Background: This study standardized the parameters of a novel visual midline gauge and presents normative data, which can be used by optometrists in the assessment of visual midline.<break><break>Methods: Ninety-three participants from three age groups (18 to 44, 45 to 64, and > 65 years) without history of significant neurological or ocular problems were recruited in Waterloo, Canada and Hong Kong. In experiment 1, the perceived horizontal and vertical visual midline was measured using the gauge for 2 speeds and 2 repositioning methods. In experiment 2, the perceived midline was measured for three different distances (25, 50 and 100 cm) using a target speed and repositioning method chosen from the first experiment results. Since there was no significant difference between the two sites in any of the measures, data were combined for analysis.<break><break>Results: For experiment 1, linear mixed models showed no effect of age, speed or repositioning method and no interaction (p>0.05) for the perceived midline position (p>0.05). In experiment 2, there was no significant effect of age or distance and no interaction effects on the perceived horizontal and vertical visual midline position using the chosen speed and method (2.3 degrees per second, adjustment method). Normative data (mean and 95% ranges of the perceived visual midline for control participants) was tabulated. The measurements were found to be repeatable. A few participants were found to have significant midline shifts. <break><break>Conclusion: This study shows that the measurement of midline is tolerant of differences of target speed, testing method, test distance, and age group of the participants and that the measurements using the visual midline gauge are repeatable. It is possible for even individuals without a history of neurological or ocular disorders to have significant shifts in their perceived visual midline.

The authors have declared no competing interest.

This is not a clinical trial

This work was supported by the InnoHK initiative and the Hong Kong Special Administrative Region Government; Network for Aging Research at the University of Waterloo Emerging Scholar Mentorship Grant; and the Canadian Optometric Education Trust Fund.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Not Applicable

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This study has been reviewed and received ethics clearance through a University of Waterloo Research Ethics Board (REB 43834).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Not Applicable

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Not Applicable

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Not Applicable

Supporting data is not available in a repository as participants of this study did not give written consent for their data to be shared publicly.