Neurologists who performed the tests said they hinted that Sharon, who has been in a coma since suffering a stroke in 2006, may have a degree of consciousness and be able to hear sounds or make out pictures.
Video courtesy of jn1.tv
"It's encouraging to find these signs because it opens up the possibility of some meaningful communication," said Paul Matthews, a professor of neurology at Imperial College London.
Until recently, he said, it had been assumed that many comatose patients diagnosed as being in a "vegetative state" had no meaningful awareness of their surroundings.
Yet progress in the use of functional magnetic resonance imaging (fMRI) scanners – machines that measure brain activity by detecting changes in blood flow – and in reading the signals they give has begun to change that view.
Compared with other "vegetative state" patients who have had similar tests and been described in scientific papers, Sharon's responses were very faint, said Martin Monti, a cognitive psychologist from the University of California Los Angeles, who co-led the American-Israeli team that scanned Sharon's brain.
There is little or no likelihood of a rapid recovery by Sharon, a former general, hawkish defense chief and leader of the right-wing Likud party who suffered his stroke weeks after leaving Likud to found a centrist group to pursue peace with the Palestinians.
But with more tests and research to devise a way for him to signal whether he is processing external information, it is possible that he, like others, could one day respond to questions about his state of mind and whether he is in pain.
"If it turns out that these signals are more robust, then there's no doubt that we could use things like brain-computer interfaces (to communicate)," Monti told Reuters in an interview as he travelled from Israel back to the United States.
The science behind such possibilities has made significant advances in recent years, allowing some patients previously thought to be completely unaware to show they are in fact conscious and able – with help – to communicate.
A study published by Monti and colleagues in 2010 in the New England Journal of Medicine (NEJM) was among the first to show how fMRI scans allowed doctors to detect awareness and communicate with patients in a long-term coma.
"The technology is still very new and we haven't by any means optimized this kind of work yet," said Steve Williams, of King's College London's center for neuroimaging sciences.
"But if you do see something, it must give so much hope. And if you can get robust and consistent activation of the brain ... then you can take it a lot further."
The experiments use imagery tasks – such as asking a patient to imagine playing tennis or walking from room to room in their home. The fMRI scanner maps the distinctive activity of the brain when each task is performed.
The patient is then asked to equate one of the imagery tasks to the answer "yes" and another to the answer "no", and apply them to simple yes-or-no questions.
These tests "give us a lot of information about how we can contact these patients and stimulate them", said Alon Friedman, a neurological director at Israel's Soroka Medical Center in Beersheba, who worked with Monti on Sharon's scans.
Of the 54 patients described in the NEJM study, all diagnosed as being in a vegetative or minimally conscious state, five were able to "willfully modulate their brain activity", the researchers said, and one was able to answer yes-or-no questions during the scan.
EEG caps to aid communication
In the 84-year-old Sharon's case, evidence that he was able to perform the imagery tasks "was there but was not as strong as we have seen in other patients", Monti said.
"We're very cautious. There is a little evidence that he might have been doing some of the things we asked of him, but it was extremely weak and faint so it is difficult to interpret."
If further tests on Sharon were to detect a more robust response, scientists say the next step would be to investigate ways to make communication easier.
One possibility being explored in some vegetative state patients who show an ability to control brain processes is the use of electroencephalography (EEG) caps, which attach electrodes to the scalp and record electrical brain activity.
Research published in 2011 showed that scientists using these devices were able to communicate with people who had been considered to be in a vegetative state for more than a year.
Because the caps are mobile, they allow communication to be more frequent, since putting a patient into an fMRI scanner every time doctors want to ask a question is invasive, disruptive and expensive.
"If you can put an EEG cap on somebody's head, and the computer is trained to recognize different brain states, that's like having a language," Monti said. "You could then imagine we could proceed from there."