They're implanting mouse brains with human cells, and it's making them smarter...for now.

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In the new study, researchers led by neurologist and stem cell biologist Steven Goldman and neurobiologist Maiken Nedergaard of the University of Rochester Medical Center in New York implanted human cells called glial progenitor cells into the brains of newborn mice.

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Previously, the researchers had transplanted human glial progenitor cells into the brains of mice that had a genetic disorder mimicking multiple sclerosis. The glial progenitor cells healed the mice, allowing them to live a normal life span. That result held promise that such cell transplants might help people with neurological disorders.

The researchers also noticed something curious in the brains of mice that had received human cell transplants. "The shocker was that all the glial progenitors were human and had completely taken over the mouse progenitors," Goldman says.

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The researchers also put mice through a battery of tests, probing the animals' ability to learn mazes, distinguish new objects from old ones, and learn that a certain sound portends a mild electric shock. It took normal mice and mice with mouse cell transplants several tries to pick up on the association between the sound and the shock. Mice with human astrocytes "pretty much picked up the association immediately and got more fearful," Goldman says.

I think it says something not very nice about scientists when their measure of how a mouse's intelligence has increased is how quickly they learn to be afraid.