The human eye is affected by aberrations that degrade the retinal image and ultimately limit spatial vision (Liang & Williams, 1997; Artal, Guirao, Berrio, & Williams, 2001; Hofer, Artal, Singer, Aragón, & Williams, 2001a). The lower order aberrations, defocus and astigmatism, are corrected routinely with spectacles, contact lenses, intraocular lenses, and refractive surgery. The higher order aberrations, beyond defocus and astigmatism, have been known to exist in the eye for more than 150 years (Helmholtz, 1881). However, it has only recently been possible to correct these aberrations in the living eye. Adaptive optics (AO), a technique developed in astronomy to remove the effect of atmospheric turbulence from telescope images (Hubbin & Noethe, 1993), can also be used to correct the eye’s higher order aberrations (Liang, Williams, & Miller, 1997; Vargas-Martín, Prieto, & Artal, 1998; Fernández, Iglesias, & Artal, 2001; Hofer et al., 2001a). One application of this technology in the eye is to obtain high-resolution images of the retina to resolve individual photoreceptors in vivo (Liang et al., 1997) and to identify the photopigment in each cell (Roorda & Williams, 1999). Another important application of AO is to produce controlled wave-aberration patterns in the eye, enabling new experiments to better understand the impact of the ocular optics on vision. In particular, it is possible to address the intriguing question of whether the visual system is adapted to the particular pattern of optical aberrations of its own eye. To test this idea, subjects viewed visual stimuli with aberrations controlled with adaptive optics. The AO apparatus corrected the eye’s wave aberration and replaced it with the same wave aberration or with a rotated copy of it.

The wavefront aberration (WA) is a function that characterizes the image-forming properties of any optical system. It is defined as the optical deviation of the wavefront along a certain ray from the perfect spherical wavefront. The WA is related to the image of a point source produced by the system, the point-spread function (PSF), through an integral equation (Born & Wolf, 1985). AO permits the manipulation of the eye’s WA by introducing different optical paths over the pupil area.

An AO system consists of a wavefront sensor to measure the WA in real time and a correcting device, typically a deformable mirror, to modify the WA. Figure 1 shows a schematic diagram of the AO system at the University of Rochester used in this study. A Hartmann-Shack wavefront sensor (Liang, Grimm, Goelz, & Bille, 1994; Liang & Williams, 1997; Prieto, Vargas-Martín, Goelz, & Artal, 2000) measures the eye's WA at 30 Hz. A narrow infrared beam (810 nm, with a spectral spread of 20 nm) produced by a super-luminescent diode (SLD) is projected into the subject's retina acting as a beacon source. The irradiance on the cornea was approximately 5 μW, which is about 30 times smaller than the maximum permissible exposure for continuous viewing according to the safety standards (ANSI Z136.1, 1993). In the second pass, after the light is reflected from the retina and passes through the complete system, an array of 177 lenslets produces an image of spots on a charged-coupled device (CCD) camera. The locations of the spots in this image provide the local slopes of the ocular WA. The aberrations were measured for a 6-mm pupil up to 10th radial order, corresponding to an expansion of 63 Zernike modes (Noll, 1976).

A 97-channel deformable mirror (Xinetics), with an aluminized glass faceplate and lead magnesium niobate (PMN) actuators, was used as the wavefront-correcting device. It is placed in the system optically conjugated both with the subject's pupil plane and the wavefront sensor, by using appropriate lenses and two off-axis parabolic mirrors. The AO system worked in closed-loop, with the deformable mirror driven by the measured aberration data. Typical aberrations were nearly eliminated after 5-10 iterations, thus a correction of the eye’s aberrations was completed automatically in 0.17-0.33 s.

In this experiment, besides removing the higher order aberrations in the eye on each trial, the deformable mirror also acted as an aberration generator to blur the subject’s vision either with the subject’s own aberrations or a rotated version of the aberrations. After the currently present aberrations were corrected, eight different aberration patterns were produced in each case: an average normal aberration pattern and seven similar versions rotated by 45-deg intervals. Figure 2 shows an example of the actual eight aberration patterns and the associated PSFs for one of the subjects. The desired aberrations were kept stable by the AO system working in closed-loop while subjects performed the matching experiment.

We relied on the wavefront sensor to indicate how well we succeeded in creating the same PSF at all orientations. The wavefront sensor can measure the combined wave aberration of the eye and the optical system through which the subject views the stimulus, with the exception of a beam-splitter, a mirror, and a lens. These uncommon path elements are diffraction-limited and have negligible impact on the wave aberration. We computed the Strehl ratio of the PSF generated at each orientation for every subject. These values were independent of the orientation. Specifically, there is no tendency for the image quality, assessed with these objective measures, to be any better in the zero-orientation condition.

Subjects viewed a binary noise stimulus, produced by a digital light-processing (DLP) video projector (Packer et al., 2001), through the AO system. The stimulus was viewed in 550-nm monochromatic light (shown schematically as the green light path in Figure 1). The stimulus, also shown in Figure 1, contains sharp edges at all orientations and subtended 1 deg of visual angle. The binary noise stimulus was produced from a uniform distribution filtered by taking the FFT of the noise, applying an annular filter via point-by-point multiplication, and then inverting back into the spatial domain. To get black-and-white spots rather than gray levels, the intensities were thresholded. Because the bandpass annular filter is circularly symmetric, it will eliminate all lower spatial frequencies and higher ones, which make energy at all orientations get through. A Gaussian function smoothed the edge of the field. On each trial, the computer randomly generated a different stimulus pattern.

The matching experiment was performed on the right eyes of five subjects. During the measurement, the subject's head was stabilized with a bite bar, and the subject's pupil was dilated and accommodation paralyzed with cyclopentholate hydrochloride (2.5%). The experiment was performed for a 6-mm diameter artificial pupil. Figure 3 shows schematically the rationale of the matching experiment.

The subject viewed the stimulus for 500 ms immediately after the deformable mirror generated the subject’s own aberrations or the rotated version. At other times, the subject viewed a uniform field.

Subjects were asked to view the stimulus through the AO 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 (Figure 3a). The yellow part in Figure 3 represents the time when the desired aberration is produced. The eye’s optics remain stable during the next 500 ms when the stimulus is presented to the subject. The subject’s task was to adjust the magnitude (the root mean square [RMS]) of the aberrations by multiplying it by a factor (F) in the rotated case to match the subjective blur of the stimulus to that seen when the wave aberration was in the normal orientation (Figure 3b). Subjects were unaware of when the stimulus was seen through the normal or rotated aberrations. In the matching process, one of the seven different rotated versions of the aberrations was randomly selected. Subjects were asked to match the blur with all eight orientations, including the normal orientation, in random order. Subjects could not tell which orientation was presented on a given trial. Because the subjects matched the blur in the normal orientation with an amplitude that was a physical match to that of normal wave aberration, there is no possibility that increased blur for other orientations.

If the rotated (uncommon) aberrations degraded the subjective image quality, a factor F smaller than 1 was re-quired, as further illustrated in Figure 4 with a blurred stimulus. If the rotated wave aberration is less damaging for vision, a factor F greater than 1 should have been required. The complete process of matching took several minutes and was repeated up to 5 times to obtain robust estimates of the adjustment factor for each rotated aberration pattern.