MONDAY, Oct. 28 (HealthScoutNews) -- That pain in your back may really be in your head. Abnormal pain-processing pathways in the brain may be the culprit in people who have lower back pain that doctors can't trace to a specific physical cause, says a study presented this weekend at the American College of Rheumatology's annual meeting in New Orleans. The University of Michigan researchers say there's no explanation for this effect, but it's similar to altered pain perception they found in people with fibromyalgia in a previous study. Using a super-fast form of magnetic resonance imaging (MRI), the researchers looked at the brains of 15 people with lower back pain. Their body scans showed no apparent physical cause, such as a ruptured disk, for their back pain. They were compared to 15 people with fibromyalgia and 15 control subjects without back pain or fibromyalgia. All the study participants were give the MRI scans for 10 minutes while varying degrees of controlled pressure was applied to the base of their left thumbnail. The researchers found it took only mild pressure to make the people with lower back pain and fibromyalgia report they felt pain. When subjected to the same amount of pressure, the people in the control group reported little pain. In the people with lower back pain and fibromyalgia, that mild pressure produced measurable responses in brain areas the process pain sensation. Those brain responses didn't occur in the control group until they were subjected to substantial pressure on their thumbs. The study notes striking differences in areas of pain-related brain activity in all three groups. People with lower back pain showed no increased activity in two brain areas that were active in both the people with fibromyalgia and those in the control group. At the same time, the people with fibromyalgia had increased activity in two other brain areas that weren't active in those with lower back pain or in the control group. The findings may help lead to a better understanding of lower back pain and pain perception in general, the researchers say.