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If you are an aging baby boomer and you've noticed it's a bit harder to drive to unfamiliar locations or to pick a new brand of olive oil at the supermarket, you can blame it on the white matter in your brain.

People's ability to make decisions in novel situations decreases with age and is associated with a reduction in the integrity of two specific white-matter pathways that connect an area in the cerebral cortex called the medial prefrontal cortex with two other areas deeper in the brain, a new study, published in the Apr. 11 issue of The Journal of Neuroscience, found.

"The evidence that this decline in decision-making is associated with white-matter integrity suggests that there may be effective ways to intervene," said Gregory R. Samanez-Larkin, the post-doctoral fellow in Vanderbilt's psychology department and Institute of Imaging Science. "Several studies have shown that white-matter connections can be strengthened by specific forms of cognitive training."

In the past, most brain-imaging research has concentrated on the grey matter. Grey matter is the part of the brain that contains the bodies of the neurons while white matter contains the cable-like axons that carry signals from one part of the brain to another. 

Recently, neuroscientists have begun looking more closely at white matter. It has been linked to the brain's processing speed and attention span, among other things. Samanez-Larkin's research is the first study to link white matter to learning and decision making.

The study involved 25 adults ranging from 21 to 85 years of age. They were asked to perform a monetary learning task. The task was designed to elicit what psychologists call probabilistic reward learning. 

"The probabilistic reward learning is a common type of decision making that we use every day," said the author. "Whenever we try to choose the best alternative based on previous experience and are uncertain of the outcome, we are relying on probabilistic reward learning."

The findings suggest that the integrity of frontostriatal white matter pathways critically supports reward learning. The findings also raise the possibility that interventions that bolster frontostriatal integrity might improve reward learning and decision making.

"The protocols for DTI have improved substantially," said Samanez-Larkin, "In future studies we'd really like to combine fMRI and DTI to better characterize age differences in these neural circuits and examine how training might improve both structure and function."