Lab-grown “mini-brains” have, for the first time, produced human-like brain waves, and appeared to do so spontaneously. The electric patterns from in the mini-brains showed a resemblance to what’s seen in premature babies.

The lab brains belong to a type known scientifically as “organoids,” which are grown as 3D cultures of embryonic stem cells. Such cultures also have been used to create miniature versions of eyes, guts, livers, kidney, prostates, and several other organs. But the human brain is a different beast.

“It’s a very grey zone in this stage,” says neuroscientist Alysson Muotri of the University of California, San Diego in a interview with the journal Nature, “and I don’t think anyone has a clear view of the potential of this.” Muotri says he would consider canceling the project if the mini-brains began to show signs of consciousness, although that is a long way from their current state.

Muotri and her research team had been trying to coax human stem cells to recreate tissue found in the brain’s cortex, which is devoted to cognition and interpreting sensory information. Hundreds of these cortex organoids grew in cultures for 10 months. During this growth period, Muotri’s team monitored the mini-brain electrical activity through EEG readings.

Six months into their testing, Muotri’s team encountered the first surprise. These cells seemed to be hitting higher on the EEG registers than previous brain organoid efforts. Not only that, but the EEG readings themselves were strange—erratic patterns that showed a likeness to the chaos of early brain formation. Specifically, the brain organoids showed a strong resemblance to infants born prematurely at 25–39 weeks post-conception.

It’s hard to tell exactly what the recreation means. It’s “very intriguing and very amazing,” says Hongjun Song, a developmental neuroscientist at the University of Pennsylvania, speaking to Nature. But other scientists, like Sampsa Vanhatalo, a neurophysiologist at the University of Helsinki who developed the EEG machines that Muotri’s team used for testing, aren’t sure the results speak to “intentionality”—that is, the brain attempting to form a human-style brain.

The brain is famously the least understood parts of the body, the sheer complexity of human thought is something scientists have struggled with for years. Researchers know so little about how the human infant’s brain forms during these crucial growth periods. It’s completely possible that the organoids are missing the components that drive EEG growth in humans and that the electric readings are accidental.

Nevertheless, here we are. The readings exist. From here, Muotri’s team sees “novel opportunities for investigating and manipulating the role of network activity in the developing human cortex,” discussed in their paper’s abstract. Accident or not, learning how organoids respond to growth will be crucial to their future.