March 20, 2000 (Indianapolis) -- Doctors have long known that many functions of human vision decline with age, but the reasons for this are not so clear. A study published in the April edition of Nature Neuroscience indicates that age-related changes in the visual cortex, the part of the brain where visual information is processed, may bear part of the blame.
"Amazingly, no one before has looked at single cells in the aged higher-order primates to see if the visual cortex also deteriorates with age," study author Matthew T. Schmolesky tells WebMD. "We have found evidence of changes in the primary visual center of the brain that we think may, in part, underlie decreased speed and accuracy in recognizing the shape and motion of an object," says Schmolesky, a doctoral candidate in the department of neuroscience at the University of Utah.
A large majority of the cells in the visual cortex of young monkeys are known to respond to the orientation of an object. In older monkeys, the study found, the percentage of cells that are sensitive to orientation is less than half that in younger animals. This means that these monkeys might be less capable of recognizing shapes.
In human terms, that could mean you don't recognize something as quickly or might mistake it for something else. In activities such as driving, where a lot of information is coming at once, you might make more mistakes or take longer to recognize a problem.
The researchers studied neurons, or nerve cells, in four young monkeys and four older ones. The monkeys were given an anesthetic, and probes were placed in their brains to measure how often the neurons in the area that controls vision fired when presented with a certain type of stimulation. The cells' ability to respond selectively to the way a line or bar was aligned (the "orientation bias") or to the direction of its movement ("direction bias") was determined. In older monkeys, about 42% of the neurons showed significant orientation bias, compared to 90% for the younger ones.
Similarly, the percentage of cells that were strongly biased for direction was lower in the aged monkeys. The older animals' cells appeared to respond to all stimuli, which means that they were working randomly. This led the authors to suggest that the cells' decreased selectivity may be due to age-related changes in control of these cells in the brain.
"There are literally thousands of articles about what goes on in the aging eye, but we have spent very little time looking at the brain," says Schmolesky. "We also think that many of these same degenerative processes occur in other [brain] areas, including those responsible for hearing and touch perception." If researchers continue to study the aging central nervous system, he says, they may find the reasons for these study results and can then start targeting the declining processes for intervention.
"We have ample evidence that not all visual deficits are related to optics of the eye, such as cataracts blurring the lens," says James N. Ver Hoeve, PhD, who reviewed the study for WebMD. Ver Hoeve is senior scientist and director, electrodiagnostic service, in the department of ophthalmology and visual sciences at the University of Wisconsin-Madison. But "this is the first article that suggests it is the cells of the visual cortex that are affected by aging and not just the optics," he says.
- An age-related decline in the functioning of the area of the brain that processes visual information may be partly to blame for the decline in vision associated with growing older.
- In a study of monkeys, fewer cells in the brains of older animals were sensitive to the orientation or direction of visual stimuli.
- In the past, much research has focused on visual problems related to the function of the eye, but this study shows that a decline in the brain may also play a role.