Perceptual errors increase with movement duration and may contribute to hypokinesia in Parkinson's disease.

People with Parkinson's disease (PD) perceive that their movement amplitude is greater than what they actually perform. The neural mechanisms underlying one's perception of movement are believed to involve the sensorimotor integration process (SIP). How PD affects the SIP is not well understood. A previous study interrogating the SIP showed healthy adults (HAs) overestimated their limb position in the direction of movement and the error and its variance (VOE) depended on movement duration. We asked if PDs showed errors in perceived limb position and if the dependence on movement duration was different from HAs. We used an existing computational model of the SIP to explore mechanisms for the error and VOE as a function of movement duration. Twenty PDs, off medication, and 20 age-matched HAs were asked to estimate the position of their hand after performing 50, slow, non-visually guided wrist flexion or extension movements for a random period of time (<4.0 s). Both groups overestimated the amount they moved; however, the PDs' error and VOE were larger (p<0.001). HAs showed increasing error/VOE for small movement durations that reduced/stabilized for longer movement durations. PDs however showed increasing error/VOE with increasing movement duration that did not significantly improve/stabilize. The model suggested that the basis for such perceptual deficits may be abnormal proprioceptive feedback and/or processing of an abnormal internal impression (prediction) that underestimates movement amplitude. Simulation results imply that the PD's SIP could no longer effectively access sensory (proprioceptive) feedback to correct errors in other components of the SIP due to the abnormal processing of sensory feedback. We suggest from this study that an impaired perception of movement amplitude and sensory processing deficits contribute to hypokinesia in PD.