An inability to distinguish between internally-generated and externally-generated physical sensations appears to be linked to having trouble distinguishing between delusions (generated internally) and facts (that exist externally). This means that there seems to be a bond between sensory perception concept generation -- between our ability to perceive the world through our senses and our ability to organize what we understand of the world through our mental concepts.
This link was discovered by researchers studying how people play the game "tit for tat" -- hitting a person or pinching them with the exact same force as they themselves were hit or pinched with. The game is difficult for normal people for the same reason that we are unable to tickle ourselves: our brains "downgrade" the strength or impact of our own actions when compared to the actions of others. Thus our self-tickling never has the same force as when others tickle us and our hitting of another feels weaker to us than it does to them.
The sensation from a self-tickle pales in comparison to the sensation we feel when someone tickles us--and it should. Brain scans show that, compared to a real tickle, cortical areas that process sensation are less active during a self-tickle. For some reason, whenever we move, our brains predict the sensory consequences of our own actions and reduce them. So self-tickles feel lame, and my brother and I have to punch each other back harder to feel like we're playing our game by the rules. Why would our brains do this?
Two years after publishing the tit-for-tat study, Wolpert, working with a slightly different group of researchers, re-ran the experiment. They reasoned that our brains might discount the feeling of our own actions to help us differentiate between self-generated and externally generated sensations. Put another way, reducing the sensation of our own actions makes alien sensations--say, a tap on the back--more noticeable.
To test the idea, the researchers had schizophrenics play tit-for-tat against themselves. Schizophrenics have trouble recognizing their own actions--that is, they often attribute their behavior to an alien source. Some can even tickle themselves. If our brains discount the feeling of our own actions to help us differentiate between self-generated and externally generated sensations, then a group of subjects who can't make this distinction might simply be missing this sensory reduction.
In that case, reasoned Wolpert and his team, schizophrenics should be better at playing tit-for-tat by the rules. And they were. When the robot pushed on the fingers of schizophrenics they were much better at pushing back on themselves with the same amount of force the robot had applied. Their brains didn't discount the consequences of their own actions as much as the brains of healthy subjects did.
Source: Scientific American
If this merely answered the question "why can't we tickle ourselves," it would be interesting enough -- but researchers went further:
But the tale of the tit-for-tat experiment doesn't end there. This past year, Wolpert, now working at Cambridge with another group of researchers, ran the tit-for-tat study a third time. Thirty healthy subjects were recruited. They played the game against themselves and completed a short survey designed to gauge delusional thoughts. The survey asked questions like, "Do you ever feel as if you have been chosen by God in some way? and "Are you often worried that your partner may be unfaithful?"--questions that, on their own, are endorsed by about one in four people.
Wolpert and his colleagues compared the survey results to subjects' tit-for-tat performance. They found that delusional thinkers, just like schizophrenics, were better at playing tit-for-tat by the rules--they were better at pushing back on themselves with the same amount of force the robot applied. A reduced ability to discount the sensory consequences of self-generated actions was not just a consequence of schizophrenia--it seemed to be, more generally, a characteristic of deluded thinkers.
So schizophrenics and delusional thinkers have something fundamental in common: they both have a reduced ability to differentiate between self-generated stimuli and external stimuli. This means that stimuli which are entirely their own creations are, for them, just like external stimuli that are created by the external world. That might explain a lot if this problem actually underlies the mental conditions in question. To what degree are our cognitive abilities linked to our sensory perception?
Wouldn't it be interesting if this phenomenon could be nailed down enough to develop a test that could tell us with some reliability just how susceptible we are to failing to properly distinguish between the internal and the external? Imagine if we could be rated on a scale ranging from "strong separation between external and internal stimuli" and "weak separation between external and internal stimuli." This would tell people if they are susceptible to delusions -- to beliefs that are internally generated but seem just like something external.
It would be interesting... but would it have any impact on anyone? A person who tests positive for strong distinctions would feel vindicated, but of course that's no guarantee that they'll never fall for a delusion. A person who tests positive for weak distinctions would... well, what would they do? Would they really be more skeptical of their own ideas and conclusions? Would they be in denial about their own susceptibility? Is this a case where more information can help people overcome their problem or is it instead a case where the problem itself prevents people from using the information to help themselves (analogous to how a mental illness can influence a person to refuse to take medication which would help that illness)?
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