Reporting in the current issue (September) of the quarterly journal Psychology and Aging, the scientists say there is less white matter in the frontal lobes of those who struggle with focusing. The differences became apparent through the use of functional magnetic resonance (fMRI) imaging of the brains of 40 individuals ranging in age from 19 to 87.
"We found that both performance and brain-activation differences of older good performers and the older poor performers are predicted by changes in brain structure, specifically by the volume of white matter connecting the right and left hemispheres of the frontal lobes," said Arthur F. Kramer, a professor of psychology.
Participants took part in a "flanker" experiment in which they viewed a line of five keyboard arrows on a computer screen and reacted by pushing one of four buttons that corresponded with the direction the center arrow was pointing. Sometimes the participants would be distracted by changes in direction by arrows not in the center.
The experiment allowed researchers to study the ability to focus on important information and inhibit inappropriate information, Kramer said. Such focusing is important when driving a car, flying a plane or making a variety of everyday decisions.
Young people and high-functioning older adults tended to call upon tissue from the right frontal lobe -- specifically, the right middle frontal gyrus -- while some older, poorer-scoring participants also activated tissue in the left hemisphere (left middle frontal gyrus), said lead author Stan J. Colcombe, a research scientist at the Beckman Institute.
Previous research has shown similar results, followed by assumptions that other parts of the brain were activated by older people for assistance, not unlike using a cane to walk, Colcombe said. In this case, however, fMRI unveiled that the poor-performing over-60 participants were the ones using both frontal hemispheres. The older participants keeping pace with the younger group used only the right hemisphere.
Looking at the high-resolution images taken by fMRI by way of a voxel-based morphometric technique, which provides a 3D view of brain structure, the scientists examined gray and white matter. Gray matter represents neurons, or the processing units, while white matter can be thought of as the wiring that connects neurons.
No significant differences were detected in the gray matter. However, the poorer-performing older members had dramatically less white matter. Kramer and Colcombe theorize that the reduced white matter affects inhibition, the ability to turn off activation in the part of the brain not needed to complete a task.
"There is an underlying structure that supports these functions," Colcombe said. "We know that certain areas within the frontal lobes of the brain are most active in inhibitory tasks. These areas shrink with age. We are very interested in how the gray matter, the local processors, and the white matter -- the connecting inside wires -- interact."
Research in Kramer's lab conducted in 2003 showed differences in gray and white matter in parts of the brain involved in decision-making in older people. Last year, Kramer, Colcombe and colleagues documented that six months of mild exercise significantly improved brain wiring and performance.
"I think this new work fits in very well," Kramer said. "This was basic research. It didn't involve an intervention like fitness training, but we now know that the amount of white matter can predict how well a person does on a task involving inhibition control."
Other co-authors on the new paper were Kirk I. Erickson and Paige Scalf, postdoctoral researchers at the Beckman Institute.
The National Institute of Aging and the Institute for the Study of Aging funded the research.