New York: Women with severe obesity often report an underlying drive to eat continually because their brain's reward centres continue to respond to food cues even after they have eaten and are no longer hungry, a study says. The findings showed that obese study participants maintained activation in the midbrain, one of the body's most potent reward centres. The activity in the prefrontal cortex and posterior cingulate cortex significantly changed in the lean group, after eating, but not in the obese group. However, this brain activity dropped among lean participants while continuing in their obese counterparts. "Before or after the meal, they're just as excited about eating. It seems they have an instinctive drive to keep eating," said Nancy Puzziferri, Assistant Professor at the University of Texas Southwestern, in the US. Further, while the appeal of pictured food dropped by 15 per cent for lean women after they ate, the severely obese women showed only a 4 percent decline. "Lean women when full will either stop eating or just sample a food they crave. It's just not a level playing field -- it's harder for some people to maintain a healthy weight than others," Puzziferri explained. For the study, published recently in the journal Obesity, the team compared attitudes and the brain activity of 15 severely obese women (those with a body mass index greater than 35) and 15 lean women (those with a BMI under 25). Their brain activity was measured using functional magnetic resonance imaging (fMRI). After fasting for nine hours, they were asked to rate their level of hunger or fullness and then given a brain scan as they viewed pictures of food. Again, after eating, the participants went through another battery of hunger/fullness ratings and fMRI scans while being exposed to pictures of food. The obese women showed sustained "hungry" brain activation, even though they reported the same increase in satiation as their lean counterparts, the researchers concluded.