Automatic Online Motor Control Is Intact in Parkinson’s Disease With and Without Perceptual Awareness

Abstract

In the double-step paradigm, healthy human participants automatically correct reaching movements when targets are displaced. Motor deficits are prominent in Parkinson’s disease (PD) patients. In the lone investigation of online motor correction in PD using the double-step task, a recent study found that PD patients performed unconscious adjustments appropriately but seemed impaired for consciously-perceived modifications. Conscious perception of target movement was achieved by linking displacement to movement onset. PD-related bradykinesia disproportionately prolonged preparatory phases for movements to original target locations for patients, potentially accounting for deficits. Eliminating this confound in a double-step task, we evaluated the effect of conscious awareness of trajectory change on online motor corrections in PD. On and off dopaminergic therapy, PD patients (n = 14) and healthy controls (n = 14) reached to peripheral visual targets that remained stationary or unexpectedly moved during an initial saccade. Saccade latencies in PD are comparable to controls’. Hence, target displacements occurred at equal times across groups. Target jump size affected conscious awareness, confirmed in an independent target displacement judgment task. Small jumps were subliminal, but large target displacements were consciously perceived. Contrary to the previous result, PD patients performed online motor corrections normally and automatically, irrespective of conscious perception. Patients evidenced equivalent movement durations for jump and stay trials, and trajectories for patients and controls were identical, irrespective of conscious perception. Dopaminergic therapy had no effect on performance. In summary, online motor control is intact in PD, unaffected by conscious perceptual awareness. The basal ganglia are not implicated in online corrective responses.

Preservation of eye movements in Parkinson’s disease is stimulus and task specific

“…Congruently, PD patients performed corrective saccades at a comparable level to healthy controls in a saccade double-step task (Merritt et al, 2017), although they also exhibited a larger number of averaging saccades (Bhutani et al, 2013).…”

Reaching and Grasping Movements in Parkinson’s Disease: A Review

“…The fact that conscious perception of target jump interferes with automatic motor control in PD [ 101 ] has been recently contested in [ 125 ]. Indeed, when the target jump is imposed at the movement onset, the akinesia in limb motion inevitably prolongs the period from target appearance to target displacement.…”

“…This may disproportionately impact the performances of patients, since their long preparations do not favor quick re-modulation of the trajectory [ 126 ]. By using large target displacements during initial saccadic fixation to target, but still consciously perceived by the subjects, Merritt et al [ 125 ] ensured equivalent onsets of target displacement for PD and controls and demonstrated that automatic trajectory modifications were equivalent in both groups and were unaffected by dopaminergic therapy. This questioned the idea that BG actually mediate the online motor control.…”

Preservation of Eye Movements in Parkinson's Disease Is Stimulus- and Task-Specific

“…Preserved functions are also found when a movement trajectory has to be corrected online to account for a displacement of the movement target, a task that requires a sense of urgency (Desmurget et al, 2004). Congruently, PD patients performed corrective saccades at a comparable level to healthy controls in a saccade double-step task (Merritt et al, 2017), although they also exhibited a larger number of averaging saccades (Bhutani et al, 2013). In sum, there is consensus that eye movements can be spared in PD patients under some task and stimulus conditions.…”