Scientists have brought pig brains partially back to life after the animals were slaughtered, challenging the definition of ‘death’ and raising serious questions about animal ethics and consciousness in the process.

In a study published in the journal Nature, researchers slowed cell death and restored cell function (including the ability to use up sugar and oxygen and carry electrical signals if jolted with electricity) in pigs brains that had been removed from the animals some ten hours earlier. Their method also kept the brains looking, anatomically speaking, pretty similar to brains that are alive.

"What we are showing is the process of cell death is a gradual, stepwise process, and that some of those processes can be either postponed, preserved or even reversed," Nenad Sestan, lead on the study and a professor of neuroscience at Yale University, told reporters during a press briefing.

The researchers restored brain function through a device, called BrainEx, that pumps a synthetic, blood-like substance through the pigs’ brains at body temperature. The fluid contained red blood cells for carrying oxygen, glucose for powering the cells, a mix of chemicals that protect cells from damage and nerve blockers to stop the brain cells sending signals on their own.

Although some of the brain chemicals were flowing again and there was some electrical activity in some cells, the authors of the study stress that the brains weren’t ‘alive’ in the way we would normally define it. There were no patterns of electrical activity across the entire brain, meaning there was no awareness or perception, that is, the brains weren’t conscious.

The brains bridged the thin line between alive—with pumping blood, and trails of electrical activity weaving through different parts of the brain—and dead—with no blood flow or electrical activity, where the brain’s very structure starts to irreversibly disintegrate.

The researchers hope their brain-pump technology could guide new treatments for stroke or brain injuries, or improve research into brain injuries or how drugs affect the brain but first science as a discipline needs to figure out how scientists should work with these ‘partly alive’ brains.

“How do we now think about this middle category of ‘partly alive’? We didn’t think it could exist,” Nita Farahany, a bioethicist and law professor at Duke University, told The New York Times.

Most countries’ animal ethics laws only extend as far as the animal is alive. Since these brains were taken from pigs that had already been slaughtered, there was technically no ethical quandary.

But, how much electrical or chemical brain activity is needed before the animal is classed as ‘alive’ again? Or, how will researchers know at what point a brain is conscious? They’re questions that get to the very core of what it means to be conscious--questions scientists are still trying to answer.

“I think it’s very unlikely that consciousness or sentience could be restored in a several-hours-dead brain, but I’m also pretty sure that if it was, we wouldn’t know that it was,” L. Syd M Johnson, a neuroethicist at Michigan Technological University, told The Atlantic.

Other scientists have previously suggested that even with more widespread electrical activity, animals need some kind of sensory input to truly be conscious. Since these brains were disembodied, that wasn’t possible.

The fact these questions were raised points to a wider dilemma in neuroscience research: discoveries are outpacing the ethical frameworks that guide them.