Although it's far from the sort of brain transplant beloved by science fiction enthusiasts, scientists have taken one step in that direction: they have spliced a key human brain gene into mice.
In the first study designed to assess how partially "humanizing" brains of a different species affects key cognitive functions, scientists reported on Monday that mice carrying a human gene associated with language learned new ways to find food in mazes faster than normal mice.
By isolating the effects of one gene, the work sheds light on its function and hints at the evolutionary changes that led to the unique capabilities of the human brain.
For the study, scientists used hundreds of mice genetically engineered to carry the human version of Foxp2, a gene linked to speech and language. In a 2009 study, mice carrying human Foxp2 developed more-complex neurons and more-efficient brain circuits.
Building on that, neuroscientists led by Christiane Schreiweis and Ann Graybiel of the Massachusetts Institute of Technology trained mice to find chocolate in a maze. The animals had two options: use landmarks like lab equipment and furniture visible from the maze ("at the T-intersection, turn toward the chair") or by the feel of the floor ("smooth, turn right;" "nubby, turn left").
Mice with the human gene learned the route as well by seven days as regular mice did by 11, scientists reported in Proceedings of the National Academy of Sciences.
Surprisingly, however, when the scientists removed all the landmarks in the room, so mice could only learn by the feel-of-the-floor rule, the regular rodents did as well as the humanized ones. They also did just as well when the landmarks were present but the floor textiles were removed.
It was only when mice could use both learning techniques that those with the human brain gene excelled.
Language gene may increase cognitive flexibility
That suggested, Graybiel said, that what the human gene does is increase cognitive flexibility: it lets the brain segue from remembering consciously in what's called declarative learning ("turn left at the gas station") to remembering unconsciously (take a right once the floor turns from tile to carpet).
Unconscious, or procedural, learning is the kind the feel-of-the-floor cue produced: the mice didn't have to consciously think about the meaning of rough or smooth. They felt, they turned - much as people stop consciously thinking about directions on a regular route and navigate automatically.
"No one knows how the brain makes transitions from thinking about something consciously to doing it unconsciously," Graybiel said. "But mice with the human form of Foxp2 did much better."
If Foxp2 produces the cognitive flexibility to switch between forms of learning, that may help explain its role in speech and language.
When children learn to speak, they transition from consciously mimicking words they hear to speaking automatically. That suggests that switching from declarative to procedural memory, as the humanized mice did so well thanks to Foxp2, "is a crucial part of the process," Graybiel said.