title: Horizontal fixation disparity measures with nonius lines.
abstract: OBJECTIVE
  It has been reported that changes in visual direction and retinal correspondence may limit the validity of nonius lines as a subjective measure of vergence, at least in particular viewing conditions, e.g., dynamic or forced vergence. Nonius lines may be valid at larger spatial separation between fusion contour and nonius lines. Therefore, we measured fixation disparity varying the amount of a vertical gap between nonius lines.
  
  
  METHODS
  A static central fusion stimulus was given at a 100-cm viewing distance. The nonius lines were presented with various vertical gaps either continuously (and adjusted to subjective alignment) or flashed in a series of trials (in adaptive testing). Twelve subjects with normal binocular vision were tested repeatedly to identify individual vergence characteristics.
  
  
  RESULTS
  When the vertical gap between nonius lines was increased up to 7.9 deg, the amount of subjective fixation disparity tended to increase in part of the subjects, predominantly in those with an exo fixation disparity. Most subjective fixation disparity measures were correlated with each other and with tonic vergence (i.e., dark vergence tested subjectively without fusion stimulus); however, flashed nonius lines at larger nonius gaps were an exception.
  
  
  CONCLUSIONS
  We found physiologically plausible measures of idiosyncratic fixation disparity with continuous nonius lines at any amount of nonius gap or with flashed nonius lines at small gaps. In these conditions, the intersubject variability of fixation disparity was much larger than effects of the spatial separation between fusion stimulus and nonius lines.
authors.affiliation: Institut für Arbeitsphysiologie, Dortmund, Germany. Jaschinski@ifado.de
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Jaschinski
authors.forename: Wilhelm-Bernhard
authors.initials: WB
authors.lastname: Kloke
authors.forename: Stephanie
authors.initials: S
authors.lastname: Jainta
authors.forename: Julia
authors.initials: J
authors.lastname: Buchholz
mesh_terms: D000328:Adult; D003290:Convergence, Ocular / Q000502:physiology; D005133:Eye Movements / Q000502:physiology; D005403:Fixation, Ocular / Q000502:physiology*; D006801:Humans; D009801:Oculomotor Muscles / Q000502:physiology*; D010775:Photic Stimulation; D012016:Reference Values; D015357:Vision Disparity / Q000502:physiology*; D015348:Vision, Binocular / Q000502:physiology*