Significant cognitive decline in Parkinson’s disease exacerbates the reliance on visual feedback during upper limb reaches

While upper limb reaches are often made in a feed-forward manner, visual feedbackduring the movement can be used to guide the reaching hand towards a target. InParkinson’s disease (PD), there is evidence that the utilisation of this visual feedbackis increased. However, it is unclear if this is due solely to the characteristic slownessof movements in PD providing more opportunity for incorporating visual feedback tomodify reach trajectories, or whether it is due to cognitive decline impacting (feedforward)movement planning ability. To investigate this, we compared reaction timesand movement times of reaches to a target in groups of PD patients with normalcognition (PD-NC), mild cognitive impairment (PD-MCI) or dementia (PD-D), to that ofcontrols with normal cognition (CON-NC) or mild cognitive impairment (CON-MCI).Reaches were undertaken with full visual feedback (at a ‘natural’ and ‘fast-as-possible’pace); with reduced visual feedback of the reaching limb to an illuminated target; andwithout any visual feedback to a remembered target with eyes closed.PD-D exhibited slower reaction times than all other groups across conditions,indicative of less efficient movement planning. When reaching to a rememberedtarget with eyes closed, all PD groups exhibited slower movement times relative totheir natural pace with full visual feedback. Crucially, this relative slowing was mostpronounced for the PD-D group, compared to the PD-MCI and PD-NC groups,suggesting that substantial cognitive decline in PD exacerbates dependence on visualfeedback during upper limb reaches.