Sense of body ownership and body agency in schizophrenia

Abstract

Recent research suggests that embodiment sensations (sense of body ownership and sense of body agency) are altered in schizophrenia. Using a mirror box illusion setup, we tested if the anomalous embodiment experience depends on deficient processing of visuomotor synchrony, disrupted processing of movement mode, or both. The task required participants to press a lever with their index while looking at the image of the experimenter's hand moving on a similar lever. The illusion of embodiment could arise because looking toward the direction of their own hand the participant saw the reflection of the experimenter's hand visually superimposed to his own one through a mirror. During the illusion induction, we systematically varied visuomotor asynchrony (4 delays were imposed on the movement of the experimenter's hand) and the mode of movement (the participant could perform active vs. passive movements). The strength of the illusion of embodiment of the external hand was assessed with explicit judgments of ownership and agency. Patients' data showed an anomalous modulation of ownership with respect to visuomotor synchrony manipulation and an altered modulation of agency with respect to both visuomotor synchrony and movement mode manipulations. Results from the present study suggest that impairments affecting both the processing of temporal aspects of visuomotor signals and the processing of type of movement underlie anomalous embodiment sensations in schizophrenia. Hypotheses about potential deficits accounting for our results are proposed.

Figure 1

SoO and SoA scores. The graph illustrates mean ratings by group and experimental condition ((A) SoO, (B) SoA). Green and red lines represent active and passive conditions respectively. Error bars = 95% confidence intervals.

Figure 2

SoA-Control scores. The graph illustrates mean questionnaire ratings by group and experimental condition. Green and red lines represent active and passive conditions respectively. Error bars = 95% confidence intervals.

Figure 3

Experimental setup. The schematic diagram depicts the arrangement of the experimental setup when the hand subjected to the illusion was the right one. A mirror in sagittal orientation with respect to participant was placed on the table. The participant positioned his right hand behind the mirror over the lever (the master lever in case of active condition and the slave lever in case of passive condition). During each experimental condition the experimenter placed his (left) hand on the lever in front of the reflective surface. Therefore, the participant could see the reflection of experimenter’s hand spatially superimposed to his own hand when he looked toward it. We used cardboard boxed (not depicted in the figure) to limit the appearance of the experimenter’s hand on the participant’s (left) visual hemifield during the MBI induction phase. Each lever was made with a plastic bar that could move 30 degrees about a fixed pivot. By an electronic control system, delays of different duration could be applied on the onset of actuation of the rotor controlling the slave lever.