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Hand Illusion Helps Schizophrenics Connect Mind and Body

November 3, 2011 — A new study provides more evidence that people with schizophrenia have a diminished sense of mind–body connection, or “body ownership,” and hints that yoga and other types of movement therapy that get patients to focus on their own body may be helpful.

Katharine N. Thakkar, PhD, from the Department of Psychology, Vanderbilt University in Nashville, Tennessee, and colleagues measured the strength of body ownership of 24 schizophrenia outpatients and 21 healthy control patients by testing their susceptibility to the rubber hand illusion (RHI).

First described in 1998, this tactile illusion is induced by simultaneously stroking a visible rubber hand and the participant’s own hidden hand.

“Watching a rubber hand being stroked while one’s own unseen hand is stroked simultaneously often leads to a sense of ownership over the rubber hand and a shift in perceived position of the real hand toward the rubber hand,” the investigators explain in their paper. “The RHI is reduced or absent when tactile stimulation of the real and rubber hands is asynchronous or when the rubber hand is spatially incongruent with the real hand,” they write.

The study was published online October 31 in PLoS One.

Weakened Sense of Self

The investigators found that patients with schizophrenia were more susceptible to the RHI than healthy control patients.

“After a while, patients with schizophrenia begin to ‘feel’ the rubber hand and disown their own hand,” author Sohee Park, PhD, also from the Department of Psychology at Vanderbilt, explained in a statement. “They also experience their real hand as closer to the rubber hand.

Dr. Sohee Park

“Healthy people get this illusion too, but weakly. Some don’t get it at all,” Dr. Park said, suggesting that people with schizophrenia have a more flexible body representation and weakened sense of self.

“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,” David M. Gray, PhD, Mellon assistant professor of philosophy at Vanderbilt, who was not involved in the study, told Medscape Medical News.

Dr. Park noted that there is significant individual variation in how people experience RHI that is related to a personality trait called schizotypy, “which is associated with psychosis-proneness.”

Before the RHI, Dr. Park’s team had participants complete a questionnaire to rate their degree of schizotypy, which is the extent to which they experience perceptual effects related to the illusion. The researchers found that participants who rated higher on the scale were more susceptible to the RHI.

The researchers also observed in both study groups that the skin temperature of the hidden hand drops by a few tenths of a degree during the RHI, although they are not sure why. “It’s almost as if the hand is disowned and rejected, no longer part of the self,” Dr. Park said.

Rubber hand illusion

Out-of-Body Experience

The researchers also report that 1 patient with schizophrenia experienced a full out-of-body experience (OBE) during the RHI. The participant reported that he was floating above his own body for about 15 minutes. This patient had another OBE when invited back for a second RHI session. These occurrences suggest a link between “body disownership and psychotic experiences,” the researchers write.

The OBE in this 1 patient that is “readily replicable” using the RHI “illustrates this temporary loss of body ownership in a spectacular way,” Dr. Park told Medscape Medical News. She emphasized, however, that “most people in the general population who experience OBEs, especially before falling asleep, are perfectly healthy, and certainly not psychotic. This one case study by itself does not prove anything.”

The researchers note that OBEs and body ownership are associated with the brain’s temporoparietal junction, suggesting that disorders in this part of the brain may also contribute to schizophrenia symptoms.

The investigators note that yoga or other forms of physical activity that focus on the body might be helpful in patients with schizophrenia. They point to 2 recent studies in which yoga was shown to be effective in attenuating symptoms and increasing social functioning in schizophrenia, “exceeding the beneficial effects of aerobic exercise.”

“If there are disturbances of body sense, it might be possible to anchor ourselves better by gaining more intimate and fine control over our body parts by practicing focused exercises,” Dr. Park told Medscape Medical News.

“Obviously a careful, clinical trial is needed to evaluate the usefulness of these programs, but physical exercise is good for everybody, so I don’t see any downside to this at all,” she said.

“In addition to yoga, I think the Alexander technique, which is used by a lot of people with back problems or other injuries, could be interesting to explore because you really get to know your body and the space it occupies,” Dr. Park said.

The study was supported in part by the National Institutes of Mental Health, National Alliance for Research on Schizophrenia and Depression, National Institute of Child Health and Human Development, and Gertrude Conaway Vanderbilt Endowed Chair. The authors and Dr. Gray have disclosed no relevant financial relationships.

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