|University of Rochester|
Adaptive Optics (AO) refers to the set of optical systems which adjusts to compensate for optical effects introduced by the medium between the object and its image. AO was first used in ground-based astronomical telescopes to remove the wavefront distortion induced by atmospheric turbulence and achieve near-diffraction limited images. Recently, the same principle has been used for high resolution retinal imaging and improving vision (Liang et al, JOSA A 1997, Yoon et al, JOSA A 2002). It is well known that the optical quality of the human eye is degraded by higher order aberrations (HOA) in addition to conventional sphere and cylinder. AO has been utilized to provide a substantial visual benefit over conventional correction methods such as spectacles and contacts. Yoon and Williams showed the significant visual benefit of correcting HOA in normal eyes using AO under normal viewing condition (white light). Contrast sensitivity when only monochromatic aberrations were corrected was improved by a factor of 2 on average at 16 and 24 cycles/deg. Moreover, by surpassing the limit imposed by the optics of the eye, AO has facilitated the non-invasive investigation of the visual system in isolation.
Eyes with abnormal corneal conditions such as keratoconus and corneal transplant have 5-6 times larger amplitudes of HOA than normal eyes, which implies that the visual benefit of HOA correction will be correspondingly larger. However, correcting large amounts of aberrations in these eyes has been very difficult due to the insufficient stroke of conventional wavefront correctors (deformable mirrors). To overcome this limitation, we have developed the large stroke adaptive optics. Our AO consists of a large-stroke deformable mirror (Imagine Eyes: MIRAO-52D) and a Shack-Hartmann wavefront sensor. The wavefront sensor is optimally designed with high dynamic range to measure highly aberrated eyes. The deformable mirror has a large surface deformation which allows us to compensate for the aberrations in eyes with abnormal corneal conditions. In addition, the highly linear response of the deformable mirror allows us to operate the closed loop at gains up to 100%. We have been able to demonstrate the feasibility of the large-stroke AO in providing almost perfect optical quality in both normal and keratoconic eyes irrespective of the different magnitudes of aberrations in the two sets of eyes. For more information, refer to Sabesan and Yoon, JOV 2009
Shown below is the time-course of AO correction in 1 normal subject for a 6-mm pupil.
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Also shown is the PSF based on the measured aberrations. During the time-course, one can notice blinks, which appear as peaks in the time course amidst time intervals of nearly perfect correction.
With the large stroke AO, we wish to study the interaction between the highly aberrated optics and the visual system in abnormal eyes. Artal et al suggested that the visual system may be adapted to the eye's particular HOA in normal eyes. This effect could be even more significant in eyes with abnormal cornea because these eyes have experienced poor optical quality for many years. Adaptation to poor optical quality might render their visual system insensitive to diffraction-limited retinal image quality. Accordingly, visual benefit achievable immediately after correction of higher order aberrations in these eyes might be limited. We have measured the visual performance after correcting aberrations to nearly diffraction limited optical quality, in normal and highly aberrated KC eyes. Interestingly, although the retinal image quality was similar in both groups, visual acuity was significantly worse in KC eyes compared to normal eyes. In addition, the magnitude of native higher order aberrations before correction might have influenced the extent of this limitation of visual performance in these eyes. (see figs below)
Such deficit in visual performance when unaccounted by optical theory might be attributed to neural factors.
Furthermore, similar to neural insensitivity for a perfect retinal image, neural compensation for blurred retinal image may also be possible, thereby reducing its impact on visual performance. We have investigated whether the neural visual system compensates for long-term visual experience with a blurred retinal image, resulting in improved visual performance in KC eyes. This was done by making normal eyes optically identical to KC eyes using AO and comparing their visual performance with KC eyes. We observed that KC eyes who had adapted to their own aberrations achieved better visual performance than normals viewing through the same aberrated optics. (See fig below)
Analogous to adaptation to aberrated image quality, an interesting and clinically relevant question to address is whether the visual system can be readapted to near-diffraction limited ocular optics. Provision of superior optical quality for a longer duration non-invasively is the primary challenge in investigating this question. Nevertheless, with customized vision correction methods rapidly gaining importance for compensating for higher order aberrations and presbyopia, the importance of neural factors governing visual performance cannot be overemphasized.