2.1.

Subjects

Three normal subjects—1, 2, and 3 (two males and one female)—participated in this study. All subjects had no previous history of ocular surgery or trauma and had normal vision or corrected vision. Clinical refraction for each subject was implemented for correction of their low-order aberrations. Table 1 lists parts of information of all subjects. During the experiment, paralysis of accommodation and dilation of pupil were achieved through administering 1% cyclopentolate. Informed consent was obtained from all subjects and the experiment procedures conformed to the tenets of the Declaration of Helsinki. Subject 1 and Subject 3 were unaware of the intention of this study, while Subject 2 was one of the authors.

Table 1

Basic information of the three subjects.

2.2.

Binocular Adaptive Optics Visual Simulator

A BAOVS was established as the experiment platform. The schematic view of our BAOVS is shown in Fig. 1. The BAOVS incorporated two identical optical channels consisting of the respective AO subsystem and visual stimulus display subsystem. The optical paths concerning AO and stimulus display are denoted by red lines and green lines, respectively; those concerning both are denoted by blue lines.

Fig. 1

Schematic view of the binocular adaptive optics visual simulator with two identical optical channels. The optical paths concerning adaptive optics (AO) and stimulus display are denoted by red lines and green lines, respectively; those concerning both are denoted by blue lines. LD, laser diode; BS, beam splitter; HS, Hartmann–Shack wavefront sensor; DM, deformable mirror; LG, lens group; TL, trial lens; IF, interference filter; L, lens; M, mirror; P, prism; O, OLED; LE, left eye; and RE, right eye.

The two identical optical channels had symmetrical optical layouts. Here, the left optical channel is taken as an example for introduction. It comprised a 37-element piezo deformable mirror made by our laboratory with a stroke of $\pm 2\mu \mathrm{m}$, a Hartmann–Shack wavefront sensor with 97 lenslets, a control system, and a visual stimulus display. A semiconductor laser diode (LD) ($\lambda =905\mathrm{nm}$, $\mathrm{\Delta }\lambda =2\mathrm{nm}$) was used to generate a near-infrared beacon for wavefront sensing. The use of near-infrared light provided more comfortable viewing conditions and higher retinal reflectance. After collimated by lens L1, the beacon light passed through a beam splitter BS1, and was directed into the eye by a reflecting prism P1. The reflected light from the eye fundus conveyed the aberrometric information. It traveled by BS1, a beam expanding lens group LG1 and a mirror M1 before reaching a deformable mirror deformable mirror (DM). Then the preaberrated light from DM was relayed to a Hartmann–Shack wavefront sensor HS, over a sequence of optics including a mirror M2, a beam constricting lens group LG2, a beam splitter BS2, and another mirror M4. DM and HS were placed in conjugate planes with the eye pupil. LG1 and LG2 were configured to match the beacon light with the deformable mirror and the wavefront sensor in size, respectively. The control system was a computer shared by both optical channels. It was responsible for processing wavefront slope data and sending driving signals to the deformable mirrors, using a direct-slope control algorithm. The control system operated at 25 Hz with a closed-loop bandwidth of about 1 Hz. Initial tilts were left uncorrected to avoid variations in the relative location of the retinal images. Correction was suspended when the subapertures of the wavefront senor were not fully filled with light (such as eye blinks). The control system would keep resending driving signals of the last frame to the deformable mirrors. The AO system had been demonstrated elsewhere in previous studies.^26,27

The visual stimuli for binocular visual testing were generated by MATLAB 6.5 (Mathworks, Natick, Massachusetts) and Psychtoolbox extensions^28,29 on another computer, and projected by two OLED displays D1, D2 (EMA-100110, eMagin Corporation). Each OLED display emitted at 540 nm (green light) with a bandwidth of 70 nm, and had a retinal subtense for $2.37\times 1.78\mathrm{deg}$. The resolution was $800\times 600\text{pixels}$ and the refresh rate was 60 Hz. A special circuit that combined two 8-bit output channels of the graphics card was used to produce gray levels of 14 bits.²⁶ The objective lens L2, reflecting prism P2, and mirror M3, associated with the aforementioned optics, were used to form images of the stimuli on the retina. Note that D1 was delivered to the right eye and D2 to the left eye. An interference filter interference filter ($550\pm 5\mathrm{nm}$) was placed between P2 and M3 to filter the stimulus light into quasimonochromatic light.

P1 was capable of translational motion for interpupillary distance adjustment between 55 and 70 mm. A chin and forehead rest were used to preclude head movements. The trial lens for compensating defocus and astigmatism was placed at TL1. The chromatic focus shift between the near-infrared beacon and the green stimulus was compensated for by axially moving the OLED displays until best focus of the subject. The OLED displays were mounted on a stepping motor for best focus adjustment.

2.3.

Experiment Procedures

Monocular contrast sensitivity function was measured before and after HOA correction (low-order aberrations were always corrected) for both eyes in the three subjects. The untested eye was occluded during testing. The stimuli were vertical sinusoidal gratings with a retinal luminance of $8\mathrm{cd}/{\mathrm{m}}^2$. Five spatial frequencies were tested ranging from 2 to 24 cycles per degree (cpd). A two-alternative forced choice procedure was adopted in which only one of two consecutive presentations of a trial contained the gratings (the other was a blank scene with background luminance). Each presentation lasted for 100 ms. The contrast of the stimuli was controlled through an adaptive staircase procedure. Three successive correct responses led to a contrast decrease by 10% of the previous value; one incorrect response brought a 10% increase.³⁰ The psychometric data were obtained from eight staircases of 80 trials each. The contrast threshold for each spatial frequency was calculated as the average of the last five turnarounds, and its reciprocal was contrast sensitivity.

The visual stimuli for testing stereopsis in this study were two-line stereograms specifying horizontal disparity, as shown in Fig. 2. The stimuli were black lines on a bright background and the retinal luminance was $8\mathrm{cd}/{\mathrm{m}}^2$ for both eyes. There were two thin vertical lines exhibited in the display for each eye. The upper reference line stood in the middle in both displays and would fuse as a line within the fixation plane. The lower pair of lines, called the test line, was horizontally shifted from the reference line in opposite directions by half the disparity. Consequently, the test line would appear to the observer to be closer or further relative to the reference line, according to the sign of the disparity that was induced. Both the reference line and the test line were 25 arc min long, and they were 4 arc min apart vertically.

Fig. 2

The two-line stereograms used in this study. To generate disparity, the test line was horizontally shifted from the reference line in opposite directions by half the disparity. The stimuli were black lines on a bright background: (a) display for LE and (b) display for RE.

The stereoacuity was obtained in the three subjects using the psychophysical method of constant stimuli. The disparities tested were 20, 80, 140, and 200 arc sec of both crossed and uncrossed disparities. The psychometric data were acquired in five sessions. Each session consisted of 80 trials presenting the eight disparities equally in random order. The time sequence of a typical trial is illustrated in Fig. 3. A trial was started with the presentation of binocular fusion stimuli. The binocular fusion stimuli were central crosses with small solid squares placed diagonally. With accurate binocular fusion, the subject would see four squares placed regularly in the four quadrants of the cross. After the subject initiated the test with a keypress, the binocular fusion stimuli disappeared and a 500 ms blank scene of the background luminance followed. Meanwhile, there was a brief “beep” sound indicating the subsequent stereograms. The stereograms were presented for the intended duration ($T_0$). Subsequent to that was another 500 ms presentation of blank scene. Then the binocular fusion stimuli recurred and the system waited for response from the subject as to whether the test line was closer or farther than the reference line. There was no audio feedback on the correctness of the response. During the test, the subject was told to focus on the presentation and try to blink between trials, which was not difficult because a trial generally lasted no longer than 2 s.

Fig. 3

The time sequence of a typical trial in stereoacuity tests. The columns “L” and “R” are visual displays for the left eye and the right eye, respectively. With accurate binocular fusion, the subject would perceive just as “B” shows.

The percentage of “far” responses was computed for each disparity and the data were fit to a logistic function.³¹ Stereoacuity threshold was calculated as the semi-interquartile range of the logistic function.^11,24 Thus, a threshold was actually on the basis of 400 presentations. The same tasks were repeated for three stimulus durations (100, 300, and 500 ms) and three optical conditions of the eyes, as follows: (1) baseline correction of defocus and astigmatism for both eyes; (2) better eye AO correction of HOAs beyond the baseline correction; and (3) binocular AO correction of HOAs beyond the baseline correction. Here, the better eye is the eye with better contrast sensitivity measured with the aforementioned procedures. Afterward, in this paper, the three optical conditions are referred to as baseline correction, better eye correction, and binocular correction, respectively.

3.1.

Binocular Real-Time Aberration Measurement and Correction

Table 2 shows the root-mean square (RMS) of HOAs over a 6 mm pupil for both eyes of the three subjects. Each data were averaged from eight measurements. Figure 4 shows the representative time course of total RMS aberration for both eyes over a 6 mm pupil (measured in subject Subject 2). The black and red lines represent RMS aberration for the right eye and the left eye, respectively. Data during eye blinks have already been removed. The native total RMS aberration was 0.48 and $0.45\mu \mathrm{m}$ in the right eye and the left eye, respectively. After AO correction was initiated ($T=10\mathrm{s}$), the residual RMS aberration of both eyes converged rapidly to a small value of about $0.1\mu \mathrm{m}$ and leveled off, demonstrating a good performance of the AO system.

Table 2

Root mean square (RMS) of higher-order aberrations (HOAs) over a 6 mm pupil in the three subjects for both eyes.

Fig. 4

The representative time course of total root mean square (RMS) aberration for both eyes. The black and red lines represent RMS aberration for the right eye and the left eye, respectively. AO is initiated at $T=10\mathrm{s}$.

3.2.

Monocular Contrast Sensitivity Before and After HOA Correction

Figure 5 shows monocular contrast sensitivity function for both eyes in the three subjects, on a log–log scale. According to the results, the better eye was the left eye for Subject 1 and the right eye for Subject 2 and Subject 3. It is evident that AO correction of HOAs improved contrast sensitivity for all spatial frequencies in all eyes. The average improvement across different eyes was by a factor of 1.34, 1.50, 1.99, 1.74, and 1.65 at 2, 4, 8, 16, and 24 cpd, respectively. The right eye of Subject 1, which had the largest amount of HOAs, also demonstrated the largest improvement of 3.57, at the middle spatial frequency of 8 cpd. The average improvements across different spatial frequencies for the left and right eyes were 1.75 and 1.86, 1.50 and 1.62, and 1.57 and 1.55 for Subject 1, Subject 2, and Subject 3, respectively. Associated with the results in Table 2, it seemed that a greater amount of HOAs brought larger improvement in contrast sensitivity for comparison between eyes of the same subject. However, it did not always follow for interindividual comparison, possibly due to the neural insensitivity or adaptation.^32,33 The data were referred to a paired $t$ test for statistical analysis. Results showed that the improvement in contrast sensitivity was significant for all spatial frequencies ($p<0.05$), indicating the validity of our optical correction.

Fig. 5

Monocular contrast sensitivity function measured with and without higher-order aberration (HOA) correction. Different rows and columns display data for different subjects and different eyes, respectively. Blue squares stand for results obtained without HOA correction (defocus and astigmatism are corrected) and green circles stand for results obtained with HOA correction.

3.3.

Stereoacuity Versus Viewing Duration under Different Optical Conditions

The stereoacuity is plotted as a function of viewing duration for the three subjects in Fig. 6. Binocular correction improved stereoacuity remarkably, by an average magnitude of 24.9, 5.7, and 3.8 arc sec at 100, 300, and 500 ms in the three subjects, respectively. This improvement was proven statistically significant for all viewing durations in a paired $t$ test ($p<0.05$). Better eye correction led to distinct results among the subjects. For Subject 1, better eye correction brought a severe degradation of stereoacuity from 102, 78, and 70 arc sec to 183, 120, and 105 arc sec at 100, 300, and 500 ms, respectively. Whereas for Subject 2 and Subject 3, better eye correction generally improved stereoacuity to a smaller extent than binocular correction did. One exception was the 500 ms case for Subject 2, in which the stereoacuity was slightly degraded by about 2 arc sec. It can be easily seen that, in all cases, stereo thresholds were always lower at longer viewing durations, suggesting a persistent effect of viewing duration on stereoacuity even when the HOAs were corrected.

Fig. 6

(a)–(c) Stereoacuity versus viewing duration under different optical conditions for the three subjects. Blue squares represent stereoacuity with baseline correction. Green circles and red triangles represent stereoacuity with binocular and better eye correction, respectively. Note that the $y$-axis labels are not consistent for the three subjects. Low thresholds suggest good stereoacuity. Error bars are standard deviations.

3.4.

Stereo Benefit upon Binocular and Better Eye HOA Correction

Figure 7 plots the stereo benefit upon binocular correction and better eye correction at different viewing durations. The benefit here corresponds to the ratio of stereoacuity with baseline correction to binocular or better eye correction. In most cases, HOA correction brought improvement in stereoacuity so that the benefit was larger than 1. In some cases for which better eye correction brought negative effects, the benefit was less than 1. As the viewing duration increased, binocular correction benefit exhibited a clear downward trend in all subjects. These benefits were commonly in the range of 1.1 to 1.3, except for the maximum benefit of 1.68 demonstrated in Subject 2 at 100 ms. As for better eye correction, the situation was also split into two parts. For Subject 2 and Subject 3, better eye correction benefit decreased slightly to above 1.0 with increasing viewing duration, just as the binocular case. However, for Subject 1, this relationship was exactly the opposite, with the benefit elevated from 0.56 at 100 to 0.67 at 500 ms. It seemed that the benefit upon HOA correction, no matter binocular or better eye, approached the level of 1.0 with increasing viewing duration. Further statistical analysis indicated that binocular correction benefit was significantly greater than 1.0 at 100 and 300 ms ($p=0.044$, 0.028, and 0.071 for 100, 300, and 500 ms, respectively). Better eye correction benefit, however, was not significantly different from 1.0 at any viewing duration ($p=0.952$, 0.788, and 0.505 for 100, 300, and 500 ms, respectively). Moreover, no significant difference was observed between any two durations for both binocular correction benefit ($p=0.231$, 0.209, and 0.097) and better eye correction benefit ($p=0.559$, 0.487, and 0.379).

Fig. 7

(a)–(c) Stereo benefit upon binocular and better eye HOA correction at different viewing durations. Binocular and better eye correction benefits are marked by green circles and red triangles, respectively. Error bars are standard deviations. The dashed lines denote the benefit level of 1.0.