Watching someone relieve an itch with a nice old scratch can cause you to drag your nails over your skin as quickly as possible.
It isn't unusual for humans to scratch. When rodents dig in, mice have the same reaction. The social phenomenon of mice scratching themselves has been found to be unusual.
The brain's visual processing hardware does not play a part in the process of being inspired to scratch.
Even if the scratching behavior isn't registered as a picture in the mouse's mind, it will still follow suit.
When the visual cortex is damaged, we can't see and respond to visual stimuli, but there are pathways that allow us to see and respond.
Recent research suggests that blind sight is a result of a subcortical pathway that takes information from the eyes to the other parts of the brain.
It's a neural shortcut that can allow someone who is completely blind to walk down a hallway while avoiding obstructions that they can't see.
There is a pathway that could alert mice to a blind itch. The test animals scratched themselves when they saw a video of another mouse doing the same thing that the researchers disabled the visual cortex for.
According to recordings of the activity of mice brain cells, there are special light-sensitive cells in the eye.
The suprachiasmatic nucleus is responsible for triggering a neurological scratching cascade. Light can be used to set the clock without the need for light stimuli to be seen as an image.
In the past, some scientists have speculated that some genes may play a role in human blind sight.
The textbook view is that they fall short of painting a scene because they don't contribute to vision in some way. There is new research on mice.
When researchers kept their visual systems intact, the animals were no longer showing contagious scratching behaviors.
Zhou-Feng Chen from Washington University in St Louis believes that the itch is important for survival.
In mice, itching is controlled through a pathway that wasn't known to be responsible for seeing things. It's possible that this type of scratching behavior is an old, protective behavior.
He compares it to a frog catching a bug. Frogs don't have the same wiring for interpreting vision as mammals, but their eyes still send information to other parts of their brain.
These cells are sensitive to movement because they can fly so fast. The sticky tongues of a frog seem to know where to go when a fly flies by.
It appears that mice can sense their companions' scratching without the need to recruit the level of neurology necessary to turn it into a visual impression.
It's not clear why contagious scratching works this way in mice, but it could give them a competitive edge.
The researchers think that contagious behavior of this sort could be a primitive form of emotional contagion. The thalamus is the brain's seat for relaying sensory information and has recently been implicated in processing emotional stimuli.
It's true that stress is a feeling of tension. Itches aren't anything if they aren't stressed.
The stress hormone cortisol was found to increase in the blood of the mice that were shown videos of other scratching mice. The stress response suggests that itch contagion is not a form of motor mimicry.
The pathway for contagious itch may be different in humans than it is in mice.
The human response probably requires the visual cortex. The itch may be an evolutionary remnant. To understand why these types of behaviors may once have been important to survival, you have to go back to the animals.
Other researchers believe that mirror neurons, brain cells that are active when we imitate actions or negative affect, have a role to play in the spread of human scratching.
Future studies will need to compare other mammals to understand these pathways.
The behavior is common in the animal world.
The study was published in a journal.
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