Diffusion MRI, meanwhile, relies on the understanding that water moving around inside a structure can reveal the structure's shape. For example, monitoring the water inside axons—those long, slender projections that carry nerve impulses but are too tiny to be detected with normal MRI—reveals each axon's position. Similarly, water leaking in unexpected places likely indicates defects in the axon's myelin coating, thought to be a factor with multiple sclerosis and other conditions. An even more advanced technique developed by Helpern and researcher Jens Jensen at New York University School of Medicine, diffusional kurtosis imaging, uses mathematical formulas to yield more detailed information about the diffusing water. In preliminary findings not yet published, DKI scans correctly identified people with mild cognitive impairment, a condition that increases the odds of later Alzheimer's. "We're looking for changes in the brain tissue microarchitecture before the brain shows evidence of shrinkage, akin to an engineer seeking out fissures inside a building's wall well before the wall begins to collapse," Helpern says. One day this may produce the holy grail of brain research, a way to identify and help people at risk for further impairment before the damage is done. Among brain researchers' other areas of exploration:
How emotion affects decision-making. Philosophers have long been stymied by the conflicting responses people give when faced with two seemingly similar moral dilemmas. Told that a runaway train is heading for five people on the track, most people find it acceptable to throw a switch and divert the train toward a single man in harm's way, yet they rebel at the idea of pushing the man off a footbridge onto the track to save the same five people. Using fMRI to image brain areas involved in decision-making, Joshua Greene, assistant professor of psychology at Harvard, has begun untangling the mystery. The prospect of actively shoving a man to his death seems to trigger an emotion-related part of the prefrontal cortex, the "ventromedial" section, while pondering the more neutral action of throwing a switch lights up areas involved with rational thought in the "dorsolateral" section nearby. Once the emotional system is engaged, choices become less reasoned. Subsequent studies of people with ventromedial damage have found that, as would be expected, they don't have the same difficulty pushing the man off the bridge.
"It's good that we have strong emotional reaction to committing an act of violence. But where that violence would actually save lives, this automatic response may not be the best," Greene says. Reworking the footbridge scenario to remove some of the emotion—say, by creating a trapdoor activated by the press of a button—appears to quiet that part of the brain and make the act more tolerable. This is no esoteric exercise: Understanding this response could help people overcome their gut-level opposition to such emotionally charged practices as physician-assisted suicide or organ donation, Greene says.
Differences in the ADHD brain. "We know that during the teenage years the brain undergoes significant structural development and becomes much more architecturally complex," says Helpern. Using DKI to scan the prefrontal cortex of a dozen teens with attention deficit hyperactivity disorder and an equal number without, Helpern's team has just revealed strikingly different developmental progress. In the prefrontal cortex, both the white matter (where the myelin-sheathed axons aiding cell communication are located) and gray matter (home of the neuron cell bodies) developed less extensively in kids with ADHD. A difference in white matter has previously been a suspect in the decision-making challenge that is a hallmark of ADHD, but Helpern thinks this finding, published in the January issue of the Journal of Magnetic Resonance Imaging, is one of the first objective measures showing that the microstructure of the gray matter is different, too. The payoff of understanding the ADHD brain should eventually be more effective therapies.