A new research from Harvard suggests that neurons aren't built as we grow but rather they are trimmed to suit the environment around us.

The researchers say that they have found evidence that neurons developed in the mice brain before birth were gradually "pruned" to adapt with the environment.

"By the time mammals – and humans would certainly be included – are first coming into the world, when they can do almost nothing, the brain is probably very wired up. Through experience, the brain works to select, out of this mass of possible circuits, a very small subset…and everything else that could have been there is gone," said the lead researcher Jeff Lichtman, the Jeremy R. Knowles Professor of Molecular and Cellular Biology.

The researchers studied mice brains before and after they were born. They found that the brain had more robust connections just before birth but these connections weakened a few weeks after birth. This suggested that the brain keeps only few neural connections alive and cuts off others.

They say that this trimming may have an association with experiences.

"We think that experience must be the engine that allows some branches to survive and the vast majority to disappear. If this were a stereotypical developmental program, you might imagine that it might trim off whole parts of the arbor, but when you look at where the ten percent of surviving branches are located, you see the arbor extends over the same area, it simply has fewer branches. It has chosen, at the terminal level, which branches to keep and which not to," said Lichtman.

The next step, the researchers said, is to study how the decisions regarding these connections are made. This could enable researchers to get an insight about autism and other disorders.

"That is one theory people have talked about, whether autism could be a disorder where connections that should have been trimmed back weren't, and as a result stimuli are much more intense than they should be. There are stories about children with autism spectrum disorders who cannot run in their bare feet on grass, because it's just too painful," said Lichtman.

The study, however, opens up more questions about why the brain chooses to work this way.

"It seems like a paradox – why would the best brains seem to be the most backward, and take the longest to figure out how to do things?" Lichtman said. He adds that humans tend to rewire according to the environment rather than keeping the same set of neuron map. This is why today's humans behave differently than humans that existed a thousand years back, but a fly then and now will probably act the same way.

The research was published June 7 in journal Neuron.