The presbyopic-correction strategy of monovision is clinically established and has been routinely practiced for decades. However, relatively few studies have investigated the relative contribution of each eye's optical quality to through-focus binocular visual performance in monovision. In this study, we used a binocular AO vision simulator to investigate the impact of modifying monovision by extending the DoF of the nondominant eye with SA. We found that modified monovision led to a substantial benefit in through-focus VA and binocular DoF as compared with traditional monovision. However, this improvement came at the cost of a reduction in CS at the anisometropic point of 1.5 D. Despite the degradation in CS for intermediate object distances, through-focus interocular image quality became more similar in modified monovision, leading to an improvement in binocular summation, particularly at 0.5 D. In addition, we found that binocular through-focus high-contrast VA in modified and traditional monovision was determined by the monocular performance of the better eye at each dioptric position. This result can provide a useful guideline for theoretical models of binocular through-focus visual performance.
The VA benefit of modified monovision was dependent on the sign and magnitude of SA induced in the nondominant eye. The sign of SA determined the defocus points of VA benefit within the vicinity of the anisometropic point. Positive SA (+0.2 μm) resulted in a significant benefit in VA at intermediate object distance (1.0 D), whereas negative SA (−0.2 μm) resulted in a significant benefit at near (2.0 D). The largest DoF was observed with modified monovision with +0.4 μm of SA. The benefit to DoF with +0.4 μm SA may be in part explained by the influence of phase transfer function on the retinal image.
57,58 As explained by Ravikumar et al.,
58 when defocus and SA are of the same sign (i.e., myopic defocus and positive SA), the phase-reversed regions of the modulation transfer function have relatively little energy and therefore a small contribution to the retinal image. Therefore, the influence of the sign of SA on through-focus image quality is determined by both the amplitude and phase of spatial frequencies in the retinal image.
The laser blended vision approach described by Reinstein et al. also takes advantage of refractive surgery–induced SA to extend monocular DoF in monovision. They found that subjects with positive postoperative SA were more likely to have near VA of 0.18 logMAR or better, as compared with those with negative postoperative SA.
43 Similarly, our study found the large magnitude of positive SA (+0.4 μm) resulted in greater benefit for near VA than negative SA. Moreover, their studies
28,29,43 support our finding that extending monocular DoF with SA in monovision provides significant benefits over traditional monovision.
In addition to near VA, extending monocular DoF in modified monovision offers the further benefit of reducing the difference in interocular retinal image quality. Ocular DoF may be extended by means of a small-aperture corneal inlay,
6–8 multifocal optical designs,
13,14 and certain combinations of Zernike primary and secondary SAs.
16,17,52 These techniques (diffraction, multifocality, higher-order aberrations) are viable options for reducing the difference in through-focus interocular retinal image quality in modified monovision.
Interocular difference in retinal image quality has been shown to compromise aspects of visual function which rely on the neural combination of two monocular channels into a single binocular percept, such as binocular summation
34,35,39,40 and stereopsis.
22,25,33,36–39 Pardhan and Gilchrist
35 described a reduction in binocular summation as anisometropia increased. Their study found that in the absence of anisometropia, the binocular summation at 6 cyc/deg was approximately 40%, similar to Campbell and Green's finding of 41%.
42 However, binocular summation degraded with anisometropia, reaching unity (no summation) at 1.5 D anisometropia, beyond which summation was <1, indicating binocular inhibition. Similarly, Loshin et al. found that binocular summation to be absent in monovision with add powers of 1.5 D and higher for mid to high spatial frequencies.
59 The results presented in this study also show distance binocular summation (at 10 cyc/deg) to be absent (near unity) with 1.5 D of anisometropia (for both traditional and modified monovision). It is important to note that the benefits in binocular summation with modified monovision may vary at additional spatial frequencies. By examining the monocular retinal image quality curves in
Figure 1b, it is clear that the interocular difference in retinal image quality reached a minimum in the neighborhood of 0.5 D. Therefore, it was not surprising to observe a peak in binocular summation at 0.5 D for traditional and modified monovision. At 0.5 D, the binocular summation improved by 19 ± 6% for the two modified monovisions as compared with traditional monovision. However, at intermediate and near object distances, no summation was observed.
In an investigation of the effect of anisometropia on distance VA, Collins et al. found that binocular acuity at distance was approximately equal to the monocular acuity of the distance-corrected eye.
23 This can be attributed to what Schor et al. described as interocular suppression of anisometropic blur.
21 The current study extends this finding from distance performance to include intermediate and near object positions. As shown in
Figure 4b, a strong correlation (
r = 0.97) between binocular and monocular (of the superior eye) through-focus VA was observed for both traditional and modified monovision. This finding was applied to the theoretical model that produced the monocular through-focus image quality curves of
Figure 1. The model produced a single-value retinal image quality metric for predicting through-focus binocular VA (
r = −0.92) in modified and traditional monovision. It should be noted that such a model may not be directly applicable to the early presbyope who may still have some degree of active accommodation. Also, the application of a single-value metric to the prediction of CS may be confounded by factors such as differences in spatial frequency content of the metric and the stimulus
58,60,61 and the neural contrast sensitivity function.
62–65 While some variability in the metric predictability among monovision conditions was found, the image convolution metric provides a robust estimate of binocular visual acuity in the presence of large amount of blur and in the case when the two eyes have significantly different image quality. A reliable metric that enables the prediction of binocular through-focus visual performance is important for the design and optimization modified monovision variables.
At the anisometropic point (1.5 D), the induction of ±0.2 μm of SA did not have a significant impact on VA. In the aberration-free case of traditional monovision, binocular VA at 1.5 D was −0.23 logMAR as compared with −0.20 ± 0.04 logMAR, the average of all modified monovision paradigms. However, CS at 1.5 D was significantly reduced in modified monovision. Modified monovision with ±0.2 μm of SA resulted in a CS reduction at 1.5 D by a factor of 2.3 ± 0.3, as compared with traditional monovision. This can be explained by examining the modulation transfer function, which shows an approximately factor of 3 decrease in contrast of a 10 cyc/deg sinusoidal grating due to +0.2 μm of SA. At best focus, positive and negative SAs have the same effect on the modulation transfer function. This result was in agreement with a study by Piers et al., in which they confirmed that CS is maximized when ocular SA is fully corrected.
66
A limitation of this study was that the experimental protocol used cyclopentolate to dilate the pupils and arrest accommodation. In natural viewing conditions, accommodative effort for viewing near objects is accompanied by pupil miosis, thereby extending ocular DoF due to diffraction. However, pupil miosis also reduces the magnitude of SA (i.e., wavefront multifocality) and can lead to changes in refraction.
67 It is important for the topic of pupil and wavefront interaction and the consequences on through-focus image quality to be addressed; however, this was not the goal of the current investigation. Furthermore, it will be important to study the effectiveness of modified monovision in relation to practical factors such as native higher order aberrations, interocular scatter, and other age-related deficits in vision.
For future work, it will be interesting to investigate the role of neural adaptation to traditional monovision and modified monovision. Collins et al. found that during the first 8 weeks of monovision contact lens wear, patients subjectively observed an improvement in some aspects of visual performance, such as walking confidence and hand-eye coordination.
68 Interestingly, this trend was not observed in objective measures (VA, stereoacuity, and blur suppression). In addition, depth perception, a key component to binocular vision, was not examined in this study. It will be interesting to investigate the impact of modified monovision on stereoacuity. It is well-known that anisometropia leads to a reduction in stereoacuity.
19,22,25,33,36–38 However, by extending the DoF of one or both eyes in modified monovision, the interocular difference in retinal image quality is reduced and may lead to an improvement in stereoacuity.
24