In a study of 11,000 pairs of twins from four countries, we have recently shown that the heritability of IQ increases linearly from childhood (about 40 percent) to adolescence (about 55 percent) to young adulthood (about 65 percent). Why? No one knows, but my guess is that the answer involves what is called genotype-environment correlation: as children grow up, they increasingly select, modify, and even create their own experiences, partly on the basis of their genetic propensities. _Plomin
An exciting new approach to studying human cognitive function involves using diffusion tensor imaging to monitor the function of the brain's white matter connectivity. The white matter connections between gray matter brain centers contribute to overall cognition -- and they can be tracked in near real time. Studying human intelligence in this way completely bypasses so-called "stereotype threat", test design bias, and other excuses commonly given for why particular groups tend not to test well on IQ tests.
The type of MRI typically used for medical scans does not show the finer details of the brain's white matter. But with a technique called diffusion tensor imaging (DTI), which uses the scanner's magnet to track the movement of water molecules in the brain, scientists have developed ways to map out neural wiring in detail. While water moves randomly within most brain tissue, it flows along the insulated neural fibers like current through a wire.
Most DTI scans break the MRI image into tiny areas and measure the diffusion of water molecules through each one in six to 12 directions, which is sufficient for detecting thick bundles of neural fibers. But places where wiring overlaps appear as a blur. Newer variations of diffusion imaging measure diffusion in 50 to 500 directions. Computer algorithms synthesize this data into a three-dimensional picture showing the most likely paths of nerve fibers through each area, and then stitch together the information from multiple points to create a wiring map.
The strength of the diffusion signal--the extent to which it reveals a clear direction--is used to gauge how organized the fibers of the white matter are. A stronger diffusion signal may indicate more fibers or thicker myelin; scientists don't yet know. But the newer diffusion imaging methods have revealed a strong correlation between the strength of this signal--what researchers refer to as the "integrity" of the white matter--and performance on a standard IQ test. "DTI turns out to be one of the most sensitive MRI measures we have for cognitive function," says Vincent Schmithorst, a neuroscientist at Cincinnati Children's Hospital.
Thompson refers to his diffusion maps as "pictures of mental speed." Previous research has repeatedly linked IQ to processing speed, and other studies show that processing speed in turn is tightly linked to the quality of one's white matter. Does that mean intelligence is determined by how fast the brain works? If so, does finding the key to processing speed in the brain mean researchers have finally found the secret to intelligence?
In reality, speed is probably not the only determinant of IQ. "One of the things that is important for IQ is frontal-lobe function, which is involved in planning, decision making, and weighing evidence," Thompson says. "I wouldn't think of those skills as being entirely reliant on mental speed."
Some of the newest theories of intelligence suggest that the crucial factor may be how efficiently information moves around the brain, rather than just how quickly. In a recent study led by Martijn P. van den Heuvel, a neuroscientist at University Medical Center Utrecht, in the Netherlands, researchers defined efficiency as the number of links it takes to get from one node to another--both in specific brain areas and all over the brain. Just as a direct flight from Paris to Chicago would be considered more efficient than one with a layover in London, a direct link between two parts of the brain would be more efficient than an indirect route.
Van den Heuvel and colleagues found that people with above-normal IQs of 120 and up had the most efficient brain networks. "Our hypothesis is that IQ is about how the human brain can integrate different types of information, how easily it can get information from one brain region to another," van den Heuvel says. "These activity patterns are highly influenced by white-matter structures in the brain, how the brain is connected."
Richard Haier and his collaborators are now working on a new method of measuring information flow around the brain using magnetoencephalography, or MEG. MEG measures the magnetic fluctuations around neurons as they fire, allowing scientists to track the millisecond-scale sequence of neural signaling in the brain as people perform different tasks, such as pressing a button in response to a light. Researchers hope to figure out how the flow of these signals differs with intelligence--whether smarter people follow the same sequence but faster, for example, or whether their brains skip a few steps in a circuit. "When you add the timing of the nodes and networks," says Jung, "then we're really talking about how the brain works in real time." __TechnologyReview
Development of such objective forms of intelligence testing was crucial, in order to put the field beyond reach of the HBD deniers such as Nisbet, Flynn, Kamin, Spelke, and the whole gang of "blank slaters." Cognitive science has grown beyond that reactionary group of obstructionists. It is time to get to work before the Idiocracy grows any more intractable and unstoppable than it already is.
Labels: brain research, human biodiversity, Intelligence