Neuroscience and intelligence refers to the various

neurological Neurology (from , "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the nervous system, which comprises the brain, the s ...

factors that are partly responsible for the variation of

intelligence Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. It can be described as t ...

within species or between different species. A large amount of research in this area has been focused on the neural basis of

human intelligence Human intelligence is the Intellect, intellectual capability of humans, which is marked by complex Cognition, cognitive feats and high levels of motivation and self-awareness. Using their intelligence, humans are able to learning, learn, Concept ...

. Historic approaches to studying the neuroscience of intelligence consisted of correlating external head parameters, for example head circumference, to intelligence. Post-mortem measures of brain weight and brain volume have also been used. More recent methodologies focus on examining correlates of intelligence within the living brain using techniques such as magnetic resonance imaging (

MRI Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and rad ...

), functional MRI (fMRI),

electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignal, bio signals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in ...

(EEG),

positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...

and other non-invasive measures of brain structure and activity. Researchers have been able to identify correlates of intelligence within the brain and its functioning. These include overall brain volume, grey matter volume, white matter volume, white matter integrity, cortical thickness and neural efficiency. Analyses of the parameters of intellectual systems, patterns of their emergence and evolution, distinctive features, and the constants and limits of their structures and functions made it possible to measure and compare the capacity of communications (~100 m/s), to quantify the number of components in intellectual systems (~10¹¹ neurons), and to calculate the number of successful links responsible for cooperation (~10¹⁴ synapses). Although the evidence base for our understanding of the neural basis of human intelligence has increased greatly over the past 30 years, even more research is needed to fully understand it. The neural basis of intelligence has also been examined in animals such as

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

ns, and

rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

Humans

Brain volume

One of the main methods used to establish a relationship between intelligence and the brain is to use measures of brain volume. The earliest attempts at estimating brain volume were done using measures of external head parameters, such as head circumference as a proxy for brain size. More recent methodologies employed to study this relationship include post-mortem measures of brain weight and volume. These have their own limitations and strengths. The advent of MRI as a non-invasive highly-accurate measure of living brain structure and function (using fMRI) made this the pre-dominant and preferred method for measuring brain volume. Overall, larger brain size and volume is associated with better cognitive functioning and higher intelligence. The specific regions that show the most robust correlation between volume and intelligence are the frontal, temporal and parietal lobes of the brain. A large number of studies have been conducted with uniformly positive correlations, leading to the generally safe conclusion that larger brains predict greater intelligence. In healthy adults, the correlation of total brain volume and IQ is approximately 0.4 when high-quality tests are used. A large scale study (n = 29k) using the UK Biobank found a correlation of .275. The strength of this relationship did not depend on sex, contradicting some earlier studies. A study using a sibling-design in two medium sized samples found evidence of causality with an effect size of .19. This study design rules out confounders that vary between families, but not those that vary within families. Less is known about variation on scales less than total brain volume. A meta-analytic review by McDaniel found that the correlation between intelligence and in vivo brain size was larger for females (0.40) than for males (0.25). The same study also found that the correlation between brain size and Intelligence increased with age, with children showing smaller correlations. It has been suggested that the link between larger brain volumes and higher intelligence is related to variation in specific brain regions: a whole-brain measure would under-estimate these links. For functions more specific than general intelligence, regional effects may be more important. For instance evidence suggests that in adolescents learning new words, vocabulary growth is associated with gray matter density in bilateral posterior supramarginal gyri. Small studies have shown transient changes in gray-matter associated with developing a new physical skill (juggling) occipito-temporal cortex Brain volume is not a perfect account of intelligence: the relationship explains a modest amount of variance in intelligence – 12% to 36% of the variance. The amount of variance explained by brain volume may also depend on the type of intelligence measured. Up to 36% of variance in verbal intelligence can be explained by brain volume, while only approximately 10% of variance in visuospatial intelligence can be explained by brain volume. A 2015 study by researcher Stuart J. Ritchie found that brain size explained 12% of the variance in intelligence among individuals. These caveats imply that there are other major factors influencing how intelligent an individual is apart from brain size. In a large meta-analysis consisting of 88 studies Pietschnig et al. (2015) estimated the correlation between brain volume and intelligence to be about correlation coefficient of 0.24 which equates to 6% variance. Taking into account measurement quality, and sample type and IQ-range, the meta-analytic association of brain volume in appears to be ~ .4 in normal adults. Researcher Jakob Pietschnig argued that the strength of the positive association of brain volume and IQ remains robust, but has been overestimated in the literature. He has stated that "''It is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as an isomorphic proxy of human intelligence differences''".

Grey matter

Grey matter Grey matter, or gray matter in American English, is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil ( dendrites and unmyelinated axons), glial cells ( astrocytes and oligodendrocytes), synapses, ...

has been examined as a potential biological foundation for differences in intelligence. Similarly to brain volume, global grey matter volume is positively associated with intelligence. More specifically, higher intelligence has been associated with larger cortical grey matter in the prefrontal and posterior temporal cortex in adults. Furthermore, both verbal and nonverbal intelligence have been shown to be positively correlated with grey matter volume across the parietal, temporal and occipital lobes in young healthy adults, implying that intelligence is associated with a wide variety of structures within the brain. There appear to be

sex differences Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consi ...

between the relationship of grey matter to intelligence between men and women. Men appear to show more intelligence to grey matter correlations in the frontal and parietal lobes, while the strongest correlations between intelligence and grey matter in women can be found in the frontal lobes and

Broca's area Broca's area, or the Broca area (, also , ), is a region in the frontal lobe of the dominant Cerebral hemisphere, hemisphere, usually the left, of the Human brain, brain with functions linked to speech production. Language processing in the brai ...

. However, these differences do not seem to impact overall Intelligence, implying that the same cognitive ability levels can be attained in different ways. One specific methodology used to study grey matter correlates of intelligence in areas of the brain is known as

voxel-based morphometry Voxel-based morphometry is a computational approach to neuroanatomy that measures differences in local concentrations of brain tissue, through a voxel-wise comparison of multiple brain images. In traditional morphometry, volume of the whole brain ...

(VBM). VBM allows researchers to specify areas of interest with great spatial resolution, allowing the examination of grey matter areas correlated with intelligence with greater special resolution. VBM has been used to correlate grey matter positively with intelligence in the frontal, temporal, parietal, and occipital lobes in healthy adults. VBM has also been used to show that grey matter volume in the medial region of the prefrontal cortex and the dorsomedial prefrontal cortex correlate positively with intelligence in a group of 55 healthy adults. VBM has also been successfully used to establish a positive correlation between grey matter volumes in the anterior cingulate and intelligence in children aged 5 to 18 years old. Grey matter has also been shown to positively correlate with intelligence in children. Reis and colleagues have found that grey matter in the prefrontal cortex contributes most robustly to variance in Intelligence in children between 5 and 17, while subcortical grey matter is related to intelligence to a lesser extent. Frangou and colleagues examined the relationship between grey matter and intelligence in children and young adults aged between 12 and 21, and found that grey matter in the

orbitofrontal cortex The orbitofrontal cortex (OFC) is a prefrontal cortex region in the frontal lobes of the brain which is involved in the cognitive process of decision-making. In non-human primates it consists of the association cortex areas Brodmann area 11, 1 ...

, cingulate gyrus, cerebellum and thalamus was positively correlated to intelligence, while grey matter in the

caudate nucleus The caudate nucleus is one of the structures that make up the corpus striatum, which is part of the basal ganglia in the human brain. Although the caudate nucleus has long been associated with motor processes because of its relation to Parkinso ...

is negatively correlated with intelligence. However, the relationship between grey matter volume and intelligence only develops over time, as no significant positive relationship can be found between grey matter volume and intelligence in children under 11. An underlying caveat to research into the relationship of grey matter volume and intelligence is demonstrated by the hypothesis of neural efficiency. The findings that more intelligent individuals are more efficient at using their neurons might indicate that the correlation of grey matter to intelligence reflects selective elimination of unused synapses, and thus a better brain circuitry.

White matter

Similar to grey matter,

white matter White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, also called Nerve tract, tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distr ...

has been shown to correlate positively with intelligence in humans. White matter consists mainly of myelinated neuronal axons, responsible for delivering signals between neurons. The pinkish-white color of white matter is actually a result of these myelin sheaths that electrically insulate neurons that are transmitting signals to other neurons. White matter connects different regions of grey matter in the cerebrum together. These interconnections make transport more seamless and allow us to perform tasks easier. Significant correlations between intelligence and the

corpus callosum The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental ...

have been found, as larger callosal areas have been positively correlated with cognitive performance. However, there appear to be differences in importance for white matter between verbal and nonverbal intelligence, as although both verbal and nonverbal measures of intelligence correlate positively with the size of the corpus callosum, the correlation for intelligence and corpus callosum size was larger (.47) for nonverbal measures than that for verbal measures (.18). Anatomical mesh-based geometrical modelling has also shown positive correlations between the thickness of the corpus callosum and Intelligence in healthy adults. White matter integrity has also been found to be related to intelligence. White matter tract integrity is important for information processing speed, and therefore reduced white matter integrity is related to lower intelligence. The effect of white matter integrity is mediate entirely through information processing speed. These findings indicate that the brain is structurally interconnected and that axonal fibres are integrally important for fast information process, and thus general intelligence. Contradicting the findings described above, VBM failed to find a relationship between the corpus callosum and intelligence in healthy adults. This contradiction can be viewed to signify that the relationship between white matter volume and intelligence is not as robust as that of grey matter and intelligence.

Cortical thickness

Cortical thickness has also been found to correlate positively with intelligence in humans. However, the rate of growth of cortical thickness is also related to intelligence. In early childhood, cortical thickness displays a negative correlation with intelligence, while by late childhood this correlation has shifted to a positive one. More intelligent children were found to develop cortical thickness more steadily and over longer periods of time than less bright children. Studies have found cortical thickness to explain 5% in the variance of intelligence among individuals. In a study conducted to find associations between cortical thickness and general intelligence between different groups of people, sex did not play a role in intelligence. Although it is hard to pin intelligence on age based on cortical thickness due to different socioeconomic circumstances and education levels, older subjects (17 - 24) tended to have less variances in terms of intelligence than when compared to younger subjects (19 - 17).

Cortical convolution

The folding of the brain’s surface, known as cortical convolution, has become more pronounced throughout human evolution. It has been suggested that this increased folding serves as a neurological foundation for some of the brain’s most unique cognitive functions. As a result, the extent of cortical convolution may play a role in shaping individual intelligence within the human species. An analysis published in 2019 found the contours of 677 children and adolescent (mean age 12.72 years) brains had a genetic correlation of almost 1 between IQ and surface area of the supramarginal gyrus on the left side of the brain.

Neural efficiency

The neural efficiency hypothesis postulates that more intelligent individuals display less activation in the brain during cognitive tasks, as measured by Glucose metabolism. A small sample of participants (N=8) displayed negative correlations between intelligence and absolute regional metabolic rates ranging from -0.48 to -0.84, as measured by PET scans, indicating that brighter individuals were more effective processors of information, as they use less energy. According to an extensive review by Neubauer & FinkNeubauer, A. C., & Fink, A. (2009). Intelligence and neural efficiency. ''Neuroscience & Biobehavioral Review''s, 33(7), 1004-1023. a large number of studies (N=27) have confirmed this finding using methods such as PET scans, EEG and fMRI. fMRI and EEG studies have revealed that task difficulty is an important factor affecting neural efficiency. More intelligent individuals display neural efficiency only when faced with tasks of subjectively easy to moderate difficulty, while no neural efficiency can be found during difficult tasks.Neubauer, A.C., Sange, G., Pfurtscheller, G., 1999. Psychometric intelligence and event-related desynchronisation during performance of a letter matching task. In: Pfurtscheller, G., Lopes da Silva, F.H. (Eds.), Event-Related Desynchronization (ERD) and Related Oscillatory EEG-Phenomena of the Awake Brain. Elsevier, Amsterdam, pp. 219–231. In fact, more able individuals appear to invest more cortical resources in tasks of high difficulty. This appears to be especially true for the Prefrontal Cortex, as individuals with higher intelligence displayed increased activation of this area during difficult tasks compared to individuals with lower intelligence.Rypma, B., Berger, J. S., Prabhakaran, V., Bly, B. M., Kimberg, D. Y., & Biswal, B. B. (2006). Neural correlates of cognitive efficiency. NeuroImage, 33(3), 969 –979. It has been proposed that the main reason for the neural efficiency phenomenon could be that individuals with high intelligence are better at blocking out interfering information than individuals with low intelligence.

Further research

Some scientists prefer to look at more qualitative variables to relate to the size of measurable regions of known function, for example relating the size of the primary

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalam ...

to its corresponding functions, that of visual performance. 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. There is an association between IQ and

myopia Myopia, also known as near-sightedness and short-sightedness, is an eye condition where light from distant objects focuses in front of, instead of on, the retina. As a result, distant objects appear blurry, while close objects appear normal. ...

. One suggested explanation is that one or several

pleiotropic Pleiotropy () is a condition in which a single gene or genetic variant influences multiple phenotypic traits. A gene that has such multiple effects is referred to as a ''pleiotropic gene''. Mutations in pleiotropic genes can impact several trait ...

gene(s) affect the size of the

neocortex The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, ...

part of the brain and eyes simultaneously.

Parieto-frontal integration theory

In 2007,

Behavioral and Brain Sciences ''Behavioral and Brain Sciences'' is a bimonthly peer-reviewed scientific journal of Open Peer Commentary established in 1978 by Stevan Harnad and published by Cambridge University Press. According to the ''Journal Citation Reports'', the journal ...

published a target article that put forth a biological model of intelligence based on 37 peer-reviewed neuroimaging studies (

Jung Carl Gustav Jung ( ; ; 26 July 1875 – 6 June 1961) was a Swiss psychiatrist, psychotherapist, and psychologist who founded the school of analytical psychology. A prolific author of over 20 books, illustrator, and correspondent, Jung was a c ...

Haier Haier Group Corporation () is a Chinese multinational home appliances and consumer electronics company headquartered in Qingdao, Shandong. Its Haier Smart Home Company affiliate, of which it owns 35%, designs, develops, manufactures and se ...

, 2007). Their review of a wealth of data from functional imaging (

functional magnetic resonance imaging Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...

and

) and structural imaging (

diffusion MRI Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast (vision), contrast ...

, in vivo magnetic resonance spectroscopy) argues that human intelligence arises from a distributed and integrated neural network comprising brain regions in the frontal and parietal lobes. A recent lesion mapping study conducted by Barbey and colleagues provides evidence to support the P-FIT theory of intelligence. Brain injuries at an early age isolated to one side of the brain typically results in relatively spared intellectual function and with IQ in the normal range.

Primates

Brain size

Another theory of

brain size The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution. Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by wei ...

in vertebrates is that it may relate to social rather than mechanical skills. Cortical size relates directly to pair-bonding lifestyle and among primates, cerebral cortex size varies directly with the demands of living in a large complex social network. Compared to other mammals, primates have significantly larger brain sizes. Additionally, most primates are found to be

polygynandrous Polygynandry is a mating system in which both males and females have multiple mating partners during a breeding season. In sexually reproducing diploid animals, different mating strategies are employed by males and females, because the cost of gam ...

, having many social relationships with others. Although inconclusive, some studies have shown that this polygynandrous statue correlates to brain size. Intelligence in chimpanzees has been found to be related to brain size, grey matter volume, and cortical thickness, as in humans.

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 The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...

is less effective. Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring

fortification A fortification (also called a fort, fortress, fastness, or stronghold) is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from Lati ...

of certain food products and laws establishing safe levels of pollutants (e.g.

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.

References

External links

Neuroscience for Kids
* * {{DEFAULTSORT:Neuroscience And Intelligence Cognitive neuroscience Factors related to intelligence Neuropsychology