Modeling pain circuits: how imaging may modify perception.

Pain is the unpleasant sensory experience following tissue damage or the threat of damage. The activation of cortical regions during noxious stimulation is believed a result of the negative affect and sensations generated by the stimulus. How a noxious event is translated into pain experience remains uncertain, and pain that occurs in the absence of a noxious event remains mysterious. Acute pain and chronic pain depend on a categorization of feeling that occurs collectively rather than individually. Capturing that process inside a brain scan is problematic. Resolving this problem requires an approach to imaging that transgress the boundaries of physical and social concepts.

Feeling the pain of others is associated with self-other confusion and prior pain experience.

Some chronic pain patients and healthy individuals experience pain when observing injury or others in pain. To further understand shared pain, we investigated perspective taking, bodily ownership and tooth pain sensitivity. First, participants who reported shared pain (responders) and those who did not (non-responders) viewed an avatar on a screen. Intermittently, 0-3 circles appeared. Sometimes the participant's and avatar's perspective were consistent, both directly viewed the same circles, and sometimes inconsistent, both directly viewed different circles. Responders were faster than non-responders to identify the number of circles when adopting a consistent perspective. Second, participants sat with their left hand hidden while viewing a rubber hand. All participants reported an illusory sensation of feeling stroking in the rubber hand and a sense of ownership of the rubber hand during synchronous stroking of the rubber and hidden hand. The responders also reported feeling the stroking and a sense of ownership of the rubber hand during asynchronous stroking. For experiment three, participants with either low, moderate, or high tooth sensitivity observed a series of images depicting someone eating an ice-popsicle. Low sensitivity participants never reported pain. In contrast, moderate and high sensitivity participants reported pain in response to an image depicting someone eating an ice popsicle (4 and 19% of the time, respectively) and depicting someone eating an ice-popsicle and expressing pain (23 and 40%, respectively). In summary, responders have reduced ability to distinguish their own and others' visual perspective and enhanced ability to integrate a foreign arm into their bodily representation. The tendency to share pain is also enhanced when an observed pain is commonly experienced by the observer. Shared pain may therefore involve reactivation of pain memories or pain schema that are readily integrated into a self perspective and bodily representation.

Pain sensation evoked by observing injury in others.

Observing someone else in pain produces a shared emotional experience that predominantly activates brain areas processing the emotional component of pain. Occasionally, however, sensory areas are also activated and there are anecdotal reports of people sharing both the somatic and emotional components of someone else's pain. Here we presented a series of images or short clips depicting noxious events to a large group of normal controls. Approximately one-third of this sample reported an actual noxious somatic experience in response to one or more of the images or clips. Ten of these pain responders were subsequently recruited and matched with 10 non-responders to take part in an fMRI study. The subjects were scanned while observing static images of noxious events. In contrast with emotional images not containing noxious events the responders activated emotional and sensory brain regions associated with pain while the non-responders activated very little. These findings provide convincing evidence that some people can readily experience both the emotional and sensory components of pain during observation of other's pain resulting in a shared physical pain experience.