A new study has shown that patients considered to be in a vegetative state may have a "rich" internal life.
Brain images from one patient appear to show an awareness of her surroundings and the ability to hear and think.
The 23-year-old was injured in a road accident 12 months earlier and has been defined as remaining in a persistent vegetative state (PVS) ever since.
However, scientists publishing their findings in the journal Science suggest that she is not in an unconscious state, or unresponsive to external stimuli, as previously thought.
By scanning the PVS patient's brain using an MRI scan, researchers were able to compare her brain activity to that of healthy volunteers when asked to perform functions such as imagining playing tennis or walking around her home.
It was found that the same parts of her brain showed activity in response to the spoken commands as happened in the volunteers.
Dr Adrian Owen, the study's lead researcher, described the results as "startling". "They confirm that, despite the diagnosis of vegetative state, this patient retained the ability to understand spoken commands and to respond to them through her brain activity, rather than through speech or movement," he said.
"Her decision to work with us by imagining particular tasks when asked represents a clear act of intent, which confirmed beyond any doubt that she was consciously aware of herself and her surroundings."
Researchers have emphasised that the results have been observed from just one patient, but there are hopes that the technique could be used to identify the level of awareness that a PVS patient has retained.
Further studies will now be carried out to determine whether it is possible to use scanning to communicate with PVS patients, who are otherwise unable to respond.