In 1953, the man known during his lifetime only as H.M. underwent neurosurgery to cure his severe epilepsy. The operation inadvertently destroyed most of his hippocampus, a brain structure nobody realized at the time was crucial to the formation of memories. From the age of 27 until his death in 2008, at 82, Henry Molaison—whose name was eventually revealed—could essentially learn nothing new at all.
No responsible person would perform that sort of surgery today, just as no responsible nation would conduct an open-air nuclear test. But the two situations, it turns out, are weirdly related. A commentary in the latest Science, based on a recent study in Cell, explains how nuclear bomb tests provide clear evidence that the hippocampus constantly generates new neurons throughout life. “At the behavioral level,” writes author Gerd Kempermann, of the Technical University of Dresden, Germany, “adult neurogenesis adds a particular type of cognitive flexibility to the hippocampus.”
Exactly what form that flexibility takes is unclear, but the fact that it was uncovered by nuclear testing is intriguing enough. It turns out — unsurprisingly — that the growth of new cells, whether in the brain or anyone else, requires carbon: the element is a fundamental building block of the proteins that make up DNA. That carbon comes from the food we eat, and the carbon in food comes, ultimately, from the atmosphere: it’s taken in by plants as they grow, and we either consume the plants themselves or the animals that eat them.
During the era of widespread open-air nuclear tests, from 1945 to 1963, levels of radioactive carbon-14 in the atmosphere, which are always present at low levels, increased significantly. That carbon worked its way into plants and up the food chain, and ultimately (and harmlessly) into our bodies.
But since the carbon is radioactive, it decays at a well-known rate. That’s how scientists date organic matter — ancient wood or ash, for example — from archeological sites. So if new brain cells were forming during the era of open-air nukes, you should be able to see the spike of C14, suitably decayed for the moment the cells were born, in their DNA. And if the C14 level corresponds to a time when the person was an adult, you know that cells are regenerating and you can even determine how prodigiously.
And that, it turns out, is just what Kirsty Spalding, of Sweden’s Karolinska Institute, and her colleagues found in autopsies. Brain cell growth was taking place—albeit it in the hippocampus alone—at a rate of about 700 neurons per day.
That being the case, you might reasonably wonder why our ability to learn and remember declines with age, given that our central memory organ is constantly growing new cells. Unfortunately, the rate of renewal doesn’t make up for the rate of cell death, which is also going on constantly — and the new cells live for a far shorter time than our original hippocampal neurons.
Still, even that limited regeneration can provide a cognitive boost. The cells grow mostly in an area of the hippocampus known as the dentate gyrus, and, writes Kemperman, “By staying ‘forever young,’ the dentate gyrus could command unique solutions to computational problems only found in the brain region central to learning, memory, and many higher cognitive functions considered essential for humans.”
That’s admittedly speculation. The precise role of the hippocampus in memory formation is still imperfectly known, despite some 50 years of study by neuroscientists focused on H.M.’s memory deficits and those of other patients with hippocampal damage. It’s even more speculative to imagine that this research could someday lead to treatment for patients like H.M. Growing new brain cells is a nifty trick — but so far, only the brain knows how to do it.