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.