Leah H. Somerville, a Harvard neuroscientist, sometimes finds herself in front of an audience of judges. They come to hear her speak about how the brain develops.
It’s a subject on which many legal questions depend. How old does someone have to be to be sentenced to death? When should someone get to vote? Can an 18-year-old give informed consent?
Scientists like Dr. Somerville have learned a great deal in recent years. But the complex picture that’s emerging lacks the bright lines that policy makers would like.
“Oftentimes, the very first question I get at the end of a presentation is, ‘O.K., that’s all very nice, but when is the brain finished? When is it done developing?’” Dr. Somerville said. “And I give a very nonsatisfying answer.”
Dr. Somerville laid out the conundrum in detail in a commentary published on Wednesday in the journal Neuron.
The human brain reaches its adult volume by age 10, but the neurons that make it up continue to change for years after that. The connections between neighboring neurons get pruned back, as new links emerge between more widely separated areas of the brain.
Eventually this reshaping slows, a sign that the brain is maturing. But it happens at different rates in different parts of the brain.
The pruning in the occipital lobe, at the back of the brain, tapers off by age 20. In the frontal lobe, in the front of the brain, new links are still forming at age 30, if not beyond.
“It challenges the notion of what ‘done’ really means,” Dr. Somerville said.
[For more of this story, written by Carl Zimmer, go to https://www.nytimes.com/2016/1...ot-so-much.html?_r=0]
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