title: Objective vs subjective measures of fixation disparity for short and long fixation periods.
abstract: OBJECTIVE
  Fixation disparity, i.e. the vergence error for stationary fusion stimuli, can be measured objectively with eye trackers and subjectively with nonius lines. Between these two measures, previous studies found differences that tended to be proportional to the amount of forced vergence, i.e. the discrepancy between vergence and accommodative stimulus. We investigate whether objective and subjective fixation disparity might be similar without forced vergence.
  
  
  METHODS
  We simultaneously measured fixation disparity with the EyeLink II system and with flashed dichoptic nonius lines in 17 subjects who observed targets at a vergence stimulus of 6 deg in an haploscope with 60 cm viewing distance.
  
  
  RESULTS
  We found individual differences in objective fixation disparity in a range of about 20 (eso) to -60 (exo) min arc which was considerably larger than the range of subjective fixation disparity. Results were similar for long fixation periods (about 15 s) and short fixation periods (about 1.5 s) between 5 deg saccadic gaze shifts. Further, objective fixation disparity was correlated with objective heterophoria, i.e. the vergence state without a fusion stimulus (r = 0.8, p < 0.001).
  
  
  CONCLUSIONS
  Subjective fixation disparity explained only about 25% of the inter-individual variability in objective fixation disparity. The discrepancy between these two measures might be explained by sensory shifts in retinal correspondence, also in the present condition without forced vergence.
authors.affiliation: Leibniz Research Centre for Working Environment and Human Factors, IfADo, Ardeystr. 67, D 44139 Dortmund, Germany. jaschinski@ifado.de
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Jaschinski
authors.forename: Stephanie
authors.initials: S
authors.lastname: Jainta
authors.forename: Wilhelm Bernhard
authors.initials: WB
authors.lastname: Kloke
mesh_terms: D000328:Adult; D003290:Convergence, Ocular / Q000502:physiology*; D006801:Humans; D015357:Vision Disparity / Q000502:physiology*; D014787:Vision Tests; D015348:Vision, Binocular / Q000502:physiology