Brain scans from a patient in a vegetative state have revealed signs of awareness, suggesting that some patients may be aware of their surroundings and able to hear and understand.
The study, which has been published in the journal Science, is based on a 23-year-old female who was left in a persistent vegetative state (PVS) following a road accident.
Doctors have previously thought that PVS patients are unconscious and unaware, with no response to external stimuli.
However, MRI brain scans which were taken five months after the female patient's accident have revealed that she understood spoken commands and was able to imagine playing tennis and walking around her home when asked to do so.
When compared with scans of healthy volunteers who were asked to imagine the same scenarios, the images revealed activity in the same parts of both the female patient's and the healthy volunteers' brains.
Lead researcher Dr Adrian Owen of the Medical Research Council's Cognition and Brain Sciences Unit described the study's 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 commented.
"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."
However, Dr Owen emphasised that the results had only be obtained from one patient, although he added that the technique could enable doctors to identify those patients with some level of awareness.
Researchers now intend to continue their studies in order to determine whether a technique could be developed to communicate with PVS patients who are aware, but unable to move or speak.