Dissecting laughter, a serious business

The bleak chamber of an MRI machine is among the least funny places on earth, but a group of German researchers is using the device to probe the origins of laughter in the human brain.

In a paper published Monday in the journal PLOS One, scientists charted which areas of the brain experienced increased blood flow when study participants were exposed to recordings of different types of laughter. What they found was that the brain responded differently to hearing certain types of laughter.

Examination of so-called “laughter perception networks” could provide clues to the evolution of the brain, according to researchers.

While primates like chimpanzees and gorillas will laugh when tickled, human laughter conveys many more social cues and carries greater consequences, according to authors.

“Laughter is an evolutionary old communication signal with high relevance for social interactions,” wrote Dirk Wildgruber, the lead author and a professor of neuroscience at Eberhard Karls University of Tubingen. “Tickling laughter is thought to be a more reflex-like behavior confined to the context of tickling and play which enforces play behavior and social bonding.”

Wildgruber and colleagues performed their experiment on 18 right-handed men and women. Each were placed in a functional magnetic resonance imaging machine, or fMRI, and asked to listen to recordings of laughter portrayed by actors.

The actors imitated tickling laughter, mocking laughter and joyous laughter, authors wrote. When researchers examined the fMRI images, they found that tickling laughter activated blood flow in a network of areas in the brain, but that these areas were different from those activated by mocking or joyous laughter.

By comparison, the mocking and joyous laughter triggered blood flow to similar areas of the brain -- areas that were associated with more complicated forms of social interaction and communication.

Authors hypothesized that there was more “mentalizing” required to interpret laughter from complex social interactions. “Tickling laughter typically occurs in a narrower spectrum of situations and incurs lower need for mentalizing,” the authors wrote.

The authors said their study was limited in several ways. One question that remained, they wrote, was whether or not the brain responded differently to laughter that was portrayed by actors and laughter that was spontaneous and genuine.

That question, they wrote, remains to be answered in future studies.

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