The effect of distractors on saccades and adaptation of saccades in strabismus

This paper reports two experiments to determine the contribution of the suppressing eye to the generation of saccadic eye movements in constant strabismus. Eye movements were recorded using a Skalar infra-red recorder. Experiment 1 tested six participants with constant strabismus, pathological suppression and no clinically demonstrable binocular single vision (BSV). We explored the effect of visual distractors presented monocularly (to either the fixing eye or the strabismic eye) and binocularly, on saccade latency and accuracy. Saccade latency significantly increased when distractors were presented to the strabismic eye compared to the no distractor condition. In all participants the effect on latency, with distractors presented to the strabismic eye, was maximum when distractors were presented towards the location of the anatomical fovea. Saccade accuracy was reduced with ipsilateral distractors to the target when presented binocularly or monocularly to the fixing eye but not affected by distractors presented to the strabismic eye. Experiment 2 investigated fast disconjugate saccade adaptations in six participants with constant strabismus, pathological suppression and no clinically demonstrable BSV and for comparison 8 with normal bifoveal BSV. Saccade disconjugacy was induced using an electronic feedback system in which the calibrated eye movement position signal could be scaled by a factor (the feedback gain) to move the target visible to one eye during binocular viewing. In all BSV participants and 3 of 6 participants with constant strabismus, saccadic adaptation occurred rapidly such that under conditions of visual feedback saccades became increasingly disconjugate. These disconjugacies persisted when normal viewing conditions were restored. The presence of an adaptive mechanism to adjust the binocular co-ordination of saccades in the presence of constant strabismus with suppression and no clinically demonstrable BSV has been demonstrated. Mechanisms that might explain such results are discussed.     

The effect of distractors on saccades and adaptation of saccades in strabismus

This paper reports two experiments to determine the contribution of the suppressing eye to the generation of saccadic eye movements in constant strabismus. Eye movements were recorded using a Skalar infra-red recorder. Experiment 1 tested six participants with constant strabismus, pathological suppression and no clinically demonstrable binocular single vision (BSV). We explored the effect of visual distractors presented monocularly (to either the fixing eye or the strabismic eye) and binocularly, on saccade latency and accuracy. Saccade latency significantly increased when distractors were presented to the strabismic eye compared to the no distractor condition. In all participants the effect on latency, with distractors presented to the strabismic eye, was maximum when distractors were presented towards the location of the anatomical fovea. Saccade accuracy was reduced with ipsilateral distractors to the target when presented binocularly or monocularly to the fixing eye but not affected by distractors presented to the strabismic eye. Experiment 2 investigated fast disconjugate saccade adaptations in six participants with constant strabismus, pathological suppression and no clinically demonstrable BSV and for comparison 8 with normal bifoveal BSV. Saccade disconjugacy was induced using an electronic feedback system in which the calibrated eye movement position signal could be scaled by a factor (the feedback gain) to move the target visible to one eye during binocular viewing. In all BSV participants and 3 of 6 participants with constant strabismus, saccadic adaptation occurred rapidly such that under conditions of visual feedback saccades became increasingly disconjugate. These disconjugacies persisted when normal viewing conditions were restored. The presence of an adaptive mechanism to adjust the binocular co-ordination of saccades in the presence of constant strabismus with suppression and no clinically demonstrable BSV has been demonstrated. Mechanisms that might explain such results are discussed.     

Disconjugate adaptation of saccades: contribution of binocular and monocular mechanisms.

We studied the effects of prism-induced disparity on static and intrasaccadic alignment in six normal human subjects. A ten diopter base-out prism, calling for convergence, was placed in front of the central field of the right eye, so that at the center the eye viewed through the prism; at left and right, outside the prism. During 15 min of training, subjects made repetitive saccades solely in the right field of vision (C-R-C sequence). This paradigm required relative divergence for centrifugal (C-R) saccades and relative convergence for centripetal (R-C) saccades, as well as increase of the amplitude for all saccades made by the right eye. We found that during training, all subjects incorporated the necessary change in alignment into the saccades. After training the resultant intrasaccadic disconjugacy persisted when tested during monocular viewing, indicating that motor learning had occurred. Subjects demonstrated increased divergence for C-R and increased convergence for R-C saccades, in accordance with the change acquired during adaptation to the prism. In addition, five subjects developed increased divergence for C-L saccades, for which they did not train. Smaller and less consistent divergence was also observed for L-C saccades. Changes in intrasaccadic alignment were accompanied by changes in the relative velocities of the two eyes' saccades and slowing of the peak velocities in both eyes during training. Static alignment showed a general tendency toward convergence that did not parallel the changes in the intrasaccadic alignment, suggesting that saccade adaptation is system-specific. The pattern of transfer of the intrasaccadic disconjugacy to saccades in the untrained field and the changes in the relative speeds of the two eyes cannot be explained by monocular adjustment of the saccades. Our results indicate that both a binocular mechanism--saccade-vergence interaction--and monocular adaptation contribute to disconjugate adaptation of saccades.     

The effect of monocular defocus on binocular contrast sensitivity

The monocular contrast sensitivity loss with defocus is well known. We measured binocular contrast sensitivity of sinewave gratings of 6 c/deg in the presence of different levels of monocular defocus. In the absence of defocus, the binocular sensitivity was about 42% higher than monocular. With increasing monocular defocus, the binocular sensitivity decreased steadily until it reached a level below the monocular, showing binocular inhibition. The clinical implications of binocular inhibition with monocular defocus are discussed.     

Comparative monocular brightness contributions in normal binocular observers

In order to find out whether equal monocular light stimuli result in equal monocular brightness sensations in normal binocular observers, one-half of a bipartite field presented to one eye was matched to the other half seen by the opposite eye while a binocular surrounding field maintained constant adaptation. Ratios of monocular luminances required for brightness matches showed an overall average mismatch of 2% (SD = 2) in 12 subjects. This value is significant at the 0.01 level, and only one subject showed a mismatch greater than 6% (more than 2 SD from the mean). It is concluded that about two-thirds of normal binocular observers would be expected to show monocular contributions to brightness that match each other within 4% or better.     

The effect of monocular versus binocular fixation on accommodative hysteresis

Changes in tonic accommodation were compared under monocular (6 or 10 D) and binocular (6 D/6 MA or 10 D/10 MA) inducing conditions in 26 visually normal young adults. Tonic accommodation was measured immediately before and after an 8 min fixation period. Statistically equivalent increases in tonic accommodation were produced under the two modes and levels of fixation. Even those individuals who showed large (greater than or equal to +0.70 D) monocularly induced changes were equally responsive under binocular fixation conditions. The results suggest that monocularly and binocularly induced accommodative hysteresis effects are similar in magnitude, regardless of the potential presence of vergence-driven accommodation under binocular but not under monocular viewing conditions.     

Colored filters and screens for studying monocular perception in binocular vision

Mutually exclusive red and green glasses can be used with white, black, or colored (red or green) screens to study monocular perception in binocular vision. An unmarked white screen viewed through the glasses is a simple test for anomalous retinal correspondence in strabismus. These patients observe a characteristic division of colors in accordance with alignment of their visual axes because the macular region of the deviating eye predominates over the functionally corresponding peripheral area of the fixating eye. This divided color field phenomenon in anomalous correspondence also can be demonstrated by having the patient note changes in the color of a white test object against a black screen or dark colorless background; however, a red or green screen adds additional refinement. The field of perception of one eye then can be plotted while the other eye maintains fixation and a full field of vision. Also, suppression scotomas and the position of the blind spot of the deviating eye can be plotted.Either white cards with red and green spots, or colored cards (red or green) with black and white spots are useful with mutually exclusive red and green glasses to test binocular perception in near vision. Three vertically aligned spots are used. Two are perceived through one filter and one through the other filter. In contrast to the conventional Worth four-light test, transparencies and retroillumination are not required, and the spots easily can be varied in size and separation. Dots are used to test bifoveal versus bimacular perception.From the John E. Weeks Institute of Ophthalmology, The University of Oregon Medical School, Portland, Oregon, U.S.A.     

Temporal dynamics of ocular aberrations: monocular vs binocular vision

The temporal dynamics of ocular aberrations are important for the evaluation of, e.g. the accuracy of aberration estimates, the correlation to visual performance, and the requirements for real-time correction with adaptive optics. Traditionally, studies on the eye's dynamic behavior have been performed monocularly, which might have affected the results. In this study we measured aberrations and their temporal dynamics both monocularly and binocularly in the relaxed and accommodated state for six healthy subjects. Temporal frequencies up to 100 Hz were measured with a fast-acquisition Hartmann–Shack wavefront sensor having an open field-of-view configuration which allowed fixation to real targets. Wavefront aberrations were collected in temporal series of 5 s duration during binocular and monocular vision with fixation targets at 5 m and 25 cm distance. As expected, a larger temporal variability was found in the root-mean-square wavefront error when the eye accommodated, mainly for frequencies lower than 30 Hz. A statistically-significant difference in temporal behavior between monocular and binocular viewing conditions was found. However, on average it was too small to be of practical importance, although some subjects showed a notably higher variability for the monocular case during near vision. We did find differences in pupil size with mono- and binocular vision but the pupil size temporal dynamics did not behave in the same way as the aberrations' dynamics.     

Binocular interactions in normal and anomalous binocular vision

Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.     

State of cyclofusion reserves in subjects with normal binocular vision

The paper for the first time presents statistically processed data on the cyclofusion reserves of subjects with normal binocular vision (25) for different age groups: adolescents aged 12-14 and adults aged 18-35 years. Two types of tests for evaluating cyclofusion under conditions of haploscopy (sinoptophore) are offered. The results can be used as a criterion for evaluating cyclofusion in various oculomotor abnormalities: squint, diplopia, and cyclotropia.     

单眼与双眼老年性白内障摘除人工晶状体植入术后的双眼视觉比较分析

目的　比较分析单眼与双眼老年性白内障摘除人工晶状体植入术后双眼视功能恢复状况。方法　对单眼和双眼人工晶状体手术患者各 6 0例分别检测矫正视力、屈光状态、双眼影像 ,以及用同视机等方法检查双眼视功能。结果　两组患者的双眼同时视功能、融合功能无显著差异。双眼手术组患者的远近立体视功能显著优于单眼组。影响单眼手术患者双眼视功能恢复的主要原因是另侧非手术眼的白内障所导致的视力低下和两眼屈光参差。结论　双眼视功能恢复水平是评价人工晶状体术后视觉质量和生活能力改善的重要指标。单眼手术和双眼手术者双眼视觉的差异主要体现在立体视功能上     

Stability of retinal correspondence in normal binocular vision

Although many studies have suggested absolute stability of retinal correspondence, perhaps more have concluded that the correspondence of adult subjects with normal binocular vision is capable of small variation, particularly with strong sustained demands upon vergence, in order for binocular vision to be maintained. In this comprehensive review of the literature, the reasons for these differences are critically discussed and areas still to be resolved are pointed out.     

The separation of monocular and binocular contrast

The contrast asynchrony is a stimulus configuration that illustrates the visual system’s separable responses to luminance and luminance contrast information (Shapiro, 2008, Shapiro et al., 2004). When two disks, whose luminances modulate in phase with each other, are each surrounded by a disk, one light and one dark, observers can see both the in-phase brightness signals and the antiphase contrast signals and can separate the two. Here we present the results of experiments in which observers viewed a similar stimulus dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony. We also explore the “window shade illusion” (Shapiro, Charles, & Shear-Heyman, 2005) dichoptically and find that when a modulating disk is presented to one eye and a horizontally split black/white annulus is presented to the other, observers perceive a “shading” motion up and down the disk. This shading can be seen in either direction in the binocular condition, but it is almost always seen as moving towards low contrast in the monocular condition. These findings indicate the presence of separable retinal and cortical networks for contrast processing at different temporal and spatial scales.