A "fingerprint of consciousness" has been found in the brains of patients who appear to be in a vegetative state but remain aware of their surroundings.
The discovery will help doctors recognise awareness in unresponsive brain-damaged patients who might otherwise be considered beyond hope.
It could even influence heart-breaking decisions of life and death which can lead to a patient having treatment withdrawn.
A patient in a vegetative state is physically awake but shows no signs of consciousness.
In contrast, coma patients are not awake and those in a minimally conscious state show clear evidence of sporadic awareness - such as blinking in response to questions.
Recent brain-scan studies have revealed that some patients classified as vegetative are able to carry out conscious tasks such as imagining playing a game of tennis.
The new UK research shows that patients with hidden awareness also seem to possess well-preserved networks of brain neurons that look similar to those of healthy individuals.
Lead scientist Dr Shrivas Chennu, from the Department of Clinical Neurosciences at Cambridge University, said: "Understanding how consciousness arises from the interactions between networks of brain regions is an elusive but fascinating scientific question.
"But for patients diagnosed as vegetative and minimally conscious, and their families, this is far more than just an academic question - it takes on a very real significance.
"Our research could improve clinical assessment and help identify patients who might be covertly aware, despite being uncommunicative."
The team used a specialised form of electroencephalography (EEG) to study networks of activity in the brains of 32 patients diagnosed as vegetative or minimally conscious, as well as a number of healthy adults.
The findings, published in the online journal Public Library of Science Computational Biology, could lead to a relatively simple and low-cost bedside awareness test.