We often talk about death as a point in time. One moment you’re alive and the next, when your heart stops beating and your lungs stop breathing, you are clinically dead. This definition tempts us to view death as a clear-cut event, like the flip of a switch.
That’s not how Jimo Borjigin, a neuroscientist at the University of Michigan, sees it. “Doctors assume that after clinical death, the brain is dead and inactive,” she says. “They use the term ‘unconscious’ again and again. But death is a process. It’s not a black-or-white line.”
In a new study, Borjigin discovered that rats show an unexpected pattern of brain activity immediately after cardiac arrest. With neither breath nor heartbeats, these rodents were clinically dead but for at least 30 seconds, their brains showed several signals of conscious thought, and strong signals to boot. This suggests that our final journey into permanent unconsciousness may actually involve a brief state of heightened consciousness.
Although the experiments were done in rats, Borjigin thinks they have implications for the near-death experiences (NDEs) reported by one in five people who are resuscitated after their hearts stop. Although they were unconscious, unresponsive and clinically dead at the time, they come back with stories of bright lights, “realer than real” memories, and meetings with people they knew. Some scientists have dismissed these accounts outright. Others have taken NDEs as proof of a religious afterlife or a consciousness that lives on outside the body, as popularised in a recent bestseller of dubious provenance.
But Borjigin’s research suggests that these experiences could just be a natural product of a dying brain. That doesn’t make them any less real, but it does root them in the natural world, without the need for a “super-“ prefix.
“The near-death experience might be considered a “final frontier” of consciousness studies,” says George Mashour, an anaesthesiologist from the University of Michigan and a co-author on the study. “It has been repeatedly proposed as a critical counter-example undermining the hypothesis that consciousness is rooted in the brain. Our study brings the phenomenon back into the realm of brain science.”
What they found
The seeds of this study began in 2007, when Borjigin, together with her husband Michael Wang, was studying the brain activity of rats that had just suffered a stroke. During the experiment, three of the animals unexpectedly died overnight. When the duo found them the next day, they noticed several dramatic peaks of strong brain activity just after at the point of death. “That stuck in my mind,” says Borjigin. “I became convinced that if something is going on in the brain after cardiac arrest, it’s got to be measurable.”
Her team implanted several electrodes across the brains of nine rats to measure their brain waves—rhythmic pulses of neural activity that are denoted by Greek letters, depending on their frequency. The rats were sedated with anaesthetic, and then killed with either a lethal injection that stopped their hearts, or a fatal dose of carbon dioxide.
As you’d expect, after their hearts stopped, most of these brainwaves weakened with time. But one set—the low-gamma waves produced when neurons fire between 25-55 times per second—became stronger for a brief period, in all of the nine rodents. “We weren’t surprised that we found brain activity but we were surprised by the high degree of it,” says Borjigin.
The activity in different parts of their brains also became more synchronised. Their low-gamma waves, in particular, became twice as synchronised when they were in their near-death state than when they were anaesthetised or awake.
These features have been linked to conscious perception in earlier studies. For example, low-gamma waves suddenly become synchronised across distant brain regions at the moment when people recognise a face among some arbitrary shapes. This makes sense—the act of recognition draws upon the brain’s visual centres, as well as areas responsible for face recognition and memory. Neurons all over the brain need to mount a global response, and fire together.
Conscious thought has also been linked to the strength of connections between the front-most areas, associated with many complex mental abilities, and those nearer the back that deal with sensory information. And sure enough, the team saw that these areas became 5-8 times more strongly connected after cardiac arrest than during either anaesthesia or their waking moments. “That’s astonishing,” says Borjigin. “It helps to explain why [humans experiencing NDEs] can ‘see’ during clinical death, and why they claim they can hear conversations during that period.”
What it means
“Near-death experiences are a physiological reality, but science and medicine haven’t taken them seriously for way too long,” says Steven Laureys, who leads the Coma Science Group at the University of Liège. “We can’t just listen to extraordinary stories from patients; we need to measure brain function. The field needs studies like these.”
Laureys compares the study of NDEs to our growing understanding of dreams. For the longest time, we only knew about dreams from the colourful stories people told when they woke up, but electrode measurements revealed their neurological underpinnings, including the existence of REM sleep when most dreams occur. “That’s the way we should go for death and NDEs.”
However, he also cautions that scientists are still arguing about which neural signals are indicators of consciousness, so decoding the patterns that Borjigin saw isn’t straightforward. “It’s terribly hard to make strong claims about what these rats actually perceived, or about possible conscious experiences,” he says. “But the study definitely shows that there is a lot more electrical activity than expected, and it’s very interesting activity. It’s tempting to link that to what we hear in patients, but we need to be very careful.”
Sam Parnia, a cardiologist from Stony Brook University Hospital, shares that view. He has studied resuscitation and near-death experiences for years and believes that comparing the rat results to the intense visions that humans recount after NDEs “is extremely premature and unsupported by evidence”.
“We have a long way to go,” admits Mashour. “We haven’t correlated the observed brain activity with a conscious experience.” The only way to get around that would be to gather electrode recordings in someone who had a near-death experience and returned to tell the tale. There are only a few possible situations when that wouldn’t be unethical—perhaps with organ donors who are undergoing cardiac death.
Meanwhile, Parnia says that there could be other explanations for the results. “After blood flow to the brain is stopped, there is an influx of calcium inside brain cells that eventually leads to cell damage and death,” he says. “That would lead to measurable electroencephalography (EEG) activity, which could be what is being measured.” This would explain why Borjigin saw the same pattern in every dying rat, while only 20 percent of people experience NDEs after a heart attack.
Parnia also notes that other EEG studies of humans during cardiac arrest haven’t found similar patterns, suggesting that these results might be due to some quirk of the experiment. But Borjigin counters that other groups have mostly placed electrodes on their patients’ scalps, with bone, flesh and skin standing between them and the underlying neurons. Her team, however, surgically implanted their electrodes right on top of the rats’ brains, making them more sensitive to subtle signals.
To her, the signals are a sign of heightened consciousness and she speculates that such spikes of activity might be a sort of built-in defence. “When the brain is in danger, it needs to be hyper-alert, so the individual can deal with a crisis,” she says.
This raises some other intriguing questions, beyond the relevance to NDEs. “We didn’t realise that brains can have heightened consciousness when oxygen and glucose are taken away,” she says. “Could this happen during our waking states, or when we’re ill, praying or meditating? If you have local fluctuations, could that give you hallucinations or artistic visions? We don’t know.”
Reference: Borjigin, Lee, Liu, Pal, Huff, Klarr, Sloboda, Hernandez, Wang & Mashour. 2013. Surge of neurophysiological coherence and connectivity in the dying brain. PNAS http://dx.doi.org/10.1073/pnas.1308285110