People who no longer recognize sarcasm or lies may be showing early signs of dementia.
The research may help doctors diagnose which type of dementia a patient has and start to treat it early, the researchers said.
"The different dementia patients tended to have difficulty identifying the form of sarcasm we used for our study," said lead researcher Katherine P. Rankin, a neuropsychologist and associate professor of neurology. Whereas many dementia patients could easily recognize a lie, patients with frontotemporal dementia were unable to pick out either sarcasm or lies.
The results of the study were scheduled to be presented Thursday at the annual meeting of the American Academy of Neurology in Honolulu. Research presented at meetings is considered preliminary until it is published in a peer-reviewed journal.
For the study, Rankin's team asked 175 people, over half of whom had dementia, to watch videos of two people talking. At different spots in the tape one person would lie or say something sarcastic. Viewers were given verbal and nonverbal clues to help pinpoint the false or insincere statements, the researchers noted.
The study participants were then asked yes/no questions about what they had seen.
People suffering from corticobasal syndrome, progressive supranuclear palsy, amyotrophic lateral sclerosis, Alzheimer's disease and frontotemporal dementia were included in the study, as well as healthy individuals.
The researchers compared their scores with the results of brain scans that measured loss of volume related to dementia.
Healthy people easily picked out the lies and sarcasm, but those with frontotemporal dementia found it hard to distinguish among lies, sarcasm and fact. Patients with other types of dementia such as Alzheimer's disease performed better on this test, Rankin's team found.
The ability to identify lies is centered in the frontal lobe of the brain, which degenerates with frontotemporal dementia, Rankin noted.
This social decline can be an early sign of frontotemporal dementia, the most common form of dementia among people under 65, according to Rankin. Other early signs of the disease can be dramatic changes in behavior or personality.
These changes are often taken as signs of depression or a midlife crisis, and not recognized for the serious condition they represent, Rankin noted.
"We wanted to know if we could use this test [to gain] a better idea of what disease the person has," she said. They discovered they can, she added.
"We want to find these people early," she explained. "We want people to recognize that these social lapses are actually a disease -- parts of their brain are being eaten away."
People who age without suffering neurodegeneration usually do not have a significant loss of their ability to recognize sarcasm and deception, the researchers pointed out.
Dr. Sam Gandy, associate director of the Mount Sinai Alzheimer's Disease Research Center in New York City, said "neuroanatomy can be surprising."
"Phenomena that sound very complex can sometimes be very discretely localized," he added. "Language, short-term memory, and recognition of 'self' are examples of clearly complex phenomena that can be dramatically affected by rather small lesions."
This study provides strong evidence that surprisingly discrete lesions -- in this case, lesions caused by neurodegenerative disorders -- can abolish perception of sarcasm and sincerity, Gandy said.