Laughter: it's catching - Nature

http://www.nature.com/news/2006/061211/full/061211-7.html

Published online: 12 December 2006; | doi:10.1038/news061211-7

Happy sounds tickle the brain to prompt a smile.

Ned Stafford

The automatic trigger to laughter helps people to build strong bonds with each other. Punchstock

Laughter is indeed infectious, according to a new study. Researchers have shown that the mere sound of giggles tickles the same area of the listener's brain that is activated when smiling. The brain's response helps to prepare the facial muscles for a good hearty laugh.

"It really seems to be true: 'Laugh and the whole world laughs with you'," says study co-author Sophie Scott, a neuroscientist at University College London in the United Kingdom.

The team of played pleasant sounds, such as laughter or cheering, and unpleasant sounds, such as screaming or retching, to volunteers. They then monitored their brains with functional magnetic resonance imaging (MRI). All the sounds triggered neural responses in the premotor cortex of the brain — an area known to prepare groups of facial muscles to respond accordingly. When a person in the study actually smiled or laughed, the neural activity moved to a primary motor cortical region.

The neural response in the premotor cortex was, on average, twice as big for pleasant sounds than for unpleasant sounds, the team reports. Since the pleasant sounds have a bigger impact on the bit of the brain that activates muscles to respond in kind, the findings suggests that pleasant sounds are more 'contagious' than unpleasant ones. The research is published this week in the Journal of Neuroscience¹.

Helpless mimicry

Scott says that these results are significant because they suggest that the laughter-triggering mechanism in the human brain is "very basic or automatic": that people are essentially helpless to control the impulse to smile or laugh when they hear pleasant sounds. A good example of this, she says, is when people in a boring meeting strenuously try to suppress the urge to giggle. As soon as one person finally emits a squeak, the whole meeting can erupt into a laugh-fest.

Humans are already known to mirror the habits or emotions of those around them, Scott says. Friends often start using the same words, assuming similar postures, and mimicking hand gestures. And the contagiousness of a good or bad mood is well known. Scott says the neural response in the brain, which automatically primes people who hear pleasant sounds to smile or laugh, is another form of mirroring behaviour that helps people to interact socially and to build strong bonds with each other.

"We usually encounter positive emotions, such as laughter or cheering, in group situations," she explains.

Forcing a smile

The work fits in with previous studies that have illustrated the link between simple stimuli and more complex emotions or feelings. Forcing a smile can actually lift a person's mood². And stimulating some bits of the brain has been shown to actually prompt laughter in at least one patient³.

As the mother of a 5-month-old baby boy, Scott says that she gets a lot of first-hand experience of this trigger mechanism. A grin from any one of her family tends to set all the others off, she says.

But while Scott's baby smiles back when smiled at, he doesn't yawn back after a tired yawn from mom, she says. Scott's team didn't include sounds or pictures of yawns in their study, but she would be interested to see how and why that action is contagious too.