It’s a major technical challenge to pin down new brain cells in humans because you can’t readily see through living skulls at the cellular level. However, none of these methods have, so far, reliably pinpointed the stem cells or progenitors capable of yielding fresh neurons–leaving room for doubt. “We nailed down active neurogenesis in the adult human brain,” Marta Paterlini, a neuroscientist at Karolinska Institutet, tells Popular Science. It’s possible that, in their efforts to identify neurons in progress, the study authors may have inadvertently included some glial stem cells in their analysis, Sorrells suggests. Perhaps down the line, adult neural stem cells could be used to help people recover after brain trauma, Paterlini suggests.