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

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Norton, Catherine E. Munro, Abigail Williams, Xavier Gallart-Palau, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6874238/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Jan, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Objective 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 to isolate perceptual biases. Methods 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. Results In both experiments, LPD did not demonstrate performance bias that was consistent with hemineglect. In Experiment one, a bias specific to LPD was demonstrated in the vertical condition. Conclusions 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. Biological sciences/Psychology/Human behaviour Biological sciences/Neuroscience/Visual system Health sciences/Neurology/Neurological disorders/Parkinsons disease Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Parkinson’s disease (PD) is associated with visual and visuocognitive abnormalities that have a significant impact on quality of life over and above the disease’s hallmark motor symptoms (Armstrong, 2017 ; Cronin-Golomb, 2010; Klepac, Trkulja, Relja, & Babic, 2008 ; Nieto-Escamez, Obrero-Gaitan & Cortes-Perez, 2023; Putcha, Jaywant & Cronin-Golomb, 2016 ; Savit & Aouchiche, 2020; Visser et al., 2008 ). Persons with PD (PwPD) whose motor symptoms are more severe on the left side of their bodies, or whose motor symptoms began on the left side (LPD), seem to demonstrate unique spatial and attentional processing changes relative to those with right motor onset (RPD) (e.g., Albert, Bernasconi, Potheegadoo & Blanke, 2025 ; DeGutis, Aul, Barthelemy, Davis, Alshuaib, Marin et al., 2023; Seichepine, Neargarder, Davidsdottir, Reynolds, & Cronin-Golomb, 2015 ; Villardita, Smirni, & Zappala,1983 ). This may stem from the fact that the right hemisphere is dominant for spatial processing, encoding both left and right visual hemifields, whereas the left hemisphere encodes mainly the right half of space (Degutis et al., 2023 ; Sheremata et al., 2010 ; Thiebaut de Schotten et al., 2005 ). PD is usually asymmetrical in its onset, and the brain pathology on the side contralateral to the initial motor symptoms remains more severe throughout the illness (Kempster, Gibb, Stern, & Lees, 1989 ). Hence, in LPD, the primary processor of left-space has been compromised, with attendant perceptual consequences including, perhaps, hemispatial neglect. For LPD, the following results are suggestive of neglect: (1) On line bisection tasks, this PD subgroup estimates the center of a horizontal line to be to the right of the actual center (Albert et al., 2025 ; Davidsdottir et al., 2008 , for men; Lee, Harris, Atkinson, & Fowler, 2001 ) or more rightward than RPD and NC at certain field positions (Laudate et al., 2013 ); (2) The initial direction of eye scanning when exploring a novel stimulus is biased toward the right (Ebersbach et al., 1996 ); (3) Objects presented on the left appear smaller than those presented on the right (Harris, Atkinson, Lee, Nithi, & Fowler, 2003 ). There is also evidence that this syndrome may affect daily living in these individuals, since LPD report bumping into doorways more often on the left than right side (Davidsdottir et al., 2005 ), which could contribute to falls or driving difficulties (Laudate et al., 2013 ; Young et al., 2010 ). Several studies found no evidence of an LPD-specific effect in terms of neglect, however. Laudate et al 2010 showed no difference between controls and LPD on line bisection midpoints in eight out of nine conditions (which differed only in their position on the screen). In addition, two studies by Norton and colleagues attempting to “explain” hemineglect in LPD at a functional level found no evidence of a left-hemifield deficit. The first measured whether space is perceptually compressed in the left hemifield in LPD (Norton et al., 2015 ) by having subjects compare the perceived thickness of bands on vertically oriented Gabor patches in the left and right hemifield, as well as measuring the perceived contrast of Gabor patches in the left and right hemifield. In both, performance of LPD did not differ from that of control participants. A second study measured sustained attention using a multiple object tracking task where the targets were systematically restricted to the left or right hemifield (Norton et al., 2016 ). Again, LPD did not perform differently than the control group. Finally, the largest sample of PD individuals on a line bisection task to our knowledge (Salazar et al., 2019 ) showed no evidence of a rightward bias for LPD (both LPD and RPD actually showed a rightward bias, but only when beginning their adjustment of a movable center hatch on the left of the line). In sum, these results raise significant doubts about whether a true hemineglect effect exists on line bisection tasks in LPD. To address this question, we used a novel psychophysical paradigm to assess line bisection. This paradigm offered two advantages over previous studies. First, it used a two alternative forced choice procedure that assessed the perception of line length apart from possible noise generated by biases and motor behaviors necessary to move a hatch-mark manually on a computer screen or draw a line on a piece of paper. Second, a very brief stimulus presentation allowed us to remove the role that biased visual exploration of the stimulus might play. In addition, we tested a subset of participants using a modified version of this paradigm that contained a fixation cross and employed eye tracking, to investigate whether any neglect effects we might find would persist when gaze was assuredly matched between groups. Experiment 1 Methods Participants Participants were 49 non-demented PwPD, including 21 LPD (12 men, 9 women, mean age = 64.2, standard deviation [SD] = 7.3), 28 RPD (15 men, 13 women, mean age = 65.5SD = 6.8) and 29 healthy control adults (HC, 11 men, 18 women, mean age = 67.6, SD = 8.7). The groups did not differ significantly in terms of age, F(2,75) = 1.27, p = .29, or years of education, F(2,75) = .93, p = .40. Participants were excluded from the study on the basis of having neurological conditions (other than PD, for the PD group), coexisting serious chronic medical illnesses (including psychiatric illness), use of psychoactive medication besides antidepressants and anxiolytics in the PD group, history of intracranial surgery (e.g., deep brain stimulation or other invasive PD treatments), traumatic brain injury, alcohol dependence or recent substance abuse. All participants received a detailed neuro-ophthalmological examination to rule out visual disorders including significant glaucoma, cataracts, or macular degeneration. The minimum Mini-Mental State Examination (MMSE; Folstein, Folstein, & McHugh, 1975 ) score for inclusion in the study was a 26 out of a possible total score of 30. The minimum score for the sample included in this study was 27. Both PD groups had a median Hoehn and Yahr (H&Y) score of 2 out of a total possible score of 5, indicating mild bilateral motor stage of disease. The range of scores for LPDs was between 1 and 4. There was only one LPD participant with a H&Y score of 4, and none with 3 or 3.5. The range of scores for RPDs was between 1 and 3. LPD and RPD did not differ significantly on their H&Y scores (Kolmogorov-Smirnov, p = .49). Scores were unavailable for 2 LPD and 3 RPD. Unified Parkinson Disease Rating Scale (UPDRS) scores were available for 17 LPD, who had a mean motor score of 19.4 (SD 8.1), and for 19 RPD, who had a mean motor score of 19.1 (SD 9.0). Because we had scores for more participants on H&Y than on UPDRS, we report H&Y scores for the participants in each experiment. Stimulus The stimulus was a long horizontal line (Fig. 1 ), 15.9 degrees in length and 7 arc min in width, with a perpendicular hatchmark 4.0 degrees in length and 7 arc min in width. The bisected line and bisecting hatchmark were both black and presented on a white background. The hatchmark was positioned along the horizontal line in one of 12 locations—1, 2.1, 4.2, 8.3 or 16.7% offset to the left or right of center. For example, a 100% offset would indicate the hatchmark being positioned on the endpoint of the line. In order to preclude saccades during the stimulus presentation, the presentation time was set at a brief duration, 83.3 msec, whereas stimulus-driven saccades are on the order of 200 msec (McPeek, Skavenski, & Nakayama, 2000 ). For the vertical condition, the stimulus was identical, except rotated 90 degrees. Procedures The task was to indicate orally whether the hatchmark was to the left or the right of the longer line's midpoint (Fig. 1 ). The hypothesis was that abnormal saccades drive neglect in LPD, and therefore eliminating saccades would also eliminate neglect. That is, LPD would not show a neglect-like pattern on this task and would perform similarly to RPD and HC. Data analysis The main dependent variable was the proportion of trials in which participants reported that the hatchmark was to the right of center for each hatch-position (8 trials each condition). From the proportion right data, a perceived midpoint (PM) was extracted using a Weibull function of the form where y is the proportion of trials for which the observer reports that the hatchmark is to right of center, x is the hatch position, and a and b are adjustable curve-fitting parameters. Neglect would manifest itself as a rightward shift in PM, and a decrease in the proportion of trials viewed as right of the midpoint, since neglect tends to shift the PM of the line to the right. Outliers were identified as those who showed a PM more than two standard deviations from the mean in their subgroup (e.g., LPD). Results Horizontal Condition Results for proportion-right data are shown in Fig. 2 a. LPD reported “right [of midpoint]” less frequently than did RPD and HC, especially at the hatch positions near the center of the line (i.e., where the stimulus was difficult to judge and uncertainty was high). However, statistical analyses did not support this being a reliable difference. When only examining right-handers, a mixed model ANOVA with group (LPD, RPD, HC) and hatch position (12 levels) showed no significant main effect for group, F(2, 65) = 1.37, p = .26, η 2 = .04. There was a main effect for hatch position, F(4.1, 266.1) = 327.8, p < .001, η 2 = .83. The interaction between group and hatch position was not significant F(8.2, 266.1) = 0.70, p = .69, η 2 = .02. Including left-handers and ambidextrous participants, the analysis remained largely unchanged. The main effect for group was not significant, F(2,74 ) = 1.52, p = .23, η 2 = .04 There was a main effect for hatch position, F(4.3,317.9) = 353.3, p < .001, η 2 = .83. The interaction between group and hatch position was not significant F(8.6,317.9) = .67, p = .73, η 2 = .02. Vertical Condition Results for vertical line bisection are shown in Fig. 2 b. Including all subjects, the main effect for group was not significant, F(2,74) = 2.38, p = .10, η 2 = .06. There was a main effect for hatch position, F(4.2, 313.6) = 514.7, p < .001, η 2 = .87. The interaction between group and hatch position was significant F(8.5,313.7) = 2.21, p = .024, η 2 = .06. When including only right-handers, the main effect for group was not significant, F(2,65) = .68, p = .51, η 2 = .02, though the main effect for hatch position remained so, F(4.1,267.7) = 470.4, p < .001, η 2 = .88, and the interaction between group and hatch position was not significant F(8.2,267.7) = 1.60, p = .12, η 2 = .05. Experiment 2 Methods Participants Participants were a subgroup of those from Experiment 1. They were 25 non-demented PwPD, including 17 LPD (10 men, 7 women, mean age = 64.2, SD = 6.9), 19 RPD (11 men, 8 women, mean age = 64.6, SD = 6.4) and 17 HC (6 men, 11 women, mean age = 64.6, SD = 9.1). The groups did not differ significantly in terms of age, F(2) = .03, p = .97 or male:female ratio χ 2 (2) = 2.5, p = .29. LPD had a Hoehn and Yahr range of 1–4, with a median of 2, and RPD had a range of 1–3 and a median of 2; the two groups did not differ significantly, Kolmogorov-Smirnov, p = .21. Stimulus and procedure The stimuli and procedures, shown in Fig. 3 , were generally similar to those described for Experiment 1. A fixation cross was used to ensure that the stimulus was processed by the same part of the retina for all participants. The stimulus was a black horizontal line 15.9 degrees in length and 7 arc min in width, with a perpendicular hatchmark 3.0 degrees in length and 7 arc min in width, both presented on a gray background. The fixation cross was positioned in the center of the screen, and participants were instructed to look directly at it each time it appeared. After 1000 msec, the horizontal section of the hatchmark was overlaid with a long horizontal line offset from center, such that the left or the right half of the line was longer than its opposite half. There were 8 different left-lengths (with right length being held constant; four decrements [-54 pixels, -27, -13, -6] and four increments [54, 27, 13 and 6] for the left half of the line), and 8 different right-lengths used. The task was to determine whether the fixated hatchmark was to the left or the right of the long horizontal line’s center. All conditions were presented in a random order, and each was repeated 8 times. Data were averaged across sides such that, for example, a decrement of 54 pixels on the left side of the line was pooled with an increment of 54 pixels on the right side of the line. Eye-tracking apparatus: Eye-tracking and recording were performed using an Applied Science Laboratories (ASL) eye-tracking system. A model D6 camera array was located underneath the stimulus monitor, and used infrared light to discern the participant’s pupil position and corneal reflection. These reflection points were monitored with Eye-Trac software to locate the position of the participant’s eye, and sampled at a rate of 120 Hz, with a maximum accuracy of 0.5 degrees of visual angle. Although the participant used binocular vision for the experiment, only their left eye was tracked. A 9-point calibration sequence was administered at the beginning of the session and as needed during testing to ensure that the equipment was accurately calculating the participant’s gaze relative to the display for the duration of the experiment. Eye-tracking data were processed with MATLAB. Trials where the participant’s gaze was more than 3 degrees of visual angle leftward or rightward of the fixation cross, or where a saccade was made during the stimulus presentation, were eliminated. Results Response as a function of line-end position The results for Experiment 2 are shown in Fig. 4 , and were analyzed using only the trials on which participants maintained fixation within 2 degrees vertically and horizontally of the fixation cross’s middle. A mixed model ANOVA, with group (RPD, HC or LPD) and line-end-position (8 levels) showed a significant effect for line-end position, F(3.7, 157.2) = 322.9, p < .001, η 2 = .80, but not for group, F(2, 42) = 1.13, p = 0.33, η 2 = 05. The interaction between group and hatch position was not significant, F(7.5, 157.2) = .26, p = .77. Using the PM data, there were no group differences, F(2, 49), = .38, p = .69,, η 2 = .02. A t-test comparing LPD and HC showed no difference, t(30) = .58, p = .56, Cohen’s d = .21. Discussion The present study used a novel psychophysical paradigm to examine line bisection performance in LPD. Across two experiments, both of which likely precluded any strategic eye movements to scan the target, we found no evidence of hemineglect as assessed by horizontal line bisection. Instead, we found that perception of line length was biased in LPD for vertical lines, where a significant interaction between hatch position and group was driven by LPD participants’ reduced proportions of trials judged as having the hatch mark above the midpoint, relative to the other groups (Fig. 2 b). Hemineglect in LPD: No direct evidence was found for hemineglect in LPD in the present study. A horizontal bias was present in LPD in the same direction as that seen in hemineglect patients with right parietal damage, but this bias was not statistically distinct from that seen in healthy control or RPD participants. The present result was obtained using a psychophysical paradigm that eliminated non-perceptual bias with its use of a two alternative forced choice procedure. Experiment one also prevented strategic eye movements to scan the target by using a brief presentation time. In Experiment two, when gaze was also controlled using a fixation cross and eye tracking, the difference in line bisection performance between HC and LPD diminished further and was still non-significant. This study was also the first to remove exploratory eye movements as a possible factor in the results of a line bisection task in PD. Since stimulus presentation was limited to 83 msec, it is unlikely that observers were able to generate any exploratory saccades during stimulus presentation. This removes reduced saccadic amplitude as a factor in the results, which is important since reduced saccadic amplitude has been demonstrated in PD (Matsumoto et al., 2011 ). The results of Experiment 1 indicate instead that a perceptual bias in vertical but not horizontal line bisection exists in LPD, but it is independent of abnormalities in saccadic functioning. In Experiment 2 an additional aspect of oculomotor functioning, gaze bias, was also precluded as a possible factor in the results. Although it is unlikely that participants were able to explore the stimulus with their eyes for more than one fixation, it is possible that PD participants had more variability or a directional bias in their gaze position when the stimulus appeared on the screen, and this could have affected their perception. Gaze bias was controlled using a fixation cross and eye tracking; trials on which fixation was not maintained were removed. Here, there was no difference between LPD and HC, and the non-significant difference was smaller than that seen for Experiment 1. The results of Experiment 2 are similar to other studies on PD that utilized psychophysical paradigms with eye tracking and a fixation cross, finding little to no group differences between LPD, RPD, and HC. When examining LPD participants’ perceptions of spatial compression, no biases were found in the data that would support the hypothesis of left hemifield neglect (Norton et al., 2015 ). Additionally, no evidence was found to support a left-hemifield deficit in sustained attention to moving objects (Norton et al 2016 ). This pattern of results across experiments suggests that when the possibility of gaze bias and biased exploratory eye movements are eliminated using a fixation cross and eye tracking, no LPD differences in the processing of left-space emerge relative to processing by RPD or control participants. A point of potential importance for future research is that we focused on LPD and found that some of these participants showed notable neglect-like biases, but so did some with RPD, as well as some HC (see PM scatter plots, Fig. 2 ). Side of motor symptom onset is likely only one factor accounting for the variance in line bisection bias in PD. Individual differences in spatial biases that occur in healthy individuals and those with PD are a potentially informative topic that to date has received little attention. Vertical line bisection A bias in LPD performance occurred with respect to the upper and lower visual fields: LPD viewed the hatchmark as above the center less frequently than RPD and HC. Vertical-axis bias has been shown previously in LPD, with conflicting results (Laudate et al., 2013 ; Lee et al., 2001 ). Lee and colleagues found that LPD estimated a vertical line’s midpoint to be below its physical center, whereas control adults and RPD did the opposite, but Laudate and colleagues ( 2013 ) did not find a difference between LPD and a healthy control group on vertical line bisection. It is unclear why the two previous studies found conflicting results, though both were based on small samples. The present data are consistent with those of Laudate and colleagues, and in the opposite direction of Lee and colleagues. Of note, the significant result did not remain when the sample was limited to right-handed individuals. Of relevance to the present discussion, in both previous studies eye movements were free to be made during an unlimited stimulus presentation time. The results of these studies are therefore ambiguous as to why LPD performed as they did (bisecting lines below or above their true vertical center), since they could be explained by eye movement differences, or by altered perception of the upper visual field of space. In the present study, LPD perceived the vertical line’s midpoint as being below its physical position, and since scanning of the target was not possible, an abnormality in processing of up- or down-space appears to exist in LPD independently of saccadic abnormalities. Possible Neural Correlates of Bias in Length Perception in PD The present results may be related to multiple forms of neuropathology that have been demonstrated in PD. Parietal neuropathology in PD has been shown in the form of decreased gray matter density (Pereira et al., 2009 , Tinaz et al., 2011 ) and cortical thinning (Filippi et al., 2020 ; Wilson et al., 2019 ). In addition, some individuals with PD, namely those with mild cognitive impairment, show hypometabolism in parietal cortex (Garcia-Garcia et al., 2012 ; Pappata et al., 2011). Because the parietal lobe is important for attention and representation of visual space (Beffara et al., 2022 ; Sheremata et al., 2010 ; Somers & Sheremata, 2013 ; Thiebaut de Schotten et al., 2005 ; Wutz, Zazio & Weisz, 2020 ), this parietal dysfunction could affect performance in a way that biases those with LPD process space and length differently. In addition, subcortical brain pathology may play a role in altered perception in PD. Where humans perceive themselves to be looking is determined by a combination of factors, including a series of presumed corollary discharges offered by the brain during each eye movement. These discharges have been proposed to be altered in PD (Diederich, Stebbins, Schiltz, & Goetz, 2014). If the movements that are executed do not match the ones that are shared in a corollary discharge (due to motor pathology in PD), it could alter individuals’ perceptual map of the world dramatically. Two brain systems that may be involved with the corollary discharge, the superior colliculus and thalamus, are affected in PD. The superior colliculus is crucial for determining location of eye movement and maintaining a map of salient locations in space (Hafed & Krauzlis, 2008 ). This area is directly connected to substantia nigra (Hikosaka et al., 1983), and has been proposed to be dysfunctional in PD (Terao et al., 2011). In addition, PD is associated with alterations in the functional connectivity and morphology of the thalamus (Chen, Y et al., 2023 ; Moustafa, McMullan, Rostron, Hewedi & Haladjian, 2017; Owens-Walton, Jakabek, Power, Walterfang, Velakoulis, van Westen et al., 2019), affecting basal ganglia-thalamocortical circuits. It is possible that alterations in thalamic connectivity and function may contribute to the manifestation of perceptual bias in PD patients, a point that necessitates further research following the findings identified here. Limitations The present work was subject to some limitations. First, it would have been useful to see if gaze bias in Experiment 1 (using eye tracking but not requiring central fixation) predicted line bisection performance on the task, but eye tracking was not used in that experiment. A second limitation relates to the line lengths used in our stimuli. Neglect in LPD previously had been shown primarily using longer line lengths than used in the present study. We used lines of approximately 16 degrees of visual angle due to constraints of a standard computer monitor. Future studies could attempt replication of the present study by using a projector or other methods to increase the visual angle of lines. Finally, replicating the present results in comparison with a task that allowed or required exploratory eye movements might reveal interesting truths about the role that eye movements play in hemineglect-like performance in LPD. Conclusions The results of the present study indicate that the perception of length is altered in LPD, particularly for vertical lines, even for briefly presented stimuli that preclude examination with exploratory eye movements. No significant hemineglect effect for LPD was found in a horizontal version of the task, despite a trend in this direction. Understanding the mechanisms underlying spatial biases in PD, as well as their impact on daily life, may offer insight into possible routes toward remediation. Declarations 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. Conflicts of interest: All authors report none. Ethics approval: 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). Consent to participate: All participants in this study provided written informed consent. Consent for publication: not applicable Code availability: Stimulus code available upon request from corresponding author (DJN). Data availability: Data are available upon request from the corresponding author (DJN) Acknowledgments: 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. References Albert, L., Bernasconi, F., Potheegadoo, J., Blanke, O. (2025). Home-based online line bisection test detects visuo-spatial neglect and pseudoneglect in Parkinson's disease. Parkinsonism and Related Disorders,130 , 107195. doi:10.1016/j.parkreldis.2024.107195. Epub 2024 Nov 8. Armstrong R. A. (2017). 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Spatial compression in visual neglect: a case study. Cortex , 27, 623-9. Harris JP, Atkinson EA, Lee AC, Nithi K, Fowler MS.(2003) Hemispace differences in the visual perception of size in left hemiParkinson's disease. Neuropsychologia , 41, 795-807. Hikosaka O, Wurtz RH. Visual and oculomotor functions of monkey substantia nigra pars reticulata. IV. Relation of substantia nigra to superior colliculus. J Neurophysiol 1983;49(5):1285-301. Kempster PA, Gibb WR, Stern GM, Lees AJ. (1989) Asymmetry of substantia nigra neuronal loss in Parkinson's disease and its relevance to the mechanism of levodopa related motor fluctuations. J Neurol Neurosurg Psychiatry , 52(1):72-6. Klepac N, Trkulja V, Relja M, Babic T. (2008). Is quality of life in non-demented Parkinson's disease patients related to cognitive performance? A clinic-based cross-sectional study. Eur J Neurol , 15, 128-33. Laudate TM, Neargarder S, Cronin-Golomb A. (2013). 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Moustafa AA, McMullan RD, Rostron B, Hewedi DH, Haladjian HH. The thalamus as a relay station and gatekeeper: relevance to brain disorders. Reviews in the Neurosciences , 28, 203-218. doi: 10.1515/revneuro-2016-0067. Nieto-Escamez, F., Obrero-Gaitán, E., & Cortés-Pérez, I. (2023). Visual Dysfunction in Parkinson's Disease. Brain sciences , 13 (8), 1173. https://doi.org/10.3390/brainsci13081173 Norton DJ, Jaywant, A., Gallart-Palau, X., & Cronin-Golomb, A. (2015). Normal discrimination of spatial frequency and contrast across visual hemifields in left-onset Parkinson's disease: evidence against perceptual hemifield biases. Vision research , 107 , 94–100. https://doi.org/10.1016/j.visres.2014.12.003 Norton DJ, Nguyen VA, Lewis MF, Reynolds GO, Somers DC, Cronin-Golomb A (2016) Visuospatial Attention to Single and Multiple Objects Is Independently Impaired in Parkinson's Disease. PLoS ONE 11(3): e0150013. https://doi.org/10.1371/journal.pone.0150013 Ogourtsova T, Korner-Bitensky N, Ptito A. (2010). Contribution of the superior colliculi to post-stroke unilateral spatial neglect and recovery. Neuropsychologia , 48, 2407-16. Owens-Walton C, Jakabek D, Power BD, Walterfang M, Velakoulis D, van Westen D, Looi JCL, Shaw M, Hansson O. Increased functional connectivity of thalamic subdivisions in patients with Parkinson's disease. PLoS One, 14 , e0222002. doi: 10.1371/journal.pone.0222002. Pappatà S 1 , Santangelo G, Aarsland D, Vicidomini C, Longo K, Bronnick K, Amboni M, Erro R, Vitale C, Caprio MG, Pellecchia MT, Brunetti A, De Michele G,Salvatore M, Barone P. (2011). Mild cognitive impairment in drug-naive patients with PD is associated with cerebral hypometabolism. Neurology 77, 1357-62 Pereira JB, Junqué C, Martí MJ, Ramirez-Ruiz B, Bargalló N, Tolosa E. (2009). Neuroanatomical substrate of visuospatial and visuoperceptual impairment in Parkinson's disease. Movement Disorders 24 , 1193-9. doi: 10.1002/mds.22560. Posner MI, Walker JA, Friedrich FJ, Rafal RD. (1984). Effects of parietal injury on covert orienting of attention. J Neurosci , 4(7):1863-74. Possin KL, Filoteo JV, Song DD, Salmon DP. (2009). Space-based but not object-based inhibition of return is impaired in Parkinson's disease. Neuropsychologia , 47, 1694-700. Putcha, D., Jaywant, A., Cronin-Golomb, A. (2016). Cognitive and perceptual impairments in Parkinson’s disease arising from dysfunction of the cortex and basal ganglia. In: The Basal Ganglia: Novel Perspectives on Motor and Cognitive Functions, J.-J. Soghomonian (ed.). Springer, pp. 189-216. Previc FH. (1998) The neuropsychology of 3-D space. Psychological Bulletin, 124, 123-64. Salazar, R. D., Moon, K. L. M., Neargarder, S., & Cronin-Golomb, A. (2019). Spatial judgment in Parkinson’s disease: Contributions of attentional and executive dysfunction. Behavioral Neuroscience , 133 (4), 350–360. https://doi.org/10.1037/bne0000329Savitt, J., & Mathews, M. (2018). Treatment of Visual Disorders in Parkinson Disease. Current treatment options in neurology , 20 (8), 30. https://doi.org/10.1007/s11940-018-0519-0 Savitt, J., & Aouchiche, R. (2020). Management of Visual Dysfunction in Patients with Parkinson's Disease. Journal of Parkinson's disease , 10 (s1), S49–S56. https://doi.org/10.3233/JPD-202103 Schenden, HE, Amick, MM, Cronin-Golomb A. (2009) Behavioral Neuroscience,123, 125–36 Seichepine, D. R., Neargarder, S., Davidsdottir, S., Reynolds, G. O., & Cronin-Golomb, A. (2015). Side and type of initial motor symptom influences visuospatial functioning in Parkinson’s disease. Journal of Parkinson’s Disease , 5 (1), 75–83. https://doi.org/10.3233/JPD-140365 Sharpe MH. (1990). Patients with early Parkinson's disease are not impaired on spatial orientating of attention. Cortex , 26, 515-24. Sheremata SL, Bettencourt KC, Somers DC. (2010). Hemispheric asymmetry in visuotopic posterior parietal cortex emerges with visual short-term memory load. J Neurosci, 30, 12581-8. Somers DC, Sheremata SL. (2013) Attention maps in the brain. Wiley Interdisciplinary Review. Cognitive Science. 4, 327-340. Terao Y 1 , Fukuda H, Yugeta A, Hikosaka O, Nomura Y, Segawa M, Hanajima R, Tsuji S, Ugawa Y. (2011) Initiation and inhibitory control of saccades with the progression of Parkinson's disease - changes in three major drives converging on the superior colliculus. Neuropsychologia . 49, 1794-806. Tinaz, S., Courtney, M. G., and Stern, C. E. (2011). Focal cortical and subcortical atrophy in early Parkinson’s disease. Mov. Disord. 26, 436–441. Thiebaut de Schotten M, Urbanski M, Duffau H, Volle E, Lévy R, Dubois B, Bartolomeo P. (2005). Direct evidence for a parietal-frontal pathway subserving spatial awareness in humans. Science. 309, 2226-8. Villardita C, Smirni P, Zappala G. (1983). Visual neglect in Parkinson's disease. Arch Neurol , 40, 737-9. Visser M, van Rooden SM, Verbaan D, Marinus J, Stiggelbout AM, van Hilten JJ. (2008). A comprehensive model of health-related quality of life in Parkinson's disease. J Neurol 255, 1580-7. Wilson, H., Niccolini, F., Pellicano, C., & Politis, M. (2019). Cortical thinning across Parkinson's disease stages and clinical correlates. Journal of the neurological sciences , 398 , 31–38. https://doi.org/10.1016/j.jns.2019.01.020 Wutz A, Zazio A, Weisz, N.J. (2020). Oscillatory Bursts in Parietal Cortex Reflect Dynamic Attention between Multiple Objects and Ensembles. Journal of Neuroscience, 40, 6927-6937. doi: 10.1523/JNEUROSCI.0231-20.2020. Young DE, Wagenaar RC, Lin CC, Chou YH, Davidsdottir S, Saltzman E, Cronin-Golomb, A. (2010). Visuospatial perception and navigation in Parkinson's disease. Vision Res, 50, 2495-504. Yu C, Klein SA, Levi DM. (2001) Surround modulation of perceived contrast and the role of brightness induction. J Vis 1, 18-31. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 09 Jan, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 11 Aug, 2025 Reviews received at journal 04 Aug, 2025 Reviewers agreed at journal 04 Aug, 2025 Reviews received at journal 20 Jul, 2025 Reviewers agreed at journal 20 Jul, 2025 Reviewers invited by journal 16 Jul, 2025 Editor assigned by journal 16 Jul, 2025 Editor invited by journal 03 Jul, 2025 Submission checks completed at journal 18 Jun, 2025 First submitted to journal 18 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Norton","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9UlEQVRIiWNgGAWjYDCCAyBUAONVgIgEEMFMQIsBTNEZIrUwwLUwthGhhe/42YMHPhgw5PFLnz/4uHBerZx5e/KxBwwV1okNOLRInslLODjDgKFYsi+Z2XjmtuPGMmeepRswnEnHqcXgQI7BYR4DhsQNZ5jZpHm3HUucIZFjJsHYdhi3lvNvDA7/gWuZA9PyD4+WG0BbGOBaGmqgWhpwa5G88cbgYI+BROLMHmZjY55jB4wleJ6lSSQcSzfGpYXvfI7xhx8VNon9PIwPH/PU1MlJsCcfk/hQYy2LSwsUSMAYhyFUAn7lKKCOBLWjYBSMglEwUgAAYidYi4M0G0gAAAAASUVORK5CYII=","orcid":"","institution":"Boston University","correspondingAuthor":true,"prefix":"","firstName":"Daniel","middleName":"J.","lastName":"Norton","suffix":""},{"id":486402563,"identity":"e8f18554-3e8e-4bc6-bfdf-eb4d7aa40bc5","order_by":1,"name":"Catherine E. Munro","email":"","orcid":"","institution":"Boston University","correspondingAuthor":false,"prefix":"","firstName":"Catherine","middleName":"E.","lastName":"Munro","suffix":""},{"id":486402564,"identity":"2d59c5b9-ea1d-45d8-9591-b41d5f8cdf03","order_by":2,"name":"Abigail Williams","email":"","orcid":"","institution":"Boston University","correspondingAuthor":false,"prefix":"","firstName":"Abigail","middleName":"","lastName":"Williams","suffix":""},{"id":486402570,"identity":"2439eb8e-d494-424a-8442-f7fb57cb14bf","order_by":3,"name":"Xavier Gallart-Palau","email":"","orcid":"","institution":"Boston University","correspondingAuthor":false,"prefix":"","firstName":"Xavier","middleName":"","lastName":"Gallart-Palau","suffix":""},{"id":486402573,"identity":"4f99d126-f72f-4816-bb9a-a8e704498abb","order_by":4,"name":"Alice Cronin-Golomb","email":"","orcid":"","institution":"Boston University","correspondingAuthor":false,"prefix":"","firstName":"Alice","middleName":"","lastName":"Cronin-Golomb","suffix":""}],"badges":[],"createdAt":"2025-06-11 18:08:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6874238/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6874238/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-025-34798-3","type":"published","date":"2026-01-09T15:58:38+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":87364973,"identity":"22423406-174b-4fd8-a452-4c39fe43879b","added_by":"auto","created_at":"2025-07-23 06:23:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":149764,"visible":true,"origin":"","legend":"\u003cp\u003eExperiment 1 line bisection task. The task begins with a blank screen that is present for 1000 msec. A horizontal line with a vertical hatchmark to the left or right of the horizontal line’s midpoint is then presented for 83 msec. The observer is instructed to verbally respond “left” or “right” after the bisected line disappears.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6874238/v1/3a64186e9e9aeb92958fc58b.png"},{"id":87364977,"identity":"3be91f3e-23fc-4f6d-9ea9-1bd099c4d8e2","added_by":"auto","created_at":"2025-07-23 06:23:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":480599,"visible":true,"origin":"","legend":"\u003cp\u003eExperiment 1 results: Negative values on x axis refer to left offset. LPD responded “rightward of center” less often than RPD and NC at hatch-positions near the midpoint of the horizontal line. RPD and NC showed a similar trend for all hatchmark positions. b) Vertical condition results from experiment 1. c) Perceived midpoints for horizontal line bisection. Outliers shown in gray and not included in analysis\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6874238/v1/9305699db7fac58849ac2aad.png"},{"id":87365897,"identity":"482a7ca5-8de1-4542-9f8a-0cbd7e4953ea","added_by":"auto","created_at":"2025-07-23 06:31:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":131973,"visible":true,"origin":"","legend":"\u003cp\u003eExperiment 2 line bisection task. The task began with a fixation cross presented in the center of the screen. After 1000 msec, a horizontal line was overlaid on the fixation cross so that the vertical portion of the cross formed a vertical hatchmark, offset to the left or right of the horizontal line’s midpoint. The horizontal line remained onscreen for 100 msec, after which participants were instructed to verbally respond “left” or “right” if the vertical hatchmark bisected the line left or right of the horizontal line’s perceived center.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6874238/v1/8926be772f7effe235268481.png"},{"id":87364985,"identity":"41309eb8-435f-40c9-84dd-594e818d07ca","added_by":"auto","created_at":"2025-07-23 06:23:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":616869,"visible":true,"origin":"","legend":"\u003cp\u003eExperiment 2 results a. Line bisection on trials for which observers maintained fixation. b. Perceived midpoints in each of the three participant groups.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6874238/v1/580f90065fa0fd585dd36e58.png"},{"id":100069348,"identity":"6a8a7b3f-206e-4d30-ab8b-32300c6e3952","added_by":"auto","created_at":"2026-01-12 16:13:08","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1976022,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6874238/v1/0b6f1d5b-3834-4c4d-b535-a7f78b9bf41d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Novel psychophysical line bisection task using brief stimulus presentation reveals no horizontal bias in left-onset Parkinson’s Disease","fulltext":[{"header":"Introduction","content":"\u003cp\u003eParkinson\u0026rsquo;s disease (PD) is associated with visual and visuocognitive abnormalities that have a significant impact on quality of life over and above the disease\u0026rsquo;s hallmark motor symptoms (Armstrong, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Cronin-Golomb, 2010; Klepac, Trkulja, Relja, \u0026amp; Babic, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Nieto-Escamez, Obrero-Gaitan \u0026amp; Cortes-Perez, 2023; Putcha, Jaywant \u0026amp; Cronin-Golomb, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Savit \u0026amp; Aouchiche, 2020; Visser et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Persons with PD (PwPD) whose motor symptoms are more severe on the left side of their bodies, or whose motor symptoms began on the left side (LPD), seem to demonstrate unique spatial and attentional processing changes relative to those with right motor onset (RPD) (e.g., Albert, Bernasconi, Potheegadoo \u0026amp; Blanke, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; DeGutis, Aul, Barthelemy, Davis, Alshuaib, Marin et al., 2023; Seichepine, Neargarder, Davidsdottir, Reynolds, \u0026amp; Cronin-Golomb, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Villardita, Smirni, \u0026amp; Zappala,1983 ). This may stem from the fact that the right hemisphere is dominant for spatial processing, encoding both left and right visual hemifields, whereas the left hemisphere encodes mainly the right half of space (Degutis et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Sheremata et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Thiebaut de Schotten et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). PD is usually asymmetrical in its onset, and the brain pathology on the side contralateral to the initial motor symptoms remains more severe throughout the illness (Kempster, Gibb, Stern, \u0026amp; Lees, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). Hence, in LPD, the primary processor of left-space has been compromised, with attendant perceptual consequences including, perhaps, hemispatial neglect.\u003c/p\u003e\u003cp\u003eFor LPD, the following results are suggestive of neglect: (1) On line bisection tasks, this PD subgroup estimates the center of a horizontal line to be to the right of the actual center (Albert et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Davidsdottir et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, for men; Lee, Harris, Atkinson, \u0026amp; Fowler, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) or more rightward than RPD and NC at certain field positions (Laudate et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013\u003c/span\u003e); (2) The initial direction of eye scanning when exploring a novel stimulus is biased toward the right (Ebersbach et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1996\u003c/span\u003e); (3) Objects presented on the left appear smaller than those presented on the right (Harris, Atkinson, Lee, Nithi, \u0026amp; Fowler, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). There is also evidence that this syndrome may affect daily living in these individuals, since LPD report bumping into doorways more often on the left than right side (Davidsdottir et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), which could contribute to falls or driving difficulties (Laudate et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Young et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSeveral studies found no evidence of an LPD-specific effect in terms of neglect, however. Laudate et al 2010 showed no difference between controls and LPD on line bisection midpoints in eight out of nine conditions (which differed only in their position on the screen). In addition, two studies by Norton and colleagues attempting to \u0026ldquo;explain\u0026rdquo; hemineglect in LPD at a functional level found no evidence of a left-hemifield deficit. The first measured whether space is perceptually compressed in the left hemifield in LPD (Norton et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) by having subjects compare the perceived thickness of bands on vertically oriented Gabor patches in the left and right hemifield, as well as measuring the perceived contrast of Gabor patches in the left and right hemifield. In both, performance of LPD did not differ from that of control participants. A second study measured sustained attention using a multiple object tracking task where the targets were systematically restricted to the left or right hemifield (Norton et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Again, LPD did not perform differently than the control group. Finally, the largest sample of PD individuals on a line bisection task to our knowledge (Salazar et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) showed no evidence of a rightward bias for LPD (both LPD and RPD actually showed a \u003cem\u003erightward\u003c/em\u003e bias, but only when beginning their adjustment of a movable center hatch on the left of the line). In sum, these results raise significant doubts about whether a true hemineglect effect exists on line bisection tasks in LPD.\u003c/p\u003e\u003cp\u003eTo address this question, we used a novel psychophysical paradigm to assess line bisection. This paradigm offered two advantages over previous studies. First, it used a two alternative forced choice procedure that assessed the perception of line length apart from possible noise generated by biases and motor behaviors necessary to move a hatch-mark manually on a computer screen or draw a line on a piece of paper. Second, a very brief stimulus presentation allowed us to remove the role that biased visual exploration of the stimulus might play. In addition, we tested a subset of participants using a modified version of this paradigm that contained a fixation cross and employed eye tracking, to investigate whether any neglect effects we might find would persist when gaze was assuredly matched between groups.\u003c/p\u003e"},{"header":"Experiment 1","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\u003ch2\u003eParticipants\u003c/h2\u003e\u003cp\u003eParticipants were 49 non-demented PwPD, including 21 LPD (12 men, 9 women, mean age\u0026thinsp;=\u0026thinsp;64.2, standard deviation [SD]\u0026thinsp;=\u0026thinsp;7.3), 28 RPD (15 men, 13 women, mean age\u0026thinsp;=\u0026thinsp;65.5SD\u0026thinsp;=\u0026thinsp;6.8) and 29 healthy control adults (HC, 11 men, 18 women, mean age\u0026thinsp;=\u0026thinsp;67.6, SD\u0026thinsp;=\u0026thinsp;8.7). The groups did not differ significantly in terms of age, F(2,75)\u0026thinsp;=\u0026thinsp;1.27, p\u0026thinsp;=\u0026thinsp;.29, or years of education, F(2,75)\u0026thinsp;=\u0026thinsp;.93, p\u0026thinsp;=\u0026thinsp;.40. Participants were excluded from the study on the basis of having neurological conditions (other than PD, for the PD group), coexisting serious chronic medical illnesses (including psychiatric illness), use of psychoactive medication besides antidepressants and anxiolytics in the PD group, history of intracranial surgery (e.g., deep brain stimulation or other invasive PD treatments), traumatic brain injury, alcohol dependence or recent substance abuse. All participants received a detailed neuro-ophthalmological examination to rule out visual disorders including significant glaucoma, cataracts, or macular degeneration. The minimum Mini-Mental State Examination (MMSE; Folstein, Folstein, \u0026amp; McHugh, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1975\u003c/span\u003e) score for inclusion in the study was a 26 out of a possible total score of 30. The minimum score for the sample included in this study was 27. Both PD groups had a median Hoehn and Yahr (H\u0026amp;Y) score of 2 out of a total possible score of 5, indicating mild bilateral motor stage of disease. The range of scores for LPDs was between 1 and 4. There was only one LPD participant with a H\u0026amp;Y score of 4, and none with 3 or 3.5. The range of scores for RPDs was between 1 and 3. LPD and RPD did not differ significantly on their H\u0026amp;Y scores (Kolmogorov-Smirnov, p\u0026thinsp;=\u0026thinsp;.49). Scores were unavailable for 2 LPD and 3 RPD. Unified Parkinson Disease Rating Scale (UPDRS) scores were available for 17 LPD, who had a mean motor score of 19.4 (SD 8.1), and for 19 RPD, who had a mean motor score of 19.1 (SD 9.0). Because we had scores for more participants on H\u0026amp;Y than on UPDRS, we report H\u0026amp;Y scores for the participants in each experiment.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\n\u003ch3\u003eStimulus\u003c/h3\u003e\n\u003cp\u003eThe stimulus was a long horizontal line (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), 15.9 degrees in length and 7 arc min in width, with a perpendicular hatchmark 4.0 degrees in length and 7 arc min in width. The bisected line and bisecting hatchmark were both black and presented on a white background. The hatchmark was positioned along the horizontal line in one of 12 locations\u0026mdash;1, 2.1, 4.2, 8.3 or 16.7% offset to the left or right of center. For example, a 100% offset would indicate the hatchmark being positioned on the endpoint of the line. In order to preclude saccades during the stimulus presentation, the presentation time was set at a brief duration, 83.3 msec, whereas stimulus-driven saccades are on the order of 200 msec (McPeek, Skavenski, \u0026amp; Nakayama, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). For the vertical condition, the stimulus was identical, except rotated 90 degrees.\u003c/p\u003e\n\u003ch3\u003eProcedures\u003c/h3\u003e\n\u003cp\u003eThe task was to indicate orally whether the hatchmark was to the left or the right of the longer line's midpoint (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The hypothesis was that abnormal saccades drive neglect in LPD, and therefore eliminating saccades would also eliminate neglect. That is, LPD would not show a neglect-like pattern on this task and would perform similarly to RPD and HC.\u003c/p\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003eData analysis\u003c/h2\u003e\n \u003cp\u003eThe main dependent variable was the proportion of trials in which participants reported that the hatchmark was to the right of center for each hatch-position (8 trials each condition). From the proportion right data, a perceived midpoint (PM) was extracted using a Weibull function of the form\u003cimg src=\"data:image/png;base64,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\" style=\"text-align: start; color: rgb(0, 0, 0); background-color: rgb(255, 255, 255); font-size: medium; font-family: \u0026quot;\u0026quot;;\"\u003ewhere \u003cem\u003ey\u003c/em\u003e is the proportion of trials for which the observer reports that the hatchmark is to right of center, \u003cem\u003ex\u003c/em\u003e is the hatch position, and \u003cem\u003ea\u003c/em\u003e and \u003cem\u003eb\u003c/em\u003e are adjustable curve-fitting parameters. Neglect would manifest itself as a rightward shift in PM, and a decrease in the proportion of trials viewed as right of the midpoint, since neglect tends to shift the PM of the line to the right. Outliers were identified as those who showed a PM more than two standard deviations from the mean in their subgroup (e.g., LPD).\u003c/p\u003e\n \n\u003c/div\u003e\n\u003ch3\u003eResults \u003c/h3\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003eHorizontal Condition\u003c/h2\u003e\u003cp\u003eResults for proportion-right data are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. LPD reported “right [of midpoint]” less frequently than did RPD and HC, especially at the hatch positions near the center of the line (i.e., where the stimulus was difficult to judge and uncertainty was high). However, statistical analyses did not support this being a reliable difference. When only examining right-handers, a mixed model ANOVA with group (LPD, RPD, HC) and hatch position (12 levels) showed no significant main effect for group, F(2, 65) = 1.37, p = .26, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .04. There was a main effect for hatch position, F(4.1, 266.1) = 327.8, p \u0026lt; .001, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .83. The interaction between group and hatch position was not significant F(8.2, 266.1) = 0.70, p = .69, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .02. Including left-handers and ambidextrous participants, the analysis remained largely unchanged. The main effect for group was not significant, F(2,74 ) = 1.52, p = .23, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .04 There was a main effect for hatch position, F(4.3,317.9) = 353.3, p \u0026lt; .001, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .83. The interaction between group and hatch position was not significant F(8.6,317.9) = .67, p = .73, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .02.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eVertical Condition\u003c/h3\u003e\n\u003cp\u003eResults for vertical line bisection are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. Including all subjects, the main effect for group was not significant, F(2,74) = 2.38, p = .10, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .06. There was a main effect for hatch position, F(4.2, 313.6) = 514.7, p \u0026lt; .001, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .87. The interaction between group and hatch position was significant F(8.5,313.7) = 2.21, p = .024, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .06. When including only right-handers, the main effect for group was not significant, F(2,65) = .68, p = .51, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .02, though the main effect for hatch position remained so, F(4.1,267.7) = 470.4, p \u0026lt; .001, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .88, and the interaction between group and hatch position was not significant F(8.2,267.7) = 1.60, p = .12, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .05.\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003cdiv id=\"Sec13\" class=\"Section4\"\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e\n\n"},{"header":"Experiment 2","content":"\u003ch2\u003eMethods\u003c/h2\u003e\u003ch2\u003eParticipants\u003c/h2\u003e\u003cp\u003eParticipants were a subgroup of those from Experiment 1. They were 25 non-demented PwPD, including 17 LPD (10 men, 7 women, mean age = 64.2, SD = 6.9), 19 RPD (11 men, 8 women, mean age = 64.6, SD = 6.4) and 17 HC (6 men, 11 women, mean age = 64.6, SD = 9.1). The groups did not differ significantly in terms of age, F(2) = .03, p = .97 or male:female ratio χ\u003csup\u003e2\u003c/sup\u003e (2) = 2.5, p = .29. LPD had a Hoehn and Yahr range of 1–4, with a median of 2, and RPD had a range of 1–3 and a median of 2; the two groups did not differ significantly, Kolmogorov-Smirnov, p = .21.\u003c/p\u003e\u003ch2\u003eStimulus and procedure\u003c/h2\u003e\u003cp\u003eThe stimuli and procedures, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e, were generally similar to those described for Experiment 1. A fixation cross was used to ensure that the stimulus was processed by the same part of the retina for all participants. The stimulus was a black horizontal line 15.9 degrees in length and 7 arc min in width, with a perpendicular hatchmark 3.0 degrees in length and 7 arc min in width, both presented on a gray background. The fixation cross was positioned in the center of the screen, and participants were instructed to look directly at it each time it appeared. After 1000 msec, the horizontal section of the hatchmark was overlaid with a long horizontal line offset from center, such that the left or the right half of the line was longer than its opposite half. There were 8 different left-lengths (with right length being held constant; four decrements [-54 pixels, -27, -13, -6] and four increments [54, 27, 13 and 6] for the left half of the line), and 8 different right-lengths used. The task was to determine whether the fixated hatchmark was to the left or the right of the long horizontal line’s center. All conditions were presented in a random order, and each was repeated 8 times. Data were averaged across sides such that, for example, a decrement of 54 pixels on the left side of the line was pooled with an increment of 54 pixels on the right side of the line.\u003c/p\u003e\u003ch2\u003eEye-tracking apparatus:\u003c/h2\u003e\u003cp\u003eEye-tracking and recording were performed using an Applied Science Laboratories (ASL) eye-tracking system. A model D6 camera array was located underneath the stimulus monitor, and used infrared light to discern the participant’s pupil position and corneal reflection. These reflection points were monitored with Eye-Trac software to locate the position of the participant’s eye, and sampled at a rate of 120 Hz, with a maximum accuracy of 0.5 degrees of visual angle. Although the participant used binocular vision for the experiment, only their left eye was tracked. A 9-point calibration sequence was administered at the beginning of the session and as needed during testing to ensure that the equipment was accurately calculating the participant’s gaze relative to the display for the duration of the experiment. Eye-tracking data were processed with MATLAB. Trials where the participant’s gaze was more than 3 degrees of visual angle leftward or rightward of the fixation cross, or where a saccade was made during the stimulus presentation, were eliminated.\u003c/p\u003e\u003ch3\u003eResults\u003c/h3\u003e\u003ch2\u003eResponse as a function of line-end position\u003c/h2\u003e\u003cp\u003eThe results for Experiment 2 are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and were analyzed using only the trials on which participants maintained fixation within 2 degrees vertically and horizontally of the fixation cross’s middle. A mixed model ANOVA, with group (RPD, HC or LPD) and line-end-position (8 levels) showed a significant effect for line-end position, F(3.7, 157.2) = 322.9, p \u0026lt; .001, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .80, but not for group, F(2, 42) = 1.13, p = 0.33, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = 05. The interaction between group and hatch position was not significant, F(7.5, 157.2) = .26, p = .77. Using the PM data, there were no group differences, F(2, 49), = .38, p = .69,, \u003cb\u003eη\u003c/b\u003e\u003csup\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sup\u003e = .02. A t-test comparing LPD and HC showed no difference, t(30) = .58, p = .56, Cohen’s d = .21.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe present study used a novel psychophysical paradigm to examine line bisection performance in LPD. Across two experiments, both of which likely precluded any strategic eye movements to scan the target, we found no evidence of hemineglect as assessed by horizontal line bisection. Instead, we found that perception of line length was biased in LPD for vertical lines, where a significant interaction between hatch position and group was driven by LPD participants\u0026rsquo; reduced proportions of trials judged as having the hatch mark above the midpoint, relative to the other groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003eHemineglect in LPD:\u003c/h2\u003e\u003cp\u003eNo direct evidence was found for hemineglect in LPD in the present study. A horizontal bias was present in LPD in the same direction as that seen in hemineglect patients with right parietal damage, but this bias was not statistically distinct from that seen in healthy control or RPD participants. The present result was obtained using a psychophysical paradigm that eliminated non-perceptual bias with its use of a two alternative forced choice procedure. Experiment one also prevented strategic eye movements to scan the target by using a brief presentation time. In Experiment two, when gaze was also controlled using a fixation cross and eye tracking, the difference in line bisection performance between HC and LPD diminished further and was still non-significant.\u003c/p\u003e\u003cp\u003eThis study was also the first to remove exploratory eye movements as a possible factor in the results of a line bisection task in PD. Since stimulus presentation was limited to 83 msec, it is unlikely that observers were able to generate any exploratory saccades during stimulus presentation. This removes reduced saccadic amplitude as a factor in the results, which is important since reduced saccadic amplitude has been demonstrated in PD (Matsumoto et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The results of Experiment 1 indicate instead that a perceptual bias in vertical but not horizontal line bisection exists in LPD, but it is independent of abnormalities in saccadic functioning.\u003c/p\u003e\u003cp\u003eIn Experiment 2 an additional aspect of oculomotor functioning, gaze bias, was also precluded as a possible factor in the results. Although it is unlikely that participants were able to explore the stimulus with their eyes for more than one fixation, it is possible that PD participants had more variability or a directional bias in their gaze position when the stimulus appeared on the screen, and this could have affected their perception. Gaze bias was controlled using a fixation cross and eye tracking; trials on which fixation was not maintained were removed. Here, there was no difference between LPD and HC, and the non-significant difference was smaller than that seen for Experiment 1. The results of Experiment 2 are similar to other studies on PD that utilized psychophysical paradigms with eye tracking and a fixation cross, finding little to no group differences between LPD, RPD, and HC. When examining LPD participants\u0026rsquo; perceptions of spatial compression, no biases were found in the data that would support the hypothesis of left hemifield neglect (Norton et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Additionally, no evidence was found to support a left-hemifield deficit in sustained attention to moving objects (Norton et al \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This pattern of results across experiments suggests that when the possibility of gaze bias and biased exploratory eye movements are eliminated using a fixation cross and eye tracking, no LPD differences in the processing of left-space emerge relative to processing by RPD or control participants.\u003c/p\u003e\u003cp\u003e A point of potential importance for future research is that we focused on LPD and found that some of these participants showed notable neglect-like biases, but so did some with RPD, as well as some HC (see PM scatter plots, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Side of motor symptom onset is likely only one factor accounting for the variance in line bisection bias in PD. Individual differences in spatial biases that occur in healthy individuals and those with PD are a potentially informative topic that to date has received little attention.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003eVertical line bisection\u003c/h2\u003e\u003cp\u003eA bias in LPD performance occurred with respect to the upper and lower visual fields: LPD viewed the hatchmark as above the center less frequently than RPD and HC. Vertical-axis bias has been shown previously in LPD, with conflicting results (Laudate et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lee et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Lee and colleagues found that LPD estimated a vertical line\u0026rsquo;s midpoint to be below its physical center, whereas control adults and RPD did the opposite, but Laudate and colleagues (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) did not find a difference between LPD and a healthy control group on vertical line bisection. It is unclear why the two previous studies found conflicting results, though both were based on small samples. The present data are consistent with those of Laudate and colleagues, and in the opposite direction of Lee and colleagues. Of note, the significant result did not remain when the sample was limited to right-handed individuals. Of relevance to the present discussion, in both previous studies eye movements were free to be made during an unlimited stimulus presentation time. The results of these studies are therefore ambiguous as to why LPD performed as they did (bisecting lines below or above their true vertical center), since they could be explained by eye movement differences, or by altered perception of the upper visual field of space. In the present study, LPD perceived the vertical line\u0026rsquo;s midpoint as being below its physical position, and since scanning of the target was not possible, an abnormality in processing of up- or down-space appears to exist in LPD independently of saccadic abnormalities.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003ePossible Neural Correlates of Bias in Length Perception in PD\u003c/h2\u003e\u003cp\u003eThe present results may be related to multiple forms of neuropathology that have been demonstrated in PD. Parietal neuropathology in PD has been shown in the form of decreased gray matter density (Pereira et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, Tinaz et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) and cortical thinning (Filippi et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Wilson et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In addition, some individuals with PD, namely those with mild cognitive impairment, show hypometabolism in parietal cortex (Garcia-Garcia et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Pappata et al., 2011). Because the parietal lobe is important for attention and representation of visual space (Beffara et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sheremata et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Somers \u0026amp; Sheremata, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Thiebaut de Schotten et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Wutz, Zazio \u0026amp; Weisz, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), this parietal dysfunction could affect performance in a way that biases those with LPD process space and length differently.\u003c/p\u003e\u003cp\u003eIn addition, subcortical brain pathology may play a role in altered perception in PD. Where humans perceive themselves to be looking is determined by a combination of factors, including a series of presumed corollary discharges offered by the brain during each eye movement. These discharges have been proposed to be altered in PD (Diederich, Stebbins, Schiltz, \u0026amp; Goetz, 2014). If the movements that are executed do not match the ones that are shared in a corollary discharge (due to motor pathology in PD), it could alter individuals\u0026rsquo; perceptual map of the world dramatically. Two brain systems that may be involved with the corollary discharge, the superior colliculus and thalamus, are affected in PD. The superior colliculus is crucial for determining location of eye movement and maintaining a map of salient locations in space (Hafed \u0026amp; Krauzlis, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). This area is directly connected to substantia nigra (Hikosaka et al., 1983), and has been proposed to be dysfunctional in PD (Terao et al., 2011). In addition, PD is associated with alterations in the functional connectivity and morphology of the thalamus (Chen, Y et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Moustafa, McMullan, Rostron, Hewedi \u0026amp; Haladjian, 2017; Owens-Walton, Jakabek, Power, Walterfang, Velakoulis, van Westen et al., 2019), affecting basal ganglia-thalamocortical circuits. It is possible that alterations in thalamic connectivity and function may contribute to the manifestation of perceptual bias in PD patients, a point that necessitates further research following the findings identified here.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003eLimitations\u003c/h2\u003e\u003cp\u003eThe present work was subject to some limitations. First, it would have been useful to see if gaze bias in Experiment 1 (using eye tracking but not requiring central fixation) predicted line bisection performance on the task, but eye tracking was not used in that experiment. A second limitation relates to the line lengths used in our stimuli. Neglect in LPD previously had been shown primarily using longer line lengths than used in the present study. We used lines of approximately 16 degrees of visual angle due to constraints of a standard computer monitor. Future studies could attempt replication of the present study by using a projector or other methods to increase the visual angle of lines. Finally, replicating the present results in comparison with a task that allowed or required exploratory eye movements might reveal interesting truths about the role that eye movements play in hemineglect-like performance in LPD.\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe results of the present study indicate that the perception of length is altered in LPD, particularly for vertical lines, even for briefly presented stimuli that preclude examination with exploratory eye movements. No significant hemineglect effect for LPD was found in a horizontal version of the task, despite a trend in this direction. Understanding the mechanisms underlying spatial biases in PD, as well as their impact on daily life, may offer insight into possible routes toward remediation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eFunding: This research was funded by the National Institute of Neurological Disorders and Stroke (F31 NS07682 to DJN; RO1NS067128 to ACG),\u0026nbsp;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.\u003c/p\u003e\n\u003cp\u003eConflicts of interest: All authors report none.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eEthics approval: 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).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eConsent to participate: All participants in this study provided written informed consent.\u003c/p\u003e\n\u003cp\u003eConsent for publication: not applicable\u003c/p\u003e\n\u003cp\u003eCode availability: Stimulus code available upon request from corresponding author (DJN).\u003c/p\u003e\n\u003cp\u003eData availability: Data are available upon request from the corresponding author (DJN)\u003c/p\u003e\n\u003cp\u003eAcknowledgments: \u0026nbsp;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\u0026rsquo;s disease support groups, as well as the Michael J. 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(2001) Surround modulation of perceived contrast and the role of brightness induction. \u003cem\u003eJ Vis\u003c/em\u003e 1, 18-31.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6874238/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6874238/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eObjective\u003c/h2\u003e\u003cp\u003eSome studies have shown that individuals with Parkinson\u0026rsquo;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 to isolate perceptual biases.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eExperiment 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.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eIn both experiments, LPD did not demonstrate performance bias that was consistent with hemineglect. In Experiment one, a bias specific to LPD was demonstrated in the vertical condition.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e\u003cp\u003eThe 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.\u003c/p\u003e","manuscriptTitle":"Novel psychophysical line bisection task using brief stimulus presentation reveals no horizontal bias in left-onset Parkinson’s Disease","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-23 06:22:58","doi":"10.21203/rs.3.rs-6874238/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-11T19:16:12+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-04T14:54:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"235459141047214488217560941122844610039","date":"2025-08-04T11:48:02+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-20T21:35:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"61700986882086326183444542110862192783","date":"2025-07-20T09:02:44+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-16T14:00:18+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-16T13:31:52+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-07-03T19:49:49+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-18T20:51:34+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-06-18T20:48:30+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7c202d23-2483-4f62-8901-93258208aa45","owner":[],"postedDate":"July 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":51637827,"name":"Biological sciences/Psychology/Human behaviour"},{"id":51637828,"name":"Biological sciences/Neuroscience/Visual system"},{"id":51637829,"name":"Health sciences/Neurology/Neurological disorders/Parkinsons disease"}],"tags":[],"updatedAt":"2026-01-12T16:04:42+00:00","versionOfRecord":{"articleIdentity":"rs-6874238","link":"https://doi.org/10.1038/s41598-025-34798-3","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-01-09 15:58:38","publishedOnDateReadable":"January 9th, 2026"},"versionCreatedAt":"2025-07-23 06:22:58","video":"","vorDoi":"10.1038/s41598-025-34798-3","vorDoiUrl":"https://doi.org/10.1038/s41598-025-34798-3","workflowStages":[]},"version":"v1","identity":"rs-6874238","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6874238","identity":"rs-6874238","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}