Human intelligence is a complex cognitive trait that has fascinated scientists and philosophers for centuries.
New research is shedding light on the neurobiological underpinnings of intelligence, from genes to cells to brain networks.
Key Facts:
- Intelligence is one of the most heritable human behavioral traits, with genetics explaining 50-80% of variability between individuals.
- Over 1000 genes likely contribute to intelligence, predominantly affecting brain development and function.
- Brain imaging shows intelligence relies on a distributed network of brain regions, not a single area.
- Larger, faster pyramidal neurons in the cortex relate to higher IQ scores and more efficient information processing.
- The structure and function of neurons integrates genetics, brain development, and information processing underlying intelligence.
Source: Frontiers in Human Neuroscience
The Genetic Basis of Intelligence
Twin studies show that genetics plays a major role in intelligence, explaining the majority of differences between people.
Researchers have sought to identify specific genes involved through genome-wide association studies (GWAS).
While early GWAS were not successful, larger studies with over 100,000 participants have now uncovered over 1000 genes associated with intelligence.
Most of these genes are non-coding and likely regulate gene expression, particularly in the brain during development.
The genes are involved in neurogenesis, synapse formation, and neuron differentiation.
A few coding genes affect synaptic function and plasticity directly.
Overall, intelligence depends on many genes working together to shape brain development and activity.
While genetics plays a large role, environment and experience still modify gene expression through epigenetics.
The heritability of intelligence increases over the lifespan, suggesting amplified genetic effects as cognition develops.
Ultimately, nature and nurture interact to produce individual differences in intelligence.
Brain Imaging Links Intelligence to Distributed Brain Regions
In addition to genetic approaches, brain imaging methods like MRI have identified anatomical correlates of intelligence.
Studies find positive correlations between IQ and gray matter volume or thickness in many higher-order association areas.
These include frontal, parietal, and temporal cortical regions that integrate information.
White matter connecting these areas also relates to intelligence.
Functional imaging shows similar patterns, with more efficient neural activity in these regions during reasoning and problem solving relating to fluid intelligence.
Overall, general intelligence maps to a distributed network of cortical areas rather than a single specialized region.
Variability in the structure and function of this network contributes to individual differences in IQ.
Differences in Neuron Structure and Function Underlie Intelligence
Genetics and neuroimaging outline intelligence at the macro-scale, but what about the micro-scale of cells and circuits?
Recently, studies have linked intelligence to the structure and function of cortical pyramidal neurons.
These excitatory neurons integrate inputs and are central to information processing.
Human pyramidal cells have much larger dendrites and receive more synapses than rodents, allowing greater integration.
Recordings from temporal cortex neurons during neurosurgery find that larger dendrites relate to higher IQ scores in the same individuals.
Computational modeling showed larger neurons can sustain faster signaling important for cognition.
Accordingly, people with higher IQs maintain faster neuronal spike firing.
This evidence suggests larger, faster pyramidal cells in association cortex enable more efficient information processing underlying intelligence.
The morphology and activity of pyramidal neurons provides a cellular basis for the distributed network observed with neuroimaging.
Larger neurons occupy greater cortical volume, have enhanced signaling, and likely connect distantly to integrate information across regions.
Variability amongst cells and circuits results in individual differences in efficiency at the micro-scale that impact cognitive performance.
Development and Plasticity of the Intelligent Brain
What shapes the structure and function of cortical neurons?
Brain imaging shows gray matter changes dynamically across the lifespan.
During childhood, dendrites grow as synapses form, increasing volume.
Then pruning refines circuits, decreasing volume and density in adolescence and adulthood.
Changes are prolonged in association areas important for cognition compared to sensory regions.
Dendritic size also varies between cortical areas, with more complex pyramidal neurons in higher-order regions.
This cortical hierarchy allows increasingly specialized information processing.
Development and plasticity adjust dendrite structure and synapses to refine circuits.
Experience and learning further modify connectivity. Intelligence likely relies on optimizing neurons and circuits for information integration.
Genes Control Neuron Structure and Function
Twin studies demonstrate intelligence is highly heritable.
The same genes that affect macro-scale brain anatomy also influence micro-scale cellular morphology.
GWAS hits are enriched in pyramidal neurons, the main integrators in cortical circuits.
Specific genes like FOXO3 linked to intelligence regulate dendrite growth.
Ultimately, genetic factors establish the anatomical scaffolding and physiological potential of neurons during development.
Environmental stimulation can then maximize or diminish this potential via synaptic plasticity and epigenetic regulation.
The interplay between genes and experience sculpts neural networks tailored to an individual’s cognitive capacities over their lifetime.
Implications: Optimizing Brains for Cognitive Performance
Research across neuroscience fields converges on key principles for biological intelligence:
- Distributed networks of association cortex regions integrate information
- Pyramidal neuron structure and function optimize information processing
- Genes guide brain development and plasticity acting via neurons
- Experience and environment modify genetic potential via epigenetics
These insights may have implications for cognitive enhancement.
Non-invasive brain stimulation that targets cortical excitability and plasticity may improve network function.
Cholinesterase inhibitors used in dementia aim to boost synaptic signaling.
Nootropics influence neuron metabolism and growth factors.
Future therapies could more precisely modulate pyramidal neuron excitability, dendrite morphology, and neurogenesis.
Genetic engineering may eventually customize brain development and plasticity.
Intelligence will remain a complex trait, but unlocking its biological basis creates opportunities for optimization.
The Human Advantage: Evolution of Intelligence
The human brain is not simply a scaled-up primate brain.
It contains distinct specializations that evolved over millions of years.
Relative to body size, humans have larger associative cortices packed with more complex pyramidal neurons.
The structure and function of these cells is further tuned by genes and experience.
It is this expansion and enhancement of cortical computation, driven by evolutionary pressures, that gives humans unmatched cognitive abilities.
Our intelligence ultimately derives from micro-scale optimizations integrated widely across macro-scale brain networks.
Research across neuroscience fields is beginning to link these levels of analysis to unravel the biological foundations of intelligence that make humans unique.
References
- Study: Genes, cells, and brain areas of intelligence
- Authors: Goriounova & Mansvelder (2019)
