There was nothing very interesting in Katherine P. Rankin’s study of sarcasm — at least, nothing worth your important time. All she did was use an M.R.I. to find the place in the brain where the ability to detect sarcasm resides. But then, you probably already knew it was in the right parahippocampal gyrus.What you may not have realized is that perceiving sarcasm, the smirking put-down that buries its barb by stating the opposite, requires a nifty mental trick that lies at the heart of social relations: figuring out what others are thinking. Those who lose the ability, whether through a head injury or the frontotemporal dementias afflicting the patients in Dr. Rankin’s study, just do not get it when someone says during a hurricane, “Nice weather we’re having.”
“A lot of the social cognition we take for granted and learn through childhood, the ability to appreciate that someone else is being ironic or sarcastic or angry — the so-called theory of mind that allows us to get inside someone else’s head — is characteristically lost very early in the course of frontotemporal dementia,” said Dr. Bradley F. Boeve, a behavioral neurologist at the Mayo Clinic in Rochester, Minn.
“It’s very disturbing for family members, but neurologists haven’t had good tools for measuring it,” he went on. “That’s why I found this study by Kate Rankin and her group so fascinating.”
Dr. Rankin, a neuropsychologist and assistant professor in the Memory and Aging Center at the University of California, San Francisco, used an innovative test developed in 2002, the Awareness of Social Inference Test, or Tasit. It incorporates videotaped examples of exchanges in which a person’s words seem straightforward enough on paper, but are delivered in a sarcastic style so ridiculously obvious to the able-brained that they seem lifted from a sitcom.
“I was testing people’s ability to detect sarcasm based entirely on paralinguistic cues, the manner of expression,” Dr. Rankin said.
In one videotaped exchange, a man walks into the room of a colleague named Ruth to tell her that he cannot take a class of hers that he had previously promised to take. “Don’t be silly, you shouldn’t feel bad about it,” she replies, hitting the kind of high and low registers of a voice usually reserved for talking to toddlers. “I know you’re busy — it probably wasn’t fair to expect you to squeeze it in,” she says, her lips curled in derision.
Although people with mild Alzheimer’s disease perceived the sarcasm as well as anyone, it went over the heads of many of those with semantic dementia, a progressive brain disease in which people forget words and their meanings.
“You would think that because they lose language, they would pay close attention to the paralinguistic elements of the communication,” Dr. Rankin said.
To her surprise, though, the magnetic resonance scans revealed that the part of the brain lost among those who failed to perceive sarcasm was not in the left hemisphere of the brain, which specializes in language and social interactions, but in a part of the right hemisphere previously identified as important only to detecting contextual background changes in visual tests.
“The right parahippocampal gyrus must be involved in detecting more than just visual context — it perceives social context as well,” Dr. Rankin said.
The discovery fits with an increasingly nuanced view of the right hemisphere’s role, said Dr. Anjan Chatterjee, an associate professor in the Center for Cognitive Neuroscience at the University of Pennsylvania.
“The left hemisphere does language in the narrow sense, understanding of individual words and sentences,” Dr. Chatterjee said. “But it’s now thought that the appreciation of humor and language that is not literal, puns and jokes, requires the right hemisphere.”
Dr. Boeve, at the Mayo Clinic, said that beyond the curiosity factor of mapping the cognitive tasks of the brain’s ridges and furrows, the study offered hope that a test like Tasit could help in the diagnosis of frontotemporal dementia.
“These people normally do perfectly well on traditional neuropsychological tests early in the course of their disease,” he said. “The family will say the person has changed dramatically, but even neurologists will often just shrug them off as having a midlife crisis.”
Short of giving such a test, he said, the best way to diagnose such problems is by talking with family members about how the person has changed over time.
After a presentation of her findings at the American Academy of Neurology’s annual meeting in April, Dr. Rankin was asked whether even those with intact brains might have differences in brain areas that explain how well they pick up on sarcasm.
“We all have strengths and weaknesses in our cognitive abilities, including our ability to detect social cues,” she said. “There may be volume-based differences in certain regions that explain variations in all sorts of cognitive abilities.”
So is it possible that Jon Stewart, who wields sarcasm like a machete on “The Daily Show,” has an unusually large right parahippocampal gyrus?
“His is probably just normal,” Dr. Rankin said. “The right parahippocampal gyrus is involved in detecting sarcasm, not being sarcastic.”
But, she quickly added, “I bet Jon Stewart has a huge right frontal lobe; that’s where the sense of humor is detected on M.R.I.”
A spokesman for Mr. Stewart said he would have no comment — not that a big-shot television star like Jon Stewart would care about the size of his neuroanatomy.
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