A patient in a vegetative state was able to pay attention to sounds in his surroundings, a study found.
An analysis of brain activity in 21 patients and eight healthy volunteers showed one of the patients could pick out individual sounds.
The University of Cambridge researchers say the finding marks an advance in understanding levels of consciousness in vegetative patients.
The team hope to develop better ways for some patients to communicate.
A vegetative or minimally conscious state can leave someone with no higher cognitive function. The trauma can be caused by a car accident or heart attack.
While patients may still be able to move their eyes and limbs, they cannot do so on command.
The patients and volunteers had their brains scanned during the experiment.
They were all played a word every second for 90 seconds. They had been told to count the number of times the word "yes" or "no" appeared during the stream of words.
Brainwave records showed one patient's brain activity was "indistinguishable" from the healthy patients, suggesting he was able to focus his attention on the words.
Three other patients showed some brain response, but it appeared to be an involuntary action.
The remaining patients showed no response at all.
Dr Srivas Chennu, from the University of Cambridge, told the BBC that the aim was to change the way patients were treated to match their level of consciousness.
He said: "We are never going to fix these patients, but where we want to get to is doctors having the best information to make decisions for the patient.
"At the moment it's a one size fits all approach. As we don't know what's going on, it's not tailor made to the patient."
In some cases this could include communication with the patient.
Dr Chennu said: "We hope to have tools to help a patient communicate and this takes us a significant step forward."
Dr Tristan Bekinschtein, from the Medical Research Council Cognition and Brain Sciences Unit, said: "Our attention can be drawn to something by its strangeness or novelty or we can consciously decide to pay attention to it.
"A lot of cognitive neuroscience research tells us that we have distinct patterns in the brain for both forms of attention, which we can measure even when the individual is unable to speak.
"These findings mean that, in certain cases of individuals who are vegetative, we might be able to enhance this ability and improve their level of communication with the outside world."