title: Visual fatigue effects on vergence dynamics in asymptomatic individuals.
abstract: OBJECTIVE
  To evaluate objectively changes in vergence dynamics following intentional visual fatiguing tasks using congruent (C) and non-congruent (NC) vergence stimulus demands.
  
  
  METHODS
  Pre- and post-task vergence dynamics to a 10.3° disparity stimulus were recorded objectively in 12 adult, asymptomatic individuals for both convergence and divergence. There were two fatigue-inducing tasks: Congruent or C-task: Subjects altered bifixation (50 cycles) between reduced Snellen charts at 2 m and at 20 cm every 3 s. Non-congruent or NC task: Subjects performed 50 cycles of vergence flipper (7Δ BO/BI), while bifixating a reduced Snellen chart at 40 cm. Pre- and post-task mean amplitude, time constant, peak velocity, and steady-state response variability were compared.
  
  
  RESULTS
  There was a significant increase in mean post-task, steady-state response variability for both the C and NC tasks. However, there were no significant group differences for either convergence or divergence between the pre- and post-mean amplitude, time constant, and peak velocity parameters for both the C and NC tasks, although post-task peak velocity was consistently reduced.
  
  
  CONCLUSIONS
  Steady-state vergence response variability was found to be the critical objective parameter to demonstrate significant fatigue effects in the group. Peak velocity was also consistently reduced to a small extent in the NC task. We speculate that these fatigue effects are of a central, and not of peripheral, origin.
authors.affiliation: Department of Biological and Vision Sciences, State College of Optometry, State University of New York (SUNY), New York, USA.
authors.forename: Preethi
authors.initials: P
authors.lastname: Thiagarajan
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Ciuffreda
mesh_terms: D000328:Adult; D000368:Aged; D001248:Asthenopia / Q000503:physiopathology*; D058070:Asymptomatic Diseases; D003290:Convergence, Ocular / Q000502:physiology; D005133:Eye Movements / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation / Q000379:methods; D013647:Task Performance and Analysis; D055815:Young Adult
keywords: asthenopia; eye movements; oculomotor control; vergence; vergence dynamics; visual fatigue