Developmental psychology - summary of chapter 4 of an Introduction to Developmental psychology by A. Slater and G. Bremner (third edition)

Developmental psychology
Chapter 4
Prenatal development

Introduction

Prenatal development: the development of human individuals before they are born.
Foetus: (by humans) the organism 12 weeks after conception until birth.
Embryo: the developing organism during the period when organs are forming. In human from first cell divisions until about 10 weeks.
Neonate: an infant less than a month old.
Postnatal development: the development of a human individual after he or she is born, particularly during early infancy.
Organogenesis: the process of organ formation in very early development. In humans this is from fist cell division until about 10 weeks.

Throughout life, normal development demands constant and complex interactions between genes, environment and the emerging organism.

The impact of prenatal experience occurs on multiple levels. From biochemical factors influencing gene expression, in the foetus’s neuronal circuitry to characteristics of the mother’s lifestyle affecting the foetal environment.
Exquisitely timed, complex interactions between the genes and environmental input affect acquisition of neuronal identity, guidance of axons to target, induction of connections between cells or synaptogenesis, and also programmed cell death or apoptosis.

The brain, the spinal cord and the emergence of mind

Processes and sequencing of brain development

Ectoderm: the outermost of the three primary germ layers of an embryo. The central nervous system and skin, among other structures, develop from ectodrem.
The other two are endoderm and mesoderm.

During he embryonic period, the central nervous system brings as cells of ectoderm, one of three germ layers. The germ layers are the foundation for organ formation.
The endoderm thickens and becomes the neural plate by day 18 of gestation. By then it is already differentiated into cells that will become forebrain, midbrain and hindbrain.
The neural plate folds to become the neural tube, and by the end of the first month the embryonic body has the basic cranial-caudal (head to feed) organization.
Cells are born, and begin extensive migration to their eventual location where the will become their final forms.
Neurogenesis and migration continue right up to about the sixth month of pregnancy, and are followed by extensive changes in individual cells that program them for the myriad tasks awaiting the emerging brain.
Despite their ultimate high level of specialization, the 10¹⁰ nerve cells that will comprise the brain originate from one single layer of identical cells in the wall of the neural tube. There is a hierarchy of control systems within the nervous system that basically determines what the foetus is doing and when. The hierarchical structure becomes more complex as the unborn infant develops. The larger the behavioral repertoire is, the greater is the need for origination by the nervous system.

Development of the cerebral cortex

Cerebral cortex: the area of the brain that is associated with complex tasks such as memory, language, and thoughts and the control and integration of movement and senses.
It is the outer six-layer ‘crust’ of the left and right hemispheres that is about as thick as a credit card.

For the first two or three months of pregnancy there is relatively little development in this crust.
Behaviors emerging before this time are largely reflexive and probably controlled via simpler circuits that begin to arise in the midbrain.
The cerebral hemispheres begin to develop from the forebrain at about 9 weeks and rapidly increase in size, expanding to form different regions that will later become highly specialized. By mid-pregnancy, the cerebral hemispheres have expanded to cover the rest of the brain.

By the fourth month of pregnancy, the cells in the cerebral hemispheres begin to proliferate and migrate. Cell migration is quite unique in the cortex, cells migrate and find their ultimate destinations I the innermost of the six layers, first with successive migrating neurons passing them to their way to the outer layers nearer the skull.

By 6 months, the surface of the cortex is not longer smooth. Rapid cell proliferation has caused the infolding that is necessary for the large surface area of the cortex to be accommodated within the skull.
Sulci: the deep narrow grooves of the outer surface of the brain.
Gyri: the prominent ridges on the outer surface of the brain.
Sulci, gyri have appeared and the frontal, parietal and occipital lobes can be differentiated.

Additional sulci and gyri develop until birth, and continues in the months after birth as well.
As the higher centers of the brain develop, and more neural inputs become active, increasingly sophisticated messages can be sent from the brain.
The process of inhibition becomes functional. The foetus can modify movement. A by-product of this process is that at about 15 weeks there is a bit of a lull in activity. This is followed by a period of reorganization of behaviors. Reflexive neuronal circuits are still in place, but these circuits are now controlled by more sophisticated nerve cells in the new higher brain centers.

By 27 weeks the number of brain cells in the cerebral cortex are thought to be mature, but at birth the brain is only about 25 per cent of this adult volume.
Additional volume comes from increases in cell body size and proliferation of dendritic spines during synaptogenesis.
Most of the growth comes from the myelination of nerve fibres.
Myelination begins in the sixth month of foetal life, and is not entirely complete until the third decade of life.

By about 24 weeks foetuses do have limited capacity to learn. They respond to the environment and begin to show a very basic form of memory, habituation of responding to repeated auditory stimulation.

By birth, the cerebral cortex consists of a large number of well-defined primary motor and sensory zones.

Behavioral organization

Foetuses’ behavior becomes progressively more organizes as gestation proceeds.
At 34 weeks, they are no longer the continually moving creatures of 13 weeks, instead they have distinct patterns of rest and activity.
Two dominant patterns of behavior have now emerged:

• Foetuses now spend most of their time either in quiet sleep or active sleep

foetuses spend about 20 to 30 per cent of their time in quiet, motionless sleep-like state with a steady heartbeat and breathing movements that are rhythmic when they occur.

For most of the rest of the time they are similarly not awake but are in a state like newborn active sleep with many different body movements and their eyes rapidly back and forth, periodically open. Heart rate and breathing patterns tend to be irregular, and they will respond to the sensory stimuli that they are naturally exposed to in their uterine environment.
During periods of active sleep foetuses may be more reactive to sounds and touch. Early neuronal networks are being stimulated both by external stimuli and also volleys of activity that the brain generates without external stimulation.
It is thought that this level op brain activity is probably necessary for adequate development and further maturation of the vital organs and the nervous system.
Foetuses make fewer general body movements now. They also make breathing movements fairly frequently, which are important for long development in readiness for birth.

38 week old term (the end of pregnancy) foetuses no longer spend quite as much time in state of active sleep.
More inhibitory pathways have developed, further reducing the amount of movement they make.
Foetuses will have longer periods when they are resting quietly in deep sleep.
Foetuses activity and rest periods alternate cyclically throughout the day.
Superimposed on this cyclical rhythm are maternnal physiological factors such as hormone levels, breathing, heart rate and uterine activity. Variations in some or all of these factors are thought to affect foetuses’ behavior over the course of the day.
In general there is a peak in activity occurring when the mother is asleep, in the late evening, and a relative lull in activity in the early hours of the morning.

Sensations

Touch

The emerge of the senses follows a set mammalian pattern of development.
Emerge is further organized in a cranial-caudal direction.

The first system to function of touch. By about 8 weeks, if the area around the lips is stroked by a hairlike wire, foetuses will respond by moving.
Within two weeks foetuses will curl their fingers in a reflexive grasp when their palm is touched.
Initially foetuses move their head and neck away from the source of facial touch, often with their mouth open. Later, they will move towards the touch.
Rooting reflex: the reflex that causes newborn babies to respond to one of their cheeks being touched by turning their head in that direction.
Once foetuses move around they will be touching the uterine wall, the umbilical cord, and also themselves. Foetuses will touch their own face more frequently than any other body part.
The foetus is provided with a wide breadth of physical sensations, which probably helps them promote further development of the physical sensation of touch.

Chemosensory system

Chemosensory system: encompasses both the gustatory (taste) ad olfactory (smell) senses.
The sensory receptors respond to molecules of the substances that contact them.

It is difficult to say exactly what foetuses taste and smell.
Molecules form the mother’s diet, and form heavy concentrations of airborne substances, can pass into her bloodstream and then into both the amniotic fluid and foetal blood.
Foetal blood is a third pathway, in addition to the mouth and nose, for the effect of experience with chemosensory stimulants.
Foetuses swallow amnoitic fluid regularly throughout the day. This fluid passes into the stomach where it will then be broken down further and sent to other organs, before it is expelled from the bladder back into the amnoitic fluid again.
During the fourth month, the plugs of tissue that were previously blocking the nostrils have gone, and the foetuses ‘inhale’, amnoitic fluid begins passing through the nose. Foetuses inhale twice as much fluid as they swallow.

During the second half of pregnancy, the constitution of amnoitic fluid becomes increasingly dependent on foetal urination. This may be particularly important for stimulation of the chemosensory system since it contains large amounts of ammonia-smelling urea in addition to molecules that have passes through the foetal digestive system.

Taste and odor molecules also travel via the placenta into the foetal circulation system.
Those within the blood have not been broken down or metabolised and are relatively undilute and consequently more intense.
Foetal blood will flow in tiny capillaries through the nose and mouth and therefore have ample opportunity to bind with olfactory and gustatory receptors.

Nearly all babies, before or after birth, show a preference for sweet substances over bitter. If the amniotic fluid tastes sweet then the foetus will swallow more regularly than if it contains bitter substances.

While some of the foetus’s ability to detect and prefer certain flavors to others may be genetically determined, other preferences may be learned in utero.
Newborn infants turn their heads in the direction of odeorants that have been present in their mother’s diet.

Colostrum: the breast fluid that precedes true milk. It is rich in minerals and antibodies, and it helps populate the newborn’s gut with ‘good’ bacteria.

The vestibular system

Vestibular system: the sensory system that contributes to balance and spatial orientation.

Foetuses move a lot around in utero.
Since mothers are moving about for much of the day, foetusese are also subject to constant passive motion and will experience positional changes relative to gravity, depending on whether the mother is standing up, sitting or lying down.
This information is sensed by the vestibular apparatus consisting of three semi-circular canals, set at right angles to each other within the foetus’s inner ear.
These canals are fluid filled and when the foetus moves, the fluid within at least one of the canals will also move, stimulating tiny hears within the canal lining.

By 25 weeks, foetuses will show a righting reflex, an it is possible that the verstbular system is in some way responsible for most babies lying head down prior to delivery.

Vestibular stimulation plays an important role in changing arousal states and this will become more apparent as time goes on.
Initially, during pregnancy, foetuses are often quiet when the mother is moving about a lot and causing a lot of vestibular stimulation. When the mother is lying down the foetuses are receiving minimal vestibular stimulation and are often at their most active.
Once foetuses are born, the parents will probably rock the baby when they are fussy or put to sleep. Vestibular stimulation!

Preterm: born prematurely.
Studies of preterm infants show lags in neurobehavioral development that may be part be due to a lack of vestibular stimulation.

Circadian rhythms: bodily cycles withing the body that occur on a 24-hour cycle such as patterns of sleeping/waking.

The visual system

Pregnancy is a time for structural formatoin of the basic components of the visual system.
There is little visual stimulation in the developing baby’s prenatal world.
The eyelids are fused shut shortly after their formation and do not open until 5-7 months of gestation, further reducing the amount of light reaching the developing retina.
This period of darkness may be necessary for proper development. After birth, visual development can proceed normally only when the system is adequately stimulated.

Development of the eyes

At about 5 weeks of postconception, two balloon like structures form on either side at the front of the brain. As they develop they become separated from the brain by a small stalk. This is where the nerve fibres will travel between the eye and the brain.
A few days later, the balloons infold to form a two-layered cup, and the retina develops from this cup.
The rods and cones develop from the inner wall of the optic cup.
The outer wall forms a pigment-containing layer that actually absorbs the light. This outer wall also goes on to develop the nutritive network of blood vessels needed by the rods and cones.

The lens of they eye begins to form at about 2 months of pregnancy.
The eyelids and muscles that move the eyes are also beginning to form around this time. The iris begins to develop.
By 3 months the eyelids have fused together. The cornea (the clear, curved part of the eye) is forming different layers, they organization of the cells and fibres in these layers is crucial in providing a strong but transparent window to the eye.
By 6 months all the muscles that move the eyeball are in place. Eye movements usually begin weeks 16-23, even though not all the muscles may be fully formed.
Even premature babies, as immature as 26 weeks gestation, are able to distinguish light from dark and are soon able to make tracking eye movement to follow an attractive moving object.

Development of the visual pathway

There is simultaneous development of the visual pathway connecting the light-sensitive cells in the eye to the brain.
By 9 weeks of pregnancy, shortly after the period of the embryo, the optic nerve has already penetrated the neural tube from its stalk, and there is partial crossing over of the fibres of the optic nerve. This allows for information from both eyes to be integrated. The crossing is complete by 15 weeks of pregnancy.
By the end of the first three months, the nerve fibres interconnect with cells in the LGN.
At about 5 months, the cells in this structure take on a very particular arrangement, six stripes appear. The cells within the stripes are highly specialized to deal with particular types of visual information and will be part of the ‘what’ and ‘where’ visual pathways for object perception. Two of the stripes respond maximally to moving stimuli and the other four cells are concerned with the what.

Development of the visual cortex

As the nerve fibres pass form the LGN they go to the visual cortex.
The visual cortex is organized like a map to the two retinas. Each point on the retinal represents a point in space within the field of vision.
The visual cortex turns the image right way up.
The striate cortex in the occipital lobe is the part of the brain concerned with many aspects of basic visual information. The surrounding brain areas are involved with perceptual processes. This is the interpretation of sensory information. Their development is less well known, but thought of to begin formation somewhat later in the last three months.

The development of the cerebral cortex is characterized by the formation of layers of varying cell densities and by about 7 months, the striate cortex attains the definitive laminar structure.
At this time, foetuses will spend some time with their eyes open and will now be making blinking movements.
Externally, foetuses’ eyes will look fully formed. There are still some minor immaturities in the gross structures of the eyes, but the major source of immaturity in foetuses visual system is within the neural structures of the eye, the retina and the pathways to the brain.
Babies of 28 weeks can easily distinguish between light and dark and have the ability to discriminate from to some extent .
By 30 weeks premature newborns are able to see patterns of fairly large size, provided that they are of sufficiently high contrast and fairly close to their eyes.

At birth babies are relatively near-sighted. The ability to focus on object across the room will develop in the first months of life.

The auditory system

The development of the auditory system begins at about 6 weeks of pregnancy. At this time, two small, inward-facing bubbles appear on either side of the back of the brain. These become the inner ear and will later contain the auditory and balance organs. The middle ear tube has also begun to develop from the pharynx or oral cavity area above the trachea.

• At 7 weeks, the external part of the ear along with the canal leading into the ear and the eardrum develop from a groove between the mouth and the heart.
• By 8 weeks of pregnancy, the inner ear begins to develop the semicircular canals that will eventually house the organs that are able to sense balance and position. A week later, the cochlea in the inner ear forms one coil, the first step in the formation of the spiral shell-like structures that will be the auditory organs.
• By 10 weeks, sensory cells are present in the semi-circular canals. The middle ear firms two soft structures that later become two of the three bones that construct sound form the outer to the inner ear.
• By 14 weeks, the vestibular system begins to work. The cochlea has become more coiled and now contains sensory cells, and the auditory nerve attaches to the cochlear duct.
• By 20 weeks the third bone of the ear is present and all three have begun to harden. Cochlear function is considered to begin around 24 weeks. At this time, the external ear is adult-shaped, but continues to grow in size until 9 years of age.

Responses to sound

The auditory system becomes mature enough between 23 and 25 weeks to detect vibroacoustic stimulation. At this point, a major immaturity is in the system’s sensors. Another immaturity is apparent within the nerve fibres that carry these messages.
However, almost all frequencies can be heard.
The sounds available to the foetus have to pass through various maternal tissues that effectively cut out the higher frequencies.

Very loud sounds result in a very vast heartrate. As foetuses get older, their response will change based on the sound intensity, how deeply they are sleeping and how familiar they are with sounds.
Foetuses will also respond to some sounds by moving their limbs, or sometimes stopping their movement.
Sounds are thought to shape permanent changes in the auditory system, and these are probably required for normal brain development.

Prenatal and transnatal auditory learning

Transnatal learning: learning that occurs during the prenatal period which is remembered during the postnatal period.

35 weeks marks an important advance in learning ability.

Learning about mother’s voice and language

Newborns prefer a low-pass filtered recording of the maternal voice compared to an unfiltered recording of his or her mothers voice.
Newborns have shown that they respond differentially to languages.
Within the first four days after birth, infants discriminate the language that their mother speaks compared to a foreign language. And 2 days old have demonstrated a preference for the maternal language compared to a foreign language.
Foetuses learn about vowels from the natural speech in their language community.
Infants respond differentially for as long as four months after birth to specific sounds that they experienced prenatally.

Risks to foetal development

Perinatal: the period just before and after birth.

Effects of exposure to psychoactive substances

Heavy maternal alcohol consumption profoundly influences foetal and child development.
Alcohol has detrimental effects on development and function of the placenta.
Cigarette smoking is bad.

Selective serotonin reuptake inhibitor (SSRI): a class of drugs typically used to treat depression or anxiety.

Nutrition and foetal development

Specific nutritional requirements must be met for healthy foetal development.
Women’s food intake and/or weight gain during pregnancy may subtly affect foetal development in ways that have implications for the child’s future medical and mental health, with some effect appearing only in adulthood.

Developmental programming: the hypothesis that prenatal conditions have detrimental effects on health into adulthood.

Effects of maternal stress

Maternal psychological stress during pregnancy has long been linked to negative birth outcomes.

Prenatal development of postnatal functions: the bridge to infancy

Most of the reflex behaviors that babies demonstrate after they are born are part of the foetal repertoire.