The effect of monocularly and binocularly induced astigmatic blur on depth discrimination is orientation dependent.
OBJECTIVE
Naturally occurring astigmatism varies according to the age of the person. Although uncorrected astigmatism may be associated with meridional amblyopia, there is little information of its effect on stereopsis. The purpose of this study was to determine the effect of astigmatism on depth discrimination and whether this was dependent on the axis of the astigmatism.


METHODS
Astigmatic blur was induced in four healthy subjects (mean age, 31.5 years; range, 22 to 42 years) using plain cylinders (-8.75 D to +11.5 D) for orientation control and Jackson cross-cylinders (0 to 12 D) for spherical neutrality. Horizontal, vertical, and oblique astigmatism was induced with five monocular and three binocular axis steps. Depth discrimination was recorded at near using Frisby, TNO, and Titmus stereoacuity tests and at distance (4 m) using the variable distance stereoacuity test. Visual acuity was recorded at 0.4 m and 4 m.


RESULTS
Visual acuity and depth discrimination degraded with increasing astigmatic blur. The effect of monocular astigmatic blur on depth discrimination and visual acuity was not dependent on the axis of orientation. For binocular astigmatic blur, the reduction in depth discrimination was dependent on the axis of the induced astigmatism (p < 0.01). The maximum effect occurred with orthogonal-oblique orientations (x45 left; x135 right), followed by against-the-rule (ATR) astigmatism; with-the-rule (WTR) astigmatism had the least effect (p < 0.001).


CONCLUSIONS
The lesser effect of WTR compared with ATR astigmatic blur on depth discrimination may reflect the contribution of horizontal compared with nonhorizontal disparity processing in stereopsis. The pronounced effect of oblique astigmatic blur may be because of the effects on horizontal and nonhorizontal disparity and interocular differential image blur.
Chen
Sean I
SI
St. Paul's Eye Unit, Royal Liverpool Broadgreen University Hospital, Prescot Street, Liverpool L7 8XP, United Kingdom.
Hove
Mfazo
M
McCloskey
Carmel L
CL
Kaye
Stephen B
SB
D000328:Adult; D001251:Astigmatism / Q000503:physiopathology*; D003867:Depth Perception; D006801:Humans; D009949:Orientation; D015348:Vision, Binocular; D015349:Vision, Monocular; D014792:Visual Acuity