title: Astigmatism impact on visual performance: meridional and adaptational effects.
abstract: OBJECTIVE
  Astigmatic subjects are adapted to their astigmatism and perceptually recalibrate upon its correction. However, the extent to which prior adaptation to astigmatism affects visual performance, whether this effect is axis dependent, and the time scale of potential changes in visual performance after astigmatism correction are not known. Moreover, the effect of possible positive interactions of aberrations (astigmatism and coma) might be altered after recalibration to correction of astigmatism.
  
  
  METHODS
  Visual acuity (VA) was measured in 25 subjects (astigmats and non-astigmats, corrected and uncorrected) under induction of astigmatism and combinations of astigmatism and coma while controlling subject aberrations. Astigmatism (1.00 diopter) was induced at three different orientations, the natural axis, the perpendicular orientation, and 45 degrees for astigmats and at 0, 90, and 45 degrees for non-astigmats. Experiments were also performed, adding coma (0.41 μm at a relative angle of 45 degrees) to the same mentioned astigmatism. Fourteen different conditions were measured using an 8-Alternative Forced Choice procedure with Tumbling E letters and a QUEST algorithm. Longitudinal measurements were performed up to 6 months. Uncorrected astigmats were provided with proper astigmatic correction after the first session.
  
  
  RESULTS
  In non-astigmats, inducing astigmatism at 90 degrees, produced a statistically lower reduction in VA than at 0 or 45 degrees, whereas in astigmats, the lower decrease in VA occurred for astigmatism induced at the natural axis. Six months of astigmatic correction did not reduce the insensitivity to astigmatic induction along the natural axis. Differences after orientation of astigmatism were also found when adding coma to astigmatism.
  
  
  CONCLUSIONS
  The impact of astigmatism on VA is greatly dependent on the orientation of the induced astigmatism, even in non-astigmats. Previous experience to astigmatism plays a significant role on VA, with a strong bias toward the natural axis. In contrast to perceived isotropy, the correction of astigmatism does not shift the bias in VA from the natural axis of astigmatism.
authors.affiliation: *BOptom, MSc †BOptom, MSc, FAAO ‡PhD Instituto de Óptica, Consejo Superior de Investigaciones Científicas, Madrid, Spain (MV, PdG, CD, LS, SM); and Essilor International, Research and Development, Vision Science Department, Saint-Maur, France (GM, MH).
authors.forename: Maria
authors.initials: M
authors.lastname: Vinas
authors.forename: Pablo
authors.initials: P
authors.lastname: de Gracia
authors.forename: Carlos
authors.initials: C
authors.lastname: Dorronsoro
authors.forename: Lucie
authors.initials: L
authors.lastname: Sawides
authors.forename: Gildas
authors.initials: G
authors.lastname: Marin
authors.forename: Martha
authors.initials: M
authors.lastname: Hernández
authors.forename: Susana
authors.initials: S
authors.lastname: Marcos
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology*; D000328:Adult; D001251:Astigmatism / Q000503:physiopathology*; D003315:Cornea / Q000503:physiopathology; D057108:Corneal Wavefront Aberration / Q000503:physiopathology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009949:Orientation; D014792:Visual Acuity / Q000502:physiology*; D055815:Young Adult