The unconscious brain is difficult to investigate. Yet we know that even when the body rests deeply in a coma — beyond the reach of sensation and thought the brain works to pump blood, move air into and out of the lungs, and digest food.

Decades ago, physicians who gave their patients anesthesia before surgery assumed they would be unable to perceive anything while unconscious. However, several studies have reported that after regaining consciousness, some patients said they heard and understood various conversations that took place while they were unconscious.

In one early study, patients experienced one or more of five states: unconsciousness, inner consciousness, perceived unconsciousness, distorted consciousness and paranormal experiences. They described hearing, understanding and responding emotionally to what was being said when it was assumed they were not aware. In another early study, 23 patients reported near-death experiences or visits, out-of-body experiences or some other paranormal experiences.

Despite some evidence showing that unconscious patients can remember conversations, little experimental research has been done. A few researchers have investigated the effect of verbal communication on severely unconscious patients.

These studies were looking at whether verbal communication could change a patient's physiological response, and they examined blood pressure, heart and respiratory rate, oxygen saturation, intracranial pressure and levels of restlessness. The results suggested that the direction of influence from conversation on the intracranial pressure is individual and may be influenced greatly by the patient's level of consciousness.

More recently, three Michigan Medicine researchers and authors of three new articles from the Center for Consciousness Science explored what happens during unconsciousness specifically, why we are conscious and how we become unconscious based on patterns of information transfer in the brain.

In the first study, the researchers analyzed different areas of the brain during sedation, surgical anesthesia and a vegetative state. The results showed that in the early stages of sedation, the information-processing timeline gets much longer with local areas of the brain becoming more tightly connected within themselves a tightening that could lead to an inability to connect with distant areas.

In the second study, the researchers explored how the brain integrates information and how it can be measured in the real world, noting that as the brain gets more modular and has more local conversations, the measure of information integration starts to decrease. They looked at how the brain network fragmentation took place and how to measure that fragmentation, which may explain why some people lose consciousness.

In the third study that examined unconsciousness across three different physiological, pharmacological and pathological states, the researchers found that during unconsciousness, disrupted connectivity in the brain and greater modularity create an environment that is not suitable for the kind of efficient information transfer that is required for consciousness.

The mind may be aware of surroundings even while under anesthesia, and these researchers are looking for a better way to quantify the depth of anesthesia in the operating room and assess consciousness in stroke patients and those with brain damage. It may be that, in some cases, consciousness can persist despite unresponsiveness.

By understanding the communication breakdown that occurs in the brain during unconsciousness, researchers may be able to precisely target or monitor these circuits to achieve safer anesthesia and restore these circuits to improve outcomes of coma.