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 Coma, from the Greek word for “deep sleep,” is marked by losses of responsiveness, voluntary movement capacity, higher-order brain functioning and a normal sleep-wake cycle. A comatose person cannot be aroused awake and retains, at most, enough brain function to carry out life-sustaining processes, including respiration. Coma patients in a persistent vegetative state, deemed “brain dead,” can’t survive without a ventilator or feeding tube.  

But consciousness is a tricky bitch. Doctors can monitor brain activity and use sensory stimulation tests to gauge awareness, but they’re still using objective measures to infer someone’s capacity to perceive both their own own mind and the outside world. In recent years, neuroscientists have discovered sporadic brain activity in unresponsive coma patients, suggesting at least some semblance of conscious awareness and emphasizing the difficulty of understanding someone else’s mental capacity with total precision.

Consider Stephanie Savage, who lay comatose for six weeks before regaining consciousness, as Vice reported in August. Savage wrote a book, detailing her experience with prolonged semi-awareness, describing her comatose state of consciousness as akin to being in a very long lucid dream.

And, of course, there are the scientific wonders — miracles, if you wish — who regain consciousness after spending over a decade in the deepest of sleeps.

Here are four very long comas:

Terry Wallis: 19 Years

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In 1984, Terry Wallis got into a car accident, incurring a head injury that sent him into a 19-year coma. He announced his return to consciousness on June 11, 2006 with the utterance of a single word: Mom.

At the time, Wallis presented an unprecedented case of coma-reversal. His brain, neuroscientists believed, had spent the 19 years rewiring itself. And, following his re-entry to the world, his brain continued to self-heal as he began to exhibit awareness of his disabled condition three years after waking up.

Sarah Scantlin: 16 Years

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In 1984, 18-year-old college freshman Sarah Scantlin was hit by a drunk driver after leaving a bar. Then, a second car ran her over, crushing her skull and sending her into a coma. While Scantlin could still breathe, doctors predicted she’d never regain consciousness. She moved into a nursing home the following year, and slowly regained the ability to swallow food but remained primarily unresponsive.

After 16 years, Scantlin began speech therapy with a nurse who had no speech-therapy training. Within a year, she learned to scream. In 2005, 21 years after her accident, she said her first word: Okay. She can communicate but still believes she’s 19-years-old when, in reality, she’s over 40.

Rom Houben: 23 Years

A car accident left Rom Houben, 23, paralyzed and comatose. Though he couldn’t express it, Houben retained consciousness and tried desperately to communicate — for 23 years. Tests, including a standardized scale that reads eye, verbal and motor responses, initially suggested Houben lay in a vegetative state. The tests were wrong, as subsequent analysis unearthed signs of consciousness. Describing Houben’s coma in a case study, one neurologist wrote that “medical advances caught up with him.” 

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"I dreamed myself away," said Rouben to, on coping with the situation. "All that time I just literally dreamed of a better life. Frustration is too small a word to describe what I felt."

Martin Pistorius: 12 Years

In the late 80s, Martin Pistorius, then a 12-year-old in South Africa, grew mysteriously ill with a suspected form of meningitis. Doctors predicted he’d die, but his parents kept their comatose son on a strict schedule. His father woke up at five a.m. to dress and bathe his son, and then returned at day’s end for supper and bedtime. His parents rotated his body every two hours to prevent bedsores.

Twelve years later, Pistorius began to exhibit awareness. Though he relies on a wheelchair to get around and a computer to assist communication, he's regained full awareness. He also believes he began to wake up about two years into his 12-year sleep, as he remembers aspects of his life when no one realized he could hear them. He felt especially sad, NPR reported, when nurses stuck him in front of a TV playing children’s shows. He hated Barney.

Pistorius is now 39 and living in England. He chronicled his 12-year mental imprisonment in a memoir, Ghost Boy.