Changes in the Brain May Account For Part of Declining Vision
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.