Scientists at Yale University in New Haven, Connecticut, have revived the disembodied brains of pigs four hours after death. Although the team stopped short of restoring consciousness, bioethicists say that its approach has profound ethical implications.

Nature’s news team answers some of your questions about the research1, and where it might lead.

Could this research lead to immortality? — Wesley Mendes

It’s unlikely. The BrainEx system developed by the Yale researchers supplied cells with oxygen and nutrients. This support restored some cellular functions, such as metabolizing sugars for energy and making proteins, for up to 36 hours. But the system can’t slow ageing or disease, and the researchers don’t know how long BrainEx can keep organs alive.

The team has not tried the technology in humans, not least because BrainEx is difficult to use without removing the brain from the skull. But further development of the system could one day offer hope of reviving people after brain death from an accident, heart attack or stroke.

What does this mean for brain transplantation? — Anonymous

Transplanting a brain from one body to another is still closer to the realm of science fiction than fact.

The researchers in the pig-brain study restored some functions to the cells within the organs. But connecting a transplanted brain to a living body would be a much more complex endeavour. One of the biggest challenges for scientists would be determining how to reconnect the brain’s stem to the body’s severed spinal cord. Making that connection would allow the brain to send electrical signals down the spine to direct the body’s function and movement, and to respond to sensory input from elsewhere in the body.

Could a disembodied brain be conscious? Could it perceive or sense things? — Albert J.

We don’t know. In this study, the researchers deliberately prevented the pig brains from regaining consciousness, by using chemicals to block neurons from firing. The team never saw any brain-wide activity that would indicate the organs could be conscious.

But the idea that a disembodied brain could regain consciousness is not impossible. In the pig-brain study, researchers removed pieces of brain tissue and applied an electrical stimulus to them. They found that neurons still had the capacity to fire in response to that external prompt. And last year, researchers at the University of California, San Diego, reported that ‘mini brains’ grown in a dish had spontaneously produced human-like brain waves for the first time. The electrical patterns produced by these brain organoids — 3D clumps of brain tissue cultured in the lab — looked similar to those seen in premature babies2.

Knowing whether a disembodied brain could perceive anything about its surroundings would require hooking it up to sensory organs such as eyes or skin. But experiments with sensory-deprivation tanks — and research into our ability to dream — suggest that brains can be conscious even without the perception of external stimuli3.

Could research like the pig-brain study change how we define consciousness? — Ego Sum

Technology such as the BrainEx system used to support the pig brains could provide a new way of modelling consciousness and brain activity in the lab — and even enable scientists to test drugs for neurological disorders in disembodied animal organs rather than in people.

The pig-brain experiments took place in the United States, which does not regulate experiments on an animal’s organs after they are removed from its body. But researchers say that any experiments that could restore brain activity to a disembodied organ should be conducted only after an ethical review.

Does the minimal brain activity seen in the pig experiment constitute a vegetative state? — Anonymous

No. Although a person in a vegetative state is not considered to be conscious, the neurons in their brain are still firing and are capable of regulating sleep–wake cycles and heart and lung function, among other processes. The pig brains in the latest study carried out metabolic functions — such as those cells use to produce energy and remove waste — but their neurons didn’t fire unless researchers stimulated them individually.

Would a possibly conscious, isolated brain have human rights? Can an unconscious brain without a body still be considered human? — T. Beyer

There are few laws governing the creation of consciousness in an organ, and ethicists are just beginning to grapple with this issue in earnest.

Still, the pig-brain experiment isn’t the first to raise questions about which entities are conscious and what rights they deserve. Ethicists are already debating whether artificially intelligent machines deserve rights, and whether the intelligence of animals such as chimpanzees and dolphins makes these animals worthy of greater rights.

What does this mean for the possibility of cryonics? — David Quintero

We don’t know whether the pig study has any implications for cryonics — the freezing of a body or brain after death in the hope of revival in the future. The pig brains were dead for four hours before the researchers infused them with the BrainEx solution that restored some cellular functions. No one has tested whether the system would work on brains that have been frozen.