Observation and imitation of object-directed hand movements in Parkinson's disease

Abstract

Action observation and imitation may facilitate movement in Parkinson's disease (PD). People with PD have been found to imitate intransitive actions similarly to neurologically healthy older adults, but their imitation of object-directed hand movements has not previously been investigated using kinematic measures. The present study examined observation and imitation of object-directed hand movements in 18 participants with PD and 21 neurologically healthy age-matched control participants. Participants observed and immediately imitated sequences showing a human hand reaching for and transferring an object between horizontal positions. Both groups significantly modulated their finger movements, showing higher vertical amplitude when imitating elevated compared to direct trajectories. In addition, movements were lower in vertical amplitude and higher in velocity when imitating the reaching segment than the transfer segment. Eye-tracking revealed that controls made smaller saccades when observing predictable than unpredictable elevated movements, but no effects of predictability on eye movements were found for the PD group. This study provides quantitative evidence that people with mild to moderate PD can imitate object-directed hand movement kinematics, although their prediction of such movements may be reduced. These findings suggest that interventions targeting object-directed actions may capitalize on the ability of people with PD to imitate kinematic parameters of a demonstrated movement.

Figure 1

Stimulus videos depicted a human hand reaching for and moving a small cube between 3 of 4 possible positions spaced 150 mm apart (example shows sequence 4-2-1), following either a direct or elevated trajectory. Participants observed and then immediately imitated the sequence but without physically manipulating an object (the object was not present in their own movement space). Note that the circles and arrows indicating the sequence are shown for illustration only and no target markers were visible during the task. Example stimulus videos are available at https://osf.io/ysbrj/.

Figure 2

Kinematic measures during imitation of object-directed actions: each measure is presented for imitation of elevated vs. direct trajectories in reach and transfer segments of the sequences. Plots show means with SEM error bars; dots represent individual participants. (A) Vertical amplitude was significantly higher for elevated vs. direct trials (indicating imitation of trajectory) and for transfer vs. reach segments. There was a non-significant trend for reduced vertical amplitude modulation in the PD group. Reference lines indicate model kinematics for the direct (red dashed line) and elevated (blue dashed line) trajectories. (B) Horizontal amplitude did not differ significantly between groups, but movements were longer in elevated vs. direct trials in the transfer segment in the PD group. (C) Peak velocity was significantly higher in the control group, as well as for direct vs. elevated trials and reach vs. transfer segments. (D) Dimensionless jerk was significantly higher in the PD group, particularly for elevated movements.

Figure 3

Saccade amplitude during observation of object-directed actions was significantly reduced for predictable vs. unpredictable transfer movements in the control group, specifically in trials with an elevated trajectory. Plots show means with SEM error bars; dots represent individual participants.