Washington: Do you portray strange behaviour and are unable to understand things like sarcasm and lies? Beware, you may be suffering from dementia, a new study has suggested.

Researchers at the University of California, San Francisco, found that people in the early stages of dementia may not be able to differentiate between sarcasm, lies and truth.

The findings could help medics diagnose neurodegenerative disorders like dementia and Alzheimer's early and provide effective treatment, the researchers said.

"If somebody has strange behaviour and they stop understanding things like sarcasm and lies, they should see a specialist who can make sure this is not the start of one of these diseases," study researcher Katherine Rankin, a UCSF neuropsychologist, was quoted as saying by LiveScience.

For their study, Rankin and her colleagues asked about 175 people, more than half of whom had a neurodegenerative disorder like dementia, to watch videos of people talking.

The videotaped people would sometimes drop in a lie or use sarcasm, which they signalled with body language and verbal cues. After watching the videos, the participants answered yes and no questions about what they'd seen.

Healthy older participants did fine at distinguishing the truth from lies. But older adults with dementia affecting their frontal lobes, the seat of judgement and self-control in the brain, had a hard time telling the difference between sarcasm, lies and truth.

People with frontotemporal dementia, which strikes the frontal lobes, had a particularly hard time, while those with Alzheimer's disease did somewhat better.

Using magnetic resonance imaging (MRI), the researchers found that the inability to detect sarcasm and lies matched up with the amount of damage in the parts of the frontal lobe responsible for that judgement.

Rankin presented the findings at the Annual Meeting of the American Academy of Neurology in Hawaii.