in the cortex necessary for perception in different modalities, and mapped territories from which movements could be elicited. Sensory and motor 'images' of the body were found, and it was noted that the amount of nerve tissue on the surface of the brain in these maps was related to detail of experience and skill in moving. Then, about 1900, methods were developed for amplifying the minute electrical impulses by which neurons excite one another. Electrodes were used to record nerve discharges, or to stimulate, and connections were followed with new precision. The tissues of the brains of many animals were mapped, showing in outline how the main parts had evolved. A common plan, evidence for common evolved principles of growth and function, became clear.

During the 1960s and 1970s, closer inspections of the complex arrangements of brain tissues were made by staining and by microelectrode techniques capable of identifying inputs to a single cell and of following its projections to the furthest tips. (See neuroanatomical techniques.) It became possible to trace where a cell at one point projects to, and backwards, to locate cell bodies that send axons to that point. With the electron microscope, nerve-cell membranes and the specialized contact points (synapses), by which selective chemical communication is established from cell to cell, can be seen. The electron microscope can also reveal the intricately folded macromolecular membranes that control nerve-cell biochemistry: the synthesis of protein or of transmitter chemicals, and other processes. Biochemical and immunohistochemical methods show up chemical differences between nerve cells and reveal the location of chemical communication points on the cell surface. The anatomical patterns revealed by these methods testify to the formation of organized cell systems by a process of development and differentiation more complex than in any other tissue. This is the classical picture of an organ designed by evolution for psychological functions that control behaviour. We are more aware now that the human brain maps are changed by environmental stimuli, and that the brain's organization is particularly sensitive to the social or interpersonal environment, especially in early childhood. The task now is to find out how the innate structure guides this education, and what factors are responsible for pathologies of development in essential parts of the brain and for psychological disorders. The brain is not a passive or 'plastic' network of interacting cells excited by stimuli.

4. Reflexes, motives, and consciousness: what develops?

For most of the modern scientific era, descriptions of the human brain have been dualistic or Cartesian (see Descartes, René). One set of conduction pathways is imagined to be prewired innately, before birth. These govern essential 'biological', mindless, or machine-like functions. Somehow, inside these pathways, probably at junctions between nerve cells, changes in transmission of excitation allow use and disuse to modify function. New connections make up new combinations of reflexes by conditioning, and create purposeful and conscious psychological states. This is the theory of Pavlov, based on his experiments on the formation of new connections between stimulus effects and movements or gland secretions in restrained dogs. His work gave the physiological foundation to behaviourism, the belief that the higher mental, moral, and cultural attributes of a human being are added by learning to the reflex sensorimotor biology of the brain.

But this theory does not accord with how brains actually develop. First, sensory and motor nerve cells are in a minority, even in the spinal cord. Interneurons that link input to output, and determine how such links will be grouped into coordinating systems, are hundreds of times more numerous, and they develop first. Everywhere in the brain these intermediate integrative cells, some small, some reaching far in the brain, form systems that can generate states of intention, attention, and emotional feeling. Interneurons are capable of discharging impulses spontaneously that may be transmitted widely through the nerve net. This is not an automatic sensorimotor reflex system.

In fetal birds and mammals, flexibly coordinated movements and rhythmic sequences of action occur before sensory cells are connected to the brain, or in pieces of the central nervous system that have been cut off from sensory nerves by surgery. Rhythmic movement programmes are set up among the interneurons, largely in the brain stem. On the input side, too, the axons of sense cells — from the retina, from the touch cells of the skin, etc. — grow millions of fine branches that end among already organized and active central networks. These afferent terminals segregate themselves in such a way that they map the body and the outside world into the prewired systems of the brain. How the human brain parts grow before birth suggests that the interacting nerve cells make up and coordinate basic rules for object perception, for purposeful movement patterns, and for motive states that construct time and space for action, and that might become conscious of other bodies and their behaviours, without benefit of experience. This is confirmed by detailed analysis of what infants can do shortly after they are born.

In the 1940s, Roger Sperry proved that surgically rearranged nerve networks of growing fish and amphibia could re-form well-ordered contacts, without benefit of learning. He ruled out 'contact guidance' and 'electrical field' hypotheses, and concluded, as Cajal had done, that some unknown chemical 'landmarks' guided the nerve fibre tips to the right location. This 'chemoaffinity' theory has been upheld by subsequent studies, but no gene code can possibly specify, on its own, this cell-to-cell precision of nerve mappings in such well-ordered arrays as the projections of the visual field. Ordering processes must involve intercellular effects that 'emerge' in the activity traffic of the growing brain and body.

In 1949, Donald Hebb identified a fundamental principle governing the acquisition of adaptive cell assemblies in impressionable brain tissues, such as the cerebral cortex. Cells that fire together wire together. Synchrony of excitation strengthens synaptic connections and connects cells in new functional groups. As a result, a given cell with its synapse-carrying dendrites can participate in a number of assemblies and perform in different functions, for example making representations of the meanings of different words. There is now much direct evidence of the formation of neuronal assemblies that store skills and experiences by changes in the wiring of the brain caused by use, but guided by motivated 'interests' of the mind.

5. The embryo brain (conception to 8 weeks)

The human brain starts as a slab of identical-looking blocklike cells in the upper layer of a disclike embryo one millimetre long (Fig. 1a, 16 days). These cells become irreversibly different from other cells about 3 weeks after gestation. Different parts are already designated to form specific relations in the future with sense organs and muscles in the body. Brain and body of the embryo have bilateral symmetry, but hidden differences exist between the chemistry of left and right halves. Brain and spinal cord roll into a hollow cylinder (Fig. 1a, 22 days), then, in a few days, cell multiplication occurs round the central cavity, and cells migrate outwards to form rudimentary brain nuclei or cell colonies in the wall of the tube. Some cells become neurons and others change into non-neural support cells (glia). At about 4 weeks, ventral cells in brain stem and cord grow axons out to the muscles of the trunk, limbs, and viscera, the anterior-most motor cells terminating in muscles of the eyes, face, and mouth. Sensory cells grow shortly after motor cells, projecting their input to the dorsal half of the central nervous system (Fig. 1c).

As embryo nerve cells migrate and form patterned aggregates to make up functional circuits, they communicate by biochemical expressions of regulator genes that can switch on other genes governing nerve-cell development. There are message-emitting and message-receiving loci on the cell surfaces. These respond to hormones and growth substances that are produced by cells in many parts of the body. Once the network of nerve connections is formed, conduction of nerve impulses adds power and precision to this cell-to-cell communication. The electrical excitation of nerve-cell membranes causes adjustments in their biochemistry as intercellular chemical messages are turned off or on, and this builds the adaptive organization of primary brain systems.

In this way, the intricate interconnected anatomy of the brain is formed as a result of the multiplying effects of many choices in cell differentiation that are governed by genes acting in the context of development. To effect this, there is much negotiation — an editing of connections, with removal of many, so that the remainder establish functional relations inside an overall array oriented approximately by chemical field markers of the body set out early in the embryo period. The multiplication, migration, and survival of neuroblasts, the growth and pruning away of axons, and the reinforcement or suppression of synapses are decided by the molecular compositions of the surrounding media and nerve-cell membranes. Trophic or inhibitory factors are produced by many types of cell, including non-neural supporting cells (glia) in the central nervous system. The glia make up a scaffold that guides migrating neuroblasts and the creeping of axon growth cones, and they assist in the maturation of axons and synapses. Within this tapestry of interacting elements, nerve cells take many forms and become differentiated chemically. They come to contribute different patterns of electrical and chemical activity to the system. This is how the brain begins its 'self-formation' or 'autopoiesis'.

Soon the brain is much larger than the cord, its conspicuous dorsal lobes receiving axons from the nose, eyes, ears, and mouth (Fig. 1b and e). The hypothalamus, the foremost ventral component of the brain stem, is the 'head ganglion' of the viscera (Fig. 1d and f). It will control appetites and aversions, and will act as a coordinator between activities of the central nervous system and the endocrine glands that secrete hormones controlling growth, metabolic activity, and sexual development. Among the first neurons inside the brain to send axons down to the cord and upward to the forebrain hemispheres are the core interneurons of the 'affective nervous system', which will transmit a spectrum of messenger chemicals to other neurons and maintain the balance of integrative states in the brain throughout its development (Fig. 1d and f). By the end of the embryo period (eight weeks), when the cells of the future neocortex of the cerebral hemispheres first appear, the brain stem has an elaborate system of projections that will influence the migration and differentiation of cortical neurons (Fig. 1f). Neurochemicals regulate nerve-cell growth and differentiation everywhere in the young brain. They become the media of dynamic emotional states, which will be expressed in special movements, innervated by the cranial nerves (Fig. 1f). In the mature brain, the affective nervous system modulates perception, the formation and recall of memories, and motor coordination.


Fig. 1. The human embryo and its nervous system. a. The first 3 weeks. The neural tube closes and the brain end enlarges. b. The body, with head and sense organs, is clear at 4 weeks. c. The first axons grow from the spinal cord, and bipolar sensory cells grow axons into the cord and dendrites to receptive endings. d. After 5 weeks integrative pathways fill the brain stem, while the cerebral hemispheres are rudimentary. e. At the end of the embryo period, the body is still like that of other vertebrates. f. The advanced embryo brain has an identifiable core reticular system and cranial nerve nuclei (here numbered). Basic elements of the visceral brain, motive systems, special senses, and the emotional motor system are present: smelling and tasting (1); seeing (2); looking and focusing vision, crying (3, 4, 6); face expressions, vocalizing, speech, facial feeling (5, 7); hearing (8); vocalizing, chewing, breathing, gasping, coughing, licking, sucking, expressions of voice, speaking (9, 10, 11, 12).

6. The fetal brain (8 to 40 weeks)

By two months after gestation, the developing human fetus has elaborately adapted special sense organs in the form of eyes, face with nose and mouth, vestibular semicircular canals and cochlea, hands, and feet (Fig. 2a). The cerebral hemispheres appear in the early fetus and increase rapidly in size in mid-fetal stages as nerve cells multiply in the cortex of the hemispheres, which are at this stage thin membranous sacs distended by liquid (Fig. 2b). A second fourfold increase in bulk of the cortex takes place in the last two months of gestation and continues into the first few months of infancy (Fig. 2c). This is due, not to multiplication of cortical neurons, but to their branching and to the formation of the connections that integrate the experience-sensitive cortical tissues with the rest of the brain to make conscious perception, voluntary action, and intelligent learning possible.

The cells of the neocortex, generated round the cavities of the cerebral hemispheres (ventricles), enter the cortex between 15 and 25 weeks after gestation (Fig. 2c). Immature cells (neuroblasts) migrate in waves to make a layer called the 'cortical plate', later arrivals passing through earlier migrants so that the youngest cells are in layers towards the outside of the hemispheres. This flow of neuroblasts into the cortical plate proceeds at different rates in different areas, the last territories to receive a full complement of potential neurons, at six months (25 weeks) after gestation, being areas that will attain mature tissue structure years later, towards adolescence. Cells in cortex and subcortex that are dedicated to one psychological role start as neighbours in the periventicular germ layer, or derive from the same stem neuroblast cell.

Late migrating components of the hemispheres tend to left–right asymmetry (see Fig. 4). How hemispheric asymmetry begins and how it relates to gene-controlled asymmetries in the chemistry and immunological properties of cells widespread in the brain remains to be worked out. It is known that abnormal migration, in the left hemisphere, of one late contingent of cortical and thalamic neurons related to the perisylvian area, where temporal and parietal lobes meet, can lead to language disorders, including dyslexia, which makes it very difficult for a person of otherwise normal intelligence to learn to generate the high-speed processes required for reading and writing. This is an example of a brain development long before birth that shows up only years later, in school perform-ance.

In the 40 weeks of gestation a human brain grows to a two-thirds-sized likeness of the adult brain (Figs. 2c and 3b). At birth all the typically human cortical areas and nuclear masses of the brain stem are there, containing a million million (1,000,000,000,000) nerve cells in total. But this amazing fact is misleading in one important respect. The formation of intercellular connections, which take up little extra space but upon which the function of the brain depends, is far from complete, especially in the neocortex. There is a huge post-natal manufacture of fine branches as nerve cells form effective contacts (synapses) with dendrites of other cells (Fig. 2c). Each mature cortical cell is estimated to have, on average, about 10,000 synapses, the greater part of which develop a few months after birth. The total number of synapses in the cortex of one person (10¹⁵) is about 200,000 times the population of humans on earth. Prolific branching of dendrites and formation of synaptic contacts can be seen in microscope slides of the cerebral cortex from infants, and comparable developments occur in the even more intricate anatomy of the cerebellum of toddlers and young children.

How might specific connections be selected accurately to govern mental processes in this teeming array of minute living elements? On the selection of the right connections depends the development of skill in motor coordination, refinement of perception, retention of memories of all kinds, formation of vocabulary, and development of increasingly critical and precise patterns of thought. The facts of brain growth do not imply that mental and physical abilities governed by the brain simply expand and elaborate independently of stimuli. Nor do they give the brain a passive submissiveness to experience and diffuse response to exercise. True, the environment matters.

Even in fetal stages the selection of nerve connections depends upon stimuli from the intra-uterine environment. Serious mental difficulties in childhood can arise if the mother is severely undernourished or under extreme mental stress during critical brain growth periods in gestation. But the process of gaining experience is at all stages an active one that the growing brain acts to direct. Chemical communication of the fetus's body with the mother's through the placenta affects brain development, and refinement of brain structures responds to gustatory, mechanical, touch, and auditory stimulation. The fetus begins to contribute to this stimulation by moving to change posture, to displace limbs, and to swallow amniotic fluid, but it is vulnerable, and may be permanently damaged by infection, or by the mother taking drugs or alcohol while she is pregnant.

In the last 3 months of gestation, a fetus has a well-developed subcortical auditory system that can hear and learn to recognize the expressive rhythms and tones of the mother's voice. After birth, stimuli that identify her by sight and communicate her feelings are sought and actively taken up by a baby (Fig. 3b). Those stimuli that are assimilated, especially those that come from the mother and other caregiving companions, cause selections to be made from among rival adaptive alternatives in brain structure. The ground rules for recognizing other persons and for sympathizing with their emotions and interests from their expressions are innate in the sense that they are formulated earlier, before stimuli have any effect. The learning takes place as part of a developmental strategy that must be ascribed to a continuous regulated unfolding of nerve-cell interactions from the embryo to the adult. While there is a huge excess of nerve cells and connections in the newborn brain, it is not chaotic, and the elimination to create adaptive systems is neither random nor the statistical consequence of emerging patterns of relative 'use' by stimulation.


Fig. 2. In the fetus, the cerebral hemispheres and cerebellum form. a. By 2 months all the special senses of the head, and the face, hands, and feet are well formed and clearly human. b. At 4 months the cerebral hemispheres and thalamus are swelling rapidly, but cortical cells are undeveloped. c. In the mid-fetal period (20 weeks) there is a great production of neurons in the cortex. After birth there is a multiplication of glia cells as dendrites grow. The newborn brain has proportionally small parietal (P), frontal (F), and temporal (T) lobes, and cerebellum (C), compared to the adult.

7. The newborn infant's brain

At birth the human being enters a new world. New forces on a body no longer floating, new levels and qualities of sound, new material taken in by mouth, air breathed by the lungs and bringing in new substances and micro-organisms, new stimulation of the surface of the body, and the immense range of new visual information; all these give the unfinished circuits of the cortex a fresh set of criteria for selective retention of functional nerve connections. More changes occur in the cellular structure of the cortex in the first six months after birth than at any other time in development. Dendrites branch out from large cortical cells and receive astronomical numbers of synapses (Fig. 3a).

Stimuli to the infant are regulated both by the infant's movements and by the behaviour of caretakers who mediate between the environment and the infant, and the immature brain has powerful control of this caretaker behaviour, especially through emotional expression. Indeed, the most precociously mature functions of a young child's brain are those that communicate needs, feelings, and motives to other persons, and that lead them to present the world to the child in precisely regulated ways (Fig. 3b).

The brain seeks stimulation of particular kinds that will facilitate developing circuitry in the brain. For example, selection of the circuits necessary for binocular stereoptic perception of depth, which happens in a human baby about six months after birth, depends both on the coincidence of stimulus patterns from the two eyes on cells in layer 4 of the visual cortex, and on activation of these cells by inputs to the cortex from the reticular formation of the brain stem. Controlled rotations of the eyes in precise unison bring corresponding stimuli in register. The visual cortex of girl babies gains binocular discrimination about two weeks ahead of boy babies, which suggests that sex hormones have an influence on the readying of these areas for visual information — possibly it is testosterone, produced in male infants for a few weeks after birth, that delays the process of cortical cell maturation.

In the mature brain, the cortical sheet is composed of columnar territories of uniform size. Interconnections between its parts, and with structures deeper in the brain, are arranged so they link columns into ordered systems (Fig 3c). This ordered anatomy becomes clear in infancy, when redundant axons are eliminated and synaptic fields are sorted out, a process that forms the basis for sharp perceptions and precisely skilled movements. From four months after birth the massive interhemispheric bridge (corpus callosum) gains in bulk as its fibres receive a sheath of myelin (see split-brain and the mind). Interhemispheric communication matures in step with the development of mature synaptic arrays in different cortical areas over the first decade or so of childhood.


Fig. 3. a. The visual cortex of a baby shows a huge development in the first 6 months after birth.



b. Infants are ready to communicate with other persons by a multi-channel system of senses and expressions. This links the baby's brain activities to those of the adult expressed in touches, vocalizations, face expressions, and gestures.



c. The functions and memories of the cerebral cortex develop over many years, all learning of knowledge and skills being regulated by emotions and in communication.

8. Age-related changes in functions of cortex in different lobes of the hemispheres

Different parts of a child's brain mature at different times and have different 'sensitive periods' when they are dependent on appropriate stimulation. The biggest developments in perception and in the formation of cognitive strategies occur in infancy or early childhood (Fig. 3c). That is the time when serious deprivation, such as that due to blindness, deafness, limblessness, or lack of affectionate care, can distort the growth of the brain most seriously. For example, defects in the optics or motor control of an eye (as in squint) cause the cortical connections of that eye to be pushed out of action by the more coherent and regulated input of the other eye. Deprivation of all humane care and emotional support may cause mental deformity — reflected in anatomical abnormalities in the brain.

The cerebral cortex has different relations with sense organs and the muscles of the body in different regions (Fig. 3c). The greater part of all main lobes, frontal, parietal, and temporal, has strong reciprocal connections with subcortical motivating systems that determine exploratory activities, planning of complex sequences of action, and integration of awareness for all the senses and the whole body.

Appearance of new functions at particular ages continues throughout life, along with maturation and ageing in the rest of the body, but at a slowing pace. Temporal, parietal, and frontal lobes of the brain grow disproportionately after infancy, changing the shape of the brain (Fig. 3c). Their surges of development correlate with changes in motivation for social life and for seeking experience; they are centres of desire and curiosity for knowledge and for relationships. In early infancy the right hemisphere and orbitofrontal and inferior temporal cortices develop in relation to the affective regulation of the mother–child relationship and developments in communication with other partners. The lateral prefrontal cortex expands conspicuously in the last months of the first year and into the second year with developments both in intelligence for exploration and combining of objects, and in protolanguage (the use of gesture and voice to make preverbal utterances or 'acts of meaning'). Developments in superior temporal and prefrontal cortex are both significant during the first weeks as the child's social understanding increases and recognition of others' states of mind increases (Fig. 3c).

Changes are observed at the end of infancy, in middle childhood, in adolescence, in middle life, and in old age. The developmental programme of the brain expresses changes in motivation, and this affects who is communicated with, and what is experienced, learned, and remembered. Transformation of psychological processes as new brain parts arrive at functional maturity confers a plasticity of function so that a child can partly recover from loss of brain tissue by injury or disease (see plasticity in the nervous system). The developments also explain why injury of a given part of the cortex can have different effects in children and adults.

9. How language and other meanings fit in: a brain with personality and for all cultures

Anthropologists and psychologists consider language the hallmark of humankind. It permits symbolic communication without which traditions of belief and understanding could not be built up. It powerfully aids processes of thought and reasoning. There was, therefore, some surprise when brain scientists discovered that territories of the left temporal and parietal lobes, which are essential for language understanding in the great majority of adult humans (Fig. 4), are already asymmetrical in a human fetus of 24 weeks gestational age — 4 months before birth. Although quite indeterminate as to which language it will learn, the human brain is set to acquire some language long before it hears a single word. Structures anticipating functions in consciousness and communication are seen in various cortical regions of the right hemisphere, which seems, indeed, to be slightly ahead of the left hemisphere in development during fetal stages. These underlie this hemisphere's subsequent superiority in perception of form, in visuo-constructive skills, and in other schematic processes that are of obvious importance in the development of technology and habits of interpersonal and cooperative life. These 'non-verbal' kinds of human mental ability may be more directly related to whole body awareness and self-related conceptions of the world than the more calculating, logically programmed 'propositional' skills of the left hemisphere. The right hemisphere starts life in infancy ahead of the left and it plays a lead role in the development of an affectionate attachment between the infant and caregiver. It is more sensitive to emotion in the voice and apparently to the expression of song and music, and it may recognize the affection of a caregiver more strongly.

It is now clear that the anatomy and function of human cerebral cortices are very variable. People differ in the pattern of their mental abilities because their brains grow in different forms. Males tend to differ from females, left-handers from right-handers, and architects from psychologists or lawyers. Some of this diversity of human minds, and their temperaments and aptitudes, will be 'preprogrammed' in a great variety of outcomes of gene expression in nerve-tissue development, but the same processes are set to respond to intra-uterine and post-natal environments. Among the influential factors are hormones, especially sex hormones, nutrition, and state of health of the mother during pregnancy, chemical and immunological factors, viral infections, epilepsy, and trauma. A pregnant woman who takes psychoactive drugs or alcohol or who smokes heavily can be changing the development of her child's brain. Particular risks are associated with birth — the brain of a fetus or premature infant is sensitive to its chemical and physical environment, which must be regulated inside narrow limits. On the other hand, a premature newborn's state of body and brain benefits from the holding, touching, and affectionate musical speech of 'intuitive parenting'.

Brains grow in conversation and in emotion. Throughout a long life, growing and learning human brains require, and care about, cultivation by intimate and morally regulated communication. Thus are built personal and family histories, and the fellow feeling, rivalry, and powerful collaborations and conflicts of human society.

(Published 2004)

— Colwyn Trevarthen

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