Conventional wisdom suggests that part of the reason it can be difficult to make decisions is because of the number of choices we often have. But a recent study suggests that decision-making ability becomes measurably more difficult as our brains get older.
According to the Vanderbilt University study, aging affects the microstructural integrity of our brain's white matter, the central nervous system material that modulates the distribution of action potentials-electrical impulses that flow along a kind of subway system that, in this case, facilitates decisiveness between different brain regions.
Grey matter has been the cerebral celebrity in neuroscience in the last 20 years or so, gaining most of the attention because it's the part of the brain that houses neurons. These core components of the nervous system are associated with cognition, a concept that readily supports popular discourse about behavior and motivation.
White matter, on the other hand, was once so overlooked that it was thought to be passive tissue. It consists mainly of glial cells and myelinated axons and is now known to be essential to brain function. It has been linked to an individual's attention span and the brain's processing speed, and this is the first study to link white matter with decision making.
Researchers found two specific white-matter pathways associated with decision making that demonstrated a reduction in integrity in the study's older subjects when compared with younger subjects.
One pathway connects an area in the cerebral cortex called the medial prefrontal cortex, which is widely credited with decision making, with the thalamus, an area of the brain known for relaying signals and regulating consciousness, sleep, and alertness. The other pathway connects the medial prefrontal cortex with the ventral striatum, a reward center associated with emotional and motivational aspects of behavior.
Subjects in the study ranged in age from 21 to 85 years old. Each was given a task designed to elicit probabilistic reward learning, the name of a common type of decision making we use whenever trying to choose the best alternative based on prior experience while being uncertain of the outcome.
Participants' brains were scanned using an MRI technique called diffusion tensor imaging, or DTI. The process maps the water trapped by myelin sheaths that support axons and creates a signal that transmits the density, diameter, and amount of myelination, the measures of which determine the integrity.
Functional MRI has been the major brain-scanning method used in neuroscience, coinciding with the focus on grey matter since fMRI measures variation in oxygen consumption in grey matter.
The study's lead author, Gregory R. Samanez-Larkin, acknowledged in the paper that the sample size of 25 participants was small. He suggested that future research might determine whether changes in white matter precede changes in learning or vice versa. "These findings build upon and extend existing documented associations between age differences in white matter and a wide range of cognitive variables including processing speed, task-switching, top-down attention, and implicit sequence learning," he wrote (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3744863/).
Samanez-Larkin told interviewers that the demonstrated decline in decision making suggests that there may be effective ways to intervene. He said cognitive training has been shown to strengthen white-matter connections.
In one such study, researchers at the National Institutes of Health recently found that diffusion patterns within white matter could be structurally altered with reasoning training that included juggling, meditation, and working memory (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3424489/).
Written by Will Hector