Novel psychophysical line bisection task using brief stimulus presentation reveals no horizontal bias in left-onset parkinson’s disease

Some studies have shown that individuals with Parkinson’s disease (PD) with motor-symptom onset on the left side of the body (LPD) show mild neglect-like performance on some tasks, but others have not. Individuals with PD onset on the right side of the body (RPD) have not shown these effects. To clarify whether a bias in the perception of length exists in LPD, we administered a novel line bisection experiment, using psychophysical methods and no motor demands beyond a verbal response to isolate perceptual biases. Experiment 1 used a psychophysical procedure to test 21 LPD, 29 RPD and 28 age-matched healthy control adults (HC) on horizontal line bisection. A vertical line bisection condition was included as a control. Experiment 2 repeated the horizontal condition in a subset of participants, using eye-tracking and a fixation cross to preclude gaze bias. In both experiments, LPD did not demonstrate performance bias that was consistent with hemineglect. In Experiment 1, a bias specific to LPD was demonstrated in the vertical condition. The present results suggest that any neglect-like perceptual shifts occurring in LPD do not occur when psychophysically isolating perceptual decisions from higher-order processes that may be at play in other tasks.