THE first images to reveal the quality of the brain's wiring show that more aspects of intelligence are inherited than previously known. The finding hints at ways to boost intelligence.
It is clear that intelligence is at least partly genetically determined. This was supported by the discovery in 2001 that the volume of the brain's grey matter, made up of "processor" cells, is heritable and correlates with certain elements of IQ. The amount of white matter, which provides the connections between these processors, has since been shown to be heritable too (Journal of Neuroscience, vol 26, p 10235).
Now it seems that the quality of these connections, which is governed by the integrity of the protective myelin sheath that encases them, is also largely genetic, and correlates with IQ.
Paul Thompson and colleagues at the University of California, Los Angeles, scanned the brains of 23 sets of identical twins and the same number of fraternal twins, using a type of magnetic resonance imaging called HARDI. MRI scans typically show the volumes of different tissues in the brain by measuring the amount of water present. HARDI measures the amount of water that is diffusing through white matter, a measure of the integrity of myelin sheathing, and therefore the speed of nerve impulses. "It's like a picture of your mental speed," says Thompson.
By comparing brain maps of identical twins, which share the same genes, with fraternal twins, which share about half their genes, the team calculate that myelin integrity is genetically determined in many brain areas important for intelligence. This includes the corpus callosum, which integrates signals from the left and right sides of the body, and the parietal lobes, responsible for visual and spatial reasoning and logic (see above). Myelin quality in these areas was also correlated with scores on tests of abstract reasoning and overall intelligence.
Just because intelligence is strongly genetic, that doesn't mean it cannot be improved. "It's just the opposite," says Richard Haier, of the University of California, Irvine, who works with Thompson. "If it's genetic, it's biochemical, and we have all kinds of ways of influencing biochemistry."
Myelin integrity is an especially promising target for manipulation, because, unlike the volume of grey matter, it changes throughout life. That it can change may seem surprising given its heritability. One explanation is that genes drive us to interact with our environment in ways that can lead to changes in myelin integrity, says Thompson.
Identifying the genes that promote high-integrity myelin could lead to ways to enhance the genes' activity or artificially add the proteins they code for. This may in turn provide therapies for multiple sclerosis, autism and attention deficit disorder, which are associated with degraded myelin. Intelligence enhancement in people who just want help passing an exam, say, is also "within the realm of possibility", Thompson reckons.
Medical treatments are still a long way off, warns Naomi Friedman, a behavioural geneticist at the University of Colorado in Boulder: "There'll be interactions between genes and environment that are going to have to be disentangled."
References:
http://www.newscientist.com/article/mg20126993.300-highspeed-brains-are-in-the-genes.html
http://www.nature.com/neuro/journal/v4/n12/abs/nn758.html
http://www.jneurosci.org/cgi/content/abstract/29/7/2212
By George - 42061560
