Brain development notes

Neurons and basic brain activity

• Basic units of brain activity: neurons (nerve cells).
• There are billions of neurons in the human brain.
• Each neuron stores and transmits information that guides our behavior.
• Major function: communication between neurons to support brain functioning and behavior.

Prenatal brain development (brain development — pre-natal)

• Most neurons are formed before birth.
• Period of greatest activity in brain cell production occurs between $10$ and $26$ weeks after conception.
• Cell division: during this period the brain grows at a rate of $250{,}000$ neurons per minute.
• Cell migration: once formed in the neural tube, neurons migrate to their destinations in the brain.
• Both processes are very sensitive to negative influences from harmful environmental agents (e.g., alcohol).

Postnatal brain development

• Substantial brain development occurs before birth, but brain continues developing throughout the lifespan.
• New branches grow on existing cells.
• Communication between cells becomes more efficient.
• Connections between cells are formed and broken (synapses turn over).
• Experience guides the two processes that shape brain functioning:
  • Synaptogenesis
  • Synaptic pruning
• Synapses are formed and broken throughout life (e.g., in any act of learning).
• Quote: “The principal activities of brains are making changes in themselves.” — Marvin L. Minsky

Synaptogenesis

• Timeline illustrated: Birth → 6 months → 2 years (focus on early-life synapse formation).
• Synaptogenesis refers to the formation of synapses between neurons as the brain develops.

Synaptic pruning (auditory)

• Experiential tune-up: Between $6$ and $10$ months, infants’ ability to recognize acoustic differences in their language improves, while discrimination of contrasts in other languages decreases (Kuhl, Williams, Lacerda, & Steven, 1992; Werker & Tees, 1984).
• One of the first studies compared English /ba/ vs /da/ and native-American /ki/ vs /qi/.
• Study design (Werker & Tees, 1984): infants rewarded for turning their heads when the sound changed; adults asked to press a button.
• Results:
  • The /ki/ - /qi/ contrast was perceived by native-American adults and English infants $6$–$8$ months.
  • English adults, babies $10$–$12$ months, and ≈50% of infants $8$–$10$ months could not perceive the distinction.

Synaptic pruning (visual)

• Experiential tune-up: Between $6$ and $10$ months, infants’ ability to discriminate non-human faces decreases (Pascalis, de Hannn, & Nelson, 2002).
• Experimental design: infants and adults were shown pairs of identical faces (human or monkey), followed by pairs including a familiar and a novel face.
• Results:
  • Adults and 9-month-olds looked longer at novel human faces, but not at novel monkey faces.
  • 6-month-olds looked longer at the novel face, for both humans and monkeys.

Brain plasticity

• Some functions that are unlearned early in life can be learned later.
  • Example: Japanese children lose the ability to discriminate “r-l” sounds in English around $6$ months, but can become native speakers if immersed in an English environment by age $3$.
• Brain plasticity: the brain’s ability to reorganize its structure or function.
  • Applications: learning new tasks; relocating lost functions after brain injury.
  • Plasticity is greatest early in life but is present in older children and adults (e.g., adults recovering from stroke).

What is a sensitive or critical period?

• A time window when a particular skill is most easily acquired.
• A period when environmental effects on the developing brain are especially strong.
• Critical/Sensitive Periods: This does not mean the skill cannot be acquired after the period ends; learning may be more difficult and may not reach the same level of expertise.
• Examples of a critical period in development (discussed in lectures):
  • Language acquisition (evidence from cases like deaf children exposed late to sign language; Genie’s language development after delayed exposure).

Language acquisition and critical periods

• Evidence of critical periods in language:
  • Deaf children not exposed to sign language promptly show delays in language development.
  • Genie (famous case): not exposed to language until age $13$; after nearly four years of language exposure, grammar resembled that of a child around age $2.0$–$2.5$ years.

Beyond critical periods

• Brain development continues beyond early life; certain developments are pronounced from later childhood into adulthood.
• Specialization and interhemispheric communication evolve:
  • Cerebral cortex is the largest brain structure and is divided into two hemispheres.
  • Each hemisphere controls movements and sensations of the opposite side of the body.

Lateralization of the cerebral cortex

• Lateralization: specialization of the two hemispheres.
• Left hemisphere: better at processing information in a sequential, detailed, piece-by-piece way.
• Right hemisphere: better at processing information in a holistic, integrative way.
• Demonstrations/exemplars: tasks and lesions show differential hemispheric involvement; a common test asks which letter is perceived when presented with a certain pattern, depending on LH or RH damage.

Left vs Right brain: functional specialization

• Both hemispheres are typically involved in learning.
• People may be described as more “left-brained” (analytical, piece-by-piece) or “right-brained” (holistic), though in reality both participate in most tasks.

Communication between hemispheres

• The two hemispheres are connected by the corpus callosum.
• The corpus callosum is a large bundle of fibers that extends between hemispheres.
• It develops through adolescence.
• Because of constant communication, the hemispheres are not independent.
• They may interfere with certain tasks if miscoordinated; but for most learning, integration across hemispheres is crucial.

Write with your left hand vs right hand

• Demonstration of lateralization and interhemispheric coordination: writing with each hand engages different hemispheric control.

Central idea: interhemispheric communication and task complexity

• Generally, communication between hemispheres is critical for successful functioning.
• It is only harmful if trying to perform different motions with the two limbs simultaneously.
• In cognitive/learning tasks, integrating input from both hemispheres and involving different brain regions is important.
• The more complex the task, the more crucial interhemispheric communication becomes.

Key points so far

• Early brain development provides a foundational basis for learning, but brain development continues throughout adulthood.
• Synaptogenesis and synaptic pruning shape brain functions and underlie learning processes.
• Two hemispheres become highly specialized but are coordinated in their work.
• Biological factors (e.g., timing of developmental windows) interact with environmental factors to influence brain development.
• Sensitive/critical periods illustrate how biological constraints limit the age-appropriate timing of environmental effects on brain development.

Brain development and environmental input: impoverished environments

• Animal models show effects of impoverished environment.
• Study of chimpanzees raised in darkness (first $16$ months): later they were unable to learn simple patterns and colors.
• Neurons in the visual cortex had $70\%$ fewer synapses than in normally raised chimpanzees.

Brain development and environmental input: enriched environments (animal models)

• Study of rats raised in stimulated environments:
  • Increased weight of the cerebral cortex.
  • Increased number of synaptic connections.
  • Larger neuronal cell bodies.
  • Increased amounts of a particular brain enzyme that enhances learning.

Enriched environments: human evidence

• London cabbies study: activity of the hippocampus increases with spatial experience.

Enriched environments and human outcomes after brain injury

• Research on humans: children born with brain injury
  • Those with large injuries who experienced enriched input show better language outcomes than those with medium injuries and poor input.