Monday, November 01, 2010

Intelligence is mostly a matter of white matter

In 2001, Thompson showed that intelligence is correlated with volume in the frontal cortex, a result consistent with a number of studies that have linked intelligence to overall brain size.
But size is a crude measure: while larger brains may be smarter on average, it’s not clear if that’s because they have more nerve cells, more connections between cells, or more of the fibers that carry neural signals.
Studies of Albert Einstein’s brain, for example, have found that it was typical in size, or even a bit on the small side.
But what if the key to intelligence is neither an individual area of the brain nor its total volume but the network over which information is transmitted and integrated?
The neural signals travel from nodes near the back of the brain, where sensory data is collected and synthesized, to those in the frontal lobes, which are responsible for decision making and planning. The connections between these nodes are just as critical as the nodes themselves. “If the nodes of a network
aren’t communicating effectively and efficiently, then the network won’t function efficiently,”.
By volume, gray matter makes up roughly half the human brain.
The other half is white matter, consisting of fi lament-like neural projections wrapped in a fatty material called myelin; such a high proportion of white matter appears to be unique to humans.
As we evolved from worms to humans, the number of non-neural cells in the brain increased 50 times more than the number of neurons.
What gives us our cognitive capacity is not actually the number of neurons, which can vary tremendously between human individuals, but rather the quality of our connections.
Thanks to their layer of insulation, which prevents leakage of electrical impulses, myelinated nerve fibers can send signals about 100 times as fast as unmyelinated ones.
The myelin also allows more information to be sent per second by reducing the waiting time between signals. The result is that neurons can process 3,000 times as much information as would otherwise be possible. That
capacity is crucial for speaking and processing language.
Is intelligence 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?
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.
The crucial factor may be how efficiently information moves around the brain, rather than just how quickly.
People with above-normal IQs of 120 and up have the most efficient brain networks.
If white matter plays a key role in intelligence, is there a way to enhance it?
It’s likely that the quality of white matter is at least partly genetically determined and, therefore, difficult to change.
But environmental factors also play a role. Rodents raised in a stimulating environment have more white matter. And research suggests that the apparent IQ difference between people who were breast-fed and bottle-fed as babies may arise because breast milk contains omega-3s, fatty acids involved in the production of myelin; as a result, some baby formula now includes these compounds.
Scientists haven’t yet studied white matter enough to know how to improve it directly, especially in healthy people. But exercise, diet, and mental activity have all been shown to boost brain health and decrease the risk of dementia, a disorder that has been linked to white-matter damage.
Learning more about the role of white matter in intelligence will give scientists a fuller picture of how brain anatomy influences cognition.
This could be a boon for physicians working with Alzheimer’s patients or others suffering from diseases that cause cognitive damage.
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