Lecture 26: Typical and Atypical Brain Development

Overview

This lecture explores typical brain development, aging, intelligence, atypical development, and sensory substitution, integrating cutting-edge research on brain plasticity and neural adaptability. The focus includes neuronal correlates of intelligence, developmental dynamics, and functional implications of cortical reorganization, especially in atypical populations.

Typical Brain Development

• Developmental Processes:
  • Progressive: Growth of neurites (spines, dendrites, axons) and synaptic connections.
  • Regressive: Programmed cell death, axon pruning, and synapse elimination.
  • About 50% of neurons present at birth are eliminated through programmed cell death during early development.
• Synaptic Density:
  • Peaks around two years of age, followed by pruning during childhood and adolescence.
  • Pruning increases neural efficiency by refining neural circuits.
• Myelination:
  • Sensory and motor regions myelinate early, while prefrontal cortex development extends into the late 20s.
  • Myelination enhances conduction speed and supports higher-order cognitive functions.
• Glucose Utilization:
  • Peaks between ages 2 and 4, correlating with synaptic density.
  • Declines in adolescence as neural circuits become more efficient.
• Timing of Psychiatric Disorders:
  • Many disorders, such as schizophrenia and bipolar disorder, emerge during adolescence, coinciding with the development of the prefrontal cortex.

Aging and Brain Plasticity

• Cortical Decline:
  • Cortical thickness and volume decrease linearly after age 20.
  • Functional efficiency may mitigate the impact of structural decline in some individuals.
• Plasticity and Resilience:
  • Studies on aerobic exercise demonstrate that lifestyle interventions can increase hippocampal volume and enhance cognitive function in middle-aged and older adults.
  • Research suggests that the brain retains capacity for positive structural and functional changes throughout life.

Intelligence

• Components:
  • Fluid Intelligence (Gf): Problem-solving and reasoning, correlates with working memory.
  • Crystallized Intelligence (Gc): Knowledge and skills accumulated over a lifetime.
• Stability Across Life:
  • Intelligence measures show high test-retest reliability across decades.
  • For example, a 45-minute test administered at ages 11 and 79 had a correlation of 0.63.
• Brain-Intelligence Correlations:
  • Larger brain volumes weakly correlate with higher intelligence (r ~0.30–0.40).
  • The parieto-frontal integration theory identifies interactions between frontal and parietal regions as central to intelligence.
• Lesion Studies:
  • Damage to specific brain regions (e.g., TPJ, prefrontal areas) disrupts working memory, processing speed, and general intelligence measures.

Atypical Brain Development

• Attention Deficit Hyperactivity Disorder (ADHD):
  • Associated with reduced cortical thickness in prefrontal and parietal regions.
  • Smaller brain volumes persist across childhood and adulthood, as shown in longitudinal studies.
• Autism Spectrum Disorder (ASD):
  • Characterized by increased cortical and subcortical volumes, linked to impaired synaptic pruning.
  • Excessive connectivity and persistent neural exuberance disrupt functional integration.
• Environmental Impacts:
  • Romanian orphan studies revealed that severe social deprivation leads to widespread cortical thinning and elevated ADHD symptoms.
  • Cortical changes correlated with reduced attention and cognitive deficits in deprived environments.

Sensory Substitution in Atypical Development

• Concept:
  • Sensory substitution involves using one sensory modality to compensate for the loss of another, relying on the brain’s plasticity to repurpose cortical areas.
• Adaptations in Blind Individuals:
  • Visual Cortex Recruitment:
    • Studies show blind individuals use primary visual areas (e.g., V1) for non-visual tasks, such as tactile or auditory spatial navigation.
    • For example, blind individuals using Tongue Display Units (TDUs) activate the visual cortex while performing route-finding tasks.
  • Motion Processing:
    • Auditory motion cues (e.g., Doppler effect) engage area MT, a region typically associated with visual motion perception.
    • This suggests MT functions as a general motion-detection hub, independent of sensory modality.
• Adaptations in Deaf Individuals:
  • Language Plasticity:
    • Deaf individuals proficient in American Sign Language (ASL) recruit Broca’s and Wernicke’s areas for processing signed gestures, demonstrating functional reorganization of auditory regions.
  • Auditory Cortex Repurposing:
    • Studies show Heschl’s gyrus (primary auditory cortex) is activated by visual and somatosensory stimuli in congenitally deaf individuals.
    • Multimodal integration occurs, enhancing sensitivity to non-auditory inputs.
• Functional Implications:
  • Sensory substitution technologies and training reveal the brain’s capacity to adapt functionally to environmental and sensory changes, emphasizing latent plasticity.

Neural Prosthetics

• Technological Advancements:
  • Devices like TDUs and auditory-to-visual conversion tools help individuals with sensory impairments regain functionality.
• Functional Outcomes:
  • Prosthetics demonstrate the brain’s ability to adapt to novel inputs, emphasizing its potential for rehabilitation and assistive technologies.

Ethical Considerations in Brain Research

• Plasticity vs. Determinism:
  • Neuroplasticity highlights the brain’s capacity for change, countering deterministic interpretations of trauma or deprivation.
  • Findings must be framed to avoid stigmatizing individuals with atypical brain structures or conditions.
• Applications of Neuroimaging:
  • Advances in MRI and cortical thickness analysis allow for precise measurements of individual differences.
  • Ethical concerns arise in areas like neural marketing, fMRI-based lie detection, and potential misuse of brain measures in legal contexts.

Conclusion

This lecture highlights the brain’s remarkable adaptability through processes like plasticity, sensory substitution, and cognitive resilience. The discussions of atypical development emphasize how sensory deficits drive cortical reorganization, offering insights into both the challenges and possibilities for rehabilitation and neural enhancement.