Binocular summation for reflexive eye movements

Different rules for binocular combination of luminance flicker in cortical and subcortical pathways

How does the human brain combine information across the eyes? It has been known for many years that cortical normalization mechanisms implement 'ocularity invariance': equalizing neural responses to spatial patterns presented either monocularly or binocularly. Here, we used a novel combination of electrophysiology, psychophysics, pupillometry, and computational modeling to ask whether this invariance also holds for flickering luminance stimuli with no spatial contrast. We find dramatic violations of ocularity invariance for these stimuli, both in the cortex and also in the subcortical pathways that govern pupil diameter. Specifically, we find substantial binocular facilitation in both pathways with the effect being strongest in the cortex. Near-linear binocular additivity (instead of ocularity invariance) was also found using a perceptual luminance matching task. Ocularity invariance is, therefore, not a ubiquitous feature of visual processing, and the brain appears to repurpose a generic normalization algorithm for different visual functions by adjusting the amount of interocular suppression.

Refining Clinical Quantification of Depth of Suppression in Amblyopia through Synoptophore Measurement

BACKGROUND

Amblyopia is associated with unbalanced suppression between the two eyes. Existing clinical measures of suppression, such as the Worth 4 Dot test, provide qualitative information about suppression but cannot precisely quantify it. The Synoptophore, a well-established instrument in binocular vision clinics, has historically been used to gauge suppression qualitatively as well but has the capability to quantify suppression. We extended the capability of the Synoptophore through the development of a systematic protocol of illumination manipulation to quantify suppression in amblyopia.

METHODS

Twenty-six previously treated adult amblyopes underwent our protocol on the Synoptophore to measure the illumination balance needed to obtain fusion responses. Separately, these same amblyopes were tested with Worth 4 Dot as it is classically performed in the United States, utilizing different test distances and room illuminations to qualify the suppression response.

RESULTS

Smaller, more central targets revealed larger magnitudes of suppression for both the Synoptophore and Worth 4 Dot tests (Synoptophore: χ25,26 = 25.538, p < 0.001; Worth 4 Dot: χ23,26 = 39.020, p < 0.001). There was a significant correlation between the two tests for depth of suppression measurements (rΤ > 0.345, p < 0.036), with more sensitivity measured by the Synoptophore, as suppression could be graded on a quantitative scale. Strabismic amblyopes demonstrated more suppression than non-strabismic amblyopes (z > 2.410, p < 0.016). Additionally, depth of suppression was correlated with interocular difference in both visual acuity (rΤ = 0.604, p < 0.001) and stereoacuity (rΤ = 0.488, p = 0.001).

CONCLUSIONS

We extended the utility of the Synoptophore by measuring its illuminance outputs and developing a suppression testing protocol that compared favorably with Worth 4 Dot (clinic standard) while improving upon the latter through more sensitive quantification of suppression.

Effectiveness of SMILE Combined with Micro-Monovision in Presbyopic Patients: A Pilot Study

Binocular summation along all defocus range after a micro-monovision procedure has scarcely been studied. The aim of this pilot study was to evaluate the efficacy of SMILE combined with different levels of micro-monovision in presbyopic patients and to assess the binocular summation effect on contrast sensitivity defocus curves (CSDC) at the 6-month follow-up. Efficacy was assessed on the basis of visual acuity (VA) and stereopsis at far, intermediate, and near distances. Patient-reported outcomes (PROs) and binocular CSDC were also evaluated. Six patients completed the study with a programmed median anisometropia of 0.81 Diopter. The median binocular uncorrected VA was better than 0 logMAR at the three evaluated distances, and stereopsis was not impaired in any patient, achieving a median of ≤119 arcsec at any distance. CSDC increased binocularly after surgery, significantly in the range of -2 to -3 D (p < 0.05). No clinically relevant changes were observed in PROs compared with the preoperative period, and all patients achieved spectacle independence at intermediate/near distance and were likely or very likely to undergo the same surgery. In conclusion, micro-monovision with SMILE could be an effective procedure, with results that might be comparable to other laser correction techniques specifically designed for presbyopia correction.

Ocular-following responses in school-age children

Ocular following eye movements have provided insights into how the visual system of humans and monkeys processes motion. Recently, it has been shown that they also reliably reveal stereoanomalies, and, thus, might have clinical applications. Their translation from research to clinical setting has however been hindered by their small size, which makes them difficult to record, and by a lack of data about their properties in sizable populations. Notably, they have so far only been recorded in adults. We recorded ocular following responses (OFRs)–defined as the change in eye position in the 80–160 ms time window following the motion onset of a large textured stimulus–in 14 school-age children (6 to 13 years old, 9 males and 5 females), under recording conditions that closely mimic a clinical setting. The OFRs were acquired non-invasively by a custom developed high-resolution video-oculography system, described in this study. With the developed system we were able to non-invasively detect OFRs in all children in short recording sessions. Across subjects, we observed a large variability in the magnitude of the movements (by a factor of 4); OFR magnitude was however not correlated with age. A power analysis indicates that even considerably smaller movements could be detected. We conclude that the ocular following system is well developed by age six, and OFRs can be recorded non-invasively in young children in a clinical setting.

Horizontal Heterophoria Modifications by Means of Thin Proprioceptive Stimulations Applied on the Foot Sole: A Randomised Study

Some authors have demonstrated that proprioceptive stimuli applied on the feet soles can interfere on the ocular muscles. However, these studies do not clarify possible functional differences between the dominant eye and the non-dominant eye. The purpose of this randomised study is to establish if the positioning of an Internal Heel Wedge (IHW) and an External Heel Wedge (EHW) can modify horizontal heterophoria, determine dissimilar behaviours between the dominant eye and the non-dominant eye. Forty-two healthy subjects, with a right dominant eye, were tested. The 1.5 mm-thick proprioceptive stimuli were shaped out of a cork half-moon. The experimental group was divided into two groups: IHW group and EHW group. Both groups performed the "Baseline" (without mechanical stimulation) and "After 15'" (following a fitting period of 15 minutes on a treadmill with mechanical stimulation) trials. The control group performed the same trials without any podalic stimulation. Meaningful changes were observed on the horizontal heterophoria of the non-dominating eye with an IHW. Non-statistically significant variations were observed with an EHW and in the Control group. A thin heel wedge applied on the foot sole was able to generate functional changes in the non-dominant eye and could help health professionals develop increasingly personalised rehabilitation programmes.

Interocular velocity cues elicit vergence eye movements in mice

We stabilize the dynamic visual world on our retina by moving our eyes in response to motion signals. Coordinated movements between the two eyes are characterized as version when both eyes move in the same direction and vergence when the two eyes move in opposite directions. Vergence eye movements are necessary to track objects in three dimensions. In primates they can be elicited by intraocular differences in either spatial signals (disparity) or velocity, requiring the integration of left and right eye inputs. Whether mice are capable of similar behaviors is not known. To address this issue, we measured vergence eye movements in mice using a stereoscopic stimulus known to elicit vergence eye movements in primates. We found that mice also exhibit vergence eye movements, although at a low gain and that the primary driver of these vergence eye movements is interocular motion. Spatial disparity cues alone are ineffective. We also found that the vergence eye movements we observed in mice were robust to silencing visual cortex and to manipulations that disrupt the normal development of binocularity in visual cortex. A sublinear combination of motor commands driven by monocular signals is sufficient to account for our results.NEW & NOTEWORTHY The visual system integrates signals from the left and right eye to generate a representation of the world in depth. The binocular integration of signals may be observed from the coordinated vergence eye movements elicited by object motion in depth. We explored the circuits and signals responsible for these vergence eye movements in rodent and find these vergence eye movements are generated by a comparison of the motion and not spatial visual signals.

Center-surround velocity-based segmentation: Speed, eccentricity, and timing of visual stimuli interact to determine interocular dominance

We used a novel method to capture the spatial dominance pattern of competing motion fields at rivalry onset. When rivaling velocities were different, the participants reported center-surround segmentation: The slower stimuli often dominated in the center while faster motion persisted along the borders. The size of the central static/slow field scaled with the stimulus size. The central dominance was time-locked to the static stimulus onset but was disrupted if the dynamic stimulus was presented later. We then used the same stimuli as masks in an interocular suppression paradigm. The local suppression strengths were probed with targets at different eccentricities. Consistent with the center-surround segmentation, target speed and location interacted with mask velocities. Specifically, suppression power of the slower masks was nonhomogenous with eccentricity, providing a potential explanation for center-surround velocity-based segmentation. This interaction of speed, eccentricity, and timing has implications for motion processing and interocular suppression. The influence of different masks on which target features get suppressed predicts that some "unconscious effects" are not generalizable across masks and, thus, need to be replicated under various masking conditions.

Binocular Summation for Reflexive Eye Movements: A Potential Diagnostic Tool for Stereodeficiencies

Purpose

Stereoscopic vision, by detecting interocular correlations, enhances depth perception. Stereodeficiencies often emerge during the first months of life, and left untreated can lead to severe loss of visual acuity in one eye and/or strabismus. Early treatment results in much better outcomes, yet diagnostic tests for infants are cumbersome and not widely available. We asked whether reflexive eye movements, which in principle can be recorded even in infants, can be used to identify stereodeficiencies.

Methods

Reflexive ocular following eye movements induced by fast drifting noise stimuli were recorded in 10 adult human participants (5 with normal stereoacuity, 5 stereodeficient). To manipulate interocular correlation, the stimuli shown to the two eyes were either identical, different, or had opposite contrast. Monocular presentations were also interleaved. The participants were asked to passively fixate the screen.

Results

In the participants with normal stereoacuity, the responses to binocular identical stimuli were significantly larger than those induced by binocular opposite stimuli. In the stereodeficient participants the responses were indistinguishable. Despite the small size of ocular following responses, 40 trials, corresponding to less than 2 minutes of testing, were sufficient to reliably differentiate normal from stereodeficient participants.

Conclusions

Ocular-following eye movements, because of their reliance on cortical neurons sensitive to interocular correlations, are affected by stereodeficiencies. Because these eye movements can be recorded noninvasively and with minimal participant cooperation, they can potentially be measured even in infants and might thus provide an useful screening tool for this currently underserved population.

How Ocular Dominance and Binocularity Are Reflected by the Population Receptive Field Properties

Purpose

The neural substrate of binocularity and sighting ocular dominance in humans is not clear. By utilizing the population receptive field (pRF) modeling technique, we explored whether these phenomena are associated with amplitude and pRF size differences.

Methods

The visual field maps of 13 subjects were scanned (3-T Skyra) while viewing drifting bar stimuli. Both eyes (binocular condition), the dominant eye and the nondominant eye (two monocular conditions) were stimulated in separate sessions. For each condition, pRF size and amplitude were assessed. Binocular summation ratios were calculated by dividing binocular by mean monocular values (amplitude and pRF size).

Results

No differences in pRF size were seen between the viewing conditions within each region, that is, either between binocular and monocular or between dominant and nondominant viewing conditions. Binocular amplitudes were higher than the monocular amplitudes, but similar among the dominant and nondominant eyes. Binocular summation ratios derived from amplitudes were significantly higher than one (∼1.2), while those ratios derived from pRF size were not. These effects were found in all studied areas along the visual hierarchy, starting in V1.

Conclusions

Neither the amplitude nor the pRF size show intereye difference and therefore cannot explain the different roles of the dominant and the nondominant eyes. Binocular, as compared to monocular vision, resulted in higher amplitudes, while receptive fields' sizes were similar, suggesting increased binocular response intensity as the basis for the binocular summation phenomenon. Our results could be applicable in imaging studies of monocular disease and studies that deal with nondisparity binocularity effects.