Asymmetric eye movements during fixation challenge Hering’s Law of a unitary neural command

Asymmetric eye movements during fixation challenge Hering’s Law of a unitary neural command | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Asymmetric eye movements during fixation challenge Hering’s Law of a unitary neural command Arvind Chandna, Devashish Singh, Scott Watamaniuk, Stephen Heinen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4426926/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Visual perception is largely supported by tiny fixational eye movements (FEMs), the control of which is a mystery. Understanding eye movement control has been impeded by researchers recording one eye, assuming the other behaves symmetrically as if “yoked” (Hering’s Law). However, recent work revealed asynchronous smooth eye movements during vergence, challenged Hering’s Law, suggesting FEMs might also be decoupled. We assessed FEMs of both eyes during binocular and monocular viewing. During binocular viewing, eye movement variability (BCEA) was larger in one eye, implying the eyes were independently controlled. During monocular viewing, covered eyes’ BCEAs were larger than viewing ones. Eyes with larger BCEAs drifted faster, suggesting poorer eye control. In contrast, microsaccades were always binocular, yet larger in viewing eyes with larger BCEAs, indicating independent control improved their drift correction. Our results support independent control of smooth eye movements and elucidate the genesis of ocular misalignment relevant to strabismus intervention. Biological sciences/Neuroscience/Oculomotor system/Saccades Health sciences/Diseases/Eye diseases/Vision disorders Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Ocular fixation is employed in many studies of visual processing and during strabismus assessment to keep the eyes stable at a fixed location. Yet, fixation is a misnomer, as during fixation the eyes are virtually never still. Miniature eye movements known as “fixational eye movements” (FEMs) continuously occur during fixation. The most studied FEMs are microsaccades, rapid movements of the eyes that resemble large saccades, but are limited in magnitude to less than 1 deg, or even less than 0.5 deg, as this smaller size constrains them to the foveola, a one-degree diameter region of the central fovea (for discussion see [ 1 ]). Microsaccades are thought to prevent retinal fading (e.g., [ 2 ]), and to direct the eyes back to the fixation point or to image features in micro scenes [ 3 ] as large saccades do during normal viewing. The other dominant type of FEM is a slow, smooth rotation of the eye known as “drift” [ 4 , 5 , 6 ].While drift is less studied than microsaccades, there is evidence that it sharpens images by moving them across ganglion cell receptive fields at a velocity appropriate to emphasize high spatial frequency content and improve acuity [ 7 , 8 ]. During studies of ocular fixation in neurotypicals, only one eye is generally recorded under the assumption of Hering’s Law, i.e. that the eyes are yoked, and therefore the other rotates identically [ 9 ]. Hering’s Law states that there is equal innervation to the eyes, and hence a unitary neural command is thought to drive all eye movements. Since this theory guides most research on the behavior and physiology of the oculomotor system, classic models of eye movement control are constructed with a single input and a single output. The assumption of symmetric eye movements extends to the clinic in that it guides intervention in constant and intermittent strabismus [ 10 , 11 , 12 , 13 ]. However, in studies of amblyopia and strabismus in which both eyes are recorded there are differences in fixation characteristics between the amblyopic and the non-amblyopic fellow eye [ 14 , 15 , 16 ], suggesting the eyes are not yoked in these disorders, and possibly not at all. In strabismus with amblyopia the amblyopic eye is more unstable and both eyes were more unstable than controls [ 17 ] and even in strabismus without amblyopia the unaligned eye is more unstable [ 17 , 18 ].Reports of the cause of fixation instability in strabismus and amblyopia are varied and include slower drift speeds, increased drift amplitude and velocity, disconjugate saccades, increased saccadic intrusions or a combination of these factors. The type and severity of amblyopia and strabismus, and even methodological differences may have a bearing on the results[ 14 , 15 , 17 , 19 , 20 , 21 ] While these results raise doubts about the validity of Hering’s Law. However, the absence of a systematic investigation of binocular fixation in neurotypicals leaves open the possibility that Hering’s Law is correct. An alternative to Hering’s Law that could explain non-yoked behavior was proposed by Helmholtz (1867), who suggested that the eyes are controlled independently and we learn to move them together in early childhood [ 22 ]. Despite that Helmholtz’s opposing theory was postulated contemporaneously with Hering’s, critical experiments differentiating these theories are lacking, and Hering’s theory has managed to prevail for over 150 years. However, recent results of experiments comparing oculomotor behavior of both eyes during monocular viewing challenge Hering’s Law [ 23 ]. This study found asymmetric behavior between the two eyes during smooth pursuit of a midline target, evidence that the eyes are not yoked. Furthermore, the covered eye’s rotation was idiosyncratic between observers, and even within an observer from trial to trial and therefore, could not be predicted from recording only the viewing eye. That one eye’s behavior can be predicted from the other eye is the standard assumption that guides most oculomotor studies. Here we investigate in neurotypicals whether the magnitude of the area of fixation excursion and the component FEMs are the same or different for each eye during fixation. We employed the similar methodology as in Chandna et al. (2021) of recording both eyes simultaneously during binocular and monocular viewing. Interestingly, we found differences in the variability and speed of ocular drift, and the magnitude of microsaccades between the two eyes, yet microsaccades always occurred in both eyes at the same time and in the same direction. The results suggest that neither Hering’s nor Helmholtz’s theories of oculomotor control are completely correct, nor are they completely wrong. Instead, the data are consistent with a hybrid model which generates yoked conjugate eye movements that in turn interact with independent slow control [ 24 ]. Methods Subjects. Eight participants (7 naïve, 5 female, ages 25–42) were tested. All participants were interviewed about their clinical history including detailed vision history. All participants underwent comprehensive clinical testing including visual acuity with a LogMAR chart, stereoacuity with the TNO Stereotest, and the “hole in the card” ocular sighting dominance test. Participants all had corrected normal visual acuity of 20/25 or better; normal stereoacuity; and no history of eye disease including strabismus, amblyopia or early onset eye disease which may have led to anomalous visual development. The study was approved by Smith-Kettlewell Eye Research Institute’s Institutional Review board in accordance with the mandates set forth by the Declaration of Helsinki. All participants were informed of the study prior to participating and gave their written consent to the experiment. Stimuli. Participants fixated on a modified, digital version of the letter array used in our previous pursuit vergence experiments [ 23 ](Fig. 1 ). They were instructed and encouraged to maintain their gaze at the cross-over point of the central X and were reminded to keep the target clear throughout the experimental procedure. The stimulus was displayed on a monitor positioned on the midline 67 cm away from the participant. It was created and displayed using Psychtoolbox [ 25 , 26 , 27 ] and Tobii Pro Lab using Titta [ 28 ], an open-source software. Eye position recording. All data was recorded on the Tobii Pro Spectrum [ 29 ] using Titta [ 28 ]. Subjects were placed in a chin-rest to minimize head-motion related eye movements during the fixation task. Eye position data wer recorded at 1200 Hz from each eye even under monocular viewing through an infra-red (IR)-pass filter (Fig. 1 ). Titta’s nine-point calibration and five-point validation routines were used for each calibration before the first binocular block for every subject and after a break if needed by the participant. Trials impacted by heavy recording noise and those that suffered significant data loss (more than ~ 67% of the trial) were excluded. To compensate for excluded trials, an additional block of data was taken from the participant (as long as they were willing and able to do so). Experimental Design. Participants were instructed to fixate on the midline target for 20 seconds during trials under either monocular or binocular viewing (Fig. 1 ). Two blocks of data were acquired from each participant, with three trials in each block. In the first block, the first trial condition was binocular viewing. In the next trial, monocular occlusion was randomly assigned to one eye and fixation was repeated (monocular viewing). In the third trial, fixation was performed with the other eye occluded. The second block also had three trials, but now they were performed in reverse order; starting with each eye occluded and ending with binocular viewing. Occlusion. Participants were occluded using an IR-pass occluder, cutoff ~ 750 nm (ePlastics, San Diego, CA) affixed to one side of a pair of eye glass frames (right or left eye occluded) [ 23 ]. Data Analysis. Data analysis was performed using MATLAB (The MathWorks Ink., Natick MA). For all participants, the first 5 seconds of the 20 second fixation block was excluded from analysis to avoid errant eye movements related to acquiring fixation. Horizontal and vertical eye velocities were calculated offline from the recorded eye position signals by differentiating and filtering the raw position data (2-pole Butterworth non-causal filter, cutoff = 25 Hz). Saccade detection was done using the Engbert-Kliegl-Merganthaler method for saccade detection.[ 30 , 31 ] Horizontal and vertical eye velocity were processed through an in-house program to validate the location of saccades and subsequently remove them from velocity traces. Using custom MATLAB code, we analyzed and visualized each eye movement trial. Saccades were confirmed by visual inspection of all eye velocity and position traces by trained researchers. Any saccades that occurred near blinks were excluded as were any erroneously detected saccades, either due to blinks or noise (181 saccades excluded, 11.67%). Post-hoc analysis was done on microsaccades only, by excluding saccades with magnitudes greater than 2 deg consistent with previous studies (e.g., [ 2 , 32 , 33 ]). This procedure resulted in the exclusion of 14 (0.9%) saccades. Drift rate was quantified by calculating the instantaneous radial speed between successive pairs of position samples and then averaged over the entire trial after removing saccades and blinks. Bivariate Contour Ellipse Area (BCEA) was calculated with a 95 percent confidence interval using the following equation: $${\left(\frac{x}{{\sigma }_{x}}\right)}^{2}+{\left(\frac{y}{{\sigma }_{y}}\right)}^{2}=5.991$$ 1 Where x and y represent the horizontal and vertical coordinates for each point in the data set, and \({\sigma }_{x}\) and \({\sigma }_{y}\) represent the standard deviation of the x and y components. The value 5.991 is the scale of the ellipse and corresponds to the two-degree of freedom chi-square distribution value for the 95 percent confidence interval. The ellipse was then transformed by determining the eigenvectors based on the covariance of the data and rotating the data by the angle between the largest eigenvector and the x axis so that it is aligned with that eigenvector. The BCEA plot was overlaid on a fixation density heat map obtained by creating a 2-D contour plot using the frequency distribution of the x and y coordinates of the eye position data (see [ 34 ]). All t-tests were paired two-tailed tests, unless otherwise stated. Results This study was conducted to investigate fixation behavior of both eyes under binocular and monocular viewing. We wished to determine whether the eyes behaved the same or differently motivated by results of previous work showing asynchronous and unpredictable behavior of an occluded eye during midline pursuit [23]. Surprisingly, the eyes’ movements were not identical during binocular viewing, the standard protocol used to characterize eye movements, in fixation experiments where only one eye is measured. Fig 2 shows raw eye position traces for each eye from one observer (S1) during binocular fixation. Ocular “drift” is dissimilar between the eyes in both horizontal and vertical directions. However, microsaccades consistently occur at the same time and in the same direction in the two eyes. The differences in ocular behavior suggest that the drift was independently controlled for the two eyes. However, an alternative explanation is that uncorrelated neural noise at the level of the output circuitry controlling the eye muscles was responsible for the differential eye movement behavior. To determine whether the observed differences in ocular behavior were due to independent control or neural noise, we covered one of the eyes effectively decoupling the influence of the stimulus from the yoking-like behavior observed in oculomotor experiments. If uncorrelated noise was creating the differential ocular behavior, covering an eye should not change the general character of that eye’s behavior. However, if the eyes are controlled independently, the independent control mechanism for the covered eye should suffer without a visual anchor. When covering an eye, we found striking differences in the ocular behavior relative to binocular viewing in terms of the excursion variability and drift speed, as well as differences in microsaccade magnitude. Fixation variability To quantify the differences in excursion variability between the eyes, we computed the 95% bivariate contour ellipse area (BCEA), a 2-dimensional measure of standard deviation (see Methods), for each eye during monocular and binocular viewing. Figure 3A shows BCEAs (red dashed lines) superimposed on fixation density plots (see Methods) for each eye of a typical observer in each condition. During binocular viewing, the BCEA is more compact compared to BCEA’s during monocular viewing, demonstrating better control of fixation. However, note that even during binocular viewing the right and left eye BCEAs are not identical with the right eye’s BCEA being larger than left eye’s BCEA for this observer. During monocular viewing a strikingly different pattern emerged. The covered eye now showed much greater variability than it did during binocular viewing. Even more surprisingly, the monocular viewing eye’s BCEA was larger than that of the same eye during binocular viewing. Figure 3B summarizes the BCEAs for monocular and binocular viewing for each subject, as well as the group means. To characterize the differences in BCEA during binocular viewing, we compared the eye with the smaller BCEA in each subject to the eye with the larger BCEA and found their difference to be significant ( Fig 3B ) (mean smaller BCEA = 0.498±0.08, mean larger BCEA = 0.668±0.11, t(7)=-4.76, p < .01). During monocular viewing, we expected that the covered eye would wander more, since it would be less able to maintain its position without a visual target to anchor it. Yoked control proposed by Hering would allow the viewing eye’s use of the target to keep the covered eye stable, but the Chandna et al. (2021) finding that the covered eye’s behavior was not determined by the target implied less stability here. Consistent with this, the covered eyes’ BCEAs were significantly larger than those of the fellow viewing eyes (mean viewing BCEA = 0.829±0.05, mean covered BCEA = 1.681±0.07, t(7)=-2.10, p < .05). This result suggests that the covered eye was more poorly controlled when it was unable to view the fixation stimulus, and furthermore that its control was not coupled to that of the viewing eye . Finally, the BCEA’s of the monocular viewing eyes were significantly larger than the BCEAs during binocular viewing (mean smaller BCEA = 0.498±0.08, mean monocular viewing BCEA = 0.829±0.05, t(7)=5.93, p < .001; mean larger BCEA = 0.668±0.11, mean monocular viewing BCEA = 0.829±0.05, t(7)=3.11, p < .01). Apparently, the monocular viewing eye was more poorly controlled than it was when both eyes viewed the fixation stimulus. Therefore, even though it appears that different circuitry controls each eye, a benefit was gleaned from both eyes viewing the target. Drift Speed A larger BCEA might result from the eye being more poorly controlled, leading to a less stable retinal image. Further evidence for poorer control with larger BCEAs was discovered when we investigated the relationship between BCEA size and the speed of the eyes’ drift. Figure 4 shows mean drift speed in the different conditions for each observer superimposed on the group mean. Overall, drift speed was no different between binocular and monocular viewing (p = .97) and varied considerably across observers. Moreover, the drift speeds did not differ between the two eyes during binocular viewing (p = .45). While overall the covered eye moved faster than the monocular viewing eye this result did not reach significance (mean viewing = 0.879±0.04, mean covered = 0.954±0.07, t(7)=-1.91, p = .097). Despite failing to reach significance, there appeared to be a rough relationship between BCEA and drift speed. To explore this, we correlated these measures for each eye in both viewing conditions. Figure 5 shows that the correlation during binocular viewing for each eye was weak and did not reach statistical significance (Pearson correlation, eye with smaller BCEA: t(6)=1.71, p=0.14 p= 0.1588; eye with larger BCEA: t(6)=1.29, p=0.24). However, for both eyes during monocular viewing the correlations between drift speed and BCEA were significant (Pearson correlation, covered eye: t(6)=4.10, p=0.0064; viewing eye: t(6)=3.60, p=0.011). The relatively constant relationship between a smaller BCEA and slow drift speed supports the idea that when the eyes are better controlled, smaller BCEAs result. Conversely, strong correlations between drift speed and BCEA during monocular viewing suggest that the eyes were more poorly controlled than they were during binocular viewing. Microsaccades One function of microsaccades is to correct for the eye’s displacement from a target, presumably because drift moved it away.[34] However, experiments in previous studies were performed when only one eye was recorded, assuming drift and corrective microsaccades would operate the same in the unrecorded eye (see [35] for a review). The current study provides only limited support for this assumption. Regarding microsaccade rate, some degree of yoking is apparent, as every microsaccade we recorded occurs in both eyes. However, the size of microsaccades can vary between the two eyes, so the eyes do not appear to be strictly yoked during microsaccades, yoking is merely apparent when they are triggered. Figure 6 shows microsaccade rate and size across conditions. Fig 6A shows that microsaccade rate was identical for the two eyes during binocular viewing, as saccades were always binocular. This was also true for the covered and viewing eyes during monocular viewing. It has been reported previously that a microsaccade in one eye is always accompanied by a microsaccade in the other eye during binocular viewing [36], and apparently this is also the case when only one eye views the target. Interestingly, there was a tendency for more microsaccades to occur during monocular than binocular viewing, but this difference did not reach significance (mean binocular = 0.717±0.13, mean monocular = 0.928±0.17, t(7) = -2.66, p = 0.11) . Fig 6B plots the magnitude of microsaccades in the different conditions for all observers. Microsaccade magnitude was marginally significantly different between the eyes in the binocular condition with the eye with the smaller BCEA having smaller microsaccades (mean smaller BCEA = 0.391±0.034, mean larger BCEA = 0.441±0.045, t(7)=-2.32, p = .053). During monocular viewing, saccade magnitudes were also slightly larger in the covered than the viewing eyes (mean viewing = 0.404±0.029, mean covered = 0.425±0.030, t(7)=-2.291, p = .056). This difference is curious though, as it suggests that microsaccades, while apparently always occurring in both eyes, are not completely conjugate when an eye is covered and even when viewing is binocular. Microsaccades are thought to correct for error induced in their position when drift carries the eyes away from a target. [34] If so, larger microsaccades might be required to correct error when BCEAs are larger, because larger BCEAs indicate that an eye has strayed further from the target. To determine this, we correlated BCEA size with microsaccade magnitude for each eye in the different conditions ( Fig 7 ). As expected, microsaccade magnitude was well correlated with BCEA size and significantly so during binocular viewing, both for the larger BCEA (t(6)=2.97, p=0.025) as well as for the smaller one (t(6)=4.93, p=0.0026). During monocular viewing, microsaccade magnitude was also significantly correlated with BCEA size for the viewing eye (t(6)=3.46, p=0.013). Curiously though, this correlation was not present for the covered eye (t(6)=0.57, p=0.59). We think this is because the microsaccades in the covered eye did not have a visual target to correct to, and therefore the drift and microsaccades were decoupled in that eye. Instead, it appeared that the covered eye’s microsaccades were driven by the microsaccades in the viewing eye, as these were highly correlated (R=0.989; t(6)=16.25 p=0.000). Since the viewing eyes drift was apparently indirectly driving the microsaccades in the covered eye, it is not surprising that these two values were highly correlated as well (R=0.774; t(6)=2.99 p=0.024). Discussion Previous work used monocular occlusion to demonstrate that the eyes appear not to be yoked for vergence.[23] Here, we employed binocular viewing and monocular occlusion to test if yoked eye movement control is present or not during fixational eye movements. We found that during binocular fixation BCEAs in the two eyes significantly differed in size, possibly due to different noise in the output of the eye control circuits. Covering an eye resulted in a much larger BCEA in that eye than it showed during binocular viewing, suggesting noise is not the sole cause of BCEA differences, rather the eyes are independently controlled. Interestingly, the BCEAs of the monocular viewing eyes were also larger than they were during binocular viewing. Larger BCEAs were apparently the result of poorer ocular control as they resulted in less ocular stability evidenced by faster drift speed. While microsaccades were always binocular, their magnitudes could differ in the two eyes. For viewing eyes, microsaccade magnitude correlated with BCEA size, consistent with them correcting for the larger-BCEA eyes drifting farther. Covered eye microsaccades were not correlated with BCEA size, as their size mirrored those in the viewing eye in the same trial, presumably because these microsaccades were correcting drift in the viewing eye. A previous study cast doubt on yoked eye commands and Hering’s Law, but for midline vergence. [23] In that study, observers used smooth pursuit to follow a physical target that moved along a track aligned on the midline in a lit room. While binocular eye movements followed the target with appropriate vergence, covering an eye resulted in completely unpredictable behavior of the covered eye. The covered eye occasionally showed weak vergence, or barely any movement at all, but most commonly it moved in a conjugate fashion. While Hering’s Law would describe the vergence and conjugate signals needed to generate all of these eye movements, it does not predict them as it merely theorizes underlying signals that can reorient the eyes together to any location in 3-D space. [9] However, Hering’s Law posits that a single signal drives vergence, and we found that a covered eye was delayed (often on the order of seconds) relative to the viewing eye, and it is not possible to drive the eyes at different times with a single signal. The present results provide evidence that yoked eye commands according to Hering’s Law does not operate during fixational eye movements, at least for slow drift, and instead the eyes are independently controlled. We found that even during binocular viewing the BCEAs of the two eyes had different sizes suggesting the eyes were moving differently. While these differences were small, they were magnified when we covered an eye. During monocular viewing, not only was the BCEA in the covered eye much larger than the viewing one, the viewing eye’s BCEA was larger than it was when both eyes were viewing. This is a curious finding that may arise from summation in the visual system that has been shown to occur when both eyes view a target relative to when just one eye views it..[4,37,38,39] We think the differences in BCEA size were due to how well each eye was controlled, i.e., eyes with larger BCEAs were more poorly controlled. This explanation was supported by the finding that in general faster drift accompanied larger BCEAs. While weak during binocular viewing, a strong positive correlation between BCEA size and drift speed was found during monocular viewing in which the eyes strayed farther. These results suggest that using the word “drift” to describe the slow movements of the eyes is a misnomer, instead these smooth movements are controlled. Generation of drift has been described as “slow control” in the past [40], although the idea of independent drift control has not been previously advanced. It is curious that the idea of yoked eye commands has survived for almost 150 years, despite that the theory of independent control was introduced at about the same time [22] and recent evidence supports it. [41] We can think of several reasons why this might be the case. One is that in laboratory experiments that present a target on a tangent screen, the eyes simply move as if they are yoked. This certainly should happen if both eyes follow the stimulus optimally. But here, not only are the eye movements correlated with each other, they are also correlated with the stimulus, which could certainly be driving both eyes. Our manipulation of covering an eye breaks the correlation between that eye and the stimulus, revealing that the stimulus is at least partly responsibly for putative “yoked” behaviour. The other reason that Hering’s theory has dominated for so long is that saccades may actually be at least partially yoked, and at least triggered by a unitary conjugate command. In support of saccade yoking, microsaccades in our study were always generated simultaneously in both eyes during binocular as well as monocular viewing. Other work has specifically investigated the yoked nature of microsaccades during binocular viewing for thousands of microsaccades and failed to find a single monocular microsaccade. [36] While microsaccades in our study were always generated by both eyes, there were systematic differences in their amplitude under both monocular and binocular viewing. Specifically, larger saccades were present for eyes with larger BCEAs. This suggests to us that while these microsaccades may be initiated with a single command, at some point in their execution, they are controlled independently, allowing them to correct for different drift excursions of each eye. Curiously however, BCEA size and saccade magnitude were uncorrelated in the covered eyes. But saccade magnitude was correlated in the covered and viewing eyes during monocular viewing. These findings suggest the covered-eye saccades defaulting to being driven by the same signal that was sent to the viewing eye when no visual target was present. There is evidence that independent control signals coexist with conjugate signals in the abducens nucleus in the brainstem’s final oculomotor path, which could support modulation of a conjugate signal by independent eye commands.[41, 42] We proposed a model in which independent control modulates unitary conjugate signals that potentially explains the similarity and difference in binocular microsaccades observed here. [24] It might be argued that the differences we see in drift between the two eyes are merely caused by neural noise. However, if that noise was present in a yoked command, it would have to be present on both eyes, yielding identical BCEAs. Even if there was a unitary command for executing an eye movement, it would have to branch at some point to drive the two eyes and the six extraocular muscles that control each of them. There are multiple oculomotor nuclei in the brainstem that carry separate eye or muscle signals, and noise could be introduced at that stage of processing. Thus, we think that the observed differences between the two eyes resulted from independent mechanisms that controlled the eyes with varying precision. A question arises as how the slow movements in the covered eye are driven. If the eyes were yoked, a covered eye would simply be driven by the same command that drove the viewing eye. However, given our results questioning yoking, an unknown mechanism is driving the covered eye. One alternative is that there is a “virtual” target created in the brain by the seeing eye’s input, and the virtual target drives the covered eye, which our model proposes.[24] Without direct access to the visual signal, the virtual target would afford a less precise signal to which the occluded eye has access. The differences in BCEA behavior during binocular viewing will have implications for strabismus management. Intermittent strabismus often precedes constant strabismus with significant implications for management choice of non-surgical versus surgical intervention.[43,44,45,46,47] Current observational clinical methods do not reliably predict this deterioration. Measuring fixation behavior under binocular and monocular viewing with analysis of FEMs could help with these predictions. Differences in BCEAs, drift speed and microsaccades for each eye at presentation and measured over time may provide an early indicator of worsening of intermittent strabismus triggering timely and less-invasive intervention and provide indicators of improvement after intervention leading to improved outcomes. Declarations Author Contribution S.H., A.C., S.W. and D.S. contributed to the conceptual framework of the manuscript and its experimental design. D.S. and S.W. prepared the figures within manuscript. D.S. collected and analyzed data presented within manuscript. Acknowledgement We acknowledge the NEI, Smith-Kettlewell Eye Research Institute, Wright State University and Alder-Hey Children's Hospital for supporting this manuscript. Data Availability Data is only available upon request to the corresponding author. We are taking this precaution to protect the identity of the subjects that participated in the human behavior study that resulted in this manuscript. 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Fixation instability in amblyopia: Oculomotor disease biomarkers predictive of treatment effectiveness. Progress in brain research, 249 , 235–248. Murray, J., Gupta, P., Dulaney, C., Garg, K., Shaikh, A. G., & Ghasia, F. F. (2022). Effect of viewing conditions on fixation eye movements and eye alignment in amblyopia. Investigative ophthalmology & visual science, 63 (2), 33–33. Von Helmholtz, H. Handbuch der Physiologischen Optik: Mit 213 in den Text Eingedruckten Holzschnitten und 11 Tafeln . Vol. 9. (Voss, 1867). Chandna, A., Badler, J., Singh, D., Watamaniuk, S., & Heinen, S. (2021). A covered eye fails to follow an object moving in depth. Scientific Reports, 11 (1), 10983. Heinen, S., Chandna, A., Singh, D., Watamaniuk, S. (2023). A new binocular control model (ISMYS) generates miniature eye movements of fixation. Society for Neuroscience (SFN) Annual Meeting, Washington D.C., November, 2023. Brainard DH (1997) The psychophysics toolbox. Spat Vis 10:433–436. CrossRef Medline Pelli DG (1997) The VideoToolbox software for visual psychophysics: transforming numbers into movies. Spat Vis 10:437–442. CrossRef Medline Kleiner M, Brainard D, Pelli D, Ingling A, Murray R, Broussard C (2007) What’s new in Psychtoolbox-3. Perception 36:1–16. Niehorster, D. C., Andersson, R., & Nyström, M. (2020). Titta: A toolbox for creating PsychToolbox and Psychopy experiments with Tobii eye trackers. Behavior research methods, 52 , 1970–1979. Tobii. (2018). Most Advanced Eye Tracking System - TOBII Pro Spectrum. Tobii. https://www.tobii.com/products/eye-trackers/screen-based/tobii-pro-spectrum Engbert R, Kliegl R. Microsaccades uncover the orientation of covert attention. Vision Res 43:1035–1045, 2003. Engbert R, Mergenthaler K. Microsaccades are triggered by low retinal image slip. Proc Natl Acad Sci 103: 7192–7197, 2006. Thaler, L., Schütz, A. C., Goodale, M. A., & Gegenfurtner, K. R. (2013). What is the best fixation target? The effect of target shape on stability of fixational eye movements. Vision research, 76 , 31–42. Betta, E., & Turatto, M. (2006). Are you ready? I can tell by looking at your microsaccades. Neuroreport, 17(10), 1001–1004. Cherici, C., Kuang, X., Poletti, M., & Rucci, M. (2012). Precision of sustained fixation in trained and untrained observers. Journal of vision, 12 (6), 31–31. Poletti, M., & Rucci, M. (2016). A compact field guide to the study of microsaccades: Challenges and functions. Vision research, 118 , 83–97. Fang, Y., Gill, C., Poletti, M., & Rucci, M. (2018). Monocular microsaccades: Do they really occur?. Journal of vision, 18 (3), 18–18. Campbell, F. W., & Green, D. G. (1965, October 9). Monocular versus binocular visual acuity. Nature , 208, 191–192 Blake, R., Sloane, M., & Fox, R. (1981). Further developments in binocular summation. Perception & Psychophysics, 30 , 266–276. Frisén, L. A. R. S., & Lindblom, B. E. R. T. I. L. (1988). Binocular summation in humans: evidence for a hierarchic model. The Journal of Physiology, 402 (1), 773–782. Steinman, R. M. Haddad, G. M. Skavenski, A. A. Wyman, D. (1973). Miniature eye movement. Science, 181, 810–819. Zhou, W., & King, W. M. (1998). Premotor commands encode monocular eye movements. Nature, 393 (6686), 692–695. Sylvestre, P. A., & Cullen, K. E. (2002). Dynamics of abducens nucleus neuron discharges during disjunctive saccades. Journal of neurophysiology, 88 (6), 3452–3468. Wright, K. W. (2006). INTERMITTENT EXOTROPIA. Handbook of Pediatric Strabismus and Amblyopia, 266 von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. St Louis, USA: Mosby-Year Book Inc, 2002. Hiles DA, Davies GT, Costenbader FD. Long-Term observations on un- operated intermittent exotropia. Arch Ophthalmol 1968;80:436–442. Ludwig IH, Parks MM, Getson PR, et al. Rate of deterioration in accommodative esotropia correlated to the AC/A relationship. J Pediatr Ophthalmol Strabismus. 1988;25:8–12. Fu, Valeria LN, David R. Stager, and Eileen E. Birch. "Progression of intermittent, small-angle, and variable esotropia in infancy." Investigative ophthalmology & visual science 48.2 (2007): 661–664. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4426926","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":309054775,"identity":"53024c81-77bc-49bd-836b-0c3a429fa878","order_by":0,"name":"Arvind Chandna","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAArklEQVRIiWNgGAWjYJCCAwxsNgwSzCRqSSNRCwMD22EGCaIV888+Y3i4oOy8vWQ7A+Pjil9EaJE4l2NweMa524mzmRmYDc/2EWPNGbaEw7xttxPkmBnYJBt7iNAhD9Fyzp54LQZnmA8AtRxgnA3S0vCDCC2GIC0855ITZzYzNhs2NhChRe4MY/NnnjI7e4nzhw8+bPhDhBYkwNjAwNhGmhYQINGWUTAKRsEoGBkAAG9mMulC+vb6AAAAAElFTkSuQmCC","orcid":"","institution":"Smith Kettlewell Eye Research Institute","correspondingAuthor":true,"prefix":"","firstName":"Arvind","middleName":"","lastName":"Chandna","suffix":""},{"id":309054778,"identity":"144572d5-69e2-4ae6-8ce6-197a7ca364ec","order_by":1,"name":"Devashish Singh","email":"","orcid":"","institution":"Smith Kettlewell Eye Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Devashish","middleName":"","lastName":"Singh","suffix":""},{"id":309054780,"identity":"a459e7ca-3191-437c-9e9a-853d7629a714","order_by":2,"name":"Scott Watamaniuk","email":"","orcid":"","institution":"Wright State University","correspondingAuthor":false,"prefix":"","firstName":"Scott","middleName":"","lastName":"Watamaniuk","suffix":""},{"id":309054788,"identity":"7330d541-938c-4359-b927-5571b1a6231b","order_by":3,"name":"Stephen Heinen","email":"","orcid":"","institution":"Smith Kettlewell Eye Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Stephen","middleName":"","lastName":"Heinen","suffix":""}],"badges":[],"createdAt":"2024-05-15 18:42:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4426926/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4426926/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57925947,"identity":"46d48a30-f7bd-47ab-9eb3-20e722fd9f18","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":55042,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental paradigm. Observers fixate on an \"X” located on the midline and centered in a multi-letter array. Viewing was binocular, or monocular in each eye implemented with occlusion of the fellow eye with IR-pass filters. Timeline shows the sequence of data collection.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/f4054b5cbeb30f59e48ef51c.png"},{"id":57925948,"identity":"b712c5fe-23f6-425d-bbe4-723de4e297b3","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":65600,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRaw horizontal and vertical eye position traces during binocular viewing\u003c/strong\u003e. Red trace right eye and blue trace left eye. Saccades are in the blue bands. Drift is different between the two eyes, while microsaccades are binocular. Data from subject S1.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/38399f02a40569203d9283fd.png"},{"id":57925953,"identity":"78a66696-c61f-440e-abdd-4a179d3d25b3","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":193158,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFixation variability differs between the eyes in binocular and monocular viewing. \u003c/strong\u003eA) BCEAs from single eyes of a typical observer (S1). BCEA magnitudes are inset in lower right corners. Monocular data are from only the right eye. B) Summary of BCEAs for all observers in all conditions and group means (grey bars). Eye data are shown separately. For binocular viewing, data are grouped as best and worst eyes. Significance is shown by asterisks over brackets.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/23798c94b86b23a24182cb87.png"},{"id":57925950,"identity":"de5b4055-c8c3-4191-b5d1-abf5837fb6f7","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":41836,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMean drift speed.\u003c/strong\u003e Shown are data from individual observers and group means (grey bars). Although variable across observers, overall, conditions with larger BCEAs have higher drift speeds, but not significantly.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/0c8cde95e447810b9ca76702.png"},{"id":57925949,"identity":"40a7cdb4-cdcc-47d8-b3d1-1e177b8339ae","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":44116,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBCEA is correlated with drift speed within each condition.\u003c/strong\u003e During binocular viewing the correlation between BCEA and drift speed is weak, presumably because the eyes are well controlled. Poorer control of the eyes during monocular viewing results in strong correlations between BCEA and drift speed. Data points correspond to individual observers with best fit lines for each condition.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/bbb2473a2632980d3ad8848f.png"},{"id":57925952,"identity":"110b6aec-8f7f-4f12-8d19-7cc387065d86","added_by":"auto","created_at":"2024-06-07 14:21:45","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":105013,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMicrosaccade rate and magnitude. \u003c/strong\u003eA)\u003cstrong\u003e \u003c/strong\u003eMicrosaccade rate under both viewing conditions for each observer and group means (grey bars). All microsaccades were binocular. B) Microsaccade magnitude for all conditions and observers. Note that microsaccades were larger in the covered eyes.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/bcff354529eaaf5b60895dad.png"},{"id":57926275,"identity":"ba94982c-0d42-4bb5-9fb0-e0be11f68def","added_by":"auto","created_at":"2024-06-07 14:29:45","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":44494,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBCEA size is correlated with saccade magnitude in viewing, but not covered eyes.\u003c/strong\u003e Fits are for the covered eyes (black) and viewing eyes (red) during monocular viewing, and larger (blue) and smaller (green) BCEAs during binocular viewing.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/6c76c604b80555798828031f.png"},{"id":65379466,"identity":"ebc77426-30c3-4504-a1fa-a6b2b30cc7d9","added_by":"auto","created_at":"2024-09-26 17:31:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":953515,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4426926/v1/c54752c3-8330-4681-aea6-105b3787eefd.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Asymmetric eye movements during fixation challenge Hering’s Law of a unitary neural command","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOcular fixation is employed in many studies of visual processing and during strabismus assessment to keep the eyes stable at a fixed location. Yet, fixation is a misnomer, as during fixation the eyes are virtually never still. Miniature eye movements known as \u0026ldquo;fixational eye movements\u0026rdquo; (FEMs) continuously occur during fixation. The most studied FEMs are microsaccades, rapid movements of the eyes that resemble large saccades, but are limited in magnitude to less than 1 deg, or even less than 0.5 deg, as this smaller size constrains them to the foveola, a one-degree diameter region of the central fovea (for discussion see [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]). Microsaccades are thought to prevent retinal fading (e.g., [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]), and to direct the eyes back to the fixation point or to image features in micro scenes [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] as large saccades do during normal viewing. The other dominant type of FEM is a slow, smooth rotation of the eye known as \u0026ldquo;drift\u0026rdquo; [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].While drift is less studied than microsaccades, there is evidence that it sharpens images by moving them across ganglion cell receptive fields at a velocity appropriate to emphasize high spatial frequency content and improve acuity [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDuring studies of ocular fixation in neurotypicals, only one eye is generally recorded under the assumption of Hering\u0026rsquo;s Law, i.e. that the eyes are yoked, and therefore the other rotates identically [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Hering\u0026rsquo;s Law states that there is equal innervation to the eyes, and hence a unitary neural command is thought to drive all eye movements. Since this theory guides most research on the behavior and physiology of the oculomotor system, classic models of eye movement control are constructed with a single input and a single output. The assumption of symmetric eye movements extends to the clinic in that it guides intervention in constant and intermittent strabismus [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, in studies of amblyopia and strabismus in which both eyes are recorded there are differences in fixation characteristics between the amblyopic and the non-amblyopic fellow eye [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], suggesting the eyes are not yoked in these disorders, and possibly not at all. In strabismus with amblyopia the amblyopic eye is more unstable and both eyes were more unstable than controls [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] and even in strabismus without amblyopia the unaligned eye is more unstable [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].Reports of the cause of fixation instability in strabismus and amblyopia are varied and include slower drift speeds, increased drift amplitude and velocity, disconjugate saccades, increased saccadic intrusions or a combination of these factors. The type and severity of amblyopia and strabismus, and even methodological differences may have a bearing on the results[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] While these results raise doubts about the validity of Hering\u0026rsquo;s Law. However, the absence of a systematic investigation of binocular fixation in neurotypicals leaves open the possibility that Hering\u0026rsquo;s Law is correct.\u003c/p\u003e \u003cp\u003eAn alternative to Hering\u0026rsquo;s Law that could explain non-yoked behavior was proposed by Helmholtz (1867), who suggested that the eyes are controlled independently and we learn to move them together in early childhood [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Despite that Helmholtz\u0026rsquo;s opposing theory was postulated contemporaneously with Hering\u0026rsquo;s, critical experiments differentiating these theories are lacking, and Hering\u0026rsquo;s theory has managed to prevail for over 150 years. However, recent results of experiments comparing oculomotor behavior of both eyes during monocular viewing challenge Hering\u0026rsquo;s Law [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. This study found asymmetric behavior between the two eyes during smooth pursuit of a midline target, evidence that the eyes are not yoked. Furthermore, the covered eye\u0026rsquo;s rotation was idiosyncratic between observers, and even within an observer from trial to trial and therefore, could not be predicted from recording only the viewing eye. That one eye\u0026rsquo;s behavior can be predicted from the other eye is the standard assumption that guides most oculomotor studies.\u003c/p\u003e \u003cp\u003eHere we investigate in neurotypicals whether the magnitude of the area of fixation excursion and the component FEMs are the same or different for each eye during fixation. We employed the similar methodology as in Chandna et al. (2021) of recording both eyes simultaneously during binocular and monocular viewing. Interestingly, we found differences in the variability and speed of ocular drift, and the magnitude of microsaccades between the two eyes, yet microsaccades always occurred in both eyes at the same time and in the same direction. The results suggest that neither Hering\u0026rsquo;s nor Helmholtz\u0026rsquo;s theories of oculomotor control are completely correct, nor are they completely wrong. Instead, the data are consistent with a hybrid model which generates yoked conjugate eye movements that in turn interact with independent slow control [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cb\u003eSubjects.\u003c/b\u003e Eight participants (7 na\u0026iuml;ve, 5 female, ages 25\u0026ndash;42) were tested. All participants were interviewed about their clinical history including detailed vision history. All participants underwent comprehensive clinical testing including visual acuity with a LogMAR chart, stereoacuity with the TNO Stereotest, and the \u0026ldquo;hole in the card\u0026rdquo; ocular sighting dominance test. Participants all had corrected normal visual acuity of 20/25 or better; normal stereoacuity; and no history of eye disease including strabismus, amblyopia or early onset eye disease which may have led to anomalous visual development. The study was approved by Smith-Kettlewell Eye Research Institute\u0026rsquo;s Institutional Review board in accordance with the mandates set forth by the Declaration of Helsinki. All participants were informed of the study prior to participating and gave their written consent to the experiment.\u003c/p\u003e \u003cp\u003e\u003cb\u003eStimuli.\u003c/b\u003e Participants fixated on a modified, digital version of the letter array used in our previous pursuit vergence experiments [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e](Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). They were instructed and encouraged to maintain their gaze at the cross-over point of the central X and were reminded to keep the target clear throughout the experimental procedure. The stimulus was displayed on a monitor positioned on the midline 67 cm away from the participant. It was created and displayed using Psychtoolbox [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] and Tobii Pro Lab using Titta [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], an open-source software.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eEye position recording.\u003c/b\u003e All data was recorded on the Tobii Pro Spectrum [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] using Titta [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Subjects were placed in a chin-rest to minimize head-motion related eye movements during the fixation task. Eye position data wer recorded at 1200 Hz from each eye even under monocular viewing through an infra-red (IR)-pass filter (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Titta\u0026rsquo;s nine-point calibration and five-point validation routines were used for each calibration before the first binocular block for every subject and after a break if needed by the participant. Trials impacted by heavy recording noise and those that suffered significant data loss (more than ~\u0026thinsp;67% of the trial) were excluded. To compensate for excluded trials, an additional block of data was taken from the participant (as long as they were willing and able to do so).\u003c/p\u003e \u003cp\u003e \u003cb\u003eExperimental Design.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eParticipants were instructed to fixate on the midline target for 20 seconds during trials under either monocular or binocular viewing (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Two blocks of data were acquired from each participant, with three trials in each block. In the first block, the first trial condition was binocular viewing. In the next trial, monocular occlusion was randomly assigned to one eye and fixation was repeated (monocular viewing). In the third trial, fixation was performed with the other eye occluded. The second block also had three trials, but now they were performed in reverse order; starting with each eye occluded and ending with binocular viewing.\u003c/p\u003e \u003cp\u003e \u003cb\u003eOcclusion.\u003c/b\u003e Participants were occluded using an IR-pass occluder, cutoff\u0026thinsp;~\u0026thinsp;750 nm (ePlastics, San Diego, CA) affixed to one side of a pair of eye glass frames (right or left eye occluded) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eData Analysis.\u003c/b\u003e Data analysis was performed using MATLAB (The MathWorks Ink., Natick MA). For all participants, the first 5 seconds of the 20 second fixation block was excluded from analysis to avoid errant eye movements related to acquiring fixation. Horizontal and vertical eye velocities were calculated offline from the recorded eye position signals by differentiating and filtering the raw position data (2-pole Butterworth non-causal filter, cutoff\u0026thinsp;=\u0026thinsp;25 Hz). Saccade detection was done using the Engbert-Kliegl-Merganthaler method for saccade detection.[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] Horizontal and vertical eye velocity were processed through an in-house program to validate the location of saccades and subsequently remove them from velocity traces. Using custom MATLAB code, we analyzed and visualized each eye movement trial. Saccades were confirmed by visual inspection of all eye velocity and position traces by trained researchers. Any saccades that occurred near blinks were excluded as were any erroneously detected saccades, either due to blinks or noise (181 saccades excluded, 11.67%). Post-hoc analysis was done on microsaccades only, by excluding saccades with magnitudes greater than 2 deg consistent with previous studies (e.g., [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]). This procedure resulted in the exclusion of 14 (0.9%) saccades. Drift rate was quantified by calculating the instantaneous radial speed between successive pairs of position samples and then averaged over the entire trial after removing saccades and blinks. Bivariate Contour Ellipse Area (BCEA) was calculated with a 95 percent confidence interval using the following equation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${\\left(\\frac{x}{{\\sigma }_{x}}\\right)}^{2}+{\\left(\\frac{y}{{\\sigma }_{y}}\\right)}^{2}=5.991$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003ex\u003c/em\u003e and \u003cem\u003ey\u003c/em\u003e represent the horizontal and vertical coordinates for each point in the data set, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{x}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{y}\\)\u003c/span\u003e\u003c/span\u003e represent the standard deviation of the x and y components. The value 5.991 is the scale of the ellipse and corresponds to the two-degree of freedom chi-square distribution value for the 95 percent confidence interval. The ellipse was then transformed by determining the eigenvectors based on the covariance of the data and rotating the data by the angle between the largest eigenvector and the x axis so that it is aligned with that eigenvector. The BCEA plot was overlaid on a fixation density heat map obtained by creating a 2-D contour plot using the frequency distribution of the x and y coordinates of the eye position data (see [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]). All t-tests were paired two-tailed tests, unless otherwise stated.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eThis study was conducted to investigate fixation behavior of both eyes under binocular and monocular viewing. We wished to determine whether the eyes behaved the same or differently motivated by results of previous work showing asynchronous and unpredictable behavior of an occluded eye during midline pursuit [23].\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Surprisingly, the eyes\u0026rsquo; movements were not identical during binocular viewing, the standard protocol used to characterize eye movements, in fixation experiments where only one eye is measured. \u003cstrong\u003eFig 2\u0026nbsp;\u003c/strong\u003eshows raw eye position traces for each eye from one observer (S1) during binocular fixation. Ocular \u0026ldquo;drift\u0026rdquo; is dissimilar between the eyes in both horizontal and vertical directions. However, microsaccades consistently occur at the same time and in the same direction in the two eyes.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe differences in ocular behavior suggest that the drift was independently controlled for the two eyes. However, an alternative explanation is that uncorrelated neural noise at the level of the output circuitry controlling the eye muscles was responsible for the differential eye movement behavior.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo determine whether the observed differences in ocular behavior were due to independent control or neural noise, we covered one of the eyes effectively decoupling the influence of the stimulus from the yoking-like behavior observed in oculomotor experiments. If uncorrelated noise was creating the differential ocular behavior, covering an eye should not change the general character of that eye\u0026rsquo;s behavior. However, if the eyes are controlled independently, the independent control mechanism for the covered eye should suffer without a visual anchor.\u003c/p\u003e\n\u003cp\u003eWhen covering an eye, we found striking differences in the ocular behavior relative to binocular viewing in terms of the excursion variability and drift speed, as well as differences in microsaccade magnitude. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFixation variability\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;To quantify the differences in excursion variability between the eyes, we computed the 95% bivariate contour ellipse area (BCEA), a 2-dimensional measure of standard deviation (see Methods), for each eye during monocular and binocular viewing.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Figure 3A shows BCEAs (red dashed lines) superimposed on fixation density plots (see Methods) for each eye of a typical observer in each condition. During binocular viewing, the BCEA is more compact compared to BCEA\u0026rsquo;s during monocular viewing, demonstrating better control of fixation. However, note that even during binocular viewing the right and left eye BCEAs are not identical with the right eye\u0026rsquo;s BCEA being larger than left eye\u0026rsquo;s BCEA for this observer. During monocular viewing a strikingly different pattern emerged. The covered eye now showed much greater variability than it did during binocular viewing. Even more surprisingly, the monocular viewing eye\u0026rsquo;s BCEA was larger than that of the same eye during binocular viewing.\u003c/p\u003e\n\u003cp\u003eFigure 3B summarizes the BCEAs for monocular and binocular viewing for each subject, as well as the group means. To characterize the differences in BCEA during binocular viewing, we compared the eye with the smaller BCEA in each subject to the eye with the larger BCEA and found their difference to be significant (\u003cstrong\u003eFig 3B\u003c/strong\u003e) (mean smaller BCEA = 0.498\u0026plusmn;0.08, mean larger BCEA = 0.668\u0026plusmn;0.11, t(7)=-4.76, p \u0026lt; .01). During monocular viewing, we expected that the covered eye would wander more, since it would be less able to maintain its position without a visual target to anchor it. Yoked control proposed by Hering would allow the viewing eye\u0026rsquo;s use of the target to keep the covered eye stable, but the Chandna et al. (2021) finding that the covered eye\u0026rsquo;s behavior was not determined by the target implied less stability here. Consistent with this, the covered eyes\u0026rsquo; BCEAs were significantly larger than those of the fellow viewing eyes (mean viewing BCEA = 0.829\u0026plusmn;0.05, mean covered BCEA = 1.681\u0026plusmn;0.07, t(7)=-2.10, p \u0026lt; .05). This result suggests that the covered eye was more poorly controlled when it was unable to view the fixation stimulus, and furthermore that its control was not coupled to that of the viewing eye\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eFinally, the BCEA\u0026rsquo;s of the monocular viewing eyes were significantly larger than the BCEAs during binocular viewing (mean smaller BCEA = 0.498\u0026plusmn;0.08, mean monocular viewing BCEA = 0.829\u0026plusmn;0.05, t(7)=5.93, p \u0026lt; .001; mean larger BCEA = 0.668\u0026plusmn;0.11, mean monocular viewing BCEA = 0.829\u0026plusmn;0.05, t(7)=3.11, p \u0026lt; .01). Apparently, the monocular viewing eye was more poorly controlled than it was when both eyes viewed the fixation stimulus. Therefore, even though it appears that different circuitry controls each eye, a benefit was gleaned from both eyes viewing the target.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDrift Speed\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA larger BCEA might result from the eye being more poorly controlled, leading to a less stable retinal image. Further evidence for poorer control with larger BCEAs was discovered when we investigated the relationship between BCEA size and the speed of the eyes\u0026rsquo; drift.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 4\u003c/strong\u003e shows mean drift speed in the different conditions for each observer superimposed on the group mean. Overall, drift speed was no different between binocular and monocular viewing (p = .97) and varied considerably across observers. Moreover, the drift speeds did not differ between the two eyes during binocular viewing (p = .45). While overall the covered eye moved faster than the monocular viewing eye this result did not reach significance (mean viewing = 0.879\u0026plusmn;0.04, mean covered = 0.954\u0026plusmn;0.07, t(7)=-1.91, p = .097). Despite failing to reach significance, there appeared to be a rough relationship between BCEA and drift speed. To explore this, we correlated these measures for each eye in both viewing \u0026nbsp;conditions. \u003cstrong\u003eFigure 5\u003c/strong\u003e shows that the correlation during binocular viewing for each eye was weak and did not reach statistical significance (Pearson correlation, eye with smaller BCEA: t(6)=1.71, p=0.14 p= 0.1588; eye with larger BCEA: t(6)=1.29, p=0.24). However, for both eyes during monocular viewing the correlations between drift speed and BCEA were significant (Pearson correlation, covered eye: t(6)=4.10, p=0.0064; viewing eye: t(6)=3.60, p=0.011). The relatively constant relationship between a smaller BCEA and slow drift speed supports the idea that when the eyes are better controlled, smaller BCEAs result. Conversely, strong correlations between drift speed and BCEA during monocular viewing suggest that the eyes were more poorly controlled than they were during binocular viewing.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMicrosaccades\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOne function of microsaccades is to correct for the eye\u0026rsquo;s displacement from a target, presumably because drift moved it away.[34] However, experiments in previous studies were performed when only one eye was recorded, assuming drift and corrective microsaccades would operate the same in the unrecorded eye (see [35] for a review). The current study provides only limited support for this assumption. Regarding microsaccade rate, some degree of yoking is apparent, as every microsaccade we recorded occurs in both eyes. However, the size of microsaccades can vary between the two eyes, so the eyes do not appear to be strictly yoked during microsaccades, yoking is merely apparent when they are triggered.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 6\u003c/strong\u003e shows microsaccade rate and size across conditions. \u003cstrong\u003eFig 6A\u003c/strong\u003e shows that microsaccade rate was identical for the two eyes during binocular viewing, as saccades were always binocular. This was also true for the covered and viewing eyes during monocular viewing. It has been reported previously that a microsaccade in one eye is always accompanied by a microsaccade in the other eye during binocular viewing [36], and apparently this is also the case when only one eye views the target. Interestingly, there was a tendency for more microsaccades to occur during monocular than binocular viewing, but this difference did not reach significance (mean binocular = 0.717\u0026plusmn;0.13, mean monocular = 0.928\u0026plusmn;0.17, t(7) = -2.66, p = 0.11) .\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Fig 6B plots the magnitude of microsaccades in the different conditions for all observers. Microsaccade magnitude was marginally significantly different between the eyes in the binocular condition with the eye with the smaller BCEA having smaller microsaccades (mean smaller BCEA = 0.391\u0026plusmn;0.034, mean larger BCEA = 0.441\u0026plusmn;0.045, t(7)=-2.32, p = .053). During monocular viewing, saccade magnitudes were also slightly larger in the covered than the viewing eyes (mean viewing = 0.404\u0026plusmn;0.029, mean covered = 0.425\u0026plusmn;0.030, t(7)=-2.291, p = .056). This difference is curious though, as it suggests that microsaccades, while apparently always occurring in both eyes, are not completely conjugate when an eye is covered and even when viewing is binocular.\u003c/p\u003e\n\u003cp\u003eMicrosaccades are thought to correct for error induced in their position when drift carries the eyes away from a target. [34] If so, larger microsaccades might be required to correct error when BCEAs are larger, because larger BCEAs indicate that an eye has strayed further from the target. To determine this, we correlated BCEA size with microsaccade magnitude for each eye in the different conditions (\u003cstrong\u003eFig 7\u003c/strong\u003e). As expected, microsaccade magnitude was well correlated with BCEA size and significantly so during binocular viewing, both for the larger BCEA (t(6)=2.97, p=0.025) as well as for the smaller one (t(6)=4.93, p=0.0026). During monocular viewing, microsaccade magnitude was also significantly correlated with BCEA size for the viewing eye (t(6)=3.46, p=0.013). Curiously though, this correlation was not present for the covered eye (t(6)=0.57, p=0.59). We think this is because the microsaccades in the covered eye did not have a visual target to correct to, and therefore the drift and microsaccades were decoupled in that eye.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eInstead, it appeared that the covered eye\u0026rsquo;s microsaccades were driven by the microsaccades in the viewing eye, as these were highly correlated (R=0.989; t(6)=16.25 \u0026nbsp;p=0.000). Since the viewing eyes drift was apparently indirectly driving the microsaccades in the covered eye, it is not surprising that these two values were highly correlated as well (R=0.774; t(6)=2.99 \u0026nbsp;p=0.024).\u0026nbsp;\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003ePrevious work used monocular occlusion to demonstrate that the eyes appear not to be yoked for vergence.[23] Here, we employed binocular viewing and monocular occlusion to test if yoked eye movement control is present or not during fixational eye movements. We found that during binocular fixation BCEAs in the two eyes significantly differed in size, possibly due to different noise in the output of the eye control circuits. Covering an eye resulted in a much larger BCEA in that eye than it showed during binocular viewing, suggesting noise is not the sole cause of BCEA differences, rather the eyes are independently controlled. Interestingly, the BCEAs of the monocular viewing eyes were also larger than they were during binocular viewing. Larger BCEAs were apparently the result of poorer ocular control as they resulted in less ocular stability evidenced by faster drift speed. While microsaccades were always binocular, their magnitudes could differ in the two eyes. For viewing eyes, microsaccade magnitude correlated with BCEA size, consistent with them correcting for the larger-BCEA eyes drifting farther. Covered eye microsaccades were not correlated with BCEA size, as their size mirrored those in the viewing eye in the same trial, presumably because these microsaccades were correcting drift in the viewing eye.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA previous study cast doubt on yoked eye commands and Hering\u0026rsquo;s Law, but for midline vergence. [23] In that study, observers used smooth pursuit to follow a physical target that moved along a track aligned on the midline in a lit room. While binocular eye movements followed the target with appropriate vergence, covering an eye resulted in completely unpredictable behavior of the covered eye. The covered eye occasionally showed weak vergence, or barely any movement at all, but most commonly it moved in a conjugate fashion. While Hering\u0026rsquo;s Law would describe the vergence and conjugate signals needed to generate all of these eye movements, it does not \u003cstrong\u003e\u003cem\u003epredict\u003c/em\u003e\u003c/strong\u003e them as it merely theorizes underlying signals that can reorient the eyes together to any location in 3-D space. [9] However, Hering\u0026rsquo;s Law posits that a single signal drives vergence, and we found that a covered eye was delayed (often on the order of seconds) relative to the viewing eye, and it is not possible to drive the eyes at different times with a single signal.\u003c/p\u003e\n\u003cp\u003eThe present results provide evidence that yoked eye commands according to Hering\u0026rsquo;s Law does not operate during fixational eye movements, at least for slow drift, and instead the eyes are independently controlled. We found that even during binocular viewing the BCEAs of the two eyes had different sizes suggesting the eyes were moving differently. While these differences were small, they were magnified when we covered an eye. During monocular viewing, not only was the BCEA in the covered eye much larger than the viewing one, the viewing eye\u0026rsquo;s BCEA was larger than it was when both eyes were viewing. This is a curious finding that may arise from summation in the visual system that has been shown to occur when both eyes view a target relative to when just one eye views it..[4,37,38,39]\u003c/p\u003e\n\u003cp\u003eWe think the differences in BCEA size were due to how well each eye was controlled, i.e., eyes with larger BCEAs were more poorly controlled. This explanation was supported by the finding that in general faster drift accompanied larger BCEAs. While weak during binocular viewing, a strong positive correlation between BCEA size and drift speed was found during monocular viewing in which the eyes strayed farther. These results suggest that using the word \u0026ldquo;drift\u0026rdquo; to describe the slow movements of the eyes is a misnomer, instead these smooth movements are controlled. Generation of drift has been described as \u0026ldquo;slow control\u0026rdquo; in the past [40], although the idea of independent drift control has not been previously advanced.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIt is curious that the idea of yoked eye commands has survived for almost 150 years, despite that the theory of independent control was introduced at about the same time [22] and recent evidence supports it. [41] We can think of several reasons why this might be the case. One is that in laboratory experiments that present a target on a tangent screen, the eyes simply move as if they are yoked. This certainly should happen if both eyes follow the stimulus optimally. But here, not only are the eye movements correlated with each other, they are also correlated with the stimulus, which could certainly be driving both eyes. Our manipulation of covering an eye breaks the correlation between that eye and the stimulus, revealing that the stimulus is at least partly responsibly for putative \u0026ldquo;yoked\u0026rdquo; behaviour. The other reason that Hering\u0026rsquo;s theory has dominated for so long is that saccades may actually be at least partially yoked, and at least triggered by a unitary conjugate command.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn support of saccade yoking, microsaccades in our study were always generated simultaneously in both eyes during binocular as well as monocular viewing. Other work has specifically investigated the yoked nature of microsaccades during binocular viewing for thousands of microsaccades and failed to find a single monocular microsaccade. [36] While microsaccades in our study were always generated by both eyes, there were systematic differences in their amplitude under both monocular and binocular viewing. Specifically, larger saccades were present for eyes with larger BCEAs. This suggests to us that while these microsaccades may be initiated with a single command, at some point in their execution, they are controlled independently, allowing them to correct for different drift excursions of each eye. Curiously however, BCEA size and saccade magnitude were uncorrelated in the covered eyes. But saccade magnitude \u003cem\u003ewas\u003c/em\u003e correlated in the covered and viewing eyes during monocular viewing. These findings suggest the covered-eye saccades defaulting to being driven by the same signal that was sent to the viewing eye when no visual target was present. There is evidence that independent control signals coexist with conjugate signals in the abducens nucleus in the brainstem\u0026rsquo;s final oculomotor path, which could support modulation of a conjugate signal by independent eye commands.[41, 42] We proposed a model in which independent control modulates unitary conjugate signals that potentially explains the similarity and difference in binocular microsaccades observed here. [24]\u003c/p\u003e\n\u003cp\u003eIt might be argued that the differences we see in drift between the two eyes are merely caused by neural noise. \u0026nbsp;However, if that noise was present in a yoked command, it would have to be present on both eyes, yielding identical BCEAs. Even if there was a unitary command for executing an eye movement, it would have to branch at some point to drive the two eyes and the six extraocular muscles that control each of them. There are multiple oculomotor nuclei in the brainstem that carry separate eye or muscle signals, and noise could be introduced at that stage of processing. Thus, we think that the observed differences between the two eyes resulted from independent mechanisms that controlled the eyes with varying precision.\u003c/p\u003e\n\u003cp\u003eA question arises as how the slow movements in the covered eye are driven. If the eyes were yoked, a covered eye would simply be driven by the same command that drove the viewing eye. However, given our results questioning yoking, an unknown mechanism is driving the covered eye. One alternative is that there is a \u0026ldquo;virtual\u0026rdquo; target created in the brain by the seeing eye\u0026rsquo;s input, and the virtual target drives the covered eye, which our model proposes.[24] Without direct access to the visual signal, the virtual target would afford a less precise signal to which the occluded eye has access.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe differences in BCEA behavior during binocular viewing will have implications for strabismus management. Intermittent strabismus often precedes constant strabismus with significant implications for management choice of non-surgical versus surgical intervention.[43,44,45,46,47] Current observational clinical methods do not reliably predict this deterioration. Measuring fixation behavior under binocular and monocular viewing with analysis of FEMs could help with these predictions. Differences in BCEAs, drift speed and microsaccades for each eye at presentation and measured over time may provide an early indicator of worsening of intermittent strabismus triggering timely and less-invasive intervention and provide indicators of improvement after intervention leading to improved outcomes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eS.H., A.C., S.W. and D.S. contributed to the conceptual framework of the manuscript and its experimental design. D.S. and S.W. prepared the figures within manuscript. D.S. collected and analyzed data presented within manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe acknowledge the NEI, Smith-Kettlewell Eye Research Institute, Wright State University and Alder-Hey Children's Hospital for supporting this manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData is only available upon request to the corresponding author. We are taking this precaution to protect the identity of the subjects that participated in the human behavior study that resulted in this manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePoletti, M., \u0026amp; Rucci, M. (2016). A compact field guide to the study of microsaccades: Challenges and functions. Vision research, \u003cem\u003e118\u003c/em\u003e, 83\u0026ndash;97.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMartinez-Conde, S., Macknik, S. L., Troncoso, X. G., \u0026amp; Dyar, T. A. 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Birch. \"Progression of intermittent, small-angle, and variable esotropia in infancy.\" Investigative ophthalmology \u0026amp; visual science 48.2 (2007): 661\u0026ndash;664.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4426926/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4426926/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eVisual perception is largely supported by tiny fixational eye movements (FEMs), the control of which is a mystery. Understanding eye movement control has been impeded by researchers recording one eye, assuming the other behaves symmetrically as if \u0026ldquo;yoked\u0026rdquo; (Hering\u0026rsquo;s Law). However, recent work revealed asynchronous smooth eye movements during vergence, challenged Hering\u0026rsquo;s Law, suggesting FEMs might also be decoupled. We assessed FEMs of both eyes during binocular and monocular viewing. During binocular viewing, eye movement variability (BCEA) was larger in one eye, implying the eyes were independently controlled. During monocular viewing, covered eyes\u0026rsquo; BCEAs were larger than viewing ones. Eyes with larger BCEAs drifted faster, suggesting poorer eye control. In contrast, microsaccades were always binocular, yet larger in viewing eyes with larger BCEAs, indicating independent control improved their drift correction. Our results support independent control of smooth eye movements and elucidate the genesis of ocular misalignment relevant to strabismus intervention.\u003c/p\u003e","manuscriptTitle":"Asymmetric eye movements during fixation challenge Hering’s Law of a unitary neural command","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-07 14:21:40","doi":"10.21203/rs.3.rs-4426926/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d5b244f9-c67f-403b-9c6c-42cb84682e8b","owner":[],"postedDate":"June 7th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":32655402,"name":"Biological sciences/Neuroscience/Oculomotor system/Saccades"},{"id":32655403,"name":"Health sciences/Diseases/Eye diseases/Vision disorders"}],"tags":[{"value":"featured","date":"2024-06-07 23:33:49"}],"updatedAt":"2024-09-26T17:23:34+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-07 14:21:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4426926","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4426926","identity":"rs-4426926","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}