Brain development: How genes and

experience make us who we are

Plan •  Genes and environment

– and their interaction

•  Plasticity (ie effects of environment) – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Plan •  Genes and environment

– and their interaction

•  Plasticity (ie effects of environment) – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Experience

Behavior

Genes

Genes and environment: the “nature versus nurture” debate

"Give me a dozen healthy infants & my own specific world to bring them up in, & I'll guarantee to take any one at random & train him to become any type of specialist I might select - doctor, lawyer, artist, merchant, chef & yes, even beggar & thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors."

- John B. Watson, 1924

Genes and environment: the “nature versus nurture” debate

”Compensatory education has been tried and it apparently has failed."

- Jensen, 1969 “The story of attempts to raise intelligence [by

environmental enrichment] is one of high hopes, flamboyant claims, and disappointing results."

- Hernstein & Murray, 1994

Behavioral genetics •  Partitions variance in a trait (eg IQ) into

genetic variance and environmental variance

•  Surprising range of traits show substantial heritability (IQ, vulnerability to most mental illnesses, personality, political orientation)

•  Clever (but imperfect) methods – Adoption studies – Twin studies

Twin studies

•  Workhorse of behavioral genetics

•  Measure correlations of a trait between twins

Twin studies

•  Workhorse of behavioral genetics

•  Measure correlations of a trait between twins

A 105 130 110 145 85 …

B 100 140 105 140 91 …

Twin studies

•  Workhorse of behavioral genetics

•  Measure correlations of a trait between twins

•  eg, at right, fairly high correlation in IQ

A 105 130 110 145 85 …

B 100 140 105 140 91 …

Twin studies

•  The correlation depends on the kind of twins – identical or fraternal –  Identical, monozygotic, share all genes – Fraternal, dizygotic, share 50% of genes

•  If a trait is strongly genetic, eg eye color, then…

•  If a trait is not genetic, eg language spoken, then…

Cortical thickness Schmidtt, Cerebral Cortex, 2008

Heritability estimates are not absolute – they describe variances in a particular population

Turkheimer (Psych Sci, 2003) twin study of intelligence, showing heritability as a function of socioeconomic status

Plan •  Genes and environment

– and their interaction

•  Plasticity – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Gene-environment interaction Caspi, Science, 2002

Gene-environment interaction Caspi, Science, 2003

Plan •  Genes and environment

– and their interaction

•  Plasticity – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

More on environmental influences:

“I want to say one word to you, just one word: Plastics Plasticity”

Plasticity

•  Neuronal connections depend on neuronal activity

“Neurons that fire together wire together” •  Small amount of neurogenesis as well •  Can actually change macroscopic size and

shape of brain!

Plasticity

•  Changes in brain as a result of experience

Maguire, PNAS, 2000

Plasticity

•  Changes in brain as a result of experience

Draganski, Nature, 2004

Plan •  Genes and environment

– and their interaction

•  Plasticity – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Limits on plasticity

•  “Critical periods,” “sensitive periods”

Critical periods

•  Vision

Critical periods

•  Language – Studies of second language acquisition

Critical periods

•  Language – Studies of second language acquisition – First language acquisition

Plan •  Genes and environment

– and their interaction

•  Plasticity – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Building a brain

•  Much known about prenatal and postnatal events at the cellular level

•  Generally hard to relate this level to psychological phenomena

Prenatal processes

•  Cell birth •  Cell migration •  Cell differentiation •  Cell maturation •  Synaptogenesis •  Cell death and

synaptic pruning •  Myelination

Postnatal development •  Synaptogenesis

continues

Postnatal development

•  Myelination continues – Enhances neuronal

conduction velocity – Continues through

young adulthood

Postnatal development •  Pruning continues

Plan •  Genes and environment

– and their interaction

•  Plasticity – and its limits (ie critical periods)

•  Lifespan brain development – Mechanisms – Regional differences

Adolescent brain development

Allstate Insurance advertisement

Adolescent brain development

•  Two systems maturing at different rates

Key Points •  Genes and environment

– Both important –  interact

•  Plasticity – Ubiquitous, evident even at macroscopic scale – but has its limits, some severe

•  Lifespan brain development – Cellular mechanisms and regional differences

Questions, comments?