Insight into dopamine-dependent planning deficits in Parkinson's disease: A sharing of cognitive & sensory resources

Cognitive and sensorimotor processes are both needed for successful planning of footsteps during complex gait situations, but the interaction between these factors during motor planning, as well as their response to dopaminergic treatment is poorly understood in Parkinson's disease (PD). In the current study, we evaluated walking and gaze behaviors of individuals with PD while planning an approach toward an obstacle to be stepped over. The obstacle clearance task was completed both ON and OFF dopaminergic medication by individuals with Parkinson's disease (n=20) and compared to healthy age-matched control participants (n=19), as well as with and without an auditory digit monitoring dual task. In this novel protocol of synchronized gaze and gait data collection, each trial was split into an early and late phase prior to the obstacle, providing a unique opportunity to examine dopamine-dependent planning deficits in PD. Interestingly, only patients in the OFF medication state showed greater deceleration in the late phase (i.e., just before the obstacle) (F(1,37)=45.42, p<0.001), as well as an increase in step time variability (also in this late phase) with the additional demands of a dual task (F(2,74)=3.49, p=0.035). Only gait deceleration between approaching phases improved with dopaminergic treatment (F(1,18)=59.20; p<0.001). Although groups showed different walking behaviors, gaze behaviors were the same for all participants, in that they planned for the obstacle more so in the early phase (p<0.05), and fixations were reduced across participants with the presence of the dual task (p<0.001). Surprisingly, the gaze behavior of the PD OFF group showed no interactions with phase or condition suggesting that the deceleration and increased variability when approaching an obstacle is the result of a greater demand for online sensory feedback that cannot be compensated for with visual strategies. We conclude that dopamine influences planning by limiting sensorimotor processing capacity, especially in the presence of increased cognitive demand in PD.