She only laughs when it hurts, is the simplest way to describe how one woman responded to being stuck with a pin while receiving treatment after a stroke. "It demonstrates the complex pathways which convey information through our brains," explained Dr Vilayanur Ramachandran of the University of California, San Diego.
Patients with brain damage could provide valuable information about the functional organisation of the brain, he said. New monitoring devices could record what parts of the brain responded to a given stimulus. Many areas of the brain have been mapped in this way and our knowledge of it is growing all the time.
Each time he pricked the woman's hand with the pin, she burst into laughter, he said. The unusual response showed that damage had been done to the connections between two brain areas, the insular cortex and the limbic system. "The former," he said, "is the area which handles painrelated signals and the latter is where, in simple terms, the person formulates a response. The limbic decides whether there is danger or whether the situation does not actually pose a genuine threat.
"Laughter is nature's `it's OK' signal," Dr Ramachandran said. If the limbic decides that there is no threat, then laughter is a common response, a signal to those nearby that there is no risk. In the patient, some part of this pathway was scrambled so, while the insular cortex may have been recording pain, the limbic interpreted it as "no big deal", with a suitable response being a burst of laughter.