The Lilliputian versions of human brains that scientists have grown in lab dishes have developed distinct structures such as the hippocampus, grown glia and other cells like those in actual brains, and produced a diverse menagerie of neurons that connect with each other and carry electrical signals. Now scientists have grown hundreds of cerebral organoids with the most complex, human-like activity yet: Though only one-fifth of an inch across, or about the size of a pea, the organoids have developed functional neural networks that generate brain waves resembling those of newborns.

The research, reported on Thursday in Cell Stem Cell, represents a significant advance in creating cerebral organoids that mimic human brain development and function.

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“I do think we need to pay attention to the ethical consequences of human neural organoids,” said law professor Henry Greely of Stanford University, co-author of a 2018 paper on the ethics of these brains-in-a-dish. If organoids acquire the ability to perceive things “we’d have to worry about pain or other negative feelings,” he said, but that worry would be more akin to concerns about animal welfare and not “humanness.”

That’s because no lab is “anywhere near making human brains in vitro,” Greely added: today’s cerebral organoids have about 2 million neurons, not humans’ 86 billion, and lack specialized regions such as the midbrain and cerebellum. The organoids therefore “just can’t, as far as I can guess, be human,” he said.