Abstract
Purpose: :
To investigate whether dynamics in ocular accommodation and higher order aberrations contribute to the reported improvements in letter acuity and changes in contrast sensitivity following blur adaptation
Methods: :
We built a compact Shack-Hartmann wavefront sensor to measure wavefront aberrations before and immediately after subjects (n = 6) adapted to 30 minutes of +2 D blurred, calibrated natural scenes. During aberrometry, subjects fixated on a high-contrast letter E generated on a CRT monitor placed 5 m away with a +2 D defocus lens in situ. Eighty aberrometry readings (4 runs of 20 readings each, with 45 seconds break between runs) for the right eye were collected for pre- and post-adaptation condition. Wave aberrations were computed over a 4 mm pupil and Zernike coefficients up to the fifth order (21 terms) were studied using a custom written program in MATLAB®. Ocular accommodation was derived from the defocus and astigmatism terms of the second-order Zernike coefficients. Pupil size was not altered pharmacologically since we wanted to investigate the role of aberrations under natural viewing conditions.
Results: :
Ocular accommodation showed no significant differences (mean difference = 0.02 D; p = 0.46) between pre- and post-adaptation conditions, suggesting that a significant relaxation in accommodation or a change in defocus is not a contributory factor in the improved visual performance resulting from blur adaptation. Similarly, neither spherical aberration (RMS difference: 0.04 µm; p = 0.52), coma (RMS difference: 0.06 µm; p = 0.49), nor combined higher order aberrations (RMS difference: 0.03 µm; p = 0.84) showed any significant changes following blur adaptation. The combined higher order aberrations incorporated third through fifth order aberrations including that of coma and spherical aberration, while coma included both the horizontal and vertical components together. The results suggest no significant optical contributions to blur-adaption, even when analyzed at the individual subject level.
Conclusions: :
Ocular accommodation and the higher order aberrations play no significant contributory role during blur adaptation. These findings support the claim that improvements in visual performance following sustained defocus-induced blur are not due to any optical contributions, but are likely due to a neural mechanism.
Keywords: aberrations • adaptation: blur • accommodation