Abstract
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.
Data availability
Data and code for experiments 1 and 2 are posted on OSF, found here: https://osf.io/hbz9v.
Code availability
Stimulus code available here: https://osf.io/hbz9v.
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Acknowledgements
Our recruitment efforts were supported, with our gratitude, by Marie Saint-Hilaire, M.D., and Cathi Thomas, R.N., M.S.N. of Boston Medical Center Neurology Associates, and by Boston area Parkinson’s disease support groups, as well as the Michael J. Fox Trial Finder. We thank Yue Chen, Ph.D., David Somers, Ph.D., Sandy Neargarder, Ph.D., Victoria Nguyen, Chelsea Toner, Laura Pistorino, and Karla Sordia for their assistance on this project.
Funding
This research was funded by the National Institute of Neurological Disorders and Stroke (F31 NS07682 to DJN; RO1NS067128 to ACG), a grant from the Supporting Structures: Innovative Partnerships to Enhance Bench Science at CCCU Member Institutions program, run by Scholarship and Christianity in Oxford, the UK subsidiary of the Council for Christian Colleges and Universities, with funding by the John Templeton Foundation and the MJ Murdock Charitable Trust. D.J.N. & A.C.G. designed the study. D.J.N wrote the main manuscript, analyzed the data and prepared the figures. X.G.P, A.W., C.E.M. & A.C.G reviewed the manuscript and offered substantive edits and helped with interpreting data in the context of existing literature.
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D.J.N. & A.C.G. designed the study. D.J.N wrote the main manuscript, analyzed the data and prepared the figures. X.G.P, A.W., C.E.M. & A.C.G reviewed the manuscript and offered substantive edits and helped with interpreting data in the context of existing literature.
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This study was performed in line with the principles of the declaration of Helsinki, and approval as granted by the Internal Review Board of Boston University (#2530E).
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Norton, D.J., Munro, C.E., Williams, A. et al. Novel psychophysical line bisection task using brief stimulus presentation reveals no horizontal bias in left-onset parkinson’s disease. Sci Rep (2026). https://doi.org/10.1038/s41598-025-34798-3
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DOI: https://doi.org/10.1038/s41598-025-34798-3
