VANDERBILT (US) — People with schizophrenia have a weakened sense of body ownership, a finding that suggests movement therapy may be useful in treating those suffering from the mental disorder.
The experiment, reported in the journal PLoS One, measured the strength of body ownership of 24 schizophrenia patients and 21 matched control subjects by testing their susceptibility to the “rubber hand illusion” or RHI. This tactile illusion, which was discovered in 1998, is induced by simultaneously stroking a visible rubber hand and the subject’s hidden hand.
“After a while, patients with schizophrenia begin to ‘feel’ the rubber hand and disown their own hand. They also experience their real hand as closer to the rubber hand,” says Sohee Park, associate professor of psychology at Vanderbilt University, who conducted the study with doctoral candidate Katharine Thakkar and research analysts Heathman Nichols and Lindsey McIntosh.
“Healthy people get this illusion too, but weakly,” Park says. “Some don’t get it at all, and there is a wide range of individual differences in how people experience this illusion that is related to a personality trait called schizotypy associated with psychosis-proneness.”
Body ownership is one of two aspects of a person’s sense of self awareness. (The other aspect is self-agency, the sense that a person is initiating his or her own actions.) The fact that schizophrenia patients are more susceptible to the rubber hand illusion suggests that they have a more flexible body representation and weakened sense of self compared to healthy people.
“What’s so interesting about Professor Park’s study is that they have found that the sense of bodily ownership does not diminish among patients with schizophrenia, but it can be extended to other objects more easily,” says David Gray, assistant professor of philosophy. He did not participate in the study but is familiar with it.
“Much of the literature concerning agency and ownership in schizophrenia focuses on the sense of lost agency over one’s own movements: But, in these cases, the sense of ownership is neither diminished nor extended.”
Before they began the procedure, the researchers gave participants a questionnaire to rate their degree of schizotypy: the extent to which they experience perceptual effects related to the illusion. Individuals who rated higher on the scale were more susceptible to the illusion.
The researchers gauged the relative strength of the RHI by asking participants to estimate the position of the index finger of their hidden hand on rulers placed on top of the box that conceals it before and after stimulation. The stronger the effect, the more the subjects’ estimate of the position of their hidden hand shifted in the direction of the rubber hand. Even the estimates of those who did not experience the effect subjectively shifted slightly.
The rubber hand illusion also has a physiological signature. Scientists don’t know why, but the temperature of the hidden hand drops by a few tenths of a degree when a person experiences the illusion. “It’s almost as if the hand is disowned and rejected, no longer part of the self,” Park says.
The experiment also produced the first case of a spontaneous, out-of-body experience in the laboratory. One participant reported he was floating above his own body for about 15 minutes. According to Park, it is extremely rare to observe spontaneous out-of-body experiences in the laboratory. When they invited the patient back for a second session, he once again had an out-of-body experience during the rubber hand procedure, proving that the experience is repeatable.
“Anomalous experiences of the self were considered to be core features of schizophrenia decades ago but in recent years much of the emphasis has been on cognitive functions such as working memory,” says Park.
According to the psychologist, out-of-body experiences and body ownership are associated with a particular area in the brain called the temporoparietal junction. Lesions in this area and stimulation by strong magnetic fields can elicit out-of-body experiences. Disorders in this part of the brain may also contribute to the symptoms of schizophrenia.
The relationship between schizophrenia and body ownership may help explain the results of a German study published in 2008 that found a 12-week exercise program reduced the symptoms and improved the behavior of a small group of patients with chronic schizophrenia when compared to a control group that did not exercise.
That study also found that the exercise increased size of the patients’ hippocampus slightly—a smaller-than-normal hippocampus is a well established symptom of schizophrenia.
“Exercise is inexpensive and obviously has a broad range of beneficial effects, so if it can also reduce the severity of schizophrenia it is all to the good,” Park says. Focused physical exercise that involves precise body control, such as yoga and dancing, could be a beneficial form of treatment for this disorder.
The study was partly funded by a grant from the National Institutes of Health and the Gertrude Conaway Vanderbilt Endowed Chair.
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