Binocular Vision and Fixational Eye Movements

Journal of Vision (2019) 19(4):9, 1–15 1

Binocular vision and fixational eye movements School of Optometry and Vision Science, University of Waterloo, Waterloo, Canada Rajkumar Nallour Present address: Envision Research Institute, Raveendran Wichita, KS, USA ([email protected]) $

School of Optometry and Vision Science, William R. Bobier University of Waterloo, Waterloo, Canada $

School of Optometry and Vision Science, Benjamin Thompson University of Waterloo, Waterloo, Canada $

The aim of this study was to assess the relationship stationary target (Helmholtz, 1925; Leigh & Zee, 1999; between binocular vision and fixation stability (FS). Martinez-Conde, Macknik, & Hubel, 2004; Steinman, Across three experiments, we investigated (a) whether Cushman, & Martins, 1982). The amplitude of these fixation was more stable during binocular versus fixational eye movements determines fixation stability monocular viewing across a range of stimulus contrasts (FS)—a measure of ocular motion extent that occurs in normal observers (n ¼ 11), (b) whether binocular during fixation. FS is often quantified using bivariate rivalry affected FS in normal observers (n ¼ 14), and (c) contour ellipse area (BCEA), larger BCEA values whether FS was affected by interocular contrast indicate less stable fixation. Normal fixational eye differences in normal observers (n ¼ 8) and patients with movements may benefit spatial vision (Kagan, 2012; anisometropic amblyopia (n ¼ 5). FS was quantified using Martinez-Conde, 2006; Martinez-Conde et al., 2004; global bivariate contour ellipse area, and microsaccades Rucci & Desbordes, 2003; Rucci & Poletti, 2015); were detected using an unsupervised cluster-detection however, abnormally large fixational eye movements method. In normal observers, binocular viewing showed can impair vision by moving the point of fixation away more stable fixation at all stimulus contrasts, and from the fovea and smearing the retinal image (Chung, binocular rivalry did not affect FS. When interocular contrast was manipulated under dichoptic viewing Kumar, Li, & Levi, 2015; Chung & Bedell, 1995; conditions, normal observers exhibited less stable Simmers, Gray, & Winn, 1999). fixation for an eye that viewed 0% contrast (no fixation A number of studies have observed less stable target). In anisometropic amblyopia, fixation was less fixation (increased amplitude of fixational eye move- stable in both eyes when the fellow eye viewed at 0% ments) in amblyopic eyes (Chung et al., 2015; contrast. No effects were observed at other interocular Ciuffreda, Kenyon, & Stark, 1991; Gonza´lez, Wong, contrast differences. Overall, binocular FS was impaired Niechwiej-Szwedo, Tarita-Nistor, & Steinbach, 2012; in both eyes in anisometropic amblyopia compared to Raveendran, Babu, Hess, & Bobier, 2014; Schor & normal observers. We conclude that binocular vision Hallmark, 1978; Shaikh, Otero-Millan, Kumar, & influences FS in normal observers but in an all-or-nothing Ghasia, 2016; Shi et al., 2012; Srebro, 1983; Subrama- fashion, whereby the presence or absence of a binocular nian, Jost, & Birch, 2013). Less stable fixation is target is important rather than the relative contrast of associated with a greater interocular acuity difference the targets in each eye. In anisometropic amblyopia, the (Gonza´lez et al., 2012) and poorer stereoacuity (Sub- fellow eye appears to control FS of both eyes under ramanian et al., 2013) in amblyopia, indicating a dichoptic viewing conditions. possible association between impaired binocular vision and reduced FS. Supporting such an association is the finding that transient FS improvements in strabismic amblyopia occurred when fixation targets were bifo- Introduction veally aligned and interocular contrast was manipulated to overcome amblyopic eye suppression and Small, involuntary eye movements (microsaccades, promote binocular combination (Raveendran et al., drifts, and tremors) occur during fixation of a 2014).

Citation: Raveendran, R. N., Bobier, W. R., & Thompson, B. (2019). Binocular vision and fixational eye movements. Journal of Vision, 19(4):9, 1–15, https://doi.org/10.1167/19.4.9.

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Three studies have reported generally more stable observers due to disrupted binocular combination fixational eye movements under binocular compared to (Pardhan & Gilchrist, 1990, 1992). monocular viewing conditions. The first involved two rhesus monkeys (Motter & Poggio, 1984), and the other two involved humans (Gonza´lez et al., 2012; Kraus- kopf, Cornsweet, & Riggs, 1960). Conceptually, General methods binocular viewing might improve FS in observers with normal vision by increasing resolution of the fixation Apparatus target through binocular summation (Campbell & Green, 1965). However, fixation targets are typically Fixational eye movements were measured under clearly visible, high-contrast images displayed for a nondichoptic and dichoptic viewing conditions. During prolonged period of time. These factors may minimize nondichoptic viewing, participants viewed a gamma binocular summation effects (Bearse & Freeman, 1994). corrected 7-in. LCD monitor (Lilliput, CA, http:// Alternatively, engagement of vergence and fusion lilliputweb.net/non-touch-screen-monitors/7-inch- mechanisms during binocular viewing may improve FS monitors/619gl-70np-c.html) at 40 cm. Dichoptic by activating interocular feedback mechanisms within viewing was achieved using a haploscope constructed oculomotor control pathways (Otero-Millan, Macknik, from two cold mirrors that transmitted infrared light & Martinez-Conde, 2014; Schor, 1979). In strabismic (Edmund Optics, NJ, http://www.edmundoptics.com/ amblyopia, reduced amblyopic eye FS appears to be optics/optical-mirrors/specialty-mirrors/cold-mirrors/ associated with a loss of foveation and suppression 1900) placed 15 cm from the eyes with the head steadied (Raveendran et al., 2014), both of which degrade fusion using a chin rest. Two 7-in. gamma-corrected, lumi- and vergence. nance-matched LCD monitors (one visible to each eye) The overall aim of this study was to further were placed laterally along each arm of the haploscope investigate the interaction between binocular vision and and set 25 cm from the mirrors, creating accommoda- FS. In Experiment 1, we measured FS for each eye of tive and vergence demands of 2.5D and 2.5MA, normal observers during monocular and binocular respectively, which were the same as the nondichoptic viewing of fixation stimuli presented within a contrast viewing condition. Therefore, in both nondichoptic and range of 100% to 0%. If improved perception of the dichoptic viewing conditions, the planes of accommo- fixation target due to binocular summation is respon- dation and vergence were fixed at 40 cm. Stimuli were sible for improved FS during binocular viewing, we presented using a MacBook, (Apple, Inc., Cupertino, would expect a greater advantage of binocular viewing CA) connected to a multidisplay adapter (Dual- at lower contrasts where binocular summation for Head2Go, Matrox Graphics Inc., Quebec, Canada, suprathreshold stimuli is more pronounced (Bearse & http://www.matrox.com/graphics/en/products/gxm/ Freeman, 1994). Alternatively, if improved FS under dh2go/analog). binocular viewing is due to the activation of vergence An infrared eye tracker (EyeLink-II, 500 Hz, SR and fusion mechanisms, binocular viewing should Research, Osgoode, Canada, http://www.sr-research. provide a relatively constant improvement in FS com/EL_II.html) was used to record fixational eye relative to monocular viewing across the whole contrast movements. Nine-point calibrations were completed range. for each eye separately. In Experiment 2, we tested whether interocular suppression affected FS in normal observers using binocular rivalry, which causes alternating suppression Participants of each eye. Binocular rivalry has been found to increase the rate of microsaccades (Sabrin & Kertesz, All participants provided informed, written consent, 1980, 1983), which may make fixation less stable. and the study was approved by the office of research Reduced FS during binocular rivalry would indicate ethics, University of Waterloo. All the procedures that interocular suppression can influence FS. involved adhered to the Declaration of Helsinki. Normal Finally, in Experiment 3, we measured FS across a observers had best corrected visual acuity of 20/20 or range of interocular contrast differences in normal better in each eye, stereoacuity of ,60 s of arc (Randot observers and participants with anisometropic ambly- test) and no strabismus. Eye dominance was assessed opia. We hypothesized that interocular contrasts using the Porta sighting test. Participants with anisome- favoring the amblyopic eye would improve amblyopic tropic amblyopia had an interocular VA difference of at eye FS by reducing interocular suppression as has been least 2 logMAR lines and a fellow eye visual acuity reported for strabismic amblyopia (Raveendran et al., 0.02 logMAR. Anisometropia was defined as an 2014). Conversely, we hypothesized that large inter- interocular refractive error difference of 1.50 diopter ocular contrast differences would impair FS in normal spherical equivalent.

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Figure 1. Visual stimuli used in Experiment 1: monocular versus binocular fixation.

Data analysis were occluded with an opaque, tight-fitting eyepatch. Each combination of stimulus contrast and viewing FS was quantified using global BCEA (Gonza´lez et condition was measured four times for each participant al., 2012; Steinman et al., 1982; Timberlake et al., 2005) in a random order. Each trial lasted 30 s. defined using the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 BCEA ¼ pv rxry ðÞ1 q ; Results where v2 is the chi-square value (two degrees of Data were analyzed using repeated-measures AN- freedom) corresponding to a probability value of 0.682 OVA with factors of viewing condition (monocular vs. (i.e., 61 SD); rx, ry correspond to standard deviations of horizontal and vertical eye positions, respectively; binocular viewing), eye (DE vs. NDE), and contrast and q corresponds to the Pearson correlation coeffi- (seven levels). BCEA values were significantly smaller cient between horizontal and vertical eye positions. (more stable fixation) for binocular than monocular BCEA provides the area of the ellipse that encompasses viewing: main effect of viewing condition, F(1, 20) ¼ 68% of eye positions within a trial. Therefore, larger 7.97, p ¼ 0.02 (Figure 2). The advantage of binocular BCEA values indicate less stable fixation. viewing was consistent across all stimulus contrasts: no In addition, microsaccades were detected using an viewing condition by contrast interaction, F(1, 20) ¼ unsupervised cluster-detection method (Otero-Millan, 1.21, p ¼ 0.42. However, there was a significant main Castro, Macknik, & Martinez-Conde, 2014). BCEA effect of contrast, F(6, 120) ¼ 13.90, p , 0.001, that was and microsaccadic amplitude data were log trans- due to larger BCEA values at 0% contrast compared to formed and analyzed using ANOVA. Tukey and t tests all other contrast levels (Tukey honestly significant were used to explore simple effects. All statistical tests difference [HSD], p , 0.001). The nonzero contrast had a critical alpha value of 0.05. levels did not vary significantly from one another (p , 0.05). There was no main effect of eye, F(1, 20) ¼ 0.02, p ¼ 0.89, and no other two-way interactions. There was a Experiment 1 three-way interaction, F(1, 20) ¼ 2.27, p ¼ 0.04, possibly due to higher variability in the NDE data as post hoc testing did not reveal the source of this interaction. Methods Unlike BCEA, microsaccadic amplitudes did not differ between monocular and binocular viewing Eleven normal observers (27 6 4 years) participated (Figure 2), F(1, 20) ¼ 0.2, p ¼ 0.67. However, in in this experiment. The fixation stimulus (Figure 1) agreement with the BCEA data, there was a significant consisted of an 8.18 frame and a 18 fixation cross main effect of contrast (Figure 2), F(6, 90) ¼ 9.128, p , presented on a gray background (50 cd/m2). The 0.001, that was due to larger amplitudes for the 0% contrast of the cross and the light portions of the frame contrast condition than all other conditions except the were presented at 0%,5%,10%,20%,40%,80%, and 100% Weber contrast levels in relation to the gray, 5% condition (Tukey HSD, p , 0.01). This implies that mean luminance background. The dark portions of the the absence of a central fixation target significantly target maintained a constant high contrast in all trials increased microsaccadic amplitudes. There was no and provided a peripheral fusion lock that was essential main effect of eye, F(1, 20) ¼ 0.03, p ¼ 0.87, and no for dichoptic stimulus presentation. interactions. Microsaccadic frequency did not vary There were three viewing conditions: (a) monocular significantly between binocular and monocular viewing fixation with the dominant eye (DE), (b) monocular conditions for both DE, F(1, 9) ¼ 0.002, p ¼ 0.962, and fixation with the nondominant eye (NDE), and (c) NDE, F(1, 9) ¼ 0.001, p ¼ 0.98 (refer to Supplementary binocular (nondichoptic) viewing. Nonviewing eyes Table S1 for mean and SEM).

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Figure 2. Monocular versus binocular fixation stability and microsaccadic amplitude. The mean FS of the DE (A) and NDE (B) during monocular (open symbols) and binocular (filled symbols) fixation. Error bars represent 61 SE. Fixation was more stable during binocular relative to monocular fixation across all contrast levels. At 0% contrast, fixation was less stable for both monocular and binocular fixation. The mean microsaccadic amplitude of the DE (C) and NDE (D) did not differ between monocular (open symbols) and binocular (filled symbols) fixation. However, microsaccadic amplitude was significantly increased while viewing 0% contrast.

Summary of Experiment 1 were presented so that the right-eye stimulus varied across conditions, whereas the left-eye stimulus re- Fixation was more stable during binocular fixation mained constant (Figure 3). Therefore, any change in compared to monocular fixation across all stimulus left-eye FS could only be due to changing binocular contrast levels (0% to 100%). However, microsaccadic interactions. We did not vary presentation conditions amplitude did not differ between monocular and binoc- according to eye dominance because Experiment 1 did ular fixation. Fixation became less stable, and micro- not identify any consistent effects of eye dominance on saccadic amplitude increased at 0% stimulus contrast. FS in observers with normal binocular vision. During binocular rivalry, participants indicated horizontal grating, vertical grating, or piecemeal percepts using buttons on a gamepad (Sidewinder, Microsoft). A Experiment 2: Effect of binocular baseline nondichoptic fusion condition was also pre- rivalry on fixation stability sented whereby a single sinusoidal grating was viewed binocularly without the haploscope. Grating orientation was changed every 4 s in the dichoptic fusion, Methods monocular stimulation, and nondichoptic fusion conditions. Conditions were presented in a random order. Fifteen normal observers (28 6 5 years) participated. Each condition lasted 40 s and was repeated six times. Visual stimuli were 100% contrast, circular sinusoidal gratings (3.68 diameter, 1.1 c/8) with a central 0.58 fixation target (Figure 3). Three dichoptic conditions Results were presented: (a) dichoptic fusion, identically oriented gratings presented dichoptically to both eyes; (b) Data were analyzed using repeated-measures AN- binocular rivalry, orthogonal dichoptic gratings; and (c) OVA. Left eye (the eye with a constant stimulus; Figure monocular stimulation, a sinusoidal grating was pre- 4) BCEA varied significantly across the four experi- sented to the left eye and a mean luminance (gray) mental conditions, F(3, 33) ¼ 8.34, p , 0.001). blank screen was presented to the right eye. The stimuli However, this main effect was due to significantly

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comparison of microsaccadic amplitude between the two eyes revealed no main effect of eye, F(1, 11) ¼ 0.6, p ¼ 0.81, and no condition by eye interaction, F(3, 33) ¼ 0.56, p ¼ 0.65, indicating no differences between the eyes for any condition. A comparison of microsaccadic frequency showed a signiﬁcant main effect of viewing condition, F(3, 33) ¼ 16.76, p , 0.001, and Tukey HSD revealed a signiﬁcant lower frequency in the nondichoptic viewing condition compared to all three dichoptic viewing conditions (p , 0.001; refer to Supplementary Table S1 for mean and SEM).

Summary of Experiment 2

Interocular suppression during binocular rivalry in observers with normal binocular vision did not inﬂuence FS. However, during the monocular stimulation condition, the eye viewing a blank screen had less stable ﬁxation than the eye viewing a target.

Experiment 3

Methods Figure 3. Visual stimuli used in Experiment 2. Eight normal observers and five participants with anisometropic amblyopia (Table 1) were recruited. smaller BCEA values in the nondichoptic fusion Amblyopia was defined as an interocular visual acuity condition compared to all other conditions (rivalry vs. difference ofatleast2 logMAR linesand afellow fixing eye nondichoptic fusion, p ¼ 0.02; dichoptic vs. non- visual acuity 0.02 logMAR. Anisometropia was defined dichoptic fusion, p , 0.001). The rivalry, monocular, as an interocular refractive error difference of 1.50DS and dichoptic fusion conditions did not differ signifi- (Gao et al., 2018; Guo et al., 2016). The visual stimuli were cantly (p . 0.05; Figure 4) indicating that interocular the same as in Experiment 1 and were presented suppression during binocular rivalry did not increase dichoptically. Contrast to one eye was fixed at 100%,and BCEA. An exploratory analysis revealed that BCEA contrast to the other eye was presented at 0%,5%,10%, values did not differ between periods of suppression 20%,40%,80%,and100%. Normal observers completed and dominance during rivalry for a particular eye. The two viewing conditions (Figure 6): (a) DE contrast was mean BCEA values of the left eye during periods of varied and NDE contrast was fixed and (b) NDE contrast suppression and dominance were 0.17 6 0.11 deg2 and 2 was varied and DE contrast was fixed. In the amblyopia 0.18 6 0.12 deg (p ¼ 0.62) and the mean BCEA values group, fellow eye contrast was varied, and amblyopic eye of the right eye during periods of suppression and contrast was fixed at 100%. In addition, a condition with 2 2 dominance were 0.17 6 0.13 deg and 0.17 6 0.12 deg amblyopic eye contrast at 0% and fellow eye contrast at (p ¼ 0.95). Intriguingly, fixation was less stable during 100% was also presented. Stimulus presentation order was periods of piecemeal compared to the periods of randomized, and each eye and contrast combination was suppression/dominance (Figure 5). presented four times. Each trial lasted 30 s. A comparison of BCEA for the left and right eyes The rationale behind recruiting only the observers revealed a significant interaction between condition and with anisometropic amblyopia was as follows. A eye, F(3, 39) ¼ 5.16, p ¼ 0.004. Post hoc pairwise previous study (Raveendran et al., 2014) showed that a analyses (Tukey HSD) revealed a significant difference lack of foveal fixation was the major factor for in BCEA between the two eyes for the monocular impaired fixation stability in the amblyopic eye of viewing condition during which the right eyes (viewing individuals with strabismic amblyopia during dichoptic a blank screen) had larger BCEAs than the left eyes viewing. In this study, our aim was to understand the (viewing a high-contrast grating), p ¼ 0.03. The eyes did role of interocular suppression on fixation stability, not not differ for any other condition. Unlike BCEA, a a lack of foveal fixation.

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Figure 4. The effect of different binocular viewing conditions on FS and microsaccadic amplitude. FS (A) and average microsaccadic amplitude (B) of the left eye (blue) and the right eye (orange) are shown for nondichoptic fusion, dichoptic fusion, binocular rivalry, and monocular stimulation (MS). Error bars represent 61 SE. The asterisk symbol represents statistical significance (p , 0.05). Results repeated measures ANOVAs. The ANOVA models had factors of contrast (seven levels of interocular In normal observers (Figure 7), BCEA values for the contrast difference) and eye (DE vs. NDE) and two viewing conditions (NDE fixed/DE varied and revealed significant contrast by eye interactions for NDE varied/DE fixed) were analyzed separately using both the NDE fixed/DE varied, F(6, 42) ¼ 3.78, p ¼

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Figure 5. Fixation stability during binocular rivalry. Mean fixation stability values of the left and right eyes during periods of dominance (horizontal stripes), suppression (blank), and piecemeal (checkered). Error bars represent 61 SE.

0.004, and NDE varied/DE fixed, F(6, 42) ¼ 3.298, p ¼ 36) ¼ 0.59, p ¼ 0.735, and NDE varied/DE fixed, F(6, 0.009, conditions. In both cases, this was due to a 36) ¼ 0.57, p ¼ 0.752 (refer to Supplementary Table S1 significant difference in BCEA between the eyes for the for mean and SEM). 0% contrast condition, whereby the eye viewing 0% contrast exhibited greater BCEA than the eye viewing 100% contrast (p , 0.001). The two eyes did not differ Observers with amblyopia in BCEA for any other interocular contrast ratios. Normal observer microsaccadic amplitudes exhibit- In the amblyopia group (Figure 8), BCEA exhib- ed a similar pattern of results (Figure 7). The ited no interaction between contrast and eye F(6, 24) interaction between condition and eye was not signif- ¼ 0.570, p ¼ 0.750, and there was no main effect of icant for the NDE fixed/DE varied condition, F(6, 36) ¼ eye, F(1, 4) ¼ 0.230, p ¼ 0.656, indicating that the eyes 1.52, p ¼ 0.20, but was significant for the NDE varied/ did not differ in BCEA for any condition. There was DE fixed condition, F(6, 36) ¼ 2.40, p ¼ 0.047, where amaineffectofcontrast,F(6, 24) ¼ 2.843, p ¼ 0.031, there was a significant difference between eyes for the whereby BCEA for both eyes was larger for the 0% 0% contrast condition only (p ¼ 0.01). However, there contrast to the fellow eye/100% contrast to the was no significant effect on microsaccadic frequency for amblyopic eye condition than any of the other both viewing conditions: NDE fixed/DE varied, F(6, conditions (p , 0.01). This suggested that BCEA was

Participant Sensory status VA VA Age Gender Refractive error (distance) (near) W4DT Stereoacuity

S1 38 F AME: þ3.50/1.00DC 3 135 AME: 0.5 AME: 0.66 D: Fusion 60 FFE: plano FFE: 0.3 FFE: 0.1 N: Fusion S2 48 F AME: þ4.00 AME: 0.46 AME: 0.46 D: Fusion 400 FFE: plano FFE: 0.10 FFE: 0.04 N: Fusion S3 26 M AME: þ3.50 AME: 0.3 AME: 0.4 D: Fusion .800 FFE: plano FFE: 0.0 FFE: 0.0 N: Fusion S4 42 M AME: þ1.50/3.25 3 170 AME: 0.4 AME: 0.9 D: Suppression .800 FFE: plano FFE: 0.02 FFE: 0.0 N: Diplopia S5 47 M FFE: plano FFE: 0.0 FFE: 0.0 D: Fusion 200 AME: þ1.75DS AME: 0.3 AME: 0.4 N: Fusion

Table 1. Clinical details of observers with anisometropic amblyopia. Notes: AME ¼ amblyopic eye; FFE ¼ fellow fixing eye. Worth four dot test (W4DT) was measured at 6 m (distance ¼ D) and 40 cm (near ¼ N). Stereoacuity was measured using preschool Randot stereo test, and the values are in arcseconds.

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determined entirely by the stimulus presented to the fellow eye. To test this theory, BCEA values for two conditions (amblyopic eye contrast at 100% and fellow eye contrast at 0% vs. amblyopic eye contrast at 100% and fellow eye contrast at 0%)were compared (Figure 9). Amblyopic-eye BCEA was smaller (more stable ﬁxation) when the amblyopic eye viewed 0% contrast and the fellow eye viewed 100% contrast than the opposite conﬁguration (p ¼ 0.03). This supports the theory that amblyopic eye BCEA is determined by the fellow eye under binocular viewing conditions. Microsaccadic amplitudes (Figure 8) also did not exhibit an interaction between contrast and eye, F(6, 24) ¼ 0.885, p ¼ 0.521, or a main effect of eye, F(1, 4) ¼ 2.137, p ¼ 0.218. There was a main effect of contrast, F(6, 24) ¼ 2.843, p ¼ 0.031, but this appeared to be due to variability across the contrast range rather than larger amplitudes only for the 0% contrast condition. Moreover, there was no effect of contrast on microsaccadic frequency, F(6, 24) ¼ 0.858, p ¼ 0.54 (refer to Supplementary Table S1 for mean and SEM).

Controls versus anisometropic amblyopia

The NDE fixed/DE varied data for normal observers were compared to the amblyopia data (amblyopic eye fixed/fellow eye varied) to assess whether FS differed between the two groups (Figure 9) using an ANOVA with factors of group (controls vs. amblyopia), eye (fellow eye/DE vs. amblyopic eye/NDE) and contrast level (seven levels). As shown in Figure 10, BCEA was larger in both eyes of the amblyopia group compared to normal observers, F(1, 11) ¼ 7.86, p ¼ 0.01. The difference between the two eyes varied significantly with contrast: significant contrast by eye interaction, Figure 6. Visual stimuli for Experiment 3. F(6, 66) ¼ 3.18, p ¼ 0.008. Specifically, there was a greater BCEA difference between the control and amblyopia groups at low contrasts. Similar trends were present for microsaccadic amplitudes although only a situation, the eye viewing 0% contrast had less stable significant main effect of contrast, F(6, 60) ¼ 6.38, p , fixation in agreement with the results of Experiment 2. 0.001, was revealed by the mixed ANOVA model (see In the amblyopia group, amblyopic eye FS appeared to Supplementary Table S2 for full statistical details). be consensually controlled by the fellow eye. Fixation However, there was no significant main effect of was less stable in both eyes of participants with contrast, F(6, 60) ¼ 1.24, p ¼ 0.29; main effect of group amblyopia compared to controls. (controls vs. amblyopia), F(1, 10) ¼ 1.88, p ¼ 0.20; and no significant interaction (contrast by group), F(6, 60) ¼ 0.513, p ¼ 0.79, for microsaccadic frequency. Discussion

Summary of Experiment 3 The objective of this study was to investigate the role of binocular vision in FS. To meet this objective, we FS was not affected by interocular contrast in conducted three experiments investigating the impact control participants except when one eye viewed 100% of binocular versus monocular viewing, binocular contrast and the other viewed 0% contrast. In this rivalry, and interocular contrast difference on FS.

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Figure 7. FS and microsaccadic amplitude of control participants. FS (A) and average microsaccadic amplitude (C) of the DE (green data points) and the NDE (red) of normal observers when the contrast was varied to the DE. The data points and error bars represent mean 6 SEM. FS was significantly different between two eyes only when one eye was presented with 0% contrast (empty green data point) and the other eye with 100% contrast. Microsaccadic amplitude did not differ between the eyes across dichoptic conditions. The results were similar when NDE contrast was varied (B and D).

Monocular versus binocular fixation dran et al., 2014) and increased microsaccadic amplitude (McCamy, Najafian Jazi, Otero-Millan, Macknik, In agreement with previous studies (Gonza´lez et al., & Martinez-Conde, 2013) in the absence of a fixation 2012; Motter & Poggio, 1984), we observed signifi- target. cantly more stable fixation under binocular compared Overall, our results demonstrate a clear binocular to monocular viewing conditions. We extended this advantage for FS that is independent of fixation previous work to show that the binocular FS advantage stimulus contrast and also occurs in the absence of a occurs across a wide range of fixation stimulus fixation stimulus. Given this pattern of results, the contrasts. The improved FS under binocular viewing engagement of vergence mechanisms may account for conditions was not due to changes in microsaccadic improved FS under binocular viewing (peripheral amplitude, as has previously been reported (Gonza´lez fusion locks were present for all stimulus contrasts). et al., 2012; Nallour Raveendran, 2013; Schulz, 1984). Notably, the binocular FS advantage was present for the 0% contrast condition, i.e., the absence of a central Binocular interaction and FS fixation target, indicating that the binocular summation of contrast for the central fixation target is not Less stable fixation correlates with interocular acuity responsible for improving FS. difference (Gonza´lez et al., 2012) and stereoacuity Apart from the 0% contrast condition, in which (Subramanian et al., 2013) in amblyopia, indicating a fixation became less stable for both binocular and possible association between impaired binocular vision monocular viewing conditions, we found no effect of and impaired FS. Therefore, we hypothesized that fixation target contrast on FS. This is consistent with a manipulations of binocular interaction would affect previous study that varied fixation stimulus contrast FS. We tested this hypothesis in Experiments 2 and 3. (Ukwade & Bedell, 1993). Our results are also in In Experiment 2, we tested whether interocular agreement with reports of reduced FS (Cherici, Kuang, suppression affected FS in normal observers. Suppres- Poletti, & Rucci, 2012; Gonza´lez et al., 2012; Raveen- sion was induced using binocular rivalry. However,

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Figure 8. Fixation stability (A) and microsaccadic amplitude (B) in observers with amblyopia when contrast presented to the fellow eye was varied. FS of both eyes was significantly reduced when the fellow eye was presented with 0% contrast. Otherwise there was no effect of interocular contrast on fixational eye movements for either eye. FFE ¼ fellow eye; AME ¼ amblyopic eye.

rivalry had no effect on FS or microsaccadic amplitude. in binocular rivalry. In Experiment 3, therefore, the Interestingly, ﬁxation was less stable during the periods effect of binocular interaction on FS was tested by of piecemeal rivalry compared to the periods of varying interocular contrast in observers with normal dominance/suppression. This effect requires further binocular vision and those with amblyopia. In ambly- exploration in future work. It could be argued that the opia, reducing the contrast of stimuli presented to the duration of binocular rivalry was too short to affect FS. fellow eye can balance monocular inputs from the Also, the chronic suppression that occurs in amblyopia amblyopic eye and the fellow eye and overcome is different from the alternating suppression that occurs suppression of amblyopic eye signals (Baker, Meese, &

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Figure 9. Effect of the presence of a target on fixation stability in observers with anisometropic amblyopia. Red bars indicate amblyopic eye (AME) data and green bars, fellow eye (FFE) data. Filled bars indicate that the eye was viewing a 100% contrast stimulus and open bars a 0% contrast stimulus. When one eye was presented with 100% contrast, the other was presented with 0% contrast. These data indicate that when the fellow eye viewed the 100% contrast fixation target, the amblyopic eye, which viewed 0% contrast, showed significantly more stable fixation than when the amblyopic eye viewed the 100% contrast fixation target and the fellow eye viewed 0% contrast.

Hess, 2008; Hess et al., 2012; Mansouri, Thompson, & was normal in all other previous studies of FS in Hess, 2008; Raveendran et al., 2014). Therefore, we amblyopia (Chung et al., 2015; Gonza´lez et al., 2012; expected amblyopic eye FS to be improved when Raveendran et al., 2014; Shi et al., 2012; Subramanian suppression was reduced by interocular contrast et al., 2013). The majority of these previous studies balancing and the FS of controls to be impaired by used monocular viewing conditions, whereas the unequal interocular contrast. Contrary to our expec- current study used dichoptic stimulus presentation. It is tations, interocular contrast difference had no effect on possible that providing a stimulus to the amblyopic eye FS or microsaccadic amplitude for either group with had a detrimental effect on fellow eye FS. Raveendran the exception of the 0% contrast conditions. For et al. (2014) also used dichoptic presentation but only controls, this result demonstrates that simply having a recruited participants with strabismic amblyopia. Dif- visible target in each eye is sufficient for optimal FS in ferences between anisometropic and strabismic ambly- agreement with Experiment 2. For the amblyopia opia may account for the differences between the fellow group, FS and microsaccadic amplitude appears to be eye results in the current study and those of Raveen- controlled by the fellow eye across the whole inter- dran et al. Additional studies are required to test these ocular contrast range with the amblyopic eye exhibiting possibilities. consensual responses. This conclusion is supported by In addition to showing that FS is not affected by the the observation that amblyopic eye fixation is more manipulation of binocular interactions, our data also stable when the fellow eye views the target and the demonstrate that FS can be independent between the amblyopic eye views a 0% contrast stimulus than vice two eyes. For control observers in Experiments 2 and 3, versa (Figure 8). The consensual responses of the having a visible fixation stimulus to one eye and a blank amblyopic eye appears to supersede any effect of or 0% stimulus contrast to the other eye led to more reduced interocular suppression on amblyopic eye FS. stable fixation in the eye with the visible stimulus Our observation that FS was impaired in both eyes although microsaccadic amplitude did not differ of the amblyopia group compared to controls is between the eyes. This interocular difference in FS consistent with one previous study (Shaikh et al., 2016) cannot be accounted for by vergence mechanisms in which the fellow eyes of the amblyopia group were because both eyes saw fusion locks. The finding that FS found to have abnormal FS. However fellow eye FS has eye-specific components is consistent with neuro-

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Figure 10. Comparison between controls and participants with anisometropic amblyopia. Comparison of FS (A) and microsaccadic amplitude (B) between the AME (red), FFE (green), DE of controls (black circle), and NDE of controls (black triangle). The FS of the AME and FFE was significantly less stable than controls. The statistical values are tabulated in Supplementary Table S2.

physiological studies demonstrating eye-specific oculo- movements that are independent between the two eyes motor control (Cullen & Van Horn, 2011; Van Horn, (Krauskopf et al., 1960). Waitzman, & Cullen, 2013). In addition, the lack of an effect on microsaccadic amplitude supports the theory that microsaccades are binocular and conjugate (Mar- tinez-Conde et al., 2004; Martinez-Conde, Macknik, Summary and conclusion Troncoso, & Hubel, 2009; Rolfs, 2009). Therefore, it is likely that ocular drifts account for this pattern of In controls, our results suggest that two components results as they are a component for fixational eye contribute to FS: a general benefit of binocular versus

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monocular viewing that may be linked to oculomotor (2015). Characteristics of fixational eye movements mechanisms engaged by binocular viewing and eye- in amblyopia: Limitations on fixation stability and specific mechanisms that require visual feedback. In acuity? Vision Research, 114, 87–99, https://doi.org/ amblyopia, amblyopic eye FS appears to be consensual 10.1016/j.visres.2015.01.016. to the fellow eye, suggesting that eye-specific mecha- Chung, S. T., & Bedell, H. E. (1995). Effect of retinal nisms are superseded by more accurate information image motion on visual acuity and contour from the fellow eye. Moreover, FS was worse in both interaction in congenital nystagmus. Vision Re- eyes for observers with anisometropic amblyopia. This search 35 could be due to noisy monocular signals and/or a , (21), 3071–3082, https://doi.org/10.1016/ disruption of oculomotor mechanisms (e.g., abnormal 0042-6989(95)00090-M. inputs to superior colliculus; Shi et al., 2012). Ciuffreda, K. J., Kenyon, R. V., & Stark, L. (1991). Fixational eye movements in amblyopia and Keywords: fixational eye movements, binocular strabismus. Journal of the American Optometric interaction, microsaccades, BCEA, binocular rivalry, Association, 50(11), 1251–1258. fixation stability, amblyopia Cullen, K. E., & Van Horn, M. R. (2011). The neural control of fast vs. slow vergence eye movements. The European Journal of Neuroscience, 33(11), Acknowledgments 2147–2154, https://doi.org/10.1111/j.1460-9568. 2011.07692.x. This research was funded by Natural Sciences and Gao, T. Y., Guo, C. X., Babu, R. J., Black, J. M., Engineering Research Council of Canada (NSERC) to Bobier, W. R., Chakraborty, A., ... Thompson, B. WRB and BT, and Canadian Optometric Education (2018). Effectiveness of a binocular video game vs Trust Fund (COETF) to RNR. The authors would like placebo video game for improving visual functions to thank Dr. Raiju Babu, Dr. Amy Chow, and Dr. in older children, teenagers, and adults with Vivek Labhishetty for their support and assistance. amblyopia. JAMA Ophthalmology, 136(2), 172– Commercial relationships: none. 181. https://doi.org/10.1001/jamaophthalmol.2017. Corresponding author: Rajkumar Nallour 6090. Raveendran. Gonza´lez, E. G., Wong, A. M. F., Niechwiej-Szwedo, Email: [email protected]. E., Tarita-Nistor, L., & Steinbach, M. J. (2012). Address: School of Optometry and Vision Science, Eye position stability in amblyopia and in normal University of Waterloo, Waterloo, Canada. binocular vision. Investigative Ophthalmology & Visual Science, 53(9), 5386–5394. Guo, C. X., Babu, R. J., Black, J. M., Bobier, W. R., References Lam, C. S. Y., Dai, S., ... Thomspon, B. (2016). Binocular treatment of amblyopia using video- games (BRAVO): Study protocol for a randomised Baker, D. H., Meese, T. S., & Hess, R. F. (2008). controlled trial. Trials, 17(504), 1–9, https://doi. Contrast masking in strabismic amblyopia: Atten- uation, noise, interocular suppression and binocu- org/10.1186/s13063-016-1635-3. lar summation. Vision Research, 48(15), 1625–1640. Helmholtz, H. (1925). Treatise on physiological optics. Bearse, M. A., Jr., & Freeman, R. D. (1994). Binocular I. New York, NY: Dover Publications, Inc. summation in orientation discrimination depends Hess, R. F., Thompson, B., Black, J. M., Machara, G., on stimulus contrast and duration. Vision Research, Zhang, P., Bobier, W. R., & Cooperstock, J. (2012). 34(1), 19–29, https://doi.org/10.1016/0042- An iPod treatment of amblyopia: An updated 6989(94)90253-4. binocular approach. Optometry, 83(2), 87–94. Campbell, F. W., & Green, D. G. (1965, October 9). Retrieved from http://www.ncbi.nlm.nih.gov/ Monocular versus binocular visual acuity. Nature, pubmed/23231369. 208, 191–192. Kagan, I. (2012). Active vision: Fixational eye move- Cherici, C., Kuang, X., Poletti, M., & Rucci, M. (2012). ments help seeing space in time. Current Biology: Precision of sustained fixation in trained and CB, 22(6), R186–R188, https://doi.org/10.1016/j. untrained observers. Journal of Vision, 12(6):31, 1– cub.2012.02.009. 16, https://doi.org/10.1167/12.6.31. [PubMed] Krauskopf, J., Cornsweet, T. N., & Riggs, L. A. (1960). [Article] Analysis of eye movements during monocular and Chung, S. T., Kumar, G., Li, R. W., & Levi, D. M. binocular fixation. Journal of the Optical Society of

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