The Relationship Between Binocular Imbalance and Myopic Shift in Unoperated Eyes After Unilateral SMILE and tPRK

Abstract

Purpose

The purpose of this study was to investigate the relationship between binocular imbalance and myopic shift in unoperated eyes after unilateral small incision lenticule extraction (SMILE) and transepithelial photorefractive keratectomy (tPRK) procedures.

Methods

This study included 51 participants who had undergone unilateral SMILE (n = 28) or tPRK (n = 23) for at least 3 months. The participants were categorized into stable and myopic shift groups based on the difference between preoperative and postoperative spherical equivalent refractive (SER) errors of unoperated eyes. Psychophysical tests were conducted only at the postoperative follow-up point. Spatial sensory eye dominance was determined by analyzing a binocular orientation combination task at spatial frequencies of 1 and 6 cycles per degree (c/d). A rotating cylinder generated a spontaneous Pulfrich phenomenon to determine the interocular delay at spatial frequencies of 0.95 and 2.95 c/d.

Results

The logrBP in the myopic shift group was significantly more negative than in the stable group at 1 c/d (P < 0.01) and 6 c/d (P < 0.01). And logrBP correlated with the difference between preoperative and postoperative SER of the unoperated eye at 1 c/d (rs = 0.513, P < 0.001) and 6 c/d (rs = 0.504, P < 0.001). In the myopic shift group, logrBP was more negative at 6 c/d than at 1 c/d (P = 0.013), but not in the stable group.

Conclusions

Patients who experience myopic shift in the unoperated eye after unilateral SMILE or tPRK tend to have stronger sensory eye dominance in that eye, with a more pronounced dominance at higher spatial frequencies.

Globally, myopia is the most common eye disease that can result in irreversible vision impairment and blindness.¹ In Asia, myopia rates have increased markedly, affecting up to 90% of teenagers and young adults.^2,3 Anisometropia refers to significantly different refractive errors (≥ 1.0 diopter [D]) between the two eyes.⁴ Traditionally, anisometropia is corrected with spectacles or contact lenses.⁵ However, laser vision correction surgery has emerged as an innovative alternative treatment. Procedures such as small incision lenticule extraction (SMILE) and transepithelial photorefractive keratectomy (tPRK)^6–10 can safely and effectively correct aniseikonia and improve stereoscopic vision in patients with anisometropia. SMILE is a lamellar procedure suited for patients with moderate myopia (<−5.0 D) and mild astigmatism (<−2.0 D), whereas tPRK is a surface ablation procedure ideal for those with high myopia and thin corneas.¹¹ Both procedures generally achieve refractive stability within 1 month postoperatively, with no significant differences between the 2 groups at this time.¹²

Unilateral myopic anisometropia involves one eye that is distinctly myopic while the other eye is essentially emmetropic.¹³ Affected patients often require only unilateral refractive surgery. However, postoperative abnormal binocular vision, such as asthenopia and diplopia,^14–17 is particularly common in patients with preoperative binocular vision dysfunctions, such as anisometropia or phoria/exophoria.^18,19 Additionally, Sella et al.²⁰ reported that, in 3 patients who underwent unilateral refractive surgery, myopia progressed more rapidly in the unoperated eye than in the operated eye within 3 to 10 years after the procedure. In our clinical practice, many patients with unilateral myopic anisometropia report blurry vision in the unoperated eye after unilateral refractive surgery. Follow-up examinations often reveal that these preoperatively emmetropic eyes exhibit a postoperative myopic shift. These phenomena highlight the need for further investigation into why myopic shift occurs in the unoperated eye after unilateral refractive surgery.

For patients with unilateral myopic anisometropia, because the surgery corrects only the myopic eye without impacting the other emmetropic eye directly, the myopic shift in the unoperated eye is likely related to binocular vision. Zhou et al.²¹ reported that patients with myopic anisometropia often exhibit some degree of binocular imbalance. Additionally, Liu et al.²² and Feng et al.²³ found that refractive surgery can temporarily affect sensory eye dominance. To understand the mechanisms underlying postoperative myopic shift in unoperated eyes after unilateral refractive surgery, the relationship between binocular imbalance and myopic shift in unoperated eyes needs to be investigated.

Accordingly, we used two different psychophysical tasks to measure binocular imbalance at both spatial and temporal levels in patients undergoing unilateral refractive surgery. Specifically, we used Zhou et al.’s binocular orientation combination task²⁴ to quantify the balance point (BP) of the eyes at two different spatial frequencies after surgery. This task reflects spatial sensory eye dominance by calculating each eye's contribution to binocular integration during measurement.²⁵ Additionally, we used a structure similar to that used by Reynaud and Hess's²⁶ motion-defined cylinders to assess interocular delay at two spatial frequencies.

Methods

Participants and Surgical Procedures

This cross-sectional study included 51 participants who had undergone unilateral SMILE (n = 28, age = 20.43 ± 3.41 years) or tPRK (n = 23, age = 21.00 ± 4.56 years) for at least 3 months (i.e. 3 months or more) at the Eye Hospital of Wenzhou Medical University. The selection between SMILE and tPRK was based on a comprehensive assessment by the clinician, considering factors such as refractive error, corneal thickness, and corneal morphology, with patients making the final decision according to their preferences. Because this study primarily focuses on the relationship between postoperative refractive status and binocular imbalance, and given that no significant differences in refractive status were observed between the two groups at the time of postoperative measurement (as shown in the Supplementary Materials), the data from both groups were combined for analysis. The Table summarizes the clinical details of all participants, with more detailed data available in the Supplementary Materials. All participants had demonstrated stable refractive status for at least 1 year preoperatively, with a best-corrected visual acuity (BCVA) of ≤ 0.0 logMAR in each eye. Participants had normal stereoscopic vision and could complete psychophysical tasks. Exclusion criteria included strabismus, amblyopia, optical opacities, ocular pathologies detected via slit-lamp examination, history of corneal surgery or ocular trauma, severe dry eye, corneal diseases, and eye infections.

Table.

Clinical Details of 51 Subjects Undergoing Unilateral SMILE and tPRK Procedures

	Operated Eye	Unoperated Eye
Sex, M/F	38/13
Age, y	20.69 ± 3.94
Pre-interocular SER difference (D)	2.97 ± 1.20
Post-interocular SER difference (D)	–0.77 ± 0.70
Pre-SER (D)	–3.19 ± 1.20	–0.22 ± 0.61
Post-SER (D)	0.34 ± 0.37	–0.43 ± 0.58
Post-pre SER difference	3.53 ± 1.29	–0.22 ± 0.26

D, diopter; F, female; M, male; Pre, preoperative; Pre-interocular SER difference, preoperative SER of unoperated eyes – preoperative SER of operated eyes, Post-interocular SER difference, postoperative SER of unoperated eyes – postoperative SER of operated eyes, Post-pre SER difference, postoperative SER – preoperative SER; Post, postoperative; SER, spherical equivalent refractive errors.

Participants were divided into two subgroups based on the difference between preoperative and postoperative spherical equivalent refractive (SER) errors after cycloplegia (SER = sphere + cylinder / 2) of the unoperated eye (SER difference = postoperative SER − preoperative SER), that is, the stable group (−0.125 D ≤ SER difference ≤ +0.125 D) and myopic shift group (SER difference ≤ −0.25 D). We selected −0.25 D as the grouping criterion for myopic shift in unoperated eyes because it is a well-established clinical threshold for refractive regression in operated eyes.^27,28 This threshold holds significant clinical value: 0.25 D represents the smallest increment in spectacle prescriptions,²⁹ the typical precision for measuring refractive errors,³⁰ and the standard interval used in defocus curve measurements.³¹ It also serves as a critical reference for assessing measurement accuracy,³² as refractive changes meeting or exceeding this value often warrant clinical attention. Although “refractive regression” specifically refers to changes caused by surgical procedures and is not applicable to unoperated eyes, we adopt the term “myopic shift” to describe the progression of myopia in these cases. To ensure consistency in quantifying refractive changes, we applied the same −0.25 D threshold in this study. The primary study outcome was the difference in logrBP between the myopic shift group and the stable group. A pilot study was conducted with 20 participants (10 in each group) to estimate the appropriate sample size. In this test, the logrBP at 6 cycles per degree (c/d) was −0.151 ± 0.271 decibel (dB) in the myopic shift group and 0.046 ± 0.110 dB in the stable group. To achieve 80% power with a significance level of 0.05, a minimum of 19 participants were required in each group. All participants were corrected of the refractive errors before psychophysical tasks.

This study was approved by the Ethics Committee of Wenzhou Medical University (institutional review board [IRB] protocol number: 2023-211-K169-01) and was conducted in accordance with the principles of the Declaration of Helsinki. All participants provided written informed consent before participation in the study.

In the SMILE procedure, the VisuMax femtosecond laser (Carl Zeiss, Oberkochen, Germany) was used to remove a stromal lenticule, leaving behind a 110-µm-thick cap. The laser energy applied at 135 nJ with 4.5-µm spot separation. The cap diameter was 7.9 mm (except for 2 patients, with a cap diameter of 7.8 mm), and the optical zone was 6.9 mm (except for 2 patients, with an optical zone of 6.7 mm). During the tPRK procedure, all surgeries were performed with the Schwind Amaris 750 Hz excimer laser (Schwind, Kleinostheim, Germany). The optical zones ranged from 5.9 to 7.6 mm, and the ablation depth ranged from 89 to 148 µm.

Apparatus

For the binocular orientation combination task, we used MATLAB R2016b (version 9.1.0; MathWorks, Inc., Natick, MA, USA) with Psychtoolbox extension 3.0.14³³ on a MacBook Pro (13-inch, 2017; Apple, Inc., Cupertino, CA, USA). The stimuli were presented dichoptically via gamma-corrected head-mounted goggles (GOOVIS Pro, AMOLED display; NED Optics, Shenzhen, China) with a refresh rate of 60 hertz (Hz), resolution of 2560 × 1600 pixels, and maximum luminance of 150 cd/m².

For the paradigm using the Pulfrich phenomenon, experiments were programmed and controlled on a Macmini computer A1347 (Apple) with Matlab R2014a (MathWorks) using the Psychophysics toolbox.³³ The dichoptic presentation was achieved using a 27-inch 3D-Ready LED monitor, LG D2792PB (LG Life Science, Seoul, South Korea), placed at a viewing distance of 90 cm. The monitor was gamma-corrected, with a maximum luminance of 250 cd/m², resolution of 1920 × 1080 pixels, and refresh rate of 60 Hz. Participants viewed the stimuli in a dark room using passive polarized 3D glasses, which reduced the luminance to approximately 43% and had a crosstalk of 1%.

Stimuli and Design

Experiment 1: Binocular Orientation Combination Task

A binocular orientation combination task was used to measure BP at spatial frequencies of 1 c/d and 6 c/d.²⁴ BP was defined as the interocular contrast ratio (dominant eye/non-dominant eye) in which the two eyes were balanced in binocular combination (i.e. have equal contribution). For each observer, the sighting dominant eye was tested using the hole-in-the-card test.³⁴ The head-mounted goggles dichoptically presented a sinusoidal grating tilted horizontally in the opposite direction to each eye in two different configurations (Fig. 1A). In the first configuration, the grating presented to the dominant eye was rotated +7.1 degrees counter-clockwise relative to the horizontal, while the grating shown to the non-dominant eye was rotated –7.1 degrees clockwise relative to the horizontal. In the second configuration, the grating presented to the non-dominant eye was rotated +7.1 degrees counter-clockwise relative to the horizontal, while the grating for the dominant eye was rotated –7.1 degrees clockwise relative to the horizontal. Consequently, the total difference of orientation between the eyes was 14.2 degrees. The size of the grating was varied at different spatial frequencies to maintain 2 cycles (2 degrees × 2 degrees at 1 c/d and 0.33 degrees × 0.33 degrees at 6 c/d; see Fig. 1A). Based on psychophysical data from practice trials, we determined a distinct set of seven interocular contrast ratios (ranging from 0 to 1) for each participant. Each condition, consisting of one orientation configuration and one interocular contrast ratio, was repeated 20 times. Thus, there were 280 trials per block at each spatial frequency (2 orientation configurations × 7 interocular contrast ratios × 20 repetitions). The interocular contrast ratios and configurations were randomized across all trials.

Figure 1.

An illustration of the binocular orientation combination task and the motion-in-depth Pulfrich task. (A) In the binocular orientation combination task, two horizontal sinusoidal gratings with equal and opposite tilts (±7.1 degrees) at spatial frequencies of 1 c/d or 6 c/d were presented to each eye separately. (B) The proportion of trials in which participants reported that DE dominated was plotted as a function of the interocular contrast ratio (DE/non-DE). A cumulative Gaussian distribution function was used to fit this psychometric curve. The BP corresponding to the 50% point of the best-fitting Gaussian function was derived from the fitting. This BP indicates the point at which the two eyes were balanced in binocular combination. BP, balance point; DE, dominant eye; non-DE, nondominant eye. (C) The stimuli, composed of Gabor patches, were presented dichoptically. The phase difference in the oscillation of the Gabor patches between the two eyes generates the percept of a motion-defined cylinder rotating in depth. When the interocular phase difference is less than 0 degrees, the cylinder appears to rotate counterclockwise; when the interocular phase difference equals 0 degrees, the percept is ambiguous, with Gabor patches moving left and right in the same plane; when the interocular phase difference is greater than 0 degrees, the cylinder appears to rotate clockwise. (D) The perceived direction as a function of the interocular phase difference was fitted with a logistic function. The midpoint of the logistic function at 0.5 performance defines the PSE. A negative value of PSE means the LE was delayed and a positive value of PSE means the RE was delayed. LE, left eye; RE, right eye.

Experiment 2: Motion-in-Depth Pulfrich Task

The paradigm for experiment 2 was the same as that used in Reynaud and Hess.²⁶ The stimulus was a structure-from-motion cylinder. The stimulus was dichoptically presented for 800 ms. The interocular phase during the oscillation remained consistent between all trajectories of Gabor patches and varied across trials to generate clear-to-ambiguous percepts of cylinder rotation in depth.^35,36 We tested two sizes of Gabor patches, 0.15 degrees and 0.45 degrees, corresponding to spatial frequencies of 2.95 c/d and 0.95 c/d, respectively. The number of Gabor patches was set at 200 for the 2.95 c/d and 66 for the 0.95 c/d. Selection of the interocular phase difference within (–1.5 degrees, –0.75 degrees, –0.375 degrees, –0.1875 degrees, –0.0938 degrees, –0.0469 degrees, –0.0234 degrees, 0 degrees, 0.0234 degrees, 0.0469 degrees, 0.0938 degrees, 0.1875 degrees, 0.375 degrees, 0.75 degrees, and 1.5 degrees), in which negative values would generate perception of a counterclockwise rotating cylinder and positive values would generate perception of a clockwise rotating cylinder. These 15 interocular phase difference values were repeated 10 times within each block, with varied interocular phase configurations randomized across different trials. Each participant completed three blocks for each spatial frequency condition in a randomized sequence.

Procedures

Experiment 1: Binocular Orientation Combination Task

Proper demonstrations were provided with detailed introductions to ensure participants understood the task. Each test trial included two parts.³⁷ The first part was an alignment phase, wherein the participants were asked to move the coordinates of stimuli with a dichoptic cross and dots to ensure perfect fusion. After alignment, the participants were asked to proceed by pressing the space bar on the keyboard to start the test phase. The participants were instructed to indicate whether the perceived cyclopean grating was oriented in a clockwise or counterclockwise direction pressing either the left or right key, respectively, on the keyboard after the stimulus presentation.

Experiment 2: Motion-in-Depth Pulfrich Task

As in a previous study,³⁸ the participants had to indicate if they saw the cylinder rotating clockwise or counterclockwise in a block-design paradigm. The perceived direction was measured using a constant stimulus method as a function of interocular spatial phase difference (Fig. 1C).

Clinical Outcome Measures

All participants underwent a series of routine preoperative and postoperative examinations for both eyes, including uncorrected visual acuity and BCVA, subjective refraction, intraocular pressure (IOP), Pentacam rotating Scheimpflug tomography (software version 1.19r11; Oculus, Wetzlar, Germany), and sighting dominant eye assessment using the hole-in-the-card test. Postoperative follow-up examinations were scheduled at least 3 months after surgery, as visual acuity typically stabilizes around this period.^39,40 After completing the psychophysical tests, participants’ subjective refraction was re-evaluated after cycloplegia.

Data Analysis

In Figure 1B, the psychometric function was fitted using a cumulative Gaussian distribution, with the probability of the fused percept orientation tilting toward the dominant eye plotted against interocular contrast ratios. BP was derived from the contrast ratio where the percept tilted toward the dominant eye 50% of the time. Two additional measures of BP were defined: (i) The rectified BP (rBP) as the contrast ratio (unoperated/operated eye), equal to BP if the sighting dominant eye was unoperated, and 1/BP if it was operated. (ii) Log-scaled rBP (logrBP), where a positive logrBP indicates that the operated eye has the stronger sensory dominance, whereas a negative logrBP indicates that the unoperated eye has the stronger sensory dominance.

In Figure 1D, the psychometric function was modeled with a logistic function. The point of subjective equality (PSE) represents the estimated midpoint, quantifying the processing delay of the left eye relative to the right eye. The rectified PSEs (rPSEs) were derived from the PSE, showing the processing delay of the unoperated eye relative to the operated eye, equal to PSE if the left eye was unoperated, and (−1) × PSE if the right eye was unoperated, with negative rPSE indicating the unoperated eye delay and positive rPSE indicating it in advance.

Statistical Analysis

SPSS version 27.0 (IBM Corporation, Armonk, NY, USA) was used for statistical analyses. Data are presented as mean ± standard deviation (SD). The Shapiro–Wilk test was performed to assess data distribution normality, and the Levene test was performed to check variance homogeneity. Parametric tests (independent sample t-test and paired samples t-test) were performed to compare normally distributed data, and nonparametric tests (Mann–Whitney U test and Wilcoxon signed-rank test) were performed for non-normally distributed data. Pearson's (r) and Spearman's (rs) correlation analyses were performed for normally and non-normally distributed continuous variables, respectively.

Results

Experiment 1: Spatial Binocular Imbalance Measured by Binocular Orientation Combination Task in the Myopic Shift Group and Stable Group

Figure 2 shows the values of logrBP at both 1 c/d and 6 c/d spatial frequencies for the myopic shift group and the stable group. At 1 c/d, the mean logrBP was −0.029 ± 0.103 dB for the myopic shift group and 0.082 ± 0.131 dB for the stable group (P < 0.01; 2-sided independent sample t-test). At 6 c/d, the mean logrBP was −0.168 ± 0.303 dB for the myopic shift group and 0.093 ± 0.246 dB for the stable group (P < 0.01; two-sided Mann–Whitney U test). logrBP was significantly more negative at 6 c/d than at 1 c/d in the myopic shift group (P = 0.013; two-tailed Wilcoxon signed-rank test), but not in the stable group. This indicates that the unoperated eye exhibited stronger sensory eye dominance at high spatial frequency in the myopic shift group. Additionally, the SD for logrBP was larger at 6 c/d than at 1 c/d in the myopic shift group, suggesting greater variability at higher spatial frequencies. The negative logrBP values indicated that the unoperated eye was the sensory dominant eye. Specifically, at 1 c/d, the unoperated eye was dominant in 16 of 28 (57.1%) patients in the myopic shift group and 7 of 23 (30.4%) patients in the stable group (P = 0.056; chi-square test). At 6 c/d, the unoperated eye was dominant in 20 of 28 (71.4%) patients in the myopic shift group and 6 of 23 (26.1%) patients in the stable group (P < 0.001; chi-square test).

Figure 2.

The logrBP at different spatial frequencies in the myopic shift and the stable groups (A), along with the correlation between the SER difference of the unoperated eye and the logrBP at spatial frequencies of 1 c/d (B) and 6 c/d (C). Data points falling in the shaded area indicate that the unoperated eye was the sensory dominant eye. Each point represents one individual observer: red points represent the SMILE group, blue points represent the tPRK group, circles denote the stable group, and triangles denote the myopic shift group. *P < 0.05, **P < 0.01. Error bars represent SEs.

Figures 2B and 2C depict the correlation between logrBP and the difference between preoperative and postoperative SER of the unoperated eye. The logrBP correlated with the SER difference at both 1 c/d and 6 c/d spatial frequencies (rs = 0.513, P < 0.001 at 1 c/d; see Fig. 2B; and rs = 0.504, P < 0.001 at 6 c/d; see Fig. 2C).

Figure 3 presents scatter plots of logrBP distributions at 1 c/d and 6 c/d for the myopic shift group and the stable group. Significant correlations were observed between logrBP at 1 c/d and 6 c/d for both the myopic shift group and the stable group (myopic shift group: rs = 0.702, P < 0.0001; see Fig. 3A; and the stable group: rs = 0.708, P < 0.001; see Fig. 3B).

Figure 3.

The distributions of logrBP at 1 c/d and 6 c/d spatial frequencies for patients in the myopic shift (A) and stable groups (B). Each point represents one individual observer: red points represent the SMILE group, blue points represent the tPRK group, circles denote the stable group, and triangles denote the myopic shift group.

Experiment 2: Temporal Binocular Imbalance Measured by Spontaneous Motion-in-Depth Pulfrich Phenomenon in the Myopic Shift Group and the Stable Group

Figure 4 shows rPSE values at both 0.95 c/d and 2.95 c/d spatial frequencies for the myopic shift group and the stable group. At 0.95 c/d, the mean rPSE was −0.004 ± 0.087 for the myopic shift group and −0.043 ± 0.055 for the stable group (P = 0.069; two-sided independent sample t-test). At 2.95 c/d, these values were −0.003 ± 0.091 and −0.018 ± 0.082, respectively (P = 0.526; two-sided independent sample t-test).

Figure 4.

The rPSE at different spatial frequencies in the myopic shift and stable groups (A), along with the correlation between the SER difference of the unoperated eye and the rPSE at spatial frequencies of 0.95 c/d (B) and 2.95 c/d (C). Data points falling in the shaded area indicate that the delayed eye was the unoperated eye. Each point represents one individual observer: purple points represent the SMILE group, green points represent the tPRK group, circles denote the stable group, and triangles denote the myopic shift group. Error bars represent SEs.

However, the negative rPSE values indicated that the unoperated eye was the delayed eye. Specifically, at 0.95 c/d, the unoperated eye was delayed in 13 of 28 (46.4%) patients in the myopic shift group and 17 of 23 (73.9%) patients in the stable group (P = 0.047; chi-square test). At 2.95 c/d, the unoperated eye was delayed in 14 of 28 (50.0%) patients in the myopic shift group and 11 of 23 (47.8%) patients in the stable group (P = 0.877; chi-square test).

Figure 5 presents scatter plots of rPSE distributions at 0.95 c/d and 2.95 c/d for the myopic shift and the stable groups. The myopic shift group showed a correlation at spatial frequencies of 0.95 c/d and 2.95 c/d, whereas the stable group did not (myopic shift group: r = 0.765, P < 0.0001; see Fig. 5A; and stable group: rs = 0.391, P = 0.065; see Fig. 5B), which potentially indicates differences in the impact mechanisms of different frequencies on the rPSE of the two groups.

Figure 5.

The distributions of rPSE at 0.95 c/d and 2.95 c/d spatial frequencies for patients in the myopic shift (A) and the stable groups (B). Each point represents one individual observer: purple points represent the SMILE group, green points represent the tPRK group, circles denote the stable group, and triangles denote the myopic shift group.

Relationship Between Binocular Imbalance and Patients’ Clinical Characteristics

We compared the clinical characteristics between the myopic shift and stable groups, and we found no significant difference (all > 0.05). To account for other factors affecting the SER difference of the unoperated eye, we used a multiple linear regression model, using logrBP (at 1 c/d or 6 c/d, respectively) as the independent variables, whereas examining the effects of sex, age, postoperative time, and changes in the sighting dominant eye (tested by the hole-in-the-card). A total of 16 patients experienced a change in sighting in the dominant eye postoperatively, with 7 in the myopic shift group and 9 in the stable group. Scatter plot analysis revealed a linear relationship between the SER difference of the unoperated eye and logrBP at both spatial frequencies (see Figs. 2B, 2C). The Durbin–Watson test confirmed data independence, and multicollinearity checks showed no issues. The model coefficient results indicated that the P values for sex, age, postoperative time, and changes in the sighting in the dominant eye were all greater than 0.05, suggesting that these variables did not have a significant impact in the model. Consequently, they do not interfere with the relationship between logrBP and the SER difference in the unoperated eye.

Discussion

To explore the relationship between binocular imbalance and myopic shift in unoperated eyes after unilateral refractive surgery, we used a binocular orientation combination task (spatial process) and a motion-in-depth Pulfrich task (temporal process) to assess binocular imbalance at various spatial frequencies in patients who had undergone unilateral SMILE or tPRK. We found that the myopic shift group had more negative logrBP and observed a positive correlation between SER differences of the unoperated eye and logrBP. This indicated that myopic shift is associated with stronger spatial sensory eye dominance postoperatively.

Our patients had myopia in the operated eye preoperatively, whereas the unoperated eye was emmetropic or did not meet the criteria for surgical treatment based on SER. Postoperatively, the unoperated eye had a more negative SER (−0.43 ± 0.58 D) than the operated eye (0.34 ± 0.37 D). Whereas Tao et al.^41,42 used a virtual reality (VR) platform to quantify binocular imbalance and demonstrated that the sensory dominant eye typically had a more negative SER, our study found no significant difference in SER between the dominant and the non-dominant eyes postoperatively (1 c/d: −0.12 ± 0.65 D in dominant eyes versus 0.02 ± 0.58 D in non-dominant eyes, P = 0.331; 6 c/d: −0.17 ± 0.65 D in dominant eyes versus 0.08 ± 0.57 D in non-dominant eyes, P = 0.09; paired t-tests), which excluded the possibility that stronger sensory eye dominance was due to a more negative refractive status. The discrepancies between our study and the findings of Tao et al. may be attributed to methodological differences. Tao et al. utilized a VR-based task, whereas we used a binocular orientation combination task to assess binocular imbalance at both low and high spatial frequencies. Furthermore, Tao et al. did not specify whether optical correction was applied during measurements, whereas our participants underwent refractive surgery and optical correction before the task. These differences in visual stimuli, task design, and correction methods may explain the observed variations.

Additionally, Zhou et al.²¹ found that binocular imbalance exists in patients with anisometropia, originating from neural factors, and that this imbalance can be improved with long-term optical corrections. Although patients in this study showed improvement in anisometropia after unilateral refractive surgery, the postoperative interocular SER difference varied between the two groups, as the myopic shift group included patients with myopic shift in the unoperated eye. To exclude the possibility that postoperative sensory eye dominance differences between the two groups were associated with interocular SER differences, we compared the postoperative interocular SER differences between the myopic shift and stable groups and found no significant difference (−0.73 ± 0.83 D in myopic group versus −0.82 ± 0.50 D in the stable group, P = 0.676; two-sided independent sample t-test).

The logrBP of the myopic shift group was more negative and variable at 6 c/d than at 1 c/d. This suggests that the unoperated eyes of the myopic shift group may exhibit stronger sensory eye dominance at higher spatial frequencies. Among those patients whose unoperated eye did not experience a myopic shift, no such significant differences were found across spatial frequencies after surgery. Previous studies have demonstrated that in patients with amblyopia, binocular imbalance is stronger at high spatial frequencies, even in cases that have been treated.^37,43–45 This may be due to lower monocular contrast sensitivity of amblyopic eyes at high spatial frequencies, differences in binocular contrast thresholds, and asymmetrical interocular suppression. However, Jiang et al.⁴⁶ found that binocular imbalance in patients with anisometropia, both before and after optical correction, correlated with differences in interocular visual acuity or SER. After correction, binocular imbalances at different spatial frequencies improved and were consistent with that of individuals with emmetropia, challenging the concept that one eye should retain more dominance after correction,²¹ and contradicting our results. The discrepancy may be explained by the more severe anisometropia (mean difference in binocular SER < −3.0 D) preoperatively in our patients.

Myopic eyes are more sensitive to lower spatial frequencies due to optical defocus, which affects the modulation transfer function of the eye. With increased myopia, the absolute sensitivity function and peak frequency of the eye shift toward lower frequencies.⁴⁷ Thus, patients in our study may have relied on the unoperated eye for high spatial frequency information before surgery. Postoperatively, we found that the spatial sensory dominant eye for high spatial frequency in the myopic shift group was predominantly the unoperated eye, whereas this trend was absent in the stable group (20/28 in the myopic shift group versus 6/23 in the stable group, P < 0.001; chi-square test). Refractive surgery can change the sensory eye dominance of individuals with anisomyopia.^22,23 Importantly, preoperative assessment of ocular dominance may be critical in patients with anisometropia, as demonstrated by Jiang et al.,⁴⁸ who showed that sensory dominance interacts with anisometropic magnitude to influence which eye becomes more myopic. Although we did not measure sensory eye dominance before surgery, we speculate that those patients with myopic shift in unoperated eyes had sensory eye dominance at high spatial frequencies that were less susceptible to surgical correction. Further prospective studies are needed to confirm this hypothesis.

In addition to spatial binocular imbalance, we used the spontaneous motion-in-depth Pulfrich phenomenon to observe temporal binocular imbalance. Burge et al.⁴⁹ found that the blurred differences between the two eyes after monocular presbyopia correction can create the Pulfrich phenomenon, that is, an illusion of movement. Gurman et al.^50,51 found that interocular delay in patients with amblyopia correlated strongly with differences in binocular visual acuity, with greater acuity differences associated with greater temporal asynchrony. Reynaud and Hess²⁶ found that reducing the contrast of Gabor patches by 60% in the fellow eye caused a shift in PSE of approximately 0.2 degrees, whereas Wu et al.³⁸ reported a mean |PSE| of 0.7 at 2.85 c/d for patients with amblyopia. However, our study found a much smaller |rPSE| than that reported by Wu et al. (as shown in the Supplementary Material). This may be due to the less severe binocular imbalance in our patients. Interestingly, when we divided patients into operated and unoperated eye groups based on their spatial sensory dominance at spatial frequencies of 1 c/d or 6 c/d, we found that the spatial sensory dominant eye processed visual information faster (as shown in the Supplementary Material). This faster processing is typically associated with brighter retinal luminance, higher contrast, more blurred images, and lower spatial frequency.^{49,50,52–54} However, interocular delay cannot be solely attributed to differences in luminance or contrast, as the visual neural system involves a complex mechanism that balances spatial and temporal perception.

This study has some limitations. As a cross-sectional study, it could not establish causal relationships between preoperative and postoperative binocular imbalance and changes in the SER of the unoperated eye. Additionally, future research should quantify the relationship between clinical ocular parameters and binocular imbalance, particularly in cases with severe myopia progression.

In conclusion, our study showed that the postoperative myopic shift in the unoperated eyes of patients who underwent unilateral refractive surgery is associated with binocular imbalance. The greater myopic shift observed may be due to stronger sensory eye dominance in unoperated eyes. Further research is needed to explore the underlying mechanisms and to evaluate how this binocular imbalance impacts postoperative visual recovery. Additionally, exploring the potential of binocular psychophysical training programs to prevent myopia progression in the unoperated eye would be an intriguing research direction.