The study of brain activity in brain-damaged patients in a minimally conscious state (MCS) suggests that they could have similar pain perception to healthy people and could need painkillers.
The study, written by Dr Steven Laureys, Coma Science Group, Cyclotron Research Center and Department of Neurology, University of Liege, Belgium, and colleagues, has been published in Lancet Neurology.
"These findings might be objective evidence of a potential pain perception capacity in patients with MCS, which supports the idea that these patients need painkilling treatment," New Scientist quoted the research team, as saying.
However, they found much less evidence that "brain-dead" coma patients in a so-called persistent vegetative state (PVS) react to pain.
The researchers discovered after taking brain scans of patients and healthy volunteers as they received mild electric shocks.
They performed the procedure on five MCS patients, 15 healthy controls and 15 "brain-dead" patients.
Patterns of brain activity were the same MCS patients and the healthy controls, who rated the pain they received as "highly unpleasant to painful".
Blood flow increased to parts of the brain that form the so-called "pain matrix", incorporating the thalamus and various parts of the cortex activated when we feel pain.
"I think it definitely means they feel pain because they activate the whole pain matrix," Laureys said.
"But what they feel is still an open question, whether they feel it the same way we do," he added.
Conversely the activity was much reduced in the PVS patients.
"The difference between patients with MCS and PVS was very striking," Laureys said.