The Neural Correlates of Human Intelligence

Humans are the most intelligent organisms to have ever lived on our planet. They have extraordinary abilities that almost no other animals possess: consciousness, self-awareness, logic, reasoning, comprehension, problem solving, and language to name a few. These abilities have allowed humans to attain remarkable evolutionary success; this success can be attributed to the development of the central nervous system. Our complex and well-developed brains set us apart from every other species on earth. However, even within our own species there are profound differences in intelligence. Both genetics and environmental factors play a role in this variability within the human species. In addition, there are distinct neuroanatomical features that can be correlated with increased or decreased intelligence. The analysis of these features is possible, in an attempt to uncover the relationship between neuroanatomy and intelligence.

There are several anatomical variables that are associated with increased scores on psychometric tests, which cover various cognitive domains like reasoning, processing speed, executive function, and memory [1]. These include total brain volume, size and shape of the frontal lobes, amount of grey and white matter, and the thickness of the cerebral cortex. The concept that bigger brains are associated with intelligence is an old idea that dates back thousands of years. There is, in fact, some truth to this. One meta-analysis found that across 1530 subjects, there is a significant correlation between total brain volume and intelligence [2]. This relationship is stronger in females compared to males, and in adults compared to children. However, these correlates could be attributed to the fact that more intelligent people may have selectively enlarged brain regions, which are relevant for specific cognitive functions. For example, certain tasks require certain areas of the brain, so measuring whole brain volume isn’t specific enough to correlate with intelligence. For this reason, it is important to look at the size of specific subregions in the brain.

 Intelligence does not reside in a single brain region. The frontal lobe, which is often associated with executive function, personality, decision-making, and abstract thinking, is commonly associated with intelligence. Although the frontal lobe is important, it is not the only area involved. A network of brain regions including the dorsolateral prefrontal cortex, the parietal lobe, the anterior cingulate cortex, and specific regions within the temporal and occipital lobes relate to individual differences in intelligence [1]. The “small-world network” model implies that intelligence requires the undisrupted movement of information among brain regions along white matter tracts. White matter tracts are collections of myelinated nerve fibers within the brain that connect regions of the brain, and are capable of fast neurotransmission. White matter integrity has been shown to be related to intelligence [1], and more white matter lesions are conducive of lower cognitive ability.

Volume and density of grey and white matter within the brain is also correlated with intelligence [3]. Quantifying grey matter gives us an estimate of the density and number of neuronal bodies and dendritic expansions in a certain area of the brain. Measuring white matter can approximate the number of axons and their degree of myelination. Due to the properties of grey and white matter, it is hypothesized that grey matter is responsible for “information processing”, while white matter allows for fast and efficient inter-neuronal communication. Increased grey and white matter volumes are correlated with higher intelligence and higher cognitive performance. Specifically, grey matter volume within the anterior cingulate cortex and the frontal cortex seems to be most significant [3]. However, there are some objections to the idea that more neurons indicate higher intelligence. Intelligence measures and glucose consumption are negatively correlated, which may suggest that more intelligent people actually use their neurons more efficiently [4]. So, having fewer neurons may not necessarily be associated with reduced cognitive performance.

Another interesting, but controversial, way to study intelligence in humans is to look at the brains of well-known geniuses. Perhaps the most widely known candidate in human history is Albert Einstein, the German physicist who developed the theory of relativity and is known for the photoelectric effect and mass-energy equivalence. After his death, Einstein’s brain was collected by Dr. Thomas Harvey, a pathologist at Princeton Hospital. Over the next 30 years, portions of Einstein’s brain were passed around to various scientists and pathologists in an attempt to understand why he was so smart. The first observation was that the size of Einstein’s brain was relatively normal for someone his age. However, some of the derived features of external neuroanatomy that are associated with higher cognitive abilities appeared to be present in Einstein’s brain. For example, he had an extraordinary prefrontal cortex, expanded primary somatosensory and motor cortices in the left hemisphere, and “unusually shaped” parietal lobes.  Einstein also appeared to have more glial cells in his left inferior parietal area. Glial cells provide support and nutrition to neurons in the brain, form myelin, and participate in signal transmission. Certain areas of the association cortex, which is responsible for incorporating and synthesizing information from multiple brain regions, had particularly high amounts of glia. On top of this, Einstein had more extensive connections between his cerebral hemispheres compared to both younger and older control groups. Unfortunately, the studies performed on Einstein’s brain have some major limitations. The findings were often not statistically significant and there were issues using age-matched controls. Many people have argued that Einstein’s brain was essentially not special and not any different from an average individual’s brain.

Although the relationship between abnormal neuroanatomy and intelligence is purely correlational, this information allows us to make important conclusions regarding “the neuroscience of intelligence.” Certain features of the human brain may be more conducive for higher cognitive abilities. Even though the cause of this is unknown, there are likely many genetic and environmental influences that control neuronal production in certain areas of the brain, the development of white matter tracts, and neuronal density.

References 

  1. Deary, I.J., Penke, L., Johnson, W. 2010. The neuroscience of human intelligence differences. Nature Reviews Neuroscience 11(2): 201-211.
  2. McDaniel, M.A. 2005. Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence 33: 337-346.
  3. Luders, E., Narr, K.L., Thompson, P.M., Toga, A.W. 2009. Neuroanatomical correlates of intelligence. Intelligence 37(2): 156-163.
  4. Haier, R.J., Siegel, B.V., Nuechterlein, K.H., Hazlett, E., Wu, J.C., Peak, J. 1988. Cortical glucose metabolic rate correlates of abstract reasoning and attention studied with Positron Emission Tomography. Intelligence 11: 199-218.

Marshal is a fourth year student at the University of Calgary majoring in Neuroscience. He is currently working on an honours thesis, studying the hypoxic effects that follow the induction of a seizure in a rat model of epilepsy. 

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