Does the Visual System Adapt to the Eye's Aberrations?

Abstract

Abstract: : Purpose: The normal visual system provides a clear image of the scene despite the fact that the eye's aberrations blur the retinal image. We used adaptive optics to determine whether the neural system sharpens the retinal image by compensating for the particular pattern of optical aberrations present in the observer's eye. Methods: If the brain adjusts for the specific aberrations of the eye, vision should be clearest when looking through the normal wave aberration rather than through an unfamiliar one. With adaptive optics, it is possible to correct and to replace the eye's wave aberration. We used the Rochester second-generation adaptive optics system to induce wave aberrations, and therefore point spread functions (PSFs), that were rotated versions of the eye's usual wave aberration by angles in 45 degrees intervals. Four normal subjects were asked to view a stimulus, containing sharp edges without any preferred orientation, through the adaptive optics system with their own aberrations or with a rotated version of their aberrations. The stimulus was seen alternatively for 500 msec with both the normal and the rotated PSF. The subject's task was to adjust the magnitude of the aberrations in the rotated case to match the subjective blur of the stimulus to that seen when the wave aberration was in the normal orientation. Results: In all the subjects tested, the RMS wavefront error of the rotated wave aberration required to match the blur with the normal wave aberration was 20 to 40% less, indicating that the subjective blur for the stimulus increased significantly when the PSF was rotated. This was true even for rotations of 180 degrees, which produce an identical modulation transfer function, but a different phase transfer function, compared with the normal orientation. A parameter that provided information on the difference between the original and rotated PSF (the maximum of the cross-correlation function) resulted in good qualitative agreement with the blur matching values for each orientation. Conclusions: These results support the hypothesis that the neural visual system is adapted to the eye's particular aberrations, so that edges appear sharp despite the modest blur in the normal retinal image. The nature and temporal characteristics of this compensation process requires further investigation. This result may have important implications for understanding the impact of aberrations on vision and will reduce the immediate benefit for the patient of attempts to surgically produce diffraction-limited eyes.

Keywords: physiological optics • refractive surgery: optical quality • spatial vision