Brief Adaptation to Astigmatism Reduces Meridional Anisotropy in Contrast Sensitivity

Abstract

Purpose: To investigate the effect of visual adaptation to orientation-dependent optical blur on meridional contrast sensitivity function in artificially imposed astigmatism.

Methods: The study adopted a top-up adapt-test paradigm. During the blur adaptation process, the 18 non-astigmatic young adult participants were briefly presented with natural scene images (first trial, 10 minutes; subsequent trials, 6 seconds). Contrast sensitivities for horizontal and vertical gratings at spatial frequencies ranging from 1 to 8 cycles per degree (cpd) were measured immediately before and after adaptation to +3.00 diopters cylinder (DC) with-the-rule or against-the-rule astigmatism. Meridional anisotropy was measured to quantify the contrast sensitivity difference between the two grating orientations.

Results: Adapting to astigmatic blur enhanced contrast sensitivity at the blurred power meridian but reduced contrast sensitivity at the least affected axis meridian. In with-the-rule conditions, contrast sensitivity for horizontal gratings was significantly increased, whereas that for vertical gratings was significantly decreased. Similarly, in against-the-rule conditions, contrast sensitivity for vertical gratings was significantly increased, whereas that for horizontal gratings was significantly decreased. These two factors together resulted in a substantial systematic reduction, averaging 34%, in meridional anisotropy of contrast sensitivity across the spatial frequency spectrum.

Conclusions: Astigmatism adaptation occurs in natural scene viewing. Brief exposure to astigmatic blur altered contrast sensitivity in the opposite direction at the two principal meridians, indicating that the mature visual system possesses functional plasticity to recalibrate the response characteristics of orientationally tuned cortical filters and thus promote substantial reductions of meridional anisotropy in astigmatic vision, to some extent counterbalancing the elongated oval shape of astigmatic blur.

Conflict of interest statement

Disclosure: T.-W. Leung, None; R.W. Li, None; C.-S. Kee, None

Figures

Figure 1.

Top-up blur adaptation paradigm. With-the-rule (WTR) and against-the-rule (ATR) astigmatism was artificially induced by imposing a +3 D cylindrical spectacle lens with the axis of astigmatism placed at 180° or 90°, respectively. The adaptation protocol began with a 10-minute exposure to astigmatically blurred natural scene images, during which a set of 16 black-and-white photographs was randomly displayed every 500 ms. Immediately after the initial adaptation, a blank gray screen was briefly displayed for 300 ms, followed by a target sine-wave grating pattern for 500 ms. The observers’ visual task was to determine the grating orientation. Starting from the second trial, the presentation sequence returned to a shorter 6-second duration for top-up readaptation. Measurements were repeated until the contrast thresholds for all four spatial frequencies (1, 2, 4, and 8 cpd) and two grating orientations (horizontal and vertical) were obtained.

Figure 2.

Contrast sensitivity and meridional anisotropy functions with full optical correction. (A) Contrast sensitivity function with full optical correction. With no artificially imposed astigmatism, there were no significant differences in contrast sensitivity between horizontal and vertical gratings (red and blue symbols, respectively) across the spatial frequencies tested. (B) Meridional anisotropy function in contrast sensitivity with full optical correction. No statistically significant meridional anisotropy in contrast sensitivity for the two grating orientations was observed across spatial frequencies. Error bars: 1 SEM.

Figure 3.

Neural adaptation to directional optical blur in WTR and ATR astigmatism (upper and lower row, respectively). (A, D, left panels) Directional astigmatic defocus produced by a +3 DC cylindrical lens. Solid black lines indicate principal power meridians with +3 D refractive power. Dotted black lines indicate principal axis meridians with zero refractive power. WTR astigmatism smeared retinal images vertically, but ATR astigmatism smeared retinal images horizontally (blue and red lines, respectively). (B, E, middle panels) Log contrast sensitivity for horizontal and vertical gratings (red and blue symbols, respectively) as a function of spatial frequency before and after astigmatic adaptation (open symbols, pre-adaptation; closed symbols, post-adaptation). Note that the pre-adaptation contrast sensitivity function for horizontal gratings was substantially decreased by WTR astigmatism, especially at spatial frequencies of 2 to 8 cpd (top panel, red dashed line) and that, similarly, the pre-adaptation contrast sensitivity function for vertical gratings was substantially decreased by ATR astigmatism (bottom panel, blue dashed line). Insets: Simulated natural scene images illustrating directional blur produced by WTR and ATR astigmatism, blurred vertically and horizontally, respectively. Simple main effects analyses (post- vs. pre-): *P < 0.05, **P < 0.01, ***P < 0.001. (C, F, right panels) Magnitude of adaptation effect is the difference in log contrast sensitivity before and after astigmatic adaptation (logCSPost – logCSPre) as a function of spatial frequency. Dashed lines indicate the mean adaptation effect across spatial frequencies. Positive values represent enhancement in contrast sensitivity; negative values, reduction in contrast sensitivity. Error bars: 1 SEM.

Figure 4.

Adaptation-related reduction of meridional anisotropy in astigmatic vision. Meridional anisotropy in contrast sensitivity before and after adaptation to WTR (A) and ATR (B) astigmatism. Adaptation to astigmatism caused a systematic reduction of meridional anisotropy (i.e., toward the value of zero; CSH = CSV) across spatial frequencies. Meridional anisotropy was defined as the logarithmic difference in contrast sensitivity for horizontal and vertical gratings before (dashed lines, open symbols) and after (solid lines, closed symbols) adaptation. Positive meridional anisotropy indicates higher contrast sensitivity for horizontal gratings compared with vertical gratings; negative meridional anisotropy, lower contrast sensitivity for horizontal gratings compared with vertical gratings. Simple main effects analyses (post- vs. pre-): *P < 0.05, **P < 0.01, ***P < 0.001. Error bars: 1 SEM.