Tracey’s latest research has investigated a key neural mechanism of chronic pain. It is situated in the brain stem, a hard-to-reach, tube-shaped mass of gray matter at the top of the spinal cord, which functions as the conduit for communication between the brain and the body. Experiments on animals had identified two mechanisms within the brain stem that, respectively, muffle and boost pain signals before they reach the rest of the brain. Since Tracey’s lab first succeeded in imaging the region, more than a decade ago, she has been able to show how these two mechanisms operate. “It can completely block the signals coming in,” she said of one, explaining that it is responsible for situations in which you don’t feel pain even though you should—for instance, when your brain is distracted by the euphoria of crossing the finish line of a marathon.

Unfortunately, in some people the mechanism that exacerbates pain is dominant. Scanning the brains of patients with diabetic nerve pain, Tracey and Segerdahl found enhanced communication from the brain stem, via the spine, to the parts of the brain known to contribute to the sensation of pain.

Although the results of this work won’t be known for many years, her brain-stem research is already on its way to a clinical application. A few years ago, in collaboration with the rheumatologist Anushka Soni, Tracey began imaging the brains of osteoarthritis patients before and after knee-replacement surgery. Roughly a fifth of patients who have knee replacements find that the operation doesn’t meaningfully reduce their pain, and, again, no one knows why. But when Tracey analyzed the scans she found that the unlucky patients had increased activity in the mechanism of the brain stem known to amplify pain signals. Their brains revealed that they had “gone chronic”; they were not just ordinary people whose knees hurt.

Although it’s not feasible to give every prospective patient a brain scan, results from fMRI experiments correlate strongly with responses to a questionnaire called painDETECT, which was developed to diagnose nerve malfunction. Such a questionnaire could predict the likely outcome of surgery, so that patients could make an informed decision about whether the procedure was worth it. Tracey is also testing, on a group of twenty-four volunteers, a compound that she hopes could dampen activity in the problematic brain-stem region. In time, patients who seem predisposed to less successful surgical outcomes may be given a drug that makes relief likelier by adjusting their brain-stem biochemistry.