Visual cues and gait improvement in Parkinson's disease: which piece of information is really important?

Visual exproprioception refers to information of the body relative to the environment and may be the critical piece of sensory information that explains why gait improvements can be achieved with visual step cues in people with Parkinson's disease (PD). The primary aim of current study was to investigate the role of visual exproprioception in the positive effect of visual cues on gait in patients with PD. Nineteen individuals with PD and 15 healthy subjects participated in this study. Four conditions of self-paced gait were tested: normal walking, without exproprioception (exproprioceptive information of lower limbs removed), visual step length cues, and visual step length cues but without exproprioception. Gaze behavior, gait parameters and the accuracy and precision of foot placement on the visual cues were recorded. Individuals with PD improved step length in both cued conditions. Both groups fixated close to 46% on visual cues necessary to accomplish the next step (ongoing step), while 54% of fixations were focused on visual cues one or more steps ahead. Also, both groups increased absolute error and error variability of the foot placement around the visual cues without vision of their feet. These results suggest that exproprioceptive information is not critical to achieve step length and overall gait benefits with visual cues in PD, but is critical for the accuracy and precision of foot placement on targets. People with PD and healthy individuals use visual information from visual cues in both on-line and feedforward fashions. In conclusion, patients with PD likely focus attention on the discrete goal of each foot hitting a visual cue placed on the floor and then use the exteroceptive information (i.e. position of next foot placement location) to plan each step individually at a cortical level.