What have they found?

The brain scans are usually done while people are responding to humorous material. You see brainwave activity spread from the sensory processing area of the occipital lobe, the bit at the back of the brain that processes visual signals, to the brain's frontal lobe. It seems that the frontal lobe is involved in recognising things as funny. The left side of the frontal lobe analyses the words and structure of jokes while the right side does the intellectual analyses required to "get" jokes. Finally, activity spreads to the motor areas of the brain controlling the physical task of laughing. We also know about these complex pathways involved in laughter from neurological illness and injury. Sometimes after brain damage, tumours, stroke or brain disorders such as Parkinson's disease, people get "stonefaced" syndrome and can't laugh.

I find it improbable that a purely psychological mass reaction would last so long and be so widespread. Sometimes people have fits of abnormal, inappropriate, unrestrained and uncontrollable laughter dissociated from any social or humour stimulus. Laughter involves motor, emotional and cognitive components, each one located in a specific part of the brain. The American neurologists Hanna and Antonio Damasio suggest that abnormal laughter occurs when structures in the basal part of the brain are damaged. The pathways that normally automatically adjust the execution of laughter to be appropriate to the stimulus for it are disrupted and the brain gets incomplete information about the cognitive and situational context of a potential stimulus--it gets it wrong about whether or not to laugh--resulting in chaotic behaviour. Based on this model, I suggest that a viral infection, probably some kind of encephalitis in the basal part of the brain, provoked the 1962 epidemic. Many of the people affected had fever, and some also had movement disturbances.

So is "normal" laughter hard-wired?
We see babies and children laughing with their mothers, and it's easy to assume that they're imitating. But we've studied children who have no opportunity to learn laughter, because they're congenitally blind, deaf, or mute, and found that they can smile and laugh even though they're unable to copy others. It's only the beginning of the story, though, and we need to know more about the differences between laughter in different people.