Pain in the brain

The brain circuits responsible for learning how to avoid pain are distinct from those responsible for learning how to pursue rewards, reports a study published online this week in Nature Neuroscience. These findings contribute to the understanding of how pain and pleasure shape learning and motivate decisions in people.

Previous studies have found that unexpected rewards, such as greater than anticipated wages, result in signals in specific brain circuits that drive learning and encourage behavior that would likely lead to similar beneficial situations.

Tor Wager and colleagues asked participants to perform a task whereby they had to choose between two options, which lead to the delivery of different amounts of painful heat or no pain at all. Using functional brain imaging and computational models of learning the authors identified a region within the periaqueductal gray (PAG) that met all the criteria for signaling discrepancies between the anticipated and received amount of pain. The PAG receives direct information from the spinal cord about pain; the authors found it also received signals about the amount of expected pain from other regions of the brain, such as the ventromedial prefrontal cortex, an area known to also signal expected rewards. In addition, Wager and colleagues found that the PAG signaled to the mid-cingulate cortex, a region critical for pain avoidance.

Thus, the authors conclude that the PAG can compare the amount of experienced pain with that of expected pain and this difference can be used by other brain regions to discourage future harmful behavior.