PSYC21022 - Week 4: The developing brain

Nature vs. nurture

• Extent to which cognition and behavior (and brain development) can be attributed to genes or environment

  • Nature = genetic blueprint (things we are born with)

  • Nurture = experiences

• History

  • Nature: Francis Galton (1874) - geniuses are born not made

  • Nurture: Freud; Skinner's behaviourism; Vygotsky

  • Middle ground (e.g., Piaget/neuroconstructivism): interaction between environment and genetic factors

Nature vs. nurture and the brain

• Human brains are similar (functionally and structurally), but our experiences are different

• Theories:

  • Blueprint analogy

  • Predetermined development

  • Probabilistic development

Blueprint analogy

• genes will shape how the brain develops (predetermine each connection between neurons) -> hard to achieve, millions of connections

Predetermined development

• genes will enable certain brain structure/functions, and this will enable experiences

Probabilistic development

• modern dominant view - different levels interact with each other (experiences can affect genes, vice versa)

Prenatal brain development

• Gestation around 38 weeks

• When babies are born, they have similar brains to adult brains

• Cell division

• After 2 weeks -> Cell specialization

  • Proliferative zones: neurons and glial cells are produced

  • During early development 250,000 neurons are produced per minute

  • Neurons migrate to their final location

• Neural tube differentiation 

Many structural features of the brain emerge from other constraints

• Folded cortex emerges from having lots of neurons

• Pattern of gyri/sulci pulled into shape by tension of axon bundles (white matter tracts)

• Hebbian learning: spontaneous electrical activity enables networks to form (e.g., electric activity from retina helps to form visual pathways)

Postnatal brain development

• Majority of neurons formed prior to birth

• Newborn born weighs 450g (vs. 1400g adult brain)

• Postnatal increase in brain size:

  • Synaptogenesis

  • Myelination

  • Glial cell proliferation

• Plasticity: experience dependent change in neural functioning

• Experience alone can lead to small but observable structural changes (e.g., juggling, driving a taxi)

• Increased grey matter: new synapses, dendrites, axon collaterals, glia cells

Synaptogenesis

• Number of synapses will increase after birth

• More synapses does not mean more efficient functioning

• Fine tuning can help more efficient functioning (redundant connections eliminated)

• Myelination

• Fatty sheath around axons

• Increases speed of info transfer

• Glial cell proliferation

• Increase in cells contribute to size increase

Plasticity

 experience dependent change in neural functioning

juggling example

• After practicing juggling, brain density increased

• After stopping practicing, brain density decreases but not all the way

Functional brain development - Functional brain plasticity

prenatal brain damage can lead to major reorganization of tracts;

• Functional brain plasticity has its limits

  • Spontaneous electrical activity enables networks to form intrauterine (prenatally) - these connections won't be fully lost

  • Opportunities for major reorganization are time-limited = critical or sensitive periods

Functional brain development - Case study: AH - no right hemisphere, only left hemisphere developed in the womb

Information from both visual fields was processed through the existing left hemisphere

Critical sensitive periods

• 2 main features:

  1. Learning takes place within a limited window

     • But opportunity/timing can be extended in lack of experience

     • Not so strict/critical

  2. This learning is hard to reverse by later experiences

     • But chicks imprinted to 1 object can generalize to similar objects (colour/shape)

     • Preference can be changed after sensitive period

Konrad Lorenz:

studied how birds recognize their mother

Filial imprinting

• the processing by which young animals learn to recognize the parent

  • Happens between 15h-3 days

  • Movement is crucial

Critical sensitive periods - 2 possible explanations:

1. Genetically programmed synaptogenesis (readies brain for learning), followed by reduced plasticity (learned info is then 'fossilized')

2. Closure of window could be initiated by learning itself, i.e., environmental cues

   • E.g., particular gene plays a role in filial imprinting, it is switched off after exposure (need environment in order for time window to be closed)

Innate knowledge

• Empiricist vs. nativist view

  • Empiricism: newborn mind is a blank state

  • Nativist: we are born with some knowledge

• Modern view

  • Innate = readiness to learn (e.g., imprinting)

  • Knowledge or behavior that arises in the absence of appropriate experience

    • Development of cat visual cortex

    • Preferences in humans - sweet taste, visual pattern

Modern view - Development of cat visual cortex

• Cat visual cortex

• Doesn't matter if cat did or did not have visual experience (same after 14 and 21 days), visual cortex develops in the same way

• But after 3 weeks, they need visual experience to form a major type of visual cortex (or else they won't be sensitive to visual info)

How can we investigate brain development? - Structure is 'easy' - prenatal ultrasound

• Structural: different types of tissue (skull, grey matter, white matter, CSF fluid) have different physical properties

  • Used to create static maps

How can we investigate brain development? - Behavioural methods

• We can infer brain development from behavior

  • E.g., preferential looking paradigm

How can we investigate brain development? - Functional neuroscience methods

• Functional: temporary changes in brain physiology associated with cognitive processing (e.g., fMRI)

• Problem

  • We ask participants to perform some kind of task (categorization, counting) or to sit still and look at images

  • Infants won't perform tasks and they won't stay still

How can we investigate brain development? - Functional neuroscience methods that can be used with infants and young children

• Electrophysiological response (electromagnetic fields generated in the brain)

  • EEG/ERPs

• Haemodynamic response (brain blood supply)

  • fMRI

  • fNIRS

How can we investigate brain development? - Infant EEG

• Infant friendly EEG systems/solutions that allow quick installation

• Infant friendly stimuli

• More breaks during the study

How can we investigate brain development? - Infant ERPs

• Some adult ERP peaks are present in infants but delayed

  • E.g., visual ERPs

  • N1/N290: perceptual and face specific component

• Some ERP components are only present in infants and toddlers

• Nc - negative central peak

  • Typically peaks between 300-700ms after stimulus onset

  • Reflect attention

  • Larger peak reflect higher attention

fMRI

• Not ideal to use with infants

  • Highly sensitive to motion artifacts

  • Loud, restrictive environment

• But there are recent attempts to use fMRI with infants

  • Custom headphones

  • Adjustable 32-channel coil

• E.g., face specific activity in the brain (yellow)

fNIRS

• Similarly to fMRI: it measures BOLD signal

• Unlike fMRI, it uses the near infrared spectrum of light

• Skin, tissue, bone are mostly transparent to NIR light

• Hb and deoxyHB absorb NIR

• Emitter: emits NIR light

• Detector: detect NIR light

• Hb and deoxyHB absorb of NIR - from the difference between emitted and detected NIR, we can compute BOLD

• Measures the concentration changes of oxyHb and deoxyHb focally, related to brain activity

• Can be an appropriate substitute for fMRI for studying brain activity related to cognitive tasks

fNIRS pros and cons

• Pros

  • fNIRS has lower spatial resolution

  • Only the surface of the cortex can be imaged

  • Often only a few sensors are used above a certain brain area

• Cons

  • Portable

  • More tolerant of movement T

Historical perspective

• some believed either genes or environment determined everything

  • Galton - importance of genes

  • Freud - experience

  • Piaget - interactionist perspective

Neuroconstructivism

emphasizes the interaction between genes and environment in shaping cognitive development

Structural brain development

investigate how the brain grows and changes, both before and after birth

Nature of developmental change

critical periods, sensitive periods, innate knowledge

Behavioural genetics

genetic influences on brain development and behaviour

Genetic code

• Genetic code of brains makes human brains similar to each other, but it does not give a blueprint for individual brain structures

• Think of the genetic code as a recipe for making a brain

Probabilistic and predetermined development

describes how genes and environment interact in shaping the brain

Predetermined development

• Genes dictate brain structure, influencing brain function, determining our experiences

• Genes have a deterministic influence on cognition

Probabilistic development

• Genes and experiences influence brain structure and function

• Brain structure and gene expression can be influenced by experiences

• Effects of genes on brain structure are probabilistic

• Even in the same environment, the same genetic program does not produce identical brain structures

• i.e., implications for identical twins - similar but not identical brains due to the probabilistic nature of development

Prenatal development - Embryonic development of NS

• Rapid cell division and differentiation forms neural tube

• Neural tube: embryo's precursor to the CNS, consisting of a set of cells arranged in a hollow cylinder

• 5 weeks - tube develops into bulges that become diff parts of brain

• Proliferative zones within tube produce neurons and glial cells (neuroblasts and glioblasts)

  • Neuroblasts: stem cells for neurons

Prenatal development - Neuronal migration

• New neurons migrate outward to their final destinations in the brain

• Process occurs…

  • Passively: older cells pushed to the surface of brain (i.e., hippocampus)

  • Actively: newer cell guidance from radial glial cells (i.e., neocortex)

• Radial glial cells: support cells that guide neurons from the neural tube to their final destination

Factors influencing brain structure

• Molecular signals influence structure, migration, survival of neurons

• Different doses of these signals determine the dimensions of brain lobes

Formation of cortex

• Develops in last few months before birth

• Genetic mutations affecting neuron production can lead to convoluted cortical surfaces

• Axon bundles place the cortical surface under tension, influencing its shape and variability

Axon guidance

• Influenced by molecular signals (like neural migration), biasing how and where axons form

• White matter tracts continue to develop a more fine pattern after birth

Spontaneous electrical activity in prenatal neurons

• Prenatal neurons exhibit spontaneous electrical activity, helping form networks in the brain based on Hebbian learning principles

• Hebbian learning: strengthening of a synapse that occurs when the presynaptic and postsynaptic neurons are active at the same time

• Spontaneous electrical activity plays a crucial role in setting up synaptic pathways for later sensory processing

Postnatal development - brain volume expands

• Most neurons are formed before birth, but brain volume expands after birth due to:

  • Synapse growth, dendrite and axon bundle growth, glial cell proliferation, myelination of nerve fibres

Postnatal development - Synaptic density changes

• Rise and fall in synapse formation (synaptogenesis) in various cortical areas during development

• Synaptic density peaks at different times in different brain regions

• Fine-tuning the brain to match environmental needs renders some connections redundant, leading to a decrease in synapse numbers

Postnatal development - Myelination and white matter volume

• Myelination, the increase in fatty sheath around axons, increases the speed of info transmission

• Increase in white matter volume over the first 2 decades of life reflects the time course of myelination

Postnatal development - Plasticity and experience-dependent changes

• Everyday experiences result in changes to the brain's structure

• Learning new skills can lead to visible changes in brain structure, like increased grey matter density

• Plasticity refers to experience-dependent changes in neural functioning, which can occur throughout life

Postnatal development - Gray matter density and cognitive ability

• Gray matter density not always a simple proxy for cognitive ability

• E.g., congenitally blind individuals may have more gray matter in their visual cortex; congenital amusia patients may have more gray matter in their auditory cortex

• Gray matter changes can result from (a) developmental pruning of synapses (thinner is better) or (b) experience-dependent changes (thicker is better)

Postnatal development - Mechanisms underlying structural changes

• cellular/molecular mechanisms that could underpin experience-related structural changes

  • Neurogenesis unlikely outside of the hippocampus

  • Other mechanisms like growing dendritic branches, synapses, axon collaterals could increase gray matter

  • Changes in glial cells, capillary vasculature, myelination can contribute to gray matter changes

  • Fractional anisotropy (in DTI) may increase as axons become more aligned or myelinated

Functional brain plasticity in rewired brains

• In response to abnormalities, the brain can reorganize itself in unique ways (no genetic blueprint for each brain)

• Case study of AH

  • AH, 10 YO girl, failed to develop a right hemisphere and right eye prenatally but had minor visual impairments

  • Visual info could reroute into the intact hemisphere, with neurons coding left AND right sides of space

• Examples of brain rewiring in animals

  • Through surgical transplants or pathway severance

  • Prenatal visual cortex transplanted into somatosensory cortex responds to touch

  • Severed pathways from the cochlear to the medial geniculate nucleus reroute visual inputs into auditory cortex

• Consequences of brain rewiring

  • Rerouted neurons can take on functional characteristics of the new region

  • Rewired areas may retain vestiges of their original connections and new representations may be less precise than normal regions

• Limits of brain plasticity

  • Neurons are not fully interchangeable

  • Major reorganization appear to be time-limited, suggesting critical/sensitive periods

Critical and sensitive periods in development

• Lorenz's study of filial imprinting in ducklings demonstrated a narrow window of opportunity for imprinting

  • Filial imprinting: process by which a young animal comes to recognize the parent

  • Movement of stimulus important for determining what object the duck will imprint to

• Critical period: time window in which appropriate environmental input is essential for learning to take place

• Sensitive period: time window in which appropriate environmental input is particularly important (not necessarily essential) for learning to take place

Critical periods have 2 features:

• Limited time window for learning

• Difficulty in reversing learning after the period ends

  • Possible in certain circumstances: chick imprinted to one object will often generalize to other objects of similar appearance

Examples of critical/sensitive periods

• Hubel and Wiesel's study on cats showed a sensitive period for visual cortex development - deprivation leading to permanent changes

• Language acquisition shows evidence of sensitive periods

  • Different components of language may have their own sensitive period (e.g., hearing, motor ability, working memory capacity)

• Feral children and language acquisition

While Genie showed some language development after being rescued, her proficiency remained poor compared to young children

Second language acquisition

• Differences in neural processing based on age of acquisition and proficiency level

• Sensitive period

Properties of the NS and sensitive periods

• Sensitive periods may be governed by a maturational timetable or self-terminating processes

• E.g., genes may control timing of sensitive periods, and NS may wait for suitable environmental cues

Innate knowledge?

• Empiricism: view that the newborn mind is a blank slate

• Nativism: view that at least some forms of knowledge are innate

• Instinct: behavior that is a product of natural selection

2 views of innate knowledge:

1. Innate as readiness for learning: some behaviours have a genetic basis but need suitable experiences to develop fully (e.g., filial imprinting, language acquisition)

2. Innate as independent of experience: certain behaviours/abilities emerge even without relevant experience (e.g., development of primary visual cortex in cats is innate but refined by experience)

Prepared learning:

• common phobias are biologically determined from evolutionary pressures

  • E.g., fear of snakes

  • Requires suitable experience

Arguments for innate abilities

• Newborns can imitate tongue protrusion without prior experience - innate ability for intermodal matching

• Some dispositions, preferences, abilities may have innate components, but the specific content of knowledge is challenging to determine

Complexity of innate knowledge

• Preferences for certain patterns (like faces) may have evolutionary roots, but specific knowledge can vary based on context and experience

• While some behaviours may appear innate, they often interact with environmental factors, making it difficult to pinpoint purely innate knowledge

Functional near infrared spectroscopy (fNIRS)

• Measures the BOLD signal

• Does not have magnetic fields

• Employs near-infrared light around 800 nanometres wavelength

• Near-infrared light is sent to the brain, scattering more strongly by oxy- and deoxyhemoglobin

  • Degree of scattering used to calculate BOLD response, indicating neural activity

• Advantages

  • Portability and tolerance to movement make fNIRS suitable for various applications

  • More affordable

• Limitations

  • Can only capture neural activity close to the scalp due to its shallow imaging depth