The Moving Rubber Hand Illusion Reveals that Explicit Sense of Agency for Tapping Movements Is Preserved in Functional Movement Disorders

Abstract

Functional movement disorders (FMD) are characterized by motor symptoms (e.g., tremor, gait disorder, and dystonia) that are not compatible with movement abnormalities related to a known organic cause. One key clinical feature of FMD is that motor symptoms are similar to voluntary movements but are subjectively experienced as involuntary by patients. This gap might be related to abnormal self-recognition of bodily action, which involves two main components: sense of agency and sense of body ownership. The aim of this study was to systematically investigate whether this function is altered in FMD, specifically focusing on the subjective feeling of agency, body ownership, and their interaction during normal voluntary movements. Patients with FMD (n = 21) and healthy controls (n = 21) underwent the moving Rubber Hand Illusion (mRHI), in which passive and active movements can differentially elicit agency, ownership or both. Explicit measures of agency and ownership were obtained via a questionnaire. Patients and controls showed a similar pattern of response: when the rubber hand was in a plausible posture, active movements elicited strong agency and ownership; implausible posture of the rubber hand abolished ownership but not agency; passive movements suppressed agency but not ownership. These findings suggest that explicit sense of agency and body ownership are preserved in FMD. The latter finding is shared by a previous study in FMD using a static version of the RHI, whereas the former appears to contrast with studies demonstrating altered implicit measures of agency (e.g., sensory attenuation). Our study extends previous findings by suggesting that in FMD: (i) the sense of body ownership is retained also when interacting with the motor system; (ii) the subjective experience of agency for voluntary tapping movements, as measured by means of mRHI, is preserved.

Keywords: bodily actions; functional movement disorders; rubber hand illusion; self-recognition; sense of agency; sense of body ownership.

Figures

FIGURE 1

Moving Rubber Hand Illusion apparatus. (A) In the Active Synchronous Congruent condition, when the participant actively moved her own index finger, the rubber hand moved in turn. This condition usually evokes the sense of agency and the sense of ownership over the rubber hand. The red arrow indicates the direction of the movement. (B) In the Passive Synchronous Congruent condition, the experimenter moved a wooden stick, thus causing both the participant’s hand and the rubber hand to move. This condition usually induces ownership but not agency over the rubber hand. (C) In the Active Synchronous Incongruent condition, the rubber hand was rotated of 180 degrees with respect to the participant’s hand. The participant moved her index finger and the rubber hand moved synchronously. This condition usually evokes agency but not ownership over the rubber hand. (D) In the Active Asynchronous Congruent condition, the rubber hand was placed in an anatomically congruent position and the participant performed active movements as in the Active Synchronous Congruent condition. In this case, however, the rubber hand was moved by the experimenter with a short delay with respect to the onset of the participant’s movement. This condition does not evoke sense of agency or sense of body ownership.

FIGURE 2

Questionnaire ratings in the two groups. Mean scores for the ownership-statements (light gray columns), ownership-controls (light gray striped columns), agency-statements (dark gray columns), and agency-controls (dark gray striped columns) in the functional movement disorders group (A) and the healthy controls group (B). Gray lines and asterisks show significant comparisons within conditions. Note that in both groups the Passive Synchronous Congruent condition elicited ownership but not agency, and that conversely the Active Synchronous Incongruent condition elicited agency but not ownership, suggesting a double dissociation between agency and ownership. Error bars indicate standard errors.

FIGURE 3

Ownership and agency across conditions in the functional movement disorders group. (A) Mean scores for the ownership-statements and (B) the agency-statements across conditions in the functional movement disorders group. Gray lines and asterisks show significant comparisons across conditions. The results showed that agency and ownership were differently elicited by each condition. Error bars indicate standard errors.

FIGURE 4

Ownership and agency across conditions in the healthy controls group. (A) Mean scores for the ownership-statements and (B) the agency-statements across conditions in the healthy controls group. Gray lines and asterisks show significant comparisons across conditions. In the functional movement disorders group, agency and ownership were differently elicited by each condition. Error bars indicate standard errors.

FIGURE 5

Ownership and agency for all the conditions in the functional movement disorders and the healthy controls groups. Mean scores for the(A) ownership-statements and (B) the agency-statements in the functional movement disorders group (FMD; black columns) and the healthy controls group (HC; gray columns). The two groups showed similar patterns of response. Error bars represent standard errors.