By Duke Medicine News and Communications

DURHAM, N.C. -- Younger and older adults depend primarily on different areas of their brain to process visual information, Duke University Medical Center researchers have found. Their brain imaging studies of visual attention revealed that young people tend to use "bottom-up" areas of the brain that are involved in processing the visual features of a scene.

By contrast, older people make greater use of brain areas involved in "top-down," or goal-directed processing. The former area is in the lower back, or occipital area of the brain, while the latter area is in the front and upper back, or parietal, brain areas that control higher functions such as speech, thought, planning for the future and interpretation of spatial orientation.

The researchers said their findings provide new insight into how the brain adjusts its thinking processes as people age. Such studies also yield clues to the underlying causes of cognitive decline with aging, they said.

The results of the Duke experiments were published online March 1, 2006, in the journal Neurobiology of Aging. The study was supported by the National Institute on Aging.

For their studies, the researchers used functional MRI (fMRI) to image the activation of gray matter during testing. And, a relatively new imaging technique called diffusion tensor imaging (DTI) allowed them to gauge the structural integrity of the fibrous white matter. The gray matter of the brain is responsible for information processing, while the white matter enables information transmission among different areas of the brain.

Functional MRI uses harmless magnetic fields and radio waves to map blood flow in brain regions, which reflects brain activity. DTI is an MRI-based technique that measures the movement of water molecules throughout tissues. DTI can determine the structural integrity of a brain region, and if the structure is not intact, its ability to propagate neural signals can be compromised.

By using the two different methods of imaging the brains of volunteer subjects during vision tasks, the researchers found that the structural integrity of the "white matter" within the brain appears to play a role in which areas of the brain are activated. Their imaging studies found that the age-related decline in the white matter was more pronounced in frontal regions of the brain.

"It appears that age-related cognitive changes are the result of combined influences on the gray matter and the white matter," said lead author David Madden, Ph.D., Duke behavioral psychologist and researcher in aging.

"We need further studies to better understand how all these different factors relate to each other, and which changes in cognition result from normal aging or something else," he continued. "Additional research combining fMRI, DTI and behavioral performance will be valuable in determining the neural mechanisms of age-related cognitive decline."

The goal of the team's experiments – which were among the first to use both fMRI and DTI to study aging– was to better understand any age-related changes in visual attention. By using two different imaging methods, the researchers could correlate attentional functioning of the gray matter as measured by fMRI with structural integrity of the white matter as measured by DTI.

The researchers recruited two groups of healthy volunteers – those between 19 and 28 and those between 60 and 82. As the volunteers took the same battery of visual tests that measured top-down attention, the researchers scanned the subjects' brains with fMRI and DTI to determine which areas of the brain were activated. The tests involved correctly locating target letters within a "slide show" of different images appearing on a computer display.

"Based on past studies, we expected greater activation in the frontal and parietal regions in the older adults, especially when top-down attention was required," Madden said. "We also hypothesized that an age-related related decline in the white matter integrity in the frontal part of the brain would correlate with changes in gray matter activation."

While the imaging studies did show the expected increase in frontal and parietal area activation on the older subjects, the team found only a limited support for the role of structural integrity of the white matter as a mediator of age differences in activation.

"We feel that results of our tests represent an important first step understanding the complex relationships between aging, brain structure and cognitive performance," Madden said. "The brain can be very plastic in how it re-directs pathways and functions in response to circumstances."

Madden and his team are currently planning future studies using more sophisticated techniques to make more detailed measurements of the way gray and white matter responds to different cognitive tasks.

Other members of the Duke team included Julia Spaniol, Barbara Bucur, James Provenzale, Roberto Cabeza, Leonard White and Scott Huettel. Wythe Whiting, Washington and Lee University, Lexington, Va., was also a member of the team.