Neuroscience and intelligence refers to the various neurological factors that are partly responsible for the variation of intelligence within species or between different species. A large amount of research in this area has been focused on the neural basis of human intelligence. 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), functional MRI (fMRI), electroencephalography (EEG), positron emission tomography 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 primates, cetaceans, and rodents.
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 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".
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