Parkinson's disease impairs the controllability of imagined action sequences.

Parkinson's Disease (PD) leads to deficits in motor control that may stem in part from impaired processing of action representations, which can be investigated using motor imagery. The controllability component of motor imagery in particular refers to the ability to precisely manipulate a motor image. Research measuring this component in PD via the Hand Laterality Judgment Task has generated inconsistent results. We sought to examine whether PD affects the controllability of motor imagery using a test that explicitly directs the use of motor imagery and involves complex action sequences. Thirty-nine people with PD on their regular medication regimen and thirty-eight controls completed the Test of Ability in Movement Imagery (TAMI). The PD group scored significantly worse than controls on the TAMI. Poorer cognitive function predicted lower TAMI scores for the PD group only. These results indicate that people with PD imagine action sequences less accurately than healthy older adults. It is possible that the disease impairs the ability to transfer sensorimotor information across sequential actions, an ability that is key to efficient motor behaviour. An alternative explanation is that motor dysfunction leads to less well-developed action representations for the actions featured in the TAMI, resulting in motor images that were of low fidelity to the target actions in the PD group. Given the documented effects of dopaminergic medication on the cognitive control process that support action sequencing, it would be of interest to determine if a similar pattern of results emerged in people with PD who are off medication.