Murder or an Accident? The Brain Knows

29 March 2010 5:39 pm

If a stranger steps on your foot, you'd probably shrug your shoulders and assure him that no harm has been done, even if your toes are throbbing like crazy. But if that stranger instead takes a swing with his fist-successfully or not—most people are unlikely to be so forgiving. Researchers now believe they've demonstrated which part of the brain allows us to make moral judgments of another person's motives, a find that could lead to a greater understanding of Asperger syndrome and other autism spectrum disorders.

Scientists already have some clues about how we judge the actions of another person. Previous research using functional magnetic resonance imaging, a method of imaging activity in the brain, has shown that an area just above the right ear called the right temporoparietal junction (RTPJ) receives more blood than usual when we read about people’s beliefs and intentions, particularly if we use the information to judge people negatively. But it's not possible to say from a simple observational study whether the brain activity is actually necessary to make such a judgment or whether making the negative judgment causes this region to become more active.

So social neuroscientist Liane Young of the Massachusetts Institute of Technology in Cambridge and colleagues decided to turn off the right temporoparietal junction and see whether people would make different judgments of others' actions. They achieved this using transcranial magnetic stimulation (TMS), a technology that uses a tightly focused magnetic field to temporarily disable individual regions of the brain.

The researchers aimed the device either at the RTPJ or at a nearby region of the brain not thought to be involved in cognitive processing. Subjects between the ages of 18 and 30 read stories containing a series of hypothetical scenarios on a computer screen. In some, person A accidentally killed person B; in others, person A intended to kill person B but failed. Subjects were asked to rate from one to seven how excusable they found person A's conduct from “not at all” to “completely.” The subjects considered some scenarios with their temporoparietal junction turned off and other scenarios with the area functioning as normal.

In both cases, volunteers judged unsuccessful attempts at murder to be more egregious than cases of accidental killing. However, subjects were significantly more forgiving of attempted murder when their right temporoparietal junctions were knocked out by TMS than when they were functioning normally, the team reports online today in the Proceedings of the National Academy of Sciences. The findings imply that activity in the region is required for us to properly judge another person's motives, say the researchers.

The team is now working with Asperger sufferers: psychologists believe that Asperger syndrome and other autism spectrum disorders are characterized by an inability to discern other people’s intentions. (This is known as lacking a “theory of mind.”) “Just as we were able to disrupt ‘theory of mind’ using TMS, autistic participants might be individuals who have naturally disrupted ‘theory of mind’ processes," speculates Young.

Sarah-Jayne Blakemore, a cognitive neurologist at University College London, says that the results may explain the changes in how we view the world as we grow up. “In humans, the temporoparietal junction continues to develop into adolescence and beyond," she says. But she cautions that disabling one region of the brain can affect others, making it difficult to say with certainty that one area alone is responsible for a given effect.