Postural Stabilization Strategies to Motor Contagion Induced by Action Observation Are Impaired in Parkinson's Disease

Abstract

Postural reactions can be influenced by concomitant tasks or different contexts and are modulated by a higher order motor control. Recent studies investigated postural changes determined by motor contagion induced by action observation (chameleon effect) showing that observing a model in postural disequilibrium induces an increase in healthy subjects' body sway. Parkinson's disease (PD) is associated with postural instability and impairments in cognitively controlled balance tasks. However, no studies investigated if viewing postural imbalance might influence postural stability in PD and if patients are able to inhibit a visual postural perturbation. In this study, an action observation paradigm for assessing postural reaction to motor contagion in PD subjects and healthy older adults was used. Postural stability changes were measured during the observation of a static stimulus (control condition) and during a point-light display of a gymnast balancing on a rope (biological stimulus). Our results showed that, during the observation of the biological stimulus, sway area and antero-posterior and medio-lateral displacements of center of pressure significantly increased only in PD participants, whereas correct stabilization reactions were present in elderly subjects. These results demonstrate that PD leads to a decreased capacity to control automatic imitative tendencies induced by motor contagion. This behavior could be the consequence either of an inability to inhibit automatic imitative tendencies or of the cognitive load requested by the task. Whatever the case, the issue about the ability to inhibit automatic imitative tendencies could be crucial for PD patients since it might increase falls risk and injuries.

Keywords: Parkinson’s disease; action observation; biological motion; chameleon effect; motor contagion; postural stabilization strategies.

Figure 1

Mean of center of pressure (CoP) displacements in the two axes across time (Bin, 4 s each). (A) The different video displayed to the observer: Cross (Control stimulus) and a point-light video displaying a model in postural imbalance (biological stimulus). (B) r Mean of CoP displacements in antero-posterior (A-P) direction for Parkinson’s disease (PD, black circles) patients and for elderly (ELD, gray circles) during the experimental conditions. (C) Mean of CoP displacements in medio-lateral (M-L) direction for PD (black circles) and for ELD (gray circles) during the experimental conditions. The biological stimulus trials are highlighted by a gray box where asterisks refer to differences between groups. Bars indicate SDs; asterisks indicate statistical significant difference (^*p < 0.05; **p < 0.001).

Figure 2

Area of center of pressure (CoP) excursions in the three stimulus conditions. The mean area of CoP displacement (computed as the surface of the confidence ellipse containing 95% of the CoP sampled positions) for each group (ELD, elderly and PD, Parkinson’s disease) during the experimental conditions (control and biological stimulus conditions) are shown. The biological stimulus trials are highlighted by a gray box where asterisks refer to differences between groups. Bars indicate SDs. Asterisks indicate statistical significant differences (^*p < 0.05; **p < 0.001).

Figure 3

Correlations between postural parameters recorded during Control-PRE (x-axis) and during Biological conditions (y-axis). Correlations of data recorded in Parkinson’s disease subjects for antero-posterior (A–P) and medio-lateral (M–L) displacements and sway area are depicted in panels (A–C), respectively. For A-P and M-L excursions, data are expressed in millimetres (mm), for sway area data are expressed in reported in squared-millimeters (mm²).