A new type of brain scanner has revealed that inherited genes have a much greater effect on intelligence than was previously thought. The scanner measures how well nerve fibres are encased in protective and insulating fatty myelin, a good covering of which results in faster nerve impulses. The research team from the University of California, Los Angeles (UCLA), published their results in The Journal of Neuroscience.
Previous studies have found that the volume of the brain's 'processor' grey matter cells, and the amount of white matter cells that provide connections between them, are both heritable traits linked to certain aspects of IQ (intelligence quotient). This new study implies that the quality of the white matter, as dictated by the integrity of the protective myelin sheath that encases it, also correlates with IQ and is largely genetically determined.
The UCLA team created the first images revealing the quality of the brain's wiring using a new type of magnetic resonance imaging (MRI) called High Angular Resolution Diffusion Imaging (HARDI). MRI scans tend to measure the volume of different areas of brain tissue by looking at the amount of water present. In contrast, HARDI scans can measure the integrity of the myelin sheathing, and therefore the speed of nerve impulses, by looking at the amount of water that is actually diffusing through the white matter. 'It's like a picture of your mental speed', chief researcher Professor Paul Thompson told New Scientist magazine.
Thompson's team scanned the brains of 23 sets of identical twins (who share all their genes), and the same number of fraternal twins (who only share about half their genes), then compared the resulting brain maps from the two groups. It was found that myelin integrity is genetically determined in many areas of the brain that are important for intelligence, including the parietal lobes (that are responsible for logic, and visual and spatial reasoning) and the corpus callosum (which integrates signals from the left and right sides of the body). The team calculated that better myelin quality in these areas correlated with better scores on tests of overall intelligence and abstract reasoning.
The level of myelin integrity, unlike the volume of grey matter, changes throughout life making it a highly promising target for manipulation. If the genes that promote high-integrity myelin can be identified this could lead to ways to enhance the activity of the genes or artificially add the proteins they produce. Although medical treatments are still a long way off this could eventually help provide therapies for a number of conditions associated with degraded myelin such as multiple sclerosis, attention deficit disorder and autism.
Thompson believes that intelligence enhancement in healthy individuals who, for example, just need help to pass an exam is also 'within the realm of possibility'.