Such applications are a long way off, and even if they never materialize, “this is already an extraordinary breakthrough,” says Nita Farahany, a bioethicist at Duke University. Although neuroscientists can study lab-grown neurons or peer at thin slices of brain tissue, these capture nothing of the three-dimensional intricacy that makes the brain, the brain. By restoring some activity to postmortem pig brains, Sestan’s team has created a much better proxy for the real thing. The irony, of course, is that “the better the proxy, the sharper the ethical dilemmas,” Farahany says.

Johnson adds that no animals died for the sake of the study: The team used brains from pigs that had been killed for food. “Thousands of sentient animals have been killed in studies searching for neuroprotective treatments that have not borne fruit,” she says. “Meanwhile, millions of animals are killed for food every year, and that’s a potentially rich source of experimental brains that would involve no additional harm.”

The study still needs to be replicated by other independent teams. And before anyone takes the technique further, or even contemplates the possibility of human trials, there are several ethical issues to consider. For example, is the team really sure that the partly revived brains have no consciousness? Latham, the Yale ethicist, feels confident. Even people under anesthesia show signs of coordinated, brainwide electrical activity, he says, so the absence of such signals strongly suggests that “we don’t even have the possibility of consciousness showing up.”

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But consciousness is still hard to define, much less measure. And no one has ever had to measure it in a brain that lacks a body. How would you assess awareness, pain, or suffering “in a brain that has restored circulation and neural function, but that’s disconnected from external sensation?” asks Steven Hyman, a neuroscientist at the Broad Institute of Harvard and MIT. “This is a very hard scientific problem and policy issue.” As Johnson says, “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.”

It’s also unclear why the pig brains never regained coordinated activity. Is it because team members waited for four hours? Is it because they only treated the brains for six hours? Was it something about the way the pigs were killed? Or is it because they added chemicals that dampen neural activity to the fluid that they pumped through the brains? (They did this because excessive firing helps to kill neurons in oxygen-starved brains.) And if that’s the case, could isolated brains gain consciousness if the blockers were removed?

Possibly, and that would certainly blur the line between living and dead. But that experiment is emphatically not in the cards. The team’s next and only step is to try BrainEx for longer periods of time. If that leads to signs of coordinated activity, “we’d have to close down the research for a while,” Latham says, “because there’s no institutional body for us to consult. We’d need to create one.” Current regulations on animal research exclude individuals that either were raised for food or have died. Nothing covers the gray area posed by an isolated brain with signs of cellular activity and that may or may not be conscious.