SAN DIEGO When biologist Alysson Muotri started tinkering with tiny balls of nerve cells in the lab more than a decade ago, his goal was simply to understand early brain development and neurological disease. He didnt realize he was stumbling into an ethical minefield. His University of California, San Diego, team found that when human stem cells were grown into so-called brain organoids in the lab, these tiny 3D structures produced regular waves of electrical activity that resembled what researchers see in a full-size human brain when they place electrodes along a persons scalp. These brain models were a useful tool to better understand conditions such as autism and schizophrenia, which could help researchers develop and test new treatments.