"If it's going on in the visual cortex, it's probably going on in other parts of the cortex," he said.
In the visual cortex, each so-called V1 neuron responds only to the sight of objects at a specific orientation or moving in a certain direction. GABA probably restricts the V1 neurons from responding to any other types of stimuli. This process helps the brain make sense out of the vast quantities of visual information coming in through the eyes.
"Its like New York City or Boston during a blackout, Leventhal said, describing what would happen if neurons werent restricted to specific responses. With all the gating mechanisms like the stoplights out, youd think traffic would move faster. But it doesnt."
The researchers recorded the activity of individual neurons in the visual cortex of old and young macaque monkeys, while showing the monkeys various images on a computer screen. The devices that monitored the neurons also held small glass tubes of substances that could be released directly onto the neurons. The substances were GABA, a GABA-enhancing compound called muscimol, and a GABA-blocking compound called bicuculline.
The GABA blocker made the neurons less selective in the young monkeys, but had no significant effect in old monkeys. Presumably, thats because the older neurons had already lost much of their selectivity, according to the researchers.
GABA and the GABA-enhancer had a relatively small effect in the young monkeys, moderately increasing the percentage of cells that were selective for particular orientations and directions. In the old monkeys, however, GABA and the GABA-enhancer had a much stronger effect, significantly increasing the percentage of highly selective cells.
Thus, the visual cortex of the older monkeys seemed to function less effecti
Contact: Lisa Onaga
American Association for the Advancement of Science