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is backed up by studies showing that the integrityand density of white matter in a person’s brain is a reliable indicator of intelligence. White matter is the other, often overlooked, kind of tissue in the brain. Gray matter gets all the attention, but 50 percent of the brain is white matter and it’s also very important. It probably gets less publicity because it doesn’t “do” as much. Gray matter is where all the important activity is generated, white matter is made up of bundles and bands of the parts that send the activity to other locations (the axons, the long bit of a typical neuron). If gray matter were the factories, white matter would be the roads needed for delivery and resupply.
    The better the white-matter connections between two brain regions, the less energy and effort is required to coordinate them and the tasks they’re responsible for, and they’re harder to find with a scanner. It’s like looking for a needle in a haystack, only instead of a haystack it’s a massive pile of slightly bigger needles, and the whole thing is in a washing machine.
    Further scanning studies suggest that the thickness of the corpus callosum is also associated with levels of general intelligence. The corpus callosum is the “bridge” between the left and right hemispheres. It’s a big tract of white matter, and the thicker it is the more connections there are between the two hemispheres, enhancing communication. If there’s a memory stored on one side that needs to be utilized by the prefrontal cortex on the other, a thicker corpus callosum makes this easier and faster. The efficiency and effectiveness of how these regions are connected seems to have a big impact on how well someone can apply their intellect to tasks and problems. As a result of this, brains that are structurally quite different (the size of certain areas, how they’re arranged in the cortex, and so on)can display similar levels of intelligence, like two game consoles made by different companies that are similarly powerful.
    Now we know efficiency is more important than power. How does that help us go about making ourselves more intelligent? Education and learning is an obvious answer. Actively exposing yourself to more facts, information and concepts means every one you remember will actively increase your crystallized intelligence, and regularly applying your fluid intelligence to as many scenarios as possible will improve matters there. This isn’t a cop-out; learning new things and practicing new skills can bring about structural changes in the brain. The brain is a plastic organ; it can and will physically adapt to the demands made of it. We met this in Chapter 2 : neurons form new synapses when they have to encode a new memory, and this sort of process is found throughout the brain.
    For example, the motor cortex, in the parietal lobe, is responsible for planning and control of voluntary movements. Different parts of the motor cortex control different parts of the body, and how much of the motor cortex is dedicated to a body part depends on how much control it needs. Not much of the motor cortex is dedicated to the torso, because you can’t do much with it. It’s important for breathing and giving your arms somewhere to connect to, but movement-wise we can turn it or bend it slightly, and that’s about it. But much of the motor cortex is dedicated to the face and hands, which require a lot of fine control. And that’s just for a typical person; studies have revealed that classically trained musicians such as violinists or pianists often have relatively huge areas of the motor cortex dedicated to fine control of the hands and fingers. 9 These people spend all their lives performingincreasingly complex and intricate movements with their hands (usually at high speeds), so the brain has adapted to support this behavior.
    Similarly, the hippocampus is needed for spatial memory (memory