Abnormal sensory eye dominance in stereoanomalous subjects

Abstract

Stereoanomalous (SA) subjects have normal visual acuity but reduced stereopsis and may have a prevalence of up to 30%. It has been suggested that, in SA subjects, an imbalance in interocular inhibition might underlie an asymmetry in sensory eye dominance (SED). Our study expands upon previous findings by examining binocular rivalry (BR) mean dominance durations, dichoptic masking (DM) thresholds and SED for a group of SA subjects compared to naïve controls. We examined BR dominance durations and DM thresholds for 15 stereonormal (SN) subjects and 10 SA subjects with normal or corrected-to-normal visual acuity. All subjects had visual acuity of 20/40 or better and less than or equal to two lines difference between eyes. Individuals who scored ≥6/9 on the Randot stereo test and <100 arcmin on the PacMan Stereo Acuity test were considered SN. We compared near-vertical and near-horizontal oriented sine-wave gratings for BR and DM in order to dissociate stereo-related mechanisms that rely on horizontal disparities from other eye-based integration mechanisms. Mean randot scores for SN subjects were 8.5/9 with a PacMan stereoacuity of 33 arcmin, and SA subjects scored 2.5/9 and 3,380 arcmin, respectively. The mean difference in SED was 0.19 for SN and 0.48 for SA when measured with a neutral density filter bar. The SA group showed a large interocular difference in BR durations that was significantly greater than normal (p = 0.004) and correlated with loss of stereoacuity. Moreover, the interocular difference for DM was similarly greater for SA subjects (p = 0.04) although a proportional difference in monocular sensitivity could partially account for this. We also found that both SN and SA subjects presented higher DM thresholds and, to some extent, sensitivity for vertical than horizontal orientations. SA subjects show an abnormal bias toward their dominant eye for both BR and DM. These data suggest that common mechanisms of monocular sensitivity and interocular inhibition may limit multiple binocular measures and provides a practical link to better understand the heterogeneity of stereopsis in amblyopia.

Introduction

Depth perception varies between individuals, and the degree of stereopsis may exist on a continuous spectrum of individuals with very precise depth perception to those with none, such as in amblyopia (Lema & Blake, 1977). Stereoanomalous (SA) subjects are individuals with normal visual acuity; however, their ability to integrate binocular disparities is limited and results in a loss of stereopsis (Richards, 1970). Thus, the term “stereoblindness” is sometimes used to refer to this condition. In an influential study, Richards (1970) claimed that up to 30% of the normal population may exhibit some form of stereoblindness although a more conservative estimate of 5% has been reported (Birch et al., 2008; Stelmach & Tam, 1996; Zaroff, Knutelska, & Frumkes, 2003). Nevertheless, SA individuals have normal or corrected-to-normal visual acuity and, thus, form a distinct population that are not defined as having amblyopia. Instead, evidence suggests that SA subjects manifest a pattern of visual performance that is neither normal nor amblyopic. For example, the reduction in functional stereopsis may potentially provide them with greater access to monocular information than normal subjects (McKee & Harrad, 1993). However, this suggestion has not been followed up with other direct tests of monocular strength and symmetry, such as binocular rivalry (BR) and dichoptic masking (DM). 

It has been proposed that stereopsis, BR, and DM may share similar underlying neural mechanisms (Chopin, Mamassian, & Blake, 2012). Rivalry has been suggested to occur when two images differ too greatly to provoke stereopsis (Blake, 1989; Blake, Yang, & Wilson, 1991) and appears to engage in a similar cortical activity pattern visualized with fMRI (Buckthought, Jessula, & Mendola, 2011). In particular, interocular suppression has been discussed in models of multiple types of binocular integration. For example, it has been shown that varying relative between-eye stimulus orientation provokes various degrees of interocular suppression during BR. Although some reports found that relative stimulus orientation had no effect on suppression during BR (Kitterle & Thomas, 1980; O'Shea & Crassini, 1981; Stuit, Cass, Paffen, & Alais, 2009), other studies have further explored the effect of varying the between-eye orientation difference during rivalrous conditions and concluded that, as the relative orientation difference decreased, the BR durations increased (Baker & Graf, 2009; Buckthought, Kim, & Wilson, 2008; O'Shea & Crassini, 1981). Moreover, varying the orientation difference between the target and mask during dichoptic monocular masking similarly affects masking thresholds (Blakemore & Campbell, 1969; Campbell & Kulikowski, 1966). Baker and Graf (2009) took a methodical approach by recording the DM thresholds over a large range of relative orientations between 30° and 90° at every 5°. Their overall findings indicated that, as the between-eye orientation difference of the mask and target decreased, the thresholds required for detection of the target increased. Maximal DM occurs when the target and mask engage in similar orientations (see also Baker & Meese, 2007; Levi, Harwerth, & Smith, 1979). Moreover, an elevated detection threshold was present for all orientations (contrary to the findings by Levi et al., 1979, that no obvious masking was observed when stimuli were orthogonal; Baker & Graf, 2009; Blakemore & Campbell, 1969; Levi et al., 1979). 

An additional approach to understanding BR is to vary the global orientation of stimuli so that any interocular relative orientation difference occurs with respect to the horizontal or vertical axis. Interestingly, near-vertical gratings provoke longer dominance durations than equivalent orientations along the horizontal orientation (Fahle, 1982). Fahle (1982) suggests that the global horizontal stimulus orientations fail to provide enough disparity, whereas the vertical orientations do. Furthermore, the visual system may have a stronger bias for the vertical orientations as they provide greater information used to produce a 3-D reconstruction of the world (Fahle, 1982). 

Another important way of relating stereopsis, BR, and DM is through the concept of sensory eye dominance (SED). One way to determine SED is to quantify the amount of interocular suppression from each eye and calculate the difference in strength between the eyes. For example, Bagolini striated lenses viewed with a sliding scale of neutral density filters over one eye can be used to assess the suppression from each eye toward the other. Thus, it can provide a measurement of SED and ultimately serve as an independent comparison to BR and DM. Furthermore, having a simple but graded measurement of SED is advantageous because the magnitude of SED varies along a continuum (Ooi & He, 2001). Importantly, high SED symmetry has been reported to correlate with good stereoacuity, binocular disparity thresholds, and time-dependent depth detection tasks in random-dot stereograms (Xu, He, & Ooi, 2010). Thus, individuals with a small SED generally demonstrate symmetrical input from both eyes with normal stereoacuity, and individuals with a larger SED may demonstrate more asymmetrical input from both eyes, resulting in weak or possibly no stereoscopic capabilities (Ooi & He, 2001; Sireteanu & Fronius, 1981; Xu, He, & Ooi, 2011). Moreover, McKee and Harrad (1993) found, in normal subjects, that the fusional suppression of the monocular information was symmetrical and that neither eye had a strong access to the monocular information. However, in SA subjects, this relationship tends to break down as the asymmetry between eyes increases with monocular access enhanced for the dominant eye and the weaker eye becoming suppressed. Therefore, having a larger SED may result in the loss of binocular integration and stereopsis (Sireteanu & Fronius, 1981). Similar effects exist for BR, with which a strong SED imbalance is correlated with the relative predominance of the dominant eye (Xu et al., 2010). Thus, rivalry may be driven by the same overall imbalance in interocular inhibition that causes a significant disruption of binocular vision (Dieter, Sy, & Blake, 2016; Xu et al., 2010). Furthermore, normal observers with symmetrical binocularity, viewing equivalent contrasts in each eye, report the perception of the stimuli as being equal (Ding, Klein, & Levi, 2013). In contrast, amblyopic subjects, for example, require more equivalent contrast presented to their nondominant eye to perceive an equivalent contrast (Ding et al., 2013). Because this occurs for low spatial frequencies typically detected normally by amblyopic eyes, it suggests an asymmetry in their binocularity rather than a loss of monocular sensitivity. 

It is also relevant to consider that, although transient binocular suppression does readily occur in normal binocular vision, e.g., during rivalry, in individuals with abnormal binocular vision (such as amblyopia), one eye is often suppressed continuously in order to avoid diplopia and blurred vision (Holopigian, 1989; von Noorden & Campos, 2001). Such asymmetrical suppression in amblyopia has been observed using continuous flash suppression (Gao et al., 2018), stimulus-orientated BR, and DM paradigms (R. Harrad, 1996; Schor, 1977). Interestingly, in some instances, amblyopes have shown less orientation tuning than normal for BR and DM (R. Harrad, 1996; Schor, 1977). In fact, when amblyopic individuals switch from a monocular to a binocular viewing condition, their acuity can be greatly reduced regardless of the stimulus presented in their fellow eye (Freeman & Jolly, 1994). Although it has been proposed that normal interocular suppression may engage mechanisms similar to those found for amblyopia, normal and amblyopic subjects do differ in terms of BR wavelength tuning (Smith, Levi, Manny, Harwerth, & White, 1985), and the underlying mechanisms may not be identical (Smith et al., 1985). 

What remains unclear is how SA subjects would perform on a similar battery of tests. We hypothesized that SA subjects would perform abnormally on tests of BR and DM and that their pattern of loss might resemble that seen for amblyopic subjects. Hence, this study compared the mean BR durations and DM thresholds for both stereonormal (SN) and SA subjects over a range of relative between-eye stimulus orientation differences as well as global orientations of near-vertical and near-horizontal oriented sine wave gratings. 

Methods

Subjects

A total of 25 subjects (18 females) including one author (PC) participated in the experiment. All subjects with the exception of two (PC and GH) were naïve to the experiment. The participants ranged between 18 and 34 years of age with a mean age of 21. Participants were recruited through the McGill Study for Pay Facebook group, Student's Society of McGill University Marketplace, Kijiji, or in person. All subjects underwent the same assessments, including a test for visual acuity, two stereoacuity tests and an SED assessment. All participants were categorized as either SN or SA depending on their stereoacuity results on the iPod stereoacuity test (for greater details about the iPod test see Hess et al., 2016) and a Randot stereoacuity test (Western Ophthalmics, Lynnwood, WA). Overall, 15 subjects were classified as SN and 10 as SA. Written informed consent was obtained, and subjects were provided with monetary compensation following the completion of the experiment. All experiments were approved by the McGill Research Ethics Board and followed the tenets of the Declaration of Helsinki. 

Classification tests

Visual acuity

Participants had their visual acuity tested using the ETDRS 2000 series chart (Precision Vision, Woodstock, IL). The chart was placed 40 cm away at eye level, and subjects were instructed to close one eye and read the letters in each row. Acuity was determined by subjects correctly identifying three or more letters on a given line but failing to correctly identify three or more letters on the following line. 

Stereoacuity

The degree of stereoacuity was measured using two different methods. The first measurement used the Randot stereoacuity test (Western Ophthalmics). The Randot stereoacuity measures fine depth discrimination. The Randot stereoacuity test ranges from 0/9 to 9/9, and scoring 0/9 indicates the inability to discriminate objects at 800 arcsec and 9/9 results in fine depth discrimination ability at 40 arcsec. If participants made two mistakes in a row, their score was based upon their previous correct response. Participants who scored six or above were classified as SN, and individuals that scored five or lower were classified as SA. The second stereoacuity measurements were made using the iPod stereoacuity test (Hess et al., 2016). The test was conducted on a Mac iPod (model A1367; Apple, Inc., Cupertino, CA) and is a stereogram test app running on a 326 pixels per inch retina display (Hess et al., 2016). Participants viewed the dichoptic stimuli through red–green anaglyph glasses with a viewing distance of 50 cm. The iPod test measured stereoacuity in arcsec and was repeated three times and averaged. Individuals who failed the Pacman iPod test were assigned a value of 5,000 arcsec. Participants who scored 100 arcsec or less were classified as SN, and individuals who scored 100 arcsec or higher were classified as SA. 

SED

The subject's dominant eye was determined through two methods. The first method was the Miles Test (hole-in-hand variant; Miles, 1930). This test determines the dominant eye by having the subject indicate which monocular view most resembles their binocular view when viewing a distant object through an aperture formed by holding both hands together (Mendola & Conner, 2007; Miles, 1930). The second method was the Bagolini Striated Glasses Test, using a neutral density filter bar to determine the relative amount of suppression in each eye (Li et al., 2011). The Bagolini Striated Glasses Test is a measurement used to test binocular function that can provide graded information on the SED strength. The Miles Test provided identical results to the Bagolini Striated Glasses Test when determining the dominant eye. However, the Bagolini Striated Glasses Test provided an enhanced assessment of both dominant and nondominant eyes; therefore, we only used that test in our analysis. In order to obtain measurements using the Bagolini striated lenses, subjects were placed in a dark room and were asked to fixate on a small light source located 50 cm away. With the striated lenses on, subjects perceived two orthogonal lines of light in the shape of an “X.” A neutral density filter bar was placed in front of one eye, resulting in the perception of a single diagonal line. The neutral density filter bar contained a filter that can move sequentially over six different density filters, ranging from darkest to lightest. The resulting filters in units of optical density (0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, 3.0, 3.3, 3.6) correspond, respectively, to percentage transmittance of (50, 25, 12.5, 6.25, 3.12, 1.56, 0.78, 0.39, 0.19, 0.10, 0.05, 0.02). At the strongest setting, the light source is not visible to the tested eye. The strength of the neutral density filter bar was decreased by participants until they perceived a faint appearance of an “X.” The measurement indicated in the neutral density filter bar was recorded at the initial return of the “X.” The “X” indicates the suppression related to the eye covered by the neutral density filter bar. This measurement was repeated for the opposite eye. The absolute difference between the two eyes was calculated by subtracting one value from the other to quantify the SED. 

General stimulus presentation apparatus

BR and DM stimuli were presented on a MacBook Pro (2.3 GHz Inter Core i7) laptop with a screen resolution of 2,880 × 1,800 with a 60 Hz refresh rate and a NVIDIA GeForce GT 750M graphics card. Psykinematix software (version 1.5; KyberVision, Montreal, Canada, psykinematix.com) was used to generate stimuli during the experiment. Screen calibration for linear output was optimized through Psykinematix and a Spyder 3 photometer. The dichoptic stimuli were displayed in a split screen arrangement by using the left and right halves of the monitor for left and right eye stimuli. Stimuli were symmetrical from the center with a viewing distance of 45 cm. Subjects were provided with 12 diopter prism goggles to facilitate and aid in fusing the two spatially different stimuli presented to each eye. An opaque divider was placed between the screen and the participant's nose, assuring that the left stimuli was presented exclusively to the left and that the right stimuli was presented exclusively to the right eye and blocking any stray light. 

General stimulus conditions

The stimuli were presented to each eye along the horizontal axis 14.5° apart with 7.25° on each side of the screen divider (Figure 1; Caldwell, 2016). In order to spatially superimpose both stimuli and achieve dichoptic stimulation, two methods were used. In addition to the prism lenses, a robust fusional cue surrounding the stimuli allowed for optimal dichoptic fusion. The robust fusional cue consisted of a square border with a uniform noise pattern of eight pixels of granularity that was of 8° in size (using the 2-D noise function in Psykinematixs). The mean luminance contrast composing the 2-D uniform noise pattern was set at 50% with the individual squares randomly varying in luminance. Each stimulus presentation was enclosed by a uniquely randomized noise pattern. Dichoptic fusion was further enhanced by incorporating a diamond fixation mark in the middle of each stimulus. The fixation marks were composed of a sine wave annular grating with a frequency of 5 c/° and a hard-edge envelope at 50% contrast. The size of the diamond fixation mark was 0.23°. 

Figure 1

 

Stimuli used for BR and DM. Subjects viewed two sinusoidal gratings presented individually to each eye through the aid of prism lenses and a divider to separate the two gratings. Fusion was aided by the surrounding frame of uniform noise pattern. (A) The orthogonal gratings varied with respect to (A) horizontal or (B) vertical axis. The between-eye orientation difference was 90°, 60°, or 30° in each case. The 90° condition (same in panels A and B) was tested once during a horizontal block and once during a vertical block.

Before commencing the experiment, participants were instructed put on the prism goggles and to fixate at the preview screen. The task and stimuli were verbally described to the participant. During a short pretest, subjects described the stimuli presented uniquely to each eye by looking through a given eye at each interval. Once subjects were able to achieve fusion, they began the experiment by pressing the space bar. During BR, participants pressed the left arrow when perceiving the left-tilted grating and the right arrow key for a predominantly right-tilted grating. For DM, the subjects pressed the left arrow key for the stimuli presented in the first interval or the right arrow key for the second interval. For all BR and DM experiments, the stimuli were counterbalanced across both eyes for each trial. 

BR

The BR stimuli were presented at a viewing distance of 45 cm. Each trial was 90 s in duration. The experiment consisted of a total of six trials broken down into two global orientations: three trials of near-horizontal and three trials of near-vertical global orientation. Each near-horizontal and near-vertical orientation condition consisted of two different relative orientations of Gabors with either 30° or 60° and a common 90° between-eye orientation difference in both orientations. Although the 90° conditions are identical for both the near-horizontal and near-vertical conditions, it was tested twice as part of the counterbalanced design. Each condition was counterbalanced for between-eye orientation differences, making a grand total of 12 trials performed. 

Stimuli

The orientated gratings were Gabor patches of 2 c/° with a Gaussian envelope of σ = 1.6°. The BR global orientation conditions were broken down into near-horizontal or near-vertical orientation with respect to their axis. Both horizontal and vertical conditions used three stimuli orientations: 90°, 60°, and 30° and, from their respective axes, (a) a between-eye dichoptic difference of 90°, in which one eye's stimulus orientation is 45° relative to the given axis and the other eye's stimulus orientation is at −45° from the axis; (b) a relative dichoptic difference of 60°, which results in one eye's stimulus orientation being 30° relative to the given axis and the other eye's stimulus orientation being −30°; and (c) a relative dichoptic difference of 30°, resulting in one eye's stimulus orientation being 15° relative to the given axis and the other eye's stimulus orientation being −15°. Each stimulus was presented with 32% contrast (Baker & Graf, 2009). At the beginning of the experiment, the subject would put on the prism glasses and view the stimuli preview screen. Subjects would verify that they had achieved fusion before commencing the experiment. The subject's task was to indicate which stimulus dominated their perception by pressing the left or right key. Pressing the left arrow key would indicate a left orientation dominating perception; likewise, pressing the right arrow key would indicate the right orientation dominating their percept. Subjects were informed to respond with a left response when that stimulus dominated at least two thirds or more of the absolute orientation. The participant's mean dominance duration was calculated and compared over the three levels of relative orientation (30°, 60°, and 90°). Individual's differences in dominance durations were recorded for each relative and global orientation with the goal of observing if dominance durations differed between conditions and subject groups. 

DM

For all masking experiments, the stimuli were presented in a two-interval, forced-choice design in which stimuli were presented for 500 ms followed by a 500 ms interstimulus interval. The staircase for the masking experiments followed a three-down, one-up rule, becoming more difficult after three correct responses and easier after one error. Each threshold was obtained after a total of six reversals, which occurred over an average of 70–80 trials. The contrast of the mask was fixed at 32% contrast, and the target contrast was controlled by the staircase. The same stimulus conditions described for BR (relative between eye dichoptic orientation of 30°, 60°, and 90° and near-horizontal and near-vertical global orientations) were used for DM. However, an additional condition with a relative orientation difference of 0° was also incorporated for the vertical and horizontal global orientations. In addition, a baseline condition with no mask was also included. 

Stimuli

DM experiments for relative orientations of 0°, 30°, 60°, and 90° between-eye differences were obtained for each participant. One interval consisted of presenting a mask to one eye (nontested eye) and a target of relative orientation of 0°, 30°, 60°, and 90° to the other eye (tested eye). The other interval, randomly presented before or after the previously described scenario, consisted of presenting the same mask to the same nontested eye while the tested eye received a mean luminance gray background. Both stimuli were surrounded by a square noise border with diamond fixation marks in the center (see Stimulus conditions). The participant's task was to indicate which interval contained the target, which was perceived as a plaid when the mask and target were presented together. The left arrow key indicated the first interval, and the right arrow key indicated the second interval. The main objective of the DM experiment is to establish the amount of contrast required, above baseline, to detect the target that is being masked by the masking stimulus over a range of Gabor patches with 0°, 30°, 60°, and 90° of relative orientations in both near-horizontal or near-vertical global orientations in a counterbalanced approach. In addition, the subject's baseline detection threshold for the target stimuli was determined in a similar fashion as described above for the target Gabor patches with 0°, 30°, 60°, and 90° of relative orientations. One interval contained a target stimulus presented to the tested eye and a mean luminance gray background was presented to the nontested eye. The other interval consisted of background luminance presented to both eyes. The subject would report which interval contained the target stimulus. The same stimulus conditions and arrow key input mentioned above also applied to the baseline threshold condition. 

Data analysis

Each experiment was analyzed separately for each subject's dominant and nondominant eye. Furthermore, horizontal and vertical stimuli were separately analyzed for each combination of eye dominance. The results were broken down into four categories: horizontal-dominant eye, horizontal-nondominant eye, vertical-dominant eye, and vertical-nondominant eye. For BR, the average mean dominance duration for each eye and global orientation were compared between the relative orientations of 30°, 60°, and 90° to establish if these conditions were significantly different. For DM, the average masked threshold for each of the four categories was compared between global orientation conditions for each eye to determine if thresholds are significantly different based upon the between-eye orientation difference in the mask and target. We also normalized (divided) these raw masking scores by the no-mask condition to report threshold elevation (e.g., Ross & Speed, 1991). 

Results

Visual acuity

All subjects had normal or corrected-to-normal visual acuity. Out of a total of 25 participants and 50 eyes, most of the eyes tested had an acuity of 20/20 or better (n = 28). The rest of the participants' eyes had an acuity of 20/25 (n = 17), 20/30 (n = 1), and 20/40 (n = 4). Only three subjects, two SN and one SA, had two lines of interocular difference, and the remaining participants had less than two lines of interocular difference. 

Stereoacuity

Subjects were classified through the administration of the Randot stereoacuity test and the iPod stereoacuity test (Hess et al., 2016). From a total of 25 subjects, 15 subjects scored between six and nine on the Randot and were classified as SN (M = 8.53; Figure 2A). The same 15 subjects scored below the 100 arcsec cutoff for the iPod stereoacuity test with responses ranging between 13.1 and 84.3 arcsec and an average of 32.8 arcsec (Figure 2B). Our SA subjects (n = 10) ranged between zero and five on the Randot stereoacuity test with a mean of 2.5 (Figure 2A). The iPod test results for our SA subjects ranged between 197.5 and 5,000 arcsec with a mean of 3,380.2 arcsec (Figure 2B). There was a strong correlation between the Randot and iPod stereoacuity test, which was statistically significant (r = −0.801, n = 25, p < 0.001; Figure 3A). Furthermore, both the Randot and iPod test cutoff scores to define SA subjects were in agreement with each other. Nevertheless, we acknowledge that this cutoff was arbitrary, and we suggest in the Discussion section that a continuous spectrum of binocularity may ultimately be the most useful concept. 

Figure 2

 

Stereoacuity and SED for SN and SA subjects. (A) The mean Randot stereoacuity scores (9–0) decrease as the subjects' stereoacuity decreases. (B) The mean stereoacuity Pod scores increase as the subjects' stereoacuity decreases. (C) The mean SED measurements defined by the absolute difference in measurements from both eyes. The mean SED measurement increased as the subjects' stereoacuity decreased. Gray dots indicate individual data that contribute to the mean result.

Figure 3

 

Plotting of individual data points for the stereoacuity measurements and SED for both SN and SA subjects. (A) The subjects (n = 25) Randot stereoacuity scores (9–0) increased as the iPod scores decreased, and this correlation was significant. The arrows on both the x- and y-axes indicate the cutoff scores that were used to label subjects as normal or anomalous. (B) The stereoacuity iPod scores tended to increase as the subjects' SED increased. (C) The SED measurement tended to increase as the subjects stereoacuity decreased.

SED

The subject's dominant eye was determined through two methods. The first method used the Miles Test (hole-in-hand variant; Miles, 1930). The second method used Bagolini lenses and a neutral density filter bar. The results between the Miles Test and Bagolini lenses were in agreement with each other. The Bagolini lenses provided us with a continuous scale measure to correlate with our results. Furthermore, the absolute difference between each eye's measurement using the Bagolini lenses (SED) was computed for each participant by subtracting the highest value by the lowest. On average, the SA subjects demonstrated a larger SED difference (M = 0.48) than the SN subjects (M = 0.2; Figure 2C). We also compared the subjects' difference in SED to their score on the Randot and iPod tests. Although these correlations did not reach significance, a trend can be observed that, as the subjects' stereoacuity improves, their SED difference generally decreases (Figure 3B and C). 

BR: SN

Overall, we observed that the mean BR durations in SN subjects did vary with the relative orientation difference between the two eyes (plotted on log axis in Figure 4A). Specifically, the mean dominance durations decreased (alternation rate increases) as the between-eye orientation difference increases. The shortest durations were for 90° of relative orientation and were significantly shorter than 60° and 30° differences, F(1.505, 21.076) = 13.415, p = 0.011, Display Formula\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicode[Times]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\(\def\bupalpha{\unicode[Times]{x1D6C2}}\)\(\def\bupbeta{\unicode[Times]{x1D6C3}}\)\(\def\bupgamma{\unicode[Times]{x1D6C4}}\)\(\def\bupdelta{\unicode[Times]{x1D6C5}}\)\(\def\bupepsilon{\unicode[Times]{x1D6C6}}\)\(\def\bupvarepsilon{\unicode[Times]{x1D6DC}}\)\(\def\bupzeta{\unicode[Times]{x1D6C7}}\)\(\def\bupeta{\unicode[Times]{x1D6C8}}\)\(\def\buptheta{\unicode[Times]{x1D6C9}}\)\(\def\bupiota{\unicode[Times]{x1D6CA}}\)\(\def\bupkappa{\unicode[Times]{x1D6CB}}\)\(\def\buplambda{\unicode[Times]{x1D6CC}}\)\(\def\bupmu{\unicode[Times]{x1D6CD}}\)\(\def\bupnu{\unicode[Times]{x1D6CE}}\)\(\def\bupxi{\unicode[Times]{x1D6CF}}\)\(\def\bupomicron{\unicode[Times]{x1D6D0}}\)\(\def\buppi{\unicode[Times]{x1D6D1}}\)\(\def\buprho{\unicode[Times]{x1D6D2}}\)\(\def\bupsigma{\unicode[Times]{x1D6D4}}\)\(\def\buptau{\unicode[Times]{x1D6D5}}\)\(\def\bupupsilon{\unicode[Times]{x1D6D6}}\)\(\def\bupphi{\unicode[Times]{x1D6D7}}\)\(\def\bupchi{\unicode[Times]{x1D6D8}}\)\(\def\buppsy{\unicode[Times]{x1D6D9}}\)\(\def\bupomega{\unicode[Times]{x1D6DA}}\)\(\def\bupvartheta{\unicode[Times]{x1D6DD}}\)\(\def\bGamma{\bf{\Gamma}}\)\(\def\bDelta{\bf{\Delta}}\)\(\def\bTheta{\bf{\Theta}}\)\(\def\bLambda{\bf{\Lambda}}\)\(\def\bXi{\bf{\Xi}}\)\(\def\bPi{\bf{\Pi}}\)\(\def\bSigma{\bf{\Sigma}}\)\(\def\bUpsilon{\bf{\Upsilon}}\)\(\def\bPhi{\bf{\Phi}}\)\(\def\bPsi{\bf{\Psi}}\)\(\def\bOmega{\bf{\Omega}}\)\(\def\iGamma{\unicode[Times]{x1D6E4}}\)\(\def\iDelta{\unicode[Times]{x1D6E5}}\)\(\def\iTheta{\unicode[Times]{x1D6E9}}\)\(\def\iLambda{\unicode[Times]{x1D6EC}}\)\(\def\iXi{\unicode[Times]{x1D6EF}}\)\(\def\iPi{\unicode[Times]{x1D6F1}}\)\(\def\iSigma{\unicode[Times]{x1D6F4}}\)\(\def\iUpsilon{\unicode[Times]{x1D6F6}}\)\(\def\iPhi{\unicode[Times]{x1D6F7}}\)\(\def\iPsi{\unicode[Times]{x1D6F9}}\)\(\def\iOmega{\unicode[Times]{x1D6FA}}\)\(\def\biGamma{\unicode[Times]{x1D71E}}\)\(\def\biDelta{\unicode[Times]{x1D71F}}\)\(\def\biTheta{\unicode[Times]{x1D723}}\)\(\def\biLambda{\unicode[Times]{x1D726}}\)\(\def\biXi{\unicode[Times]{x1D729}}\)\(\def\biPi{\unicode[Times]{x1D72B}}\)\(\def\biSigma{\unicode[Times]{x1D72E}}\)\(\def\biUpsilon{\unicode[Times]{x1D730}}\)\(\def\biPhi{\unicode[Times]{x1D731}}\)\(\def\biPsi{\unicode[Times]{x1D733}}\)\(\def\biOmega{\unicode[Times]{x1D734}}\)\({\rm{\ }}\eta _p^2\) = 0.316, using a Huyhn–Feldt correction. A post hoc test revealed that decreasing the between-eye orientation difference from 90° to 60° resulted in a slightly longer mean duration (2.757 ± 0.207 s vs. 3.405 ± 0.427 s), which was significant (p = 0.030) as well as 90° to 30° (2.757 ± 0.207 s vs. 3.517 ± 0.381 s) was significant (p = 0.010). However, the BR duration difference between 60° of orientation and 30° was not significant. 

Figure 4

 

BR averaged mean durations. (A) Results for SN subjects. As the orientation between the two eyes increases, the mean rivalry durations (seconds) decreased for all conditions. (B) Results for SA subjects. As the orientation between the two eyes increased, the mean rivalry durations decreased for all conditions except the vertical nondominant eye (NDE). Of note is a between-group difference in eye dominance (p = 0.004). Results are plotted on log axis due to the much longer rivalry durations observed for the SA subjects. Gray dots indicate individual data that contribute to the mean results.

Overall, the mean BR durations for the global orientations of near-vertical (blue bars in Figure 4) and near-horizontal (orange bars) stimuli appeared to be similar and were not significantly different following a three-factor (global orientation × relative orientation × eye dominance) repeated-measures ANOVA, F(1, 4.301) = 4.301, p = 0.251, Display Formula\({\rm{\ }}\eta _p^2\) = 0.093. Furthermore, there was no significant difference between the BR mean durations of the normal dominant versus nondominant eye using a repeated-measures ANOVA, F(1, 2.565) = 2.565, p = 0.287, Display Formula\({\rm{\ }}\eta _p^2\) = 0.081. However, a small trend can be observed, especially at 30°, in which the dominant eye has longer rivalry durations for both global orientations than the nondominant eye. 

BR: SA

For our SA subjects (Figure 4B), overall, a much greater length of dominance durations, and greater variability was observed. Nevertheless, similar trends were observed as with our SN population as we observed a general trend for longer dominance durations as the between-eye orientation difference decreases. Also, it is the vertical stimuli that produce the longest average durations in the dominant eye across all conditions. Moreover, for the SA group, the dominant eye had significantly longer mean durations than the nondominant eye across all relative and global orientations, F(1, 9) = 6.808, p = 0.028,Display Formula\({\rm{\ }}\eta _p^2\) = 0.431. These dominance durations were clearly greater for SA subjects (means ranging from 1–25 s) than in our SN group (means ranging from 2–4 s); hence, the log-Y-axis in Figure 4A and B. To further assess this statistically, we performed a three-factor (relative orientation × global orientation × eye dominance), between-group, repeated-measures ANOVA. The results yielded a significant effect for eye dominance, F(1, 23) = 10.003, p = 0.004, Display Formula\({\rm{\ }}\eta _p^2\) = 0.303. Neither main effects of relative or global orientation were significantly different between groups. Thus, SA subjects differ greatly from SN subjects in terms of mean BR dominance durations with a strong bias toward their dominant eye. In addition, SN subjects show a relative orientation tuning effect, and SA are less orientation tuned. 

Interestingly, when we examined the correlations between BR and Randot stereoacuity, we did find a significant relationship (see Table 1, Figure 5), in which the mean BR dominance durations for the dominant eye increased as the subjects' stereoacuity decreased. This correlation was significant for both SN and SA groups combined (Table 1). 

Table 1

 

Correlations of BR versus Randot. Notes: The left column shows the different testing conditions of BR with respect to their eye dominance. Results shown for dominant eye (DE) and nondominant eye. The right column shows the results from the correlation between BR versus Randot conducted to determine if BR mean durations increase as the stereoacuity decreases. Asterisks indicate statistically significant correlations, *p < 0.05 and **p < 0.01.

 

Figure 5

 

Correlations of BR and Randot scores. Results from all subjects (n = 25) for dominant eye BR mean durations (seconds) for both horizontal and vertical orientations as a function of Randot scores. (A) As the binocular rivalry (30°) mean durations decreased, the Randot scores increased. (B) As the binocular rivalry (60°) mean durations decreased, the Randot scores increased. (C) As the binocular rivalry (60°) mean durations decreased, the Randot scores increased.

DM: Normal subjects

As expected, we did find a significant decrease in DM thresholds as the between-eye orientation difference increased (0°, 30°, 60°, and 90°), F(1.760, 24.641) = 31.907, p < 0.001, Display Formula\({\rm{\ }}\eta _p^2\) = 0.695 (Figure 6A). A post hoc assessment indicated a significant difference in DM thresholds between 90° and 60° (p = 0.008), 90° and 30° (p = 0.007), 90° and 0° (p < 0.001), 60°, and 0° (p < 0.001), 30° and 0° (p < 0.001). As expected, the greatest levels of masking were seen for the 0° difference condition, the condition that saw the longest BR durations. 

Figure 6

 

DM contrast increment detection thresholds. (A) Results for SN subjects. As the orientation between the two eyes increased, the strength of the masking decreased for all conditions. (B) Results for SA subjects. Of note is a between-group difference in eye dominance (p = 0.042). Gray dots indicate individual data that contribute to the mean results.

Unlike normal rivalry, we also observed a significant difference between the near-vertical and near-horizontal global orientations, where the thresholds for horizontal were consistently lower across all conditions and eye tested (0°, 30°, 60°, and 90°) (F(1, 14) = 5.790, p = 0.031, Display Formula\({\rm{\ }}\eta _p^2\) = 0.293). This suggests that near-horizontal orientations for DM have lower inhibition present, and near-vertical orientations promote greater inhibition and masking thresholds. Furthermore, we found a significant interaction between relative orientation (90°, 60°, 30°, 0°) and global orientation (near-horizontal, near-vertical) stimuli, F(2.972, 41.603) = 4.575, p = 0.008. Once again, this indicates a difference in DM thresholds with greatest differential inhibition for near-vertical orientations at 30°, 60°. 

DM: Anomalous subjects

We first consider the raw masking results and then consider the results in terms of threshold elevation above the baseline condition. Indeed, in the presence of dichoptic masks, the thresholds for the SA subjects were different than our SN group. This effect is most prominent for the smallest degrees of orientation difference, which has the highest masking thresholds, for the nondominant eye, but abnormally low thresholds for the dominant eye (Figure 6B). Overall, the results resemble the Baker and Graf (2009) findings that, as orientation difference increased between the mask and target, subjects showed a weaker masking effect. The effect of relative orientation difference was significant, F(1.694, 15.248) = 20.452, p < 0.001, Display Formula\({\rm{\ }}\eta _p^2\) = 0.694, after using a Greenhouse–Giesser correction. A post hoc assessment indicated a significant difference in DM thresholds between 90° and 30° (p = 0.032), 60° and 30° (p < 0.022), 90° and 0° (p < 0.001), 60° and 0° (p = 0.001), 30° and 0° (p = 0.003). The effect of global orientation was not significant for the SA subjects, unlike normal subjects, although they show a trend for stronger masking thresholds for the near-vertical orientation stimuli. With regards to eye dominance, the SAs' dominant eye consistently presented lower DM thresholds than the nondominant eye across both the relative orientations and global orientations with significance, F(1, 9) = 5.563, p = 0.043 Display Formula\({\rm{\ }}\eta _p^2\) = 0.382. Consistently, a three-factor (relative orientation × global orientation × eye dominance), between-group, repeated-measures ANOVA yielded a significant effect for eye dominance, F(1, 23) = 4.653, p = 0.042, Display Formula\({\rm{\ }}\eta _p^2\) = 0.168. However, neither relative nor global orientations were significant. Overall, within-group, normal subjects demonstrated both relative and global orientation effects, and SA subjects showed a relative orientation and eye dominance effect. 

DM threshold elevation for normal subjects

We note before proceeding that, as a group, SA subjects were not significantly different from normal for the DM baseline condition with no mask that aims to measure monocular sensitivity only. Nevertheless, it is still quite informative to normalize (divide) the raw masking scores by the no-mask condition for each individual to report threshold elevation. In this case, we still found a significant decrease in DM thresholds as the relative-orientation difference increased (0°, 30°, 60°, and 90°), F(1.252, 17.530) = 25.584, p < 0.001, Display Formula\({\rm{\ }}\eta _p^2\) = 0.646 (Figure 7A). A post hoc assessment indicated a significant difference in DM threshold elevation between 90° and 60° (p = 0.008), 90° and 30° (p = 0.001), 90° and 0° (p < 0.001), 60° and 0° (p < 0.001), 30° and 0° (p < 0.001). As expected, the greatest levels of masking were seen for the 0° difference condition in both Figures 6A and 7A, the condition that saw the longest BR durations. However, we no longer find a significant difference between the near-vertical and near-horizontal global orientations (Figure 7A). 

Figure 7

 

Normalized DM contrast increment detection thresholds. (A) Normalized results for SN subjects by dividing the raw masking scores by the no-mask condition to report threshold elevation. As the orientation between the two eyes increased, the strength of the masking decreased for all conditions. (B) Results for SA subjects. Note the gray dots represent the individual data making up the mean.

DM threshold elevation for anomalous subjects

Similar to the SN, the SA also showed a significant effect of relative orientation difference for the threshold elevation, F(1.343, 12.089) = 5.950, p = 0.024, Display Formula\({\rm{\ }}\eta _p^2\) = 0.398, after using a Greenhouse–Giesser correction (Figure 7B). A post hoc assessment indicated a significant difference in dichoptic masking thresholds between 90° and 30° (p = 0.041) and 90° and 0° (p = 0.006). Again, the effect of global orientation was not significant for the SA subjects, similar to the SN subjects. Moreover, with regards to eye dominance, we no longer find a significant difference when using the masking threshold elevation calculation. 

The three-factor (relative orientation × global orientation × eye dominance), between-group, repeated-measures ANOVA yielded a significant effect only for relative orientation, F(1.313, 30.189) = 4.051, p = 0.043, Display Formula\({\rm{\ }}\eta _p^2\) = 0.150. Overall, normalizing the masking thresholds by dividing by the baseline condition, removes the effect of eye dominance and of global orientation seen in the raw masking thresholds. We further discuss this pattern of results in the next section. Finally, we also tested for correlations between DM and BR, and stereoacuity, but did not observe any significance. 

Discussion

We conducted a series of measurements to investigate the relationship between BR, DM, and eye dominance in SN and SA populations. We varied relative and global orientations and tested both eyes independently. For the BR task, we found that SN subjects showed relative orientation tuning, and SA show an extreme eye asymmetry effect with less orientation tuning. With the DM paradigm, we found that SN subjects again showed a relative orientation effect and a global orientation effect that was removed by normalization. SA subjects showed only relative tuning that was less globally tuned and an abnormally strong eye dominance effect that was removed by baseline normalization. 

Relative orientation difference in BR and DM

For our normal subjects, when decreasing the between eye orientation difference of two dichoptic gratings, the mean BR durations increased (Baker & Graf, 2009; Buckthought et al., 2008; Kitterle & Thomas, 1980; O'Shea, 1998), and the DM detection thresholds increased. Our results provide further support for the strong relationship between DM and orientation tuning (Baker & Graf, 2009; Baker & Meese, 2007; Levi et al., 1979). Additionally, our findings for DM were similar to our BR results, in which the 90° and 60° conditions, 90° and 30° conditions were significantly different, and the 60° and 30° condition was not. So, overall, both normal BR and DM show a significant effect for relative orientations in our SN subjects. Our findings are consistent with a common underlying neural mechanism shared between BR and DM. 

In a group of 10 naïve SA subjects, we did not find the normal effect of relative orientation during binocular rivalry to be significant although we observed a trend in which decreasing the between-eye orientation increased the rivalry durations. Moreover, our SA subjects had much longer mean rivalry durations than our SN group. For DM, we did find the significant normal effect whereby decreasing the between-eye orientation modestly increases DM detection thresholds. However, all masking thresholds were generally of a greater magnitude than our SN subjects. Overall, our findings indicate that the weak orientation tuning in BR and DM for SA subjects might manifest from the underlying differences in binocular function. 

Global orientation in BR and DM

In our study, we incorporated stimuli with orientation relative to the vertical and horizontal axes in order to detect any differences in global orientation. The same relative orientations were used for the horizontal and vertical global orientation conditions. BR dominance durations in our SN population for near-vertical and near-horizontal global orientation did not differ. However, the SA subjects showed a sizable difference between near-vertical and near-horizontal global orientations during BR. Although missing significance for the dominant eye, near-vertical stimuli clearly caused longer mean BR durations than near-horizontal stimuli. This finding for near-vertical global orientation in the SA subjects is reminiscent of some patterns also observed in subjects with amblyopia. First, Schor (1977) found that, at small between-eye orientation differences, the dominant eye displayed substantial suppression toward the amblyopic eye, resulting in longer dominance durations for the dominant eye. Our SA subjects also demonstrated similar behavior for the vertical durations in their dominant eye. Also, Schor noted that the absolute strength of suppression from the dominant eye was strongest for smaller vertical orientations, and suppression only marginally decreases as the orientations differences increase. Our SA subjects also exhibited similar behavior with a lack of relative orientation tuning, suggesting that the longer durations during vertical global orientations may involve similar mechanisms. 

Although normal subjects did not show an effect of global orientation for BR, they did for raw DM. This suggests that a reduced level of interocular inhibition for near-horizontal orientations than for near-vertical orientations may exist normally, but only the DM test is sensitive to it. Furthermore, this may be due in large part to sensitivity differences because the bias also occurs at baseline. Similarly, the effect of global orientation also results in higher masking thresholds in the SA subjects and suggests even higher inhibition for vertical orientations and is in line with findings from the amblyopic literature. Sireteanu and Singer (1980) found that vertical contours elicited a selective loss of acuity compared to equivalent horizontal contours in individuals with amblyopia. Sireteanu and Singer suggested that the decrease in amblyopic eye acuity for global vertical orientations is an adaptation that the visual system developed to hinder double vision around the vertical contours. In the current study, the SA subjects, as a group, did not show a significant loss of monocular sensitivity when compared to the SN, but the difference between global orientations was reduced for the normalized masking thresholds. Therefore, the disadvantage for vertical orientations may accrue from sensitivity as well as threshold elevation. Overall, our data implicates greater intraocular inhibition for SA subjects and a global orientation bias in analogy to subjects with amblyopia. 

Comparison of SED and a spectrum of binocularity

The presumed basis of a binocular visual system suggests that both eyes provide input equally to achieve optimal binocularity (Handa et al., 2004). Our normal subjects had a relatively small SED, which, on average, is approximately less than one neutral density filter in difference between their dominant and nondominant eye. Therefore, we tested both eyes independently on our BR and DM tasks. We found no significant difference between the dominant and nondominant eyes on any condition for our SN subjects. These results support our hypothesis that SN subjects exhibit a more balanced interocular inhibition during binocular integration tasks. Our SA subjects had an elevated SED in comparison to our SN group. A larger difference in absolute SED suggests the potential of higher levels of interocular inhibition between the eyes. Consistently, a significant difference between the dominant and nondominant eye emerged in both BR and raw DM experiments. This significant effect for eye dominance was also observed in a between-groups ANOVA, showing that SA subjects have an abnormal bias toward their dominant eye during BR and DM. These results further support our hypothesis that SA subjects exhibit a larger asymmetry between the dominant and nondominant eyes (McKee & Harrad, 1993). 

Although our correlation between SED and stereopsis did not reach significance, Xu et al. (2011) did find a significant correlation between SED and stereo thresholds. This correlation supports the idea that normal subjects behave differently than SA based, at least in part, upon their absolute difference in SED. As mentioned previously, balanced binocularity suggests equal or near equal input from both eyes (Li et al., 2010). Moreover, individuals with a relatively large SED from asymmetrical input alongside an elevated interocular inhibition have shown a significant reduction in stereopsis (Xu et al., 2010, 2011). In addition, the combination of having a large SED and an interocular inhibition imbalance has been suggested to be an indicator of amblyopia (Xu et al., 2010, 2011). We suggest that such interindividual variations between SED advocates for a spectrum, ranging from individuals with a small SED, low interocular inhibition, and good stereoacuity to subjects with large SED, large interocular inhibition, and little to no stereopsis, such as individuals with amblyopia (McKee & Harrad, 1993; Xu et al., 2011). As seen in our results, our normal subjects present themselves with good stereoacuity and a small SED, placing them on one end of the spectrum. Our SA subjects present themselves with weaker stereoacuity and larger SED, placing them further along the spectrum toward amblyopia. 

Interestingly, some physiology studies suggest that fewer binocular neurons are present in amblyopic animal models resulting in abnormal binocular integration (Horton, Hocking, & Adams, 1999; Schröder, Fries, Roelfsema, Singer, & Engel, 2002). Blake and Cormack (1979) suggested that SA individuals also have fewer binocular cells in the visual cortex along with more monocular cortical cells that provided them with greater utrocular access to determine which eye is being stimulated. Such utrocular discrimination is not readily reproducible among normal subjects (Blake & Cormack, 1979). If SA subjects have greater monocular input from their dominant eye and a relative lack of input to binocularly driven neurons, i.e., a milder version of strabismus but not amblyopia (McKee, Levi, & Movshon, 2003), it would be similar to conclusions from physiological studies on amblyopic animals (Hubel & Wiesel, 1968; Hubel, Wiesel, & LeVay, 1977; Kiorpes et al., 1987). In contrast, SN subjects may receive less monocularly and more binocularly driven neural inputs across both eyes, resulting in greater binocularity sensitivity (McKee et al., 2003). 

However, we acknowledge that the spectrum of stereopsis function may not be characterized solely by binocularity. The most common sensory deficits observed in amblyopic individuals are monocular losses in visual acuity, positional acuity, and contrast sensitivity (Levi, 2006). As such, many of these deficits could be suspected in SA subjects (Joly & Frankó, 2014). Indeed, multiple studies in normal subjects have shown that blurring, filtering, and decreasing the contrast in one eye does subsequently reduced their stereoacuity (Legge & Gu, 1989; Menon, Bansal, & Prakash, 1997; Westheimer & McKee, 1980). Consistently, there is a general correlation between the decrease in visual acuity and loss of stereoacuity (Levi, Knill, & Bavelier, 2015; McKee et al., 2003). Therefore, both binocularity and visual acuity are important for understanding amblyopic subjects, and we cannot exclude some monocular sensitivity loss in the SA subjects. In fact, we found that the interocular differences in masking thresholds were greatly reduced when normalized by baseline monocular sensitivity (with no grating mask). Thus, a subtle imbalance of sensitivity remains even with visual acuity scores in the normal range. 

Although models of the amblyopic visual system have been developed, it is still unclear precisely how asymmetrical monocular sensitivity interacts with potential abnormalities in interocular inhibition, and baseline monocular sensitivity is needed to distinguish these effects (Baker, Meese, & Hess, 2008; R. Harrad, 1996; R. A. Harrad & Hess, 1992; Levi et al., 1979; Meese, Georgeson, & Baker, 2006). A monocular difference in sensitivity has been long been hypothesized to account for DM in amblyopia because the dominant eye constantly receives an input that is greater than a suprathreshold contrast, and therefore, the dominant eye input always masks the weaker eye (R. A. Harrad & Hess, 1992). This suggests that, during DM, the decrease in contrast sensitivity of the amblyopic eye (nondominant eye) could explain the large deficit (Asper, Crewther, & Crewther, 2000; R. A. Harrad & Hess, 1992; Levi & Harwerth, 1977). Moreover, only some data from amblyopic subjects indicates that increased interocular suppression is needed to model the performance (Huang, Zhou, Lu, & Zhou, 2011). In the case of binocular rivalry, results have indicated a strong asymmetry in suppression favoring the dominant eye (Coren & Duckman, 1975; Sengpiel & Blakemore, 1996), but the role of monocular sensitivity versus binocularity was not distinguished. Taken together, there is a general agreement that changes in interocular suppression in amblyopic subjects at least contribute to the pronounced asymmetry in favor of their dominant eye (Baker et al., 2008). Our results are in agreement as we found that SA subjects have larger eye asymmetries in BR and DM. Also, the degree of SED as tested with BR for gratings could predict stereoacuity loss (−0.45 < r < −0.59; see Table 1, Figure 5). However, multiple factors may ultimately interact, including increased inhibition between the eyes and hypothesized elevated levels of internal noise and disarray of neurons in the visual system (Hess & Field, 1994; Levi & Klein, 2003; Sengpiel & Blakemore, 1996). In addition, the degree of monocular sensitivity loss has been shown to vary depending on the type of amblyopia (McKee et al., 2003) with strabismic subjects (especially with exotropia) showing relatively lower binocularity and higher sensitivity, perhaps in analogy with our SA subjects. 

In conclusion, our study shows that SA subjects have a significant sensory eye dominance asymmetry with bias toward their dominant eye during BR and DM. Future research incorporating a larger sample of SA subjects would be beneficial and help further our understanding of subtle distinctions between SN subjects. Moreover, further research comparing SA and amblyopic subjects is of great importance in order to determine the significant differences between contrast sensitivity, Vernier acuity, and binocularity. Such research would be beneficial in furthering our understanding of the underlying mechanisms and provide us with a practical link to better understand the heterogeneity of stereopsis. 

Acknowledgments

We are grateful to all our participants who volunteered their eyes for the study. We would like to thank Grace Ha for her assistance with data collection and Robert Hess for providing the iPod stereoacuity test. This work was supported by a Discovery Grant from the National Science and Engineering Research Council of Canada (NSERC 356445-08) to JM. 

Commercial relationships: none. 

Corresponding author: Janine D. Mendola. 

Email: . 

Address: McGill Vision Research and Department of Ophthalmology, McGill University, Montréal, QC, Canada.