A lack of empathy is a signature trait of psychopaths— fascinating in fiction, inexplicable in reality. Now, a new study on psychopathic prisoners reveals strikingly different brain patterns that may limit their ability to emotionally respond to other people's pain.

"This is the first time that neural processes associated with empathic processing have been directly examined in individuals with psychopathy, especially in response to the perception of other people in pain or distress," said lead researcher Jean Decety, a psychology professor at the University of Chicago, in a news release.

While previous studies have found differences between the brain structure of psychopathic convicts and controls, this is the first to observe neural differences in how they respond to distressing situations.

Empathy is a basic and evolutionarily ancient instinct, wrote Decety's team in the study, and sensitivity to the pain of others is one of the earliest forms of it to develop in young children. The neural circuit of empathy is believed to involve connections among outer regions of the brain like the insula, orbitofrontal cortex (OFC), and ventromedial prefrontal cortex (vmPFC), as well as inner regions like the brainstem, amygdala, and hypothalamus.

Psychopathy is a personality disorder in which people have a "callous disregard for others," according to researchers, as well as high impulsivity and aggression. It is estimated to be present in about 1 percent of Americans, and up to 30 percent of the United States prison population.

While not all people with such qualities are menaces to society, psychopaths are more likely to have committed serious crimes like rape, assault, and murder, and to commit repeat offenses. Perhaps not incidentally, some of them may also make excellent business executives.

Previous neuroimaging research on psychopaths has indicated reduced volume in some of these brain regions, as well as weakened connections among them, though it is unclear how such deficits develop. Psychopathic behavior is unlikely to be modified with existing cognitive-behavioral therapies, and the possibility that the disorder stems from intractable differences in brain structure is discouraging for researchers who hope to treat it.

In the hopes of eventually learning enough to develop effective psychological interventions, Decety's team decided to investigate the patterns of brain activity involved in psychopaths' responses to the distress of others.

Their findings, published online today in the journal JAMA Psychiatry, bring researchers slightly closer to accomplishing that goal.

80 incarcerated men aged 18 to 50 volunteered for the study, all prisoners in a medium-security correctional facility who were assessed for psychopathy levels with clinical diagnostic measures.

Prisoners watched two series of videos related to empathy while in an fMRI brain scanner. The first set included clips of one person inflicting pain on another, like intentionally slamming fingers in a car door (above), and the other set showed facial expressions of people responding to pain (below). [JAMA Psychiatry]

The participants went through functional MRI brain scans while they watched and responded to videos of individuals intentionally harming others in malicious actions like pinching, hitting, or slamming their fingers in car doors, as well as videos of pained facial expressions.

Imaging analysis indicated that while watching people get hurt, highly psychopathic participants had far less neural activation in the periaqueductal gray, vmPFC, and lateral OFC, which normally work with the amygdala in order to regulate ongoing behavior, encourage emotional learning, promote moral decision-making, and evaluate consequences. All that makes sense, since deficits in those abilities, which are vital for empathy, are characteristic of psychopathic behavior.

There was also greater activation in the dorsal striatum, posterior superior temporal sulcus (pSTS), and dorsomedial prefrontal cortex (dmPFC), regions that the researchers wrote are involved in mentalizing, or making sense of others' mental states. Since empathy makes it easy to understand another person's pain, it's possible that these three regions were compensating for that deficit.

While looking at distressed facial expressions, however, there was lower activation in those mentalizing regions, suggesting that psychopaths are less responsive to emotions expressed on others' faces.

A surprise finding was heightened activity in the insula while watching both types of videos, which surprised researchers because that region is known to play an important role in experiencing empathy and other emotions, as well as mapping bodily sensations. It's unclear how heightened insula activity might play into psychopathic tendencies- the researchers concluded that "the role of the insula in emotion and empathy is complex and far from being understood."

Decety's team concluded that "examining the neural response of individuals with psychopathy as they view others being harmed or expressing pain is an effective probe into the neural processes underlying affective and empathy deficits in psychopathy."

The study brings our understanding of psychopathy a step further- deficits and hyperactivity in certain regions may not yet fully explain how and why the disorder develops, but there do seem to be distinct differences in how psychopaths' brains respond to other people's distress.