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== Neural bases of intelligence ==
The varying techniques of imaging-based testing search for different signs of intelligence. The types of intelligence analyzed in this review were [[fluid intelligence]] (Gf), [[general intelligence]] (g), and [[crystallized intelligence]] (Gc). Early studies utilized information from patients with brain damage, noticing changes in intelligence scores that correlated to certain regions of the brain. As imaging technology has improved, so has the ability for deeper neuro-analysis. MRI studies have found that the volume of gray matter correlates to intelligence, providing evidence for generalizations made regarding brain/head-size and intelligence. Additionally, PET and fMRI studies have revealed more information regarding the functionality of certain regions of the brain. By recording and interpreting the brain activity of subjects as they complete a variety of tasks, researchers are able to draw connections between the types of task (and thus, the type of intelligence) that calls on particular areas of the brain.
In general, there have been two types of intelligence studies: psychometric and biological. Biological approaches make use of neuroimaging techniques and examine brain function. Psychometrics focuses on mental abilities. Ian Deary and associates suggest that a greater overlap of these techniques will reveal new findings.<ref name=deary1>{{cite journal|last=Deary|first=Ian J.|author2=Austin, Elizabeth J. |author3=Caryl, Peter G. |title=Testing versus understanding human intelligence.|journal=Psychology, Public Policy, and Law|date=1 January 2000|volume=6|issue=1|pages=180–190|doi=10.1037/1076-8971.6.1.180}}</ref>
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A 2009 study examined intracerebral volumetric relationships in twins. Making use of high resolution MRI data, they found strong genetic connections correlations between cerebral structures. Specifically, the study suggests that a strong correlation exists between the tissue type or spatial proximity and genes. By examining the differences or lack thereof between the brain size of twin children, the researchers drew conclusions that individuals that share genes (i.e. twins) will show similar physiological brain properties compared to genetically-unrelated individuals.<ref>{{cite journal|last=Schmitt|first=JE|author2=Wallace, GL |author3=Lenroot, RK |author4=Ordaz, SE |author5=Greenstein, D |author6=Clasen, L |author7=Kendler, KS |author8=Neale, MC |author9= Giedd, JN |title=A twin study of intracerebral volumetric relationships.|journal=Behavior Genetics|date=March 2010|volume=40|issue=2|pages=114–24|pmid=20112130|doi=10.1007/s10519-010-9332-6|pmc=3403699}}</ref> This study provides evidence of the genetic influence of the brain structure and size, which are believed to both influence intelligence in some way.
Another study in 2006 examined 100 postmortem brains, seeking a relationship between an individual’s Full Scale [[Wechsler Adult Intelligence Scale]] score and the volume of their brain regions. Prior to death, the subjects had completed the WAIS test, which measures verbal and visuospatial abilities. The factors considered important to the relationship between brain size and intelligence were age, sex and hemispheric functional lateralization. They found that general verbal ability was correlated with cerebral volume in women and right-handed men. It was not possible to find a relationship between ability and volume in with every group, however.<ref>{{cite journal|last=Witelson|first=S. F.|title=Intelligence and brain size in 100 postmortem brains: sex, lateralization and age factors|journal=Brain|date=26 October 2005|volume=129|issue=2|pages=386–398|doi=10.1093/brain/awh696|pmid=16339797|doi-access=free}}</ref>
===Neural conduction speed===
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A 2012 study from [[Washington University
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The [[lateral prefrontal cortex]] is a region of interest because those who have injuries to that part of the brain often have issues with common, every day tasks such as planning their day. The LPFC is thought to be important for "cognitive control capacity," which can be used to predict future outcomes such as success in school and the workplace. It was found by van den Heuvel et al. that higher intelligence individuals employ more efficient whole-brain network organization. This had led to the thought that cognitive control capacity may be supported by these whole-brain network properties. The 2012 study used a theoretic approach to neuroimage data known as global brain connectivity (GBC) or weighted degree centrality. GBC let the researches look closely at specific regions and their range of connectivity. It was then possible to examine each region's role in human cognitive control and intelligence. The study used fMRI to acquire data and examine each region's connectivity.<ref>{{cite journal|pmid=22745498 | doi=10.1523/JNEUROSCI.0536-12.2012 | volume=32 | issue=26 | title=Global connectivity of prefrontal cortex predicts cognitive control and intelligence | pmc=3392686 |date=June 2012 | journal=J. Neurosci. | pages=8988–99| last1=Cole | first1=M. W. | last2=Yarkoni | first2=T. | last3=Repovs | first3=G. | last4=Anticevic | first4=A. | last5=Braver | first5=T. S. }}</ref>
== Ethical implications ==
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