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.