Can old brains get full?

When it comes to learning new information, even if the mind is willing, older brains may not be physically up to the challenge. A new study suggests that old brains can become "full" -- making learning difficult as people age.

Research shows that learning becomes more difficult for older brains not because of reduced capability, but because the brain already holds so much information. In effect, it is the inability to forget old memories that limits the intake of fresh info.

The study, conducted by the Medical College of Georgia at Georgia Regents University, involved mice that were genetically altered to have brains resembling those of adult humans. Researchers found that the mice brains showed no decrease in their ability to create the synaptic connections that enable learning. But as the mice entered adulthood, their pace of learning slowed because they were unable to weaken previously existing connections in order to make new ones.

Lead author of the study, Joe Z. Tsien, told the New York Times that the difference between young and old brains is similar to a blank piece of paper and a newspaper page.

"The difference is not how dark the pen is," he said, "but that the newspaper already has writing on it."

The study focused on two proteins found in the brain -- NR2A and NR2B -- that are well known to play a role in learning. In young brains, NR2B is more common than NR2A. Once brains reach adulthood, the reverse is true.

Researchers modified the mice to produce more NR2A than NR2B, expecting the subjects to have more difficulty forming new connections in their brains. They found that the mice had no trouble creating short-term memories, but struggled to weaken older connections and thus had a limited ability to form long-term memories.

"What our study suggests," Dr. Tsien told the Times, "Is that it's not just the strengthening of connections, but the weakening of the other sets of connections that creates a holistic pattern of synaptic connectivity that is important for long-term memory formation."

The study was published in the journal "Scientific Reports."