"I would love to see if the same sort of transplant worked in older animals," says Jianhua Cang, neuroscientist at Northwestern University, in Chicago. "This work is a significant advance, but if one can do it in adult animals, it would be even more remarkable. And it opens the possibility of therapeutic potential." Cang was not involved in the current research, thought he has previously worked with the authors.

The findings could have wide-reaching implications for how we think about the nature of plasticity in the brain. Humans have a similar critical period, though in humans this phase is more extended than in mice. Infants and children with a lazy eye or a cataract will suffer permanent vision loss if the problem isn't corrected before about eight years of age, says Takao Hensch, a neuroscientist at Children's Hospital Boston, who was not involved in the current study. (During normal development, this period of plasticity is thought to be important for developing balanced input from both eyes, which is crucial for binocular vision.)

The phenomenon isn't limited to the visual system--scientists think that most parts of the cortex undergo a similar period of heightened malleability. For example, children fail to hear certain sounds after a particular age. "The classic example is kids growing up in Japan," says Hensch. "They eventually lose the ability to differentiate between 'R' and 'L' sounds."

If scientists can find a controlled way to trigger plasticity in specific parts of the brain, it would open new avenues for treatment of a variety of ailments. Adults who suffer brain damage from stroke or head trauma have some level of reorganization in the brain--enhancing that plasticity might improve recovery.

"Many psychiatric illnesses are recognized as having neurodevelopmental origins, in particular, deficits in inhibitory circuits," says Hensch. For example, many genes linked to autism may trigger an imbalance in excitation and inhibition in neural signaling, he says. "If you can restore that imbalance, you might imagine intervening during development or later in life to try to restore brain function."

Still, a long road lies ahead. To apply this type of cell transplant to humans, scientists would first need to develop a reliable source of the necessary cells, perhaps from induced pluripotent stem cell reprogramming. They would then need to show the cells can be safely transplanted into the brain. Figuring out how to properly capitalize on the newfound plasticity presents another hurdle. It's not clear whether patients would need some kind of specific training or drug treatment to properly reorganize damaged neural circuits. "For higher cognitive functions, you might need to train people cognitively in the presence of plasticity-enhancing neurons," says Hensch.

"Ideally, it would be nice to find a way to coax [the birth of new neurons in the brain] through some pharmacological or environmental means to get more of these inhibitory cells to appear," he says. "That seems like quite a challenge, but this research gives us hope it's worth trying."