Abstract
Purpose: :
To study potential shifts of the perceptual judgments of blur orientation after correction of astigmatism in astigmatic subjects. Also to evaluate the time-scale of this adaptation aftereffects and the influence of the amount of natural astigmatism.
Methods: :
A 2 AFC procedure was performed to estimate the perceived isotropic point from a series of astigmatic blurred images, by responding if the image appeared oriented along/perpendicular to a certain axis (natural astigmatism axis in astigmats or horizontal/vertical axis in non-astigmats). Test images (random noise, subtending 1.98º) were generated by convolution with astigmatism and defocus by varying the magnitude of astigmatism and defocus to maintain constant blur (B=1.5D) while modifying the orientation of blur. Images were presented monocularly through an Adaptive Optics system, which compensated the subjects’ natural aberrations. The sample was selected a priori, and classified in 3 groups (n=7 per group): non-astigmats (G1), habitually-corrected astigmats (G2) and habitually-non-corrected astigmats (G3). Astigmatism ranged from -0.75 to -1.25D in G2 and G3. Subjects performed astigmatic blur judgments, before prescription of astigmatic correction (in G3) and after 2 hours, 1 week, 1 month and 6 months of astigmatic correction wearing. The experiments were performed under natural (gray field) adaptation. Results were analyzed in terms of shifts of the isotropic point (astigmatism level for which the image did not appear oriented to the subject).
Results: :
The average isotropic point in the first session was 0.13µm for G1, 0.32µm for G2 and 0.34µm for G3, on average. In G2 and G3 the perceived isotropic point was shifted towards their axis of natural astigmatism. G1 and G2 did not shift their perceived neutral point across sessions. However, G3 shifted significantly the perceived neutral point towards more isotropic images (0.14µm, Sig<0.01) shortly (2 hours) after astigmatic correction wear. The perceived isotropic point was further reduced (0.20µm) in this group and remained constant after 6 months. In G3, the effect is correlated significantly (Sig<0.01) with the amount of natural astigmatism.
Conclusions: :
Non-corrected astigmats are naturally adapted to their astigmatism and astigmatic correction significantly changes their perception of neutral point, even after a brief period of adaptation.
Keywords: adaptation: blur • astigmatism • aberrations