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See Inside April/May/June 2009

Finding Connections: How Do the Parts of the Brain Interact?

A novel brain-imaging technique uncovers the structural connections underlying personality, behavior and disease

Neuroscience has long focused on the brain in terms of components: the visual cortex processes what we see, Broca’s area is the center for language, and so on. As our understanding of the brain has improved, however, it has become clear that a more accurate model depends on how these modules are wired together in circuits. A technique called diffusion tensor imaging (DTI) gives us a tool to probe the nature of those connections. A recent study suggests, for instance, that the more a person seeks out new experiences and relies on social approval, the stronger his or her wiring is among brain areas involved in reward, emotion and decision making.

Cognitive neuroscientist Michael Cohen and his colleagues at the University of Bonn in Germany asked 20 adults how often they sought out new experiences and relied on social approval. Then they used DTI to look at the subjects’ white matter, which ­connects disparate regions of the brain. Cognition and high-level processing happen in gray matter, found mostly in the outer layer of the brain and made up of the main cell bodies of neurons. White matter, on the other hand, is made up of the long, spindly “arms” of neurons, called axons, along which electrical signals travel. (This interior part of the brain looks white because the axons are sheathed in myelin, a white insulating protein that helps signals travel more quickly.)

Cohen’s team found that the more the subjects sought new experiences, the stronger their connections were from the hippocampus and amygdala, brain regions involved in decision making and emotion, to the ventral and mesial striatum, areas that process information related to emotion and reward. The scientists also found that the subjects who were most dependent on social approval had stronger than normal connections between the striatum and the prefrontal cortex, a brain area involved in higher-order decision making.

But what exactly do these connec­tions of varying strengths mean? DTI, which maps white matter tracts by measuring water flow along them, is not yet easy to interpret: no one knows how exactly the strength or abundance of white matter connections correlates to the quality of neuronal communi­ca­tion. But studies using the technique have already uncovered white matter’s important role in health. Malfunctioning or damaged white matter can lead to multiple sclerosis, Alzheimer’s disease and epilepsy, and a study published last year suggested that pedophiles have less white matter connecting the brain regions involved in sexual arousal.

“Never before has it been possible to link cognition and behavior to the brain’s intrinsic wiring,” says Cohen, who now splits his time between the University of Amsterdam and the University of ­Arizona. “A better under­standing of the brain’s commun­ication network will lead to a better under­standing of how the brain sup­ports cognitive, emotional and social func­tions and, perhaps more impor­tant, why disconnections between parts of the brain might contribute to patho­logies such as schizophrenia, autism and drug abuse.”

Note: This article was originally printed with the title, "Finding Connections".

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