Neuroscience and intelligence

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Several neurophysiological factors have been correlated with intelligence in humans, including the ratio of brain weight to body weight ^[1] and the size, shape and activity level of different parts of the brain.^[2] Specific features that may affect Intelligence Quotient (IQ) include the size and shape of the frontal lobes, the amount of blood and chemical activity in the frontal lobes, the total amount of gray matter in the brain, the overall thickness of the cortex and the glucose metabolic rate.

Studies have shown that intelligence is at least partly genetic. Some researchers have suggested that genetic diversity at certain gene loci may be related to the development of brain size in primates, though this idea is controversial.

Health also affects intelligence, especially in unborn children, young children and teenagers, because the brain does most of its developing before the age of 18. Factors such as sickness and malnutrition in childhood can severely impact intelligence. Research has also demonstrated that taller people have a higher average IQ, though the reasons for this are still unclear^{[citation needed]}.

Contents 1 Brain size 2 Neuroanatomy 2.1 Parieto-Frontal Integration Theory (P-FIT) 3 Genetics 3.1 Controversy 4 Height 5 Health 6 Neurobiology 7 See also 8 References

[edit] Brain size

When comparing different species the ratio of brain weight to body weight does present a correlation with intelligence, though the actual brain weight has little or no effect. For example, the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50. However, within the human species, modern studies using MRI have shown that brain size shows substantial and consistent correlation (r = .35 to .43 in various studies) with IQ among adults of the same sex [1] [2] Some scientists prefer to look at more qualitative variables such to relate the size of measurable regions of known function. For example relating the size of the primary visual cortex to its corresponding functions, that of visual performance.^[3][4].

The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy in some species. Because of this fact, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains. Skull size correlates with brain size, but is not necessarily indicative.

Brain size is a rudimentary indicator of the intelligence of a brain, and many other factors affect the intelligence of a brain. Higher ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Brain size in vertebrates may relate to social rather than mechanical skill. Cortical size relates directly to a pairbonding life style and among primates cerebral cortex size varies directly with the demands of living in a large complex social network.^[5]

Here is a list of some species, along with their rough average brain sizes:

• Homo erectus: 980 cm³
• Homo habilis: 750 cm³
• Homo floresiensis: 380 cm³
• Homo neanderthalensis: 1200-1750 cm³ skull capacity (10% greater than modern human average, when including juvenile cranial capacity)
• Homo sapiens: 1350-1400 cm³^{[citation needed]}

The cause for this increase in brain size is disputed. While it is popular to associate the increase in brain size with tools usage, this idea is fundamentally flawed and regarded as tautology: the use of tools requires a larger brain, but before the brain is improved it is impossible to use tools. Another flaw is that the early humans tool set evolved very slowly, while the brain expanded nearly 3-fold from the common ancestor of humans and chimpanzees ^[6].

Krantz suggested that the expansion was due to a gradual increase in memory requirements. He points that the only physical capability modern humans share that surpasses other land mammals is long range running. This, he postulates, was the main hunting strategy used by early humans, much as it is even today among stone-age cultures. When the climate changed and the early humans had to adapt to savannas the hunting territory increased, and a hunter that had to hunt and return to his family needed better memory^[7].

Fialkowski built upon Krantz hunting idea and came up with a very appealing thesis fondly labeled as I Run, therefore I Think. According to Fialkowski, the brain size expansion can be explained via von Neumann's idea of redundancy in computing systems. Fialkowsky postulated that the main reason for mammals to fail long distance running is excess in brain heat, which leads to the animal's collapse or death much before the human brain fails. The robustness of the human brain, according to Fialkowski, is redundancy of brain cells. Thus there is a mechanism that favors a larger brain ^[8]

The main criticism over I Run, therefore I Think was formulated by Eckhardt. He pointed out that the increase took place in the cortex, and not in brain areas that are connected with temperature control such as the thalamus. He continued to offer his own thesis - like Krantz, he claims a need for better memory was the drive behind the expansion in brain size. However, he prefers to attribute the growing need for memory to memorizing water sources in the water-scarce savanna ^[9]

A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al. 2002).

In a study of the head growth of 633 term-born children from the Avon Longitudinal Study of Parents and Children cohort, it was shown that prenatal growth and growth during infancy were associated with subsequent IQ. The study’s conclusion was that the brain volume a child achieves by the age of 1 year helps determine later intelligence. Growth in brain volume after infancy may not compensate for poorer earlier growth.^[10]

[edit] Neuroanatomy

Many different sources of information have converged on the view that the frontal lobes are critical for fluid intelligence. Patients with damage to the frontal lobe are impaired on fluid intelligence tests (Duncan et al. 1995). The volume of frontal grey (Thompson et al. 2001) and white matter (Schoenemann et al. 2005) have also been associated with general intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using Positron Emission Tomography that problem-solving tasks that correlated more highly with IQ also activate the lateral prefrontal cortex. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For an extensive review of this topic, see Gray and Thompson (2004).^[11]

In 2004, Richard Haier, professor of psychology in the Department of Pediatrics and colleagues at University of California, Irvine and the University of New Mexico used MRI to obtain structural images of the brain in 47 normal adults who also took standard IQ tests. The study demonstrated that general human intelligence appears to be correlated with the volume and location of gray matter tissue in the brain. Although the regional distribution of gray matter in humans may have a genetic basis, structural changes can also occur in response to environmental stimulation. The study also demonstrated that, of the brain's gray matter, only about 6 percent appeared to be related to IQ.^[12]

A study involving 307 children (age between six to nineteen) measuring the size of brain structures using magnetic resonance imaging (MRI) and measuring verbal and non-verbal abilities has been conducted (Shaw et al. 2006). The study has indicated that there is a relationship between IQ and the structure of the cortex—the characteristic change being the group with the superior IQ scores starts with thinner cortex in the early age then becomes thicker than average by the late teens.^[13]

[edit] Parieto-Frontal Integration Theory (P-FIT)

In 2007, Behavioral and Brain Sciences published a target article that put forth a biological model of intelligence based on 37 peer-reviewed neuroimaging studies (Jung & Haier, 2007). Their review of a wealth of data from functional imaging (functional magnetic resonance imaging and Positron Emission Tomography) and structural imaging (Diffusion MRI, voxel-based morphometry, in vivo magnetic resonance spectroscopy) argues that that human intelligence arises from a distributed and integrated neural network comprised of brain regions in the frontal and parietal lobes. ^[14]

[edit] Genetics

A 2006 University of Chicago study showed that the amount of genetic diversity at two separate gene loci (microcephalin and ASPM) indicated a signature of positive selection during the primate lineage leading to humans. Both of these genes are expressed in developing brain and are thought to regulate brain size. Loss-of-function mutations at both of these genes cause human primary microcephaly ("small brain"), a hereditary disorder causing reduced size of the cerebral cortex. The University of Chicago group caused a minor media stir when they published further papers, speculating that a common allele found at each of these two genes indicated that the evolution at these loci continued to occur into the recent past, with these two alleles being under strong positive selection. They further speculated that the two positively selected alleles were associated with distinct anthropological milestones such as the beginning of cave-painting (~37,000 years before present) and city states (~10,000 years before present). Despite these speculations, the authors clearly indicated that any specific functional consequence caused by the positively selected alleles are yet to be identified. ["Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans", SCIENCE www.sciencemag.org, 9 September 2005, Vol 309]. Nonetheless, a greater part of the controversy stemmed from the fear that genetic differentiation among human populations might indeed result in some level of cognitive differentiation among the populations, and that such a possibility will play into certain racist views. (See Links Between Brain Genes, Evolution, and Cognition Challenged, Science 314:1872)

More recent studies, including one from the University of Chicago group, failed to uncover an influence of these two alleles on brain size or intelligence.^[15][16]

[edit] Controversy

The University of Chicago group suggested that these mutations coincided with upper paleolithic technology and the development of city states. They also suggested that these mutations conferred a survival or reproductive benefit, perhaps a cognitive one and had been strongly selected to sweep through certain regions of the world.

Websites promoting white racialism quickly seized on the evolutionary findings.^{[neutrality disputed]} One magazine called the discovery "the moment the antiracists and egalitarians have dreaded". The National Review Online, wrote that the research implied that "our cherished national dream of a well-mixed and harmonious meritocracy may be unattainable."

The study began to attract considerable controversy. Many scientists criticized the authors for overinterpreting and sensationalizing their findings. The senior author would later contend that he made scientifically justified speculations, and it is the media and people who wish for or against certain outcomes of such studies due to their political convictions that exaggerated the findings of the studies. He argued that the interpretation of scientific data should not be influenced by political or moral agenda^[17][18]

[edit] Height

Epidemiological studies have shown that intelligence is positively correlated with body height in human populations. Similar associations have been found in early and late childhood and adulthood in both developed and developing countries, and associations persisted after controlling for social class and parental education. The reasons for this association between height and intelligence remain unclear, but possible explanations include that height may be a biomarker of nutritional status or general mental and physical health during development, that common genetic factors may influence both height and intelligence, or that both height and intelligence may be affected by adverse early environmental exposures. Alternatively, it may be explained by differences in brain size, which has a positive relationship with height. A large recent twin pair study of the height-intelligence relationship showed that both shared environment (59%) and shared genetics (35%) are responsible for significant portions of the observed correlation between intelligence and height.

[edit] Health

Several environmental factors related to health can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.^[19]

Improvements in nutrition, and in public policy in general, have been implicated in worldwide secular IQ increases (the Flynn effect).

[edit] Neurobiology

Other neurological parameters have been associated with IQ. Haier et al. (1995) found a correlation of -0.58 between glucose metabolic rate "GMR" (an indicator of energy use) and IQ. This suggested that intelligence is associated with more efficient brains. Others found a positive correlation between IQ and GMR (DeLeon et al. 1983; Chase et al. 1984). It seems like difference in results comes from different cognitive tasks (complicated vs. simple) that were performed by examinees (Fidelman, 1993).

[edit] See also

• Heritability of IQ
• Health and intelligence
• Intelligence quotient
• Race and intelligence
• Sex and intelligence

[edit] References

• Washington State University
• Neuroscience for Kids
• Haier, R. J., Chueh, D., Touchette, P., Lott, I., Buchsbaum, M., Macmillan, D., et al. (1995). "Brain size and cerebral glucose metabolic rate in nonspecific mental retardation and Down syndrome". Intelligence 20: 191–210. doi:10.1016/0160-2896(95)90032-2. 
• Michael A. McDaniel, Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence, Intelligence, Volume 33, Issue 4, July-August 2005, Pages 337-346. PDF
• Scans Show Different Growth for Intelligent Brains, from NY Times.

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