Hands off and on in schizophrenia

Appendage illusion points to disturbed body sense in psychotic ailment

People with schizophrenia rapidly and intensely perceive phony replicas of hands as their own, possibly contributing to this mental ailment’s signature hallucinations, a new study suggests.

HAND-Y SWITCH Simultaneous stroking of a visible rubber hand (left) and the unseen actual hand (right) of this graduate student caused him to perceive the fake appendage as his own. Schizophrenia patients experience a rubber-hand illusion especially easily and intensely, denoting a disturbance in their sense of body and self. John Russell/Vanderbilt Univ.

In a series of tests, people with schizophrenia believed a rubber hand placed in front of them was theirs if the visible fake hand and the patient’s hidden, corresponding hand were simultaneously stroked with a paintbrush.

Mentally healthy people took longer to experience a less dramatic version of this rubber-hand illusion than schizophrenia patients did, but the effect’s vividness increased among healthy volunteers who reported magical beliefs, severe social anxiety and other characteristics linked to a tendency to psychosis, psychologist Sohee Park of Vanderbilt University in Nashville and her colleagues report online October 31 in PLoS ONE.

“Schizophrenia patients may have a more flexible internal representation of their bodies and a weakened sense of self,” Park says. “Even without psychosis, the rubber-hand illusion can be more pronounced in certain personality types.”

Mental health clinicians have written for several decades about a disturbed sense of self in schizophrenia. A team led by psychiatrist Avi Peled of Sha’ar Menashe Mental Health Center in Hadera, Israel, first reported a powerful rubber-hand illusion in the illness in 2000.

In further support of disturbed body perception in schizophrenia, Park — who directed the new study with graduate student Katharine Thakkar — notes that patients thought that their stationary, unseen hands moved an average of 2 centimeters closer to the rubber hand as they felt and watched brush strokes. Healthy participants reported a weaker version of this effect.

One patient, a 55-year-old man, felt that he floated above his own body and looked down on himself during the three-minute stroking procedure. In a follow-up session, this man had another out-of-body experience, in which he and the experimenter hovered above a lab table for several minutes.

An inability to perceive one’s body as one’s own in schizophrenia prompts heightened reactions to the sight of detached body parts, such as the rubber hand, proposes neurologist Peter Brugger of University Hospital Zurich in Switzerland. That would explain why simply looking at the fake appendage evoked a rubber-hand illusion in several patients in the new study, he says.

Park’s team studied 24 schizophrenia patients and 21 volunteers who had no mental disorders. Patients lived by themselves or in group homes and received antipsychotic medication and other services at an outpatient clinic.

A right-brain region previously linked to out-of-body experiences and representation of one’s body goes awry in schizophrenia, Park hypothesizes. Yoga or other body-awareness exercises may weaken this portion of schizophrenia’s grip, she suggests.

Peled proposes that a communication breakdown among sensory and association networks throughout the brain underlies schizophrenia. This can undermine a sense of body ownership, but patients more often hear tormenting voices and retreat from social life, he says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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