Scientists have discovered a hidden sign of consciousness in comatose patients that shows they can hear and understand the world around them.

The study found bursts of organized, fast frequencies within the patient’s normal sleep patterns, revealing a level of awareness even as they were still in a coma due to their injuries.

Researchers at Columbia University analyzed 226 recent comatose patients, observing a third displayed the unique bursts – a phenomenon scientists call ‘sleep spindles.’

Brain circuits that are fundamental for consciousness are also key to how we sleep, the Columbia team explained.

Moreover, scientists said comatose patients with this type of ‘hidden consciousness’ showed signs they were already on the road to recovery from their brain injuries and many dealt with fewer disabilities later in life.

Among those displaying sleep spindles, 76 percent showed some level of consciousness before leaving the hospital and over 40 percent recovered some of their neurological function. 

Lead author Jan Claassen, a professor of neurology, said that this discovery provides new hope for families with critically injured loved ones.

‘We’re at an exciting crossroad in neurocritical care where we know that many patients appear to be unconscious, but some are recovering without our knowledge,’ said Claassen.

‘We’re starting to lift the lid a little bit and find some signs of recovery as it’s happening,’ the lead study author continued.

Scientists have discovered unique signals in the sleep patterns of comatose patients, revealing that they have a ‘hidden’ level of consciousness

The connection between certain brain circuits and sleep is important for explaining a condition called cognitive motor dissociation, where unconscious patients still show evidence of consciousness during brain scans but have no detectable ability to respond to outside stimuli – like someone talking to them.

Until now, scientists have mainly focused on EEG recordings to tell if comatose patients were still conscious in some way.

These recordings test to detect abnormalities in brain waves, based on how well the person undergoing the EEG responds to a doctor’s instructions, such as opening and closing their hand. However, Prof. Claassen noted that these recordings can lead to false-negative results.

So, the Columbia team shifted their focus to tracking the activity present in brain wave patterns during normal sleep.

‘The electrical activity during sleep looks relatively chaotic, and then occasionally in some patients, these very organized, fast frequencies appear,’ Claassen explained.

The associate professor noted that these unique bursts in the middle of all that chaos are the sleep spindles displaying hidden consciousness inside the comatose mind.

‘Spindles happen normally during sleep and they’re showing some level of organization in the brain, suggesting circuits between the thalamus and cortex needed for consciousness are intact,’ Claassen detailed.

The thalamus acts like your brain’s information relay station, while the cortex contributes to higher brain functions like memory, thinking, learning, reasoning, and problem-solving. 

The brain activity are sleep spindles and researchers said it shows unconscious patients can still hear and understand what is happing around them

According to the Cleveland Clinic, all of the information coming in from your body’s senses (except smell) has to be processed by the thalamus before it’s sent to the brain’s cortex for interpretation.

For the patients in Claassen’s study, the spindles show that they’re still hearing people talk and feel them squeeze their hand, and those sensations are still traveling to the brain’s cortex where the mind reacts to what’s happening – even if people sitting next to them can’t see a noticeable reaction.

During the study, researchers examined the overnight EEG recordings of 226 comatose patients. This group also underwent more complex testing for cognitive motor dissociation. The team noted that all of these injuries were recent and their findings likely don’t apply to those with long-term disorders causing unconsciousness.

The study in Nature Medicine focused specifically on sleep among patients in a coma after a recent brain injury. 

Results revealed that 87 patients displayed signs of both sleep spindles and cognitive motor dissociation.

Those with clear signs of the sleep spindles in their brain waves were significantly more likely to regain consciousness.

They were also more likely recover from their severe brain injury with only minor neurological deficits or a moderate disability.

Meanwhile, just 19 of the 139 comatose patients who did not experience sleep spindles or cognitive motor dissociation regained any level of consciousness.

‘I see these spindles as a way to direct more sophisticated testing to the patients most likely to benefit,’ Claassen said. ‘The techniques are not ready for use in clinical practice yet, but this is something that we’re actively working on right now.’ 

The lead study author added their study doesn’t prove that inducing sleep spindles could help brain injury patients regain consciousness, but there are signs that moving these patients to areas which promote better sleep could improve their chances of recovery.

‘If you think about the ICU environment, it is rather disruptive for a good night’s sleep. There is noise everywhere, alarms going off, clinicians touching them, 24/7. This is all for a good reason, but it’s hard to sleep in that environment,’ Claassen said.