We begin to recognize emotions in others at an early age and copy what we see. The urge to mimic the emotional behavior of others — called emotional contagion — continues throughout life.
For example, researchers know that people with autism struggle with decoding the emotional content of faces, bodies and sounds. But for most, seeing someone in distress makes "mirror neurons" in the brain that cause similar feelings.
But itching, or pruritus, is much more than an emotion, often occurring in skin diseases that can affect the entire body and caused by mechanical, thermal or chemical mediators. Such mediators include chemicals for immune response and pain relief, neuropeptides, neurotransmitters and prostaglandins.
With itching comes scratching. The itch-scratch reflex activates higher levels of our brains than the spinal-cord-level reflex that makes us pull our hand away from a hot surface.
And scratching, like laughing and yawning, is socially contagious. Because our brains are hardwired for social interaction and bonding, mimicking the actions we see in those around us is a natural way of empathizing and gaining a sense of how others are feeling.
Until recently, contagious itching had been observed in monkeys and humans, but not rodents. In a new study, researchers, headed by principal investigator Zhou-Feng Chen, Ph.D,, director of the Washington University Center for the Study of Itch in St. Louis, have discovered that socially contagious itching is hardwired in the brain.
In this study, published this month in the journal Science, the researchers identified what occurs in the brain when a mouse feels itchy after seeing another mouse scratch. Using a behavioral paradigm that does not entail previous training or reward, they found that mice scratched after observing scratching behaviors.
Researchers placed mice that scratched excessively from a chronic itch in a cage next to normal mice. Within seconds of seeing a scratchy mouse, the normal mice started scratching themselves, too.
Next, to test whether just the sight of the scratchy mouse prompted the imitation, the researchers tried the same experiment with mice watching a scratching mouse on a computer screen. The researchers weren't sure if the mice would even see the video, let alone respond to it. As before, mice that saw the scratching behavior also started scratching within seconds.
The mice showed significant increases in activity in several brain areas, including the suprachiasmatic nucleus (SCN), which receives direct visual input from the environment, suggesting that the SCN could be one of the first brain circuits involved in socially contagious itching.
The research team also investigated whether altering molecules within the SCN would affect the imitative scratching behaviors. Using various techniques, the researchers altered GRP (a protein-like molecule known to influence SCN activity) signaling.
Mice that lacked GRP signaling showed almost no imitative scratching behaviors when exposed to a scratchy mouse but still scratched themselves when exposed to a skin irritant. Artificially activating GRP signaling induced scratching in mice without the visual input of seeing a scratchy mouse.
In this study, the mice clearly displayed contagious itch behavior, suggesting that a single chemical and a single receptor mediate this uncontrollable imitative itching behavior. The discovery may help scientists understand the neural circuits that control socially contagious behaviors.
