title: Subjective fixation disparity affected by dynamic asymmetry, resting vergence, and nonius bias.
abstract: OBJECTIVE
  This study was undertaken to investigate how subjectively measured fixation disparity can be explained by (1) the convergent-divergent asymmetry of vergence dynamics (called dynamic asymmetry) for a disparity vergence step stimulus of 1° (60 arc min), (2) the dark vergence, and (3) the nonius bias.
  
  
  METHODS
  Fixation disparity, dark vergence, and nonius bias were measured subjectively using nonius lines. Dynamic vergence step responses (both convergent and divergent) were measured objectively.
  
  
  RESULTS
  In 20 subjects (mean age, 24.5 ± 4.3 years, visual acuity, ≥1.0; all emmetropic except for one with myopia, wearing contact lenses), multiple regression analyses showed that 39% of the variance in subjective fixation disparity was due to the characteristic factors of physiological vergence: dynamic asymmetry (calculated from convergent and divergent velocities), and dark vergence. An additional 23% of variance was due to the subjective nonius bias (i.e., the physical nonius offset required for perceived alignment of binocularly [nondichoptically] presented nonius lines). Together, these factors explained 62% of the interindividual differences in subjectively measured fixation disparity, demonstrating the influence of oculomotor and perceptual factors.
  
  
  CONCLUSIONS
  Clinically relevant subjective fixation disparity originates from distinct physiological sources. Dynamic asymmetry in vergence dynamics, resting vergence, and nonius bias were found to affect fixation disparity directly, not only via changes in vergence dynamics.
authors.affiliation: Department of Optometry and Vision Science, University of Latvia, Riga, Latvia. aiga.svede@lu.lv
authors.forename: Aiga
authors.initials: A
authors.lastname: Svede
authors.forename: Jörg
authors.initials: J
authors.lastname: Hoormann
authors.forename: Stephanie
authors.initials: S
authors.lastname: Jainta
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Jaschinski
mesh_terms: D000293:Adolescent; D000328:Adult; D005133:Eye Movements / Q000502:physiology*; D005246:Feedback; D005260:Female; D006801:Humans; D008297:Male; D012146:Rest; D015357:Vision Disparity; D014787:Vision Tests; D015348:Vision, Binocular / Q000502:physiology*; D014792:Visual Acuity; D055815:Young Adult