A study pinpoints an aspect of brain function that schizophrenia sufferers may lack.

Try all you want, you’ll never be able to tickle yourself. Go ahead, give it a shot. See? Nothing.

The main element of the response to being tickled is surprise—you can’t expect the sensation that is about to occur. A recent study found that healthy humans anticipate their own actions too well to get a self-induced sensation from tickling. However, this ability not to be fooled by one’s own actions may draw a line between sanity and psychosis.

Randy Flanagan, a psychologist at Queen’s University in Ontario, tried to pin down exactly what is at work when people filter out certain sensations. In his study, part of a series he is undertaking with colleagues at the University of College London, Flanagan found that a predictive sense in our brain enables us to manage the array of tactile sensations we experience, by filtering out those that we cause ourselves.

“Every time I move my eyes, my head; every time I feel something with my hand, I get masses of sensory information,” said Flanagan. “Effectively, I would be overwhelmed if I couldn’t simply take off that totally predictable information and focus my energies on the novel, unpredictable sensory information.”
Without the ability to filter out self-induced sensations, we would get distracted every time we felt the vibrations in our vocal chords when we talked or the feeling of tapping in our fingers when we typed.

Flanagan and his colleagues are doing research to determine if people who suffer from schizophrenia are unable to discern between external and internal sensations. While this may give them the ability to tickle themselves, it may also convince them that they are actually hearing someone else’s’ voice when they are actually mumbling to themselves.

Flanagan’s study, published in PLoS Biology, centered on tactile sensory cancellation. The research team devised a task where subjects, using an active finger, tapped a force sensor above a passive finger, to convey a tapping feeling in the passive finger below.

In one trial, people judged self-induced taps to be weaker than those caused by the machine due to their brains’ ability to negate some of the self-induced force. But, in a second iteration, the force sensor was moved at the last second without the subject’s knowledge so no force was actually put on the passive finger. Still, subjects reported a sensation that was weaker than the machine-induced taps, indicating that the ability to cancel out self-induced sensations is predictive. In effect, the brain senses a stimulus is coming and cancels it out instead of waiting until after the fact.

“This research is important because it demonstrates for the first time that the filtering out of self-induced sensations depends upon a predictive mechanism,” said Chris Frith, a colleague of Flanagan’s at the Institute of Neurology, University College London. “This has been suspected previously, but not proven.”

This conclusion gives scientists studying certain types of schizophrenia a clue into what parts of the brain might be malfunctioning in their patients. 

“If I move in the world or do something as a neurologically normal individual, I know what sensations are due to myself,” said Flanagan. “The suggestion is that, in certain forms of schizophrenia, there is a deficit in labeling sensory information as self-generated.”

While Flanagan focuses his energies on studying the tactile aspects of sensory cancellation, colleagues of his at the University College of London, like Frith, are looking into its direct relation to schizophrenia.

“The more we understand the mechanism for this filtering,” said Frith, “the closer we shall get to understanding what goes wrong in schizophrenia and, perhaps, how to correct it.”

Originally published March 8, 2006

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