A closer look at mechanisms underlying perceptual differences in Parkinson’s freezers and non-freezers

• PD-FOG demonstrated both visuospatial and self-motion perceptual impairments beyond that of PD-nonFOG (experiment 1).
• PD-FOG had a mismatch between actual and imagined movement that was amplified over greater distances (experiment 2).
• Imagery error predicted distance estimation errors while walking.
• This relationship suggests that self-motion deficits are independent of visuospatial impairments in PD-FOG.

Parkinson’s disease patients who suffer from freezing of gait (PD-FOG) may have sensory and/or perceptual deficits, although they are difficult to disentangle. This study evaluated whether visuospatial perception or self-motion perception were more impaired in PD-FOG, and whether distance estimation errors might be related to misperception of physical walking (compared to imagined). Finally, cognitive status was evaluated in order to evaluate whether cognitive status predicts any of the perception deficits identified. Nine PD-FOG and 15 PD-nonFOG were tested. In experiment 1, participants were shown a target, then the target was removed, before participants demonstrated the original position of the target in two different feedback conditions (pointing with a laser, or walking to its original position). In experiment 2, participants walked to a target (3, 4.5, 6 m) and then imagined walking to that same target. The time to complete both of these tasks was measured and compared. Experiment 1 found a significantly greater judgment error in PD-FOG across both conditions (p = 0.013) (compared to PD-nonFOG). Constant error revealed that both groups significantly underestimated during the self-motion condition only (p = 0.01). Interestingly, results from experiment 2 demonstrated a significant discrepancy between the time it took to imagine walking compared to their actual movement times, specifically in PD-FOG (p = 0.03). This mismatch as well as cognitive status significantly predicted judgment errors during the self-motion condition from experiment 1. Therefore, this study found evidence that PD-FOG have significantly greater sensory–perception deficits compared to PD-nonFOG. These findings have important clinical implications for further understanding FOG and developing new rehabilitative strategies for FOG symptoms.