WASHINGTON, DC—The experience of physical pain depends on nociceptors, which send information from damaged body tissue to the brain; but loneliness, rejection, and other forms of social pain also send signals that are readily detectable and often equally distressing, according to researchers at the recent Society for Neuroscience meeting who presented their findings here during a symposium on the neurobiological bases of social pain.
The sensation of social pain makes evolutionary sense, according to Naomi Eisenberger, PhD, assistant professor in the department of psychology at the University of California, Los Angeles. “Jaak Panksepp suggested that the social attachment system may have piggybacked onto the pain system to promote survival,” she said, referring to the author of AffectiveNeuroscience, a classic work on the emotional systems of the brain. The “pain” of separation, for example, produces vocalizations and other distress behaviors that involve the dorsal anterior cingulate cortex (dACC), according to Dr. Eisenberger, and such vocalizations in a rat pup separated from its mother are drastically reduced by lesioning that region. In contrast, electrically stimulating the region causes spontaneous vocalizations.
The dACC distinguishes between the affective and sensory components of pain — the physical sensation and the psychic suffering, according to Dr. Eisenberger. “A cingulotomy performed on chronic pain patients, for example, leaves patients feeling pain, but they say it no longer bothers them, suggesting that this region involves the suffering component of painful experience,” Dr. Eisenberger said.
In an effort to determine how rejection by others activates the brain, Dr. Eisenberger and her colleagues performed brain scans while subjects played “cyberball,” a video game in which the subject tosses a ball with two virtual players displayed on a screen. At some point the virtual players stop throwing the ball to the subject, producing a sense of rejection that activates the dACC and the anterior insula, a brain region that responds to the unpleasantness of painful experience. “We found that those who showed greater activity in the dorsal cingulate reported greater levels of distress,” Dr. Eisenberger said.
Physical pain and social pain also both respond to acetaminophen (Tylenol), according to another study. Subjects who took acetaminophen in a blinded fashion for three weeks reported fewer episodes of hurt feelings during that period, compared to those randomized to placebo. Also, subjects who took acetaminophen for three weeks and then had their brain scanned while playing cyberball “showed less activation in the dorsal cingulate and anterior insula compared to placebo,” Dr. Eisenberger said. “The data support the idea that social and physical pain rely on shared neural substrates.”
But a computer game may not really arouse feelings of rejection, according to neurologist Andrew Feigin, MD, associate investigator at the Center for Neurosciences at The Feinstein Institute for Medical Research at the North Shore – LIJ Health System, who was not involved with the studies.
“These findings are interesting and provocative, but from a common sense perspective I would wonder whether someone playing a game on a computer would know they're not really being jilted,” said Dr. Feinstein, who serves on the editorial advisory board for Neurology Today. “To generalize the results and say, this is what's activated in a person whose feelings are hurt — that seems like a bit of a stretch. I find it hard to believe that if I were playing a game on a computer and a virtual player stopped throwing a ball to me I would get the same feeling of being socially ostracized that I would get from being socially ostracized in real life. I'm not even sure it would be qualitatively the same.”
ROLE OF SOCIAL SUPPORT
Dr. Eisenberger also found that social support appears to alleviate pain. When 28 women received painful heat stimulation, those who could hold their partner's hand reported less pain than those who held a stranger's hand, looked at a picture of their partner or a neutral picture, or squeezed a ball.
The results of such experiments show that social pain and physical pain produce similar activity in the brain, and may both contribute to survival. “Even though experiences of social pain are distressing, they also serve a valuable function, and that is to motivate us to avoid social rejection and to maintain social connection, which is ultimately critical for survival,” Dr. Eisenberger said.
Loneliness, a chronic state of perceived social isolation, constitutes another form of social pain according to Catherine J. Norris, PhD, assistant professor of psychological and brain sciences at Dartmouth College who conducted her graduate work with University of Chicago psychology professor John Cacioppo, author of Loneliness: Human Nature and the Need for Social Connection.
Dr. Norris collected brain scans of people who varied in loneliness and found that when lonely people looked at pleasant pictures that didn't have people in them they displayed greater activity in the ventral striatum, a brain region involved in primary rewards like drugs and food, as well as social rewards like love and cooperation. Non-lonely people, in contrast, who feel embedded in social networks, showed greater activation in the ventral striatum when they looked at pictures with people in them.
“These findings suggest that non-lonely people may feel more rewarded by pleasant social scenes,” Dr. Norris said, “and lonely people feel more rewarded by pleasant non-social scenes.”
AWARENESS OF DISTRESS IN OTHERS
The awareness of distress in others appears to exist even in mice, according to Jeffrey S. Mogil, PhD, E.P. Taylor Professor of Pain Research at McGill University. He and his colleagues tested the pain response in mice by dipping their tail into hot water and recording how long it took them to pull it out. After thousands of trials they noticed what he calls the “order of testing effect.” Each subsequent mouse tested from a cage “became increasingly sensitive to pain,” he said. However, if the tested mice were not put back into the same cage, the effect disappeared, which caused the researchers to wonder if the mice were somehow communicating their distress, and thereby inducing anxiety among their cage mates who were about to be tested themselves.
Subsequent testing confirmed that the emotional contagion involved visual information, most likely from facial expressions such as cheek bulges.
In addition, the researchers found that a female mouse kept in proximity to a cage mate in pain would spend more time near that mouse, but not near an unfamiliar mouse in pain. Dr. Mogil could not explain this behavior.
People also respond to pain in others, of course, and show greater empathy for members of their own group than for others, according to Emile Bruneau, PhD, a post-doctoral fellow who studies conflict resolution in the department of brain and cognitive sciences at MIT.
He and his colleagues performed brain scans on people while they read narratives of others experiencing either physical pain, such as getting cut with a knife, or emotional pain, such as being rejected after a marriage proposal. The brain regions most sensitive to physical pain were the anterior middle cingulate cortex, the bilateral insula, secondary sensory regions, and the extrastriate body area. The brain regions most sensitive to the emotional pain of others were the medial prefrontal cortex, the precuneus, and the bilateral temporoparietal junction.
“They're completely distinct,” Dr. Bruneau said. “There were no overlapping voxels even though the stories were behaviorally related to each other.”
Young people with disruptive behavior disorders who display psychopathic traits appear to be deficient in empathy for all other people, according to Stuart F. White, PhD, a post-doctoral research fellow in the Unit of Affective Cognitive Neuroscience at the National Institute of Mental Health.
He and his colleagues performed fMRI scans on 15 such young people, along with 17 healthy controls, as they completed a task while viewing images of fearful expressions under different attention conditions. They found less amygdala response to fearful expressions among those with disruptive behavior disorders and psychopathic traits. However, there was no disruption in the activity of brain regions involved in attention, contradicting a countervailing hypothesis that disorders of attention rather than amygdala dysfunction are the primary cause of psychopathy.
Comparing social pain to physical pain requires great caution, according to pain expert Russell Portenoy, MD, chairman of the department of pain medicine and palliative care at Beth Israel Medical Center in New York City, who was not involved with the studies.
“Given the extraordinary complexity of clinical pain, the finding that there are similarities in brain activation following either noxious events or social rejection does not mean that social rejection is processed like physically painful events, or is perceived as a type of pain,” he said. “The same areas might be activated by various epiphenomena of pain, or by downstream effects produced by both experiences. It's a fascinating observation and will undoubtedly be clarified by future research.”