It's not clear whether the pea-sized brains are conscious: the team behind the breakthrough suspect they're not because the activity resembles that of preterm babies, but they cannot say for certain, opening up a new ethical dimension to this area of research.
So-called "cerebral organoids" derived from adult stem cells have been around for a decade or so but have never previously developed functional neural networks.
"If you had asked me five years ago 'Would you think that a brain organoid would ever have a sophisticated network able to generate a brain oscillation?' I would say no," Alysson Muotri, a biologist at the University of California San Diego, told AFP.
A paper published by Muotri and his colleagues in the journal Cell Press said that two factors were responsible for the breakthrough. The first was a better procedure to grow stem cells, including optimizing the culture medium formula.
The second was initially surprising, but also intuitive when the researchers thought about it: simply allowing the neurons adequate time to develop, just as babies' brains develop in the womb.
"The very early stages of human neurodevelopment are encoded in our genome," explained Muotri.
The team began to detect bursts of brain waves from organoids from about two months.
The signals were sparse at first and all at the same frequency, a pattern seen in very immature human brains. But as they grew, they produced waves at different frequencies, and the signals appeared more regularly, suggesting further development of their neural networks.