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Related Experiment Videos

Horizontal fixation disparity measures with nonius lines.

Wolfgang Jaschinski1, Wilhelm-Bernhard Kloke, Stephanie Jainta

  • 1Institut für Arbeitsphysiologie, Dortmund, Germany. Jaschinski@ifado.de

Optometry and Vision Science : Official Publication of the American Academy of Optometry
|December 1, 2005
PubMed
Summary

This study investigated fixation disparity using nonius lines with varying vertical gaps. Results show continuous nonius lines are reliable for measuring fixation disparity, with individual differences being more significant than gap size.

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Area of Science:

  • Ophthalmology
  • Vision Science
  • Neuroscience

Background:

  • Nonius lines are used to measure subjective vergence.
  • Changes in visual direction and retinal correspondence may affect nonius line validity, especially in dynamic conditions.
  • Larger spatial separation between fusion contours and nonius lines might improve validity.

Purpose of the Study:

  • To assess the validity of nonius lines for measuring fixation disparity.
  • To investigate the effect of varying vertical gaps between nonius lines on fixation disparity measurements.
  • To determine if nonius line validity is influenced by spatial separation in static viewing conditions.

Main Methods:

  • Static central fusion stimulus presented at a 100-cm viewing distance.
  • Nonius lines presented with varied vertical gaps, either continuously or flashed.

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  • Twelve subjects with normal binocular vision underwent repeated testing to characterize individual vergence.

    • Main Results:

    • Increasing the vertical gap (up to 7.9 degrees) tended to increase subjective fixation disparity in some subjects, particularly those with exo fixation disparity.
    • Most subjective fixation disparity measures correlated with each other and with tonic vergence (dark vergence).
    • Flashed nonius lines at larger gaps showed a notable exception to these correlations.

    Conclusions:

    • Physiologically plausible measures of idiosyncratic fixation disparity were found using continuous nonius lines at any gap or flashed nonius lines at small gaps.
    • In these reliable conditions, intersubject variability in fixation disparity was considerably larger than the impact of spatial separation.
    • The findings suggest that continuous nonius lines or small-gap flashed nonius lines offer valid subjective measures of vergence.