Why you can't tickle yourself?

Why you can't tickle yourself?
It’s a well-known fact that you can’t tickle yourself.
Try it; you (mostly) can’t. Brush your own fingers across the soles of your feet. You certainly feel a sensation, but it’s nothing like when someone else does it.

But why can’t you tickle yourself? If someone else can tickle you, then you should be able to tickle yourself. After all, I can feel my own touch just the same as someone else’s can’t I?

The answer is, psychologists think, that our brains have a basic function which is designed to tell whether some sensation is caused by ourselves, or whether it comes from outside, this concept is known as corollary discharge (aka efferent copy).
An efferent is an output, a  neurotransmitter going out to your muscle, and whenever you perform an action, a copy of the signal is sent to your higher processes so you're aware that you, yourself are initiating the action. If someone else tickles you, you don't receive a copy of the efferent signal and so your body doesn't account for it and you get tickled.
http://en.wikipedia.org/wiki/Efference_copy

People who are unable to send out these efferent copies develop subtle but pervasive disturbances in perception and begin to disconnect their own actions with their own bodies. It's also thought to be one of the driving factors behind the psychotic elements of schizophrenia, e.g. hallucinations.
http://thebrainbank.scienceblog.com/2012/12/24/why-cant-we-tickle-ourselves-but-schizophrenics-can/

To test this out some researchers have created a simple tickling robot. The way it works is you put your left hand on a little stick and move it around. This causes a sponge to move around on your right hand.

It turns out that when the robot works like this, people feel little, because it’s like they are causing the sponge to move themselves, through this ‘robot’. It’s like when you brush a feather duster across your own palm: your brain knows you’ve caused the sensation, so it doesn’t feel that ticklish.

But, when the robot introduces a delay of one-third of a second between their left hands moving the stick, and feeling the sponge move on their right hands, suddenly it tickles!
The reason is that the ‘robot’ has tricked the mind into thinking the source of the movement is external. And because it feels like someone else is causing the sensation, then it tickles!


References and further reading:
http://www.mitpressjournals.org/doi/abs/10.1162/089892999563607
http://www.spring.org.uk/2013/08/why-you-cant-tickle-yourself.php
http://www.ncbi.nlm.nih.gov/pubmed/10943682
Image via Reddit