Illustration by Stephen Jeffrey

BY ABOUT the age of six, the human brain is as big as it is ever going to be. That may surprise most grown-ups, who notice that children do not display the mental agility of adults (even though many fancy their own little angels are geniuses). Children can remember facts but are less good at recalling the context in which those facts are relevant. And they are easily swayed from long-term goals. Even when youngsters try their hardest they cannot wait 15 minutes for two biscuits if they can scoff one now instead.

But as people grow, their brains change. Before full volume is attained, the pruning starts. Grey matter gets picked away at different rates in different parts of the organ. Brain cells form white matter as their arms become covered in fatty sheaths that, like the plastic insulation around a metal wire, stop electrical signals leaking out as they zip along the nerve cells. As the grey matter diminishes, the white matter steadily increases. Which is why the brain can mature from an organ of overwhelmingly short-range connections into one with many long-distance links, as Bradley Schlaggar and his colleagues at Washington University, in St Louis, have found.

Dr Schlaggar likes to create diagrams of brain function using a technique called graph theory that is used, among other things, to analyse demand on power grids and the structure of the Internet. He asks volunteers to lie in brain scanners and to think about whatever they wish. Then he tries to identify which parts of the brain are simultaneously active—or almost so. Where activity exceeds certain statistical thresholds, he plots a line between those bits of the brain on his diagrams.

Networking skills

This approach has led Dr Schlaggar to suggest why it is that adults can better resist impulses that derail long-term goals in children. His work is based on an idea by his colleague Steven Petersen, who recently developed the hypothesis that two networks, rather than two areas of the brain (as the mainstream theory has it), keep the adult mind concentrated on long-term achievement.

Dr Petersen and his colleagues identified 39 regions of the brain that were active when university students applied themselves to ten different tasks, each with varying levels of surprise built in. Whether the students were listening to repetitive sounds and trying to predict when the next tone would come, or pushing the correct button if pairs of words were matched or mismatched in their meanings, some consistent synchronisation emerged. Seven of the 39 regions looked busy when the brain was pursuing a successful strategy and maintaining a consistent effort. Eleven other parts chipped in when that strategy slipped up and some innovation was needed for the student to complete the task. Dr Petersen postulated that the first seven regions form one network, which he calls the “cingulo-opercular network”, and the second 11 form another, the “frontoparietal network”.

Dr Schlaggar next wondered how the connections within these two networks might develop. So he turned to a second group, made up of children and teenagers, and asked them to think about whatever they liked while he scanned the blood flow inside the same 39 regions of their brains and calculated which parts were acting in unison.

What he found came as a shock. In the 49 children, aged seven to nine, the two networks were always bound into a single web; in the 43 adolescents, some of those connections had been undone; and in his 47 adult volunteers, aged over 21, the brain regions fired as two distinct networks. Moreover, the web of activity inside the children's heads depicted the cingulo-opercular (sustaining) network as being clamped inside the frontoparietal (rapidly adapting) one, suggesting why it is that youngsters grab one biscuit now rather than wait for two later. Both studies were published recently in the Proceedings of the National Academy of Sciences.

But why are children no good at recalling the context in which they learn facts when they retain those facts fairly well? This question was fodder for an experiment by Noa Ofen, of the Massachusetts Institute of Technology, and her colleagues, described in this week's online edition of Nature Neuroscience. Like Dr Schlaggar, Dr Ofen measured the blood flow in the brains of young people, this time aged eight to 24. Rather than letting their minds wander, though, she asked her volunteers to try to remember photographs of 250 scenes. Some were of kitchens or halls, others showed landscapes from her holidays in America and Israel.

When they came out of the scanner, Dr Ofen then tested her volunteers on how well they could recall the pictures. Mixing those images with photographs that they had not previously seen, she asked whether each one was new or familiar and, if the latter, whether the volunteers could remember something about the context in which it had appeared. Age, she found, did not help people recognise her snaps correctly. However, it did steadily enrich the details they could add about the experience of forming the memory.

The scans enabled Dr Ofen to link her findings to brain development. She noticed that, for all her volunteers, a part of the brain involved in forming memories—the medial temporal lobe—was flushed with blood whenever an image appeared in the scanner. A second region called the lateral prefrontal cortex was also active in her older volunteers, and was most active in those aged over 18 during the formation of those memories whose context they best recalled. She thus suspects that this could help to explain how fuller-bodied memories form as people grow old.

And to link her findings with the pruning of grey matter and the augmentation of white, Dr Ofen counted pixels of the two types of matter in two particularly important parts of that brain region. Richer recollections indeed came with the whitening of older age.