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Changes in the Brain May Account For Part of Declining Vision

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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.

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