Exploring the effects of degraded vision on sensorimotor performance

Purpose Many people experience unilateral degraded vision, usually owing to a developmental or age-related disorder. There are unresolved questions regarding the extent to which such unilateral visual deficits impact on sensorimotor performance; an important issue as sensorimotor limitations can constrain quality of life by restricting ‘activities of daily living’. Examination of the relationship between visual deficit and sensorimotor performance is essential for determining the functional implications of ophthalmic conditions. This study attempts to explore the effect of unilaterally degraded vision on sensorimotor performance. Methods In Experiment 1 we simulated visual deficits in 30 participants using unilateral and bilateral Bangerter filters to explore whether motor performance was affected in water pouring, peg placing, and aiming tasks. Experiment 2 (n = 74) tested the hypothesis that kinematic measures are associated with visuomotor deficits by measuring the impact of small visual sensitivity decrements created by monocular viewing on sensorimotor interactions with targets presented on a planar surface in aiming, tracking and steering tasks. Results In Experiment 1, the filters caused decreased task performance—confirming that unilateral (and bilateral) visual loss has functional implications. In Experiment 2, kinematic measures were affected by monocular viewing in two of three tasks requiring rapid online visual feedback (aiming and steering). Conclusions Unilateral visual loss has a measurable impact on sensorimotor performance. The benefits of binocular vision may be particularly important for some groups (e.g. older adults) where an inability to complete sensorimotor tasks may necessitate assisted living. There is an urgent need to develop rigorous kinematic approaches to the quantification of the functional impact of unilaterally degraded vision and of the benefits associated with treatments for unilateral ophthalmic conditions to enable informed decisions around treatment.

Nearby contours abolish the binocular advantage

That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century; however, the processes involved in combining the images from the two eyes are still not fully understood. Importantly, in natural vision, objects are rarely isolated but appear in context. It is well known that nearby contours can either facilitate or suppress detection, depending on their distance from the target and the global configuration. Here we report that at close distances collinear (but not orthogonal) flanking contours suppress detection more under binocular compared to monocular viewing, thus completely abolishing the binocular advantage, both at threshold and suprathreshold levels. In contrast, more distant flankers facilitate both monocular and binocular detection, preserving a binocular advantage up to about four times the detection threshold. Our results for monocular and binocular viewing, for threshold contrast discrimination without nearby flankers, can be explained by a gain control model with uncertainty and internal multiplicative noise adding additional constraints on detection. However, in context with nearby flankers, both contrast detection threshold and suprathreshold contrast appearance matching require the addition of both target-to-target and flank-to-target interactions occurring before the site of binocular combination. To test an alternative model, in which the interactions occur after the site of binocular combination, we performed a dichoptic contrast matching experiment, with the target presented to one eye, and the flanks to the other eye. The two models make very different predictions for abutting flanks under dichoptic conditions. Interactions after the combination site predict that the perceived contrast of the flanked target will be strongly suppressed, while interactions before the site predict the perceived contrast will be more or less veridical. The data are consistent with the latter model, strongly suggesting that the interactions take place before the site of binocular combination.

A covered eye fails to follow an object moving in depth

To clearly view approaching objects, the eyes rotate inward (vergence), and the intraocular lenses focus (accommodation). Current ocular control models assume both eyes are driven by unitary vergence and unitary accommodation commands that causally interact. The models typically describe discrete gaze shifts to non-accommodative targets performed under laboratory conditions. We probe these unitary signals using a physical stimulus moving in depth on the midline while recording vergence and accommodation simultaneously from both eyes in normal observers. Using monocular viewing, retinal disparity is removed, leaving only monocular cues for interpreting the object’s motion in depth. The viewing eye always followed the target’s motion. However, the occluded eye did not follow the target, and surprisingly, rotated out of phase with it. In contrast, accommodation in both eyes was synchronized with the target under monocular viewing. The results challenge existing unitary vergence command theories, and causal accommodation-vergence linkage.

Examining the effects of stimulus location and monocular-binocular viewing on the sound-induced flash illusion

The present study aimed to determine whether stimulus location (central or peripheral) or eye viewing condition (binocular, dominant monocular, or non-dominant monocular) had a greater magnitude of effect on perception of the sound-induced flash illusion (SIFI). Both the fission illusion (when one flash paired with two beeps is perceived as two flashes) and the fusion illusion (when two flashes paired with one beep are perceived as one flash) were measured in all location and eye viewing conditions. Analyses revealed significant fission and fusion illusions in all conditions. Additionally, we found significant differences in central and peripheral criterion levels that were driven by differences between binocular and monocular viewing conditions. Data analyses demonstrated that location of the visual stimulus had a greater magnitude of effect on the illusion than eye viewing condition. Our findings add to the growing literature supporting the mechanisms underlying central-peripheral eccentricity, and contradict the optimal integration model of the SIFI. The implications of these results would help better our understanding of the SIFI and audiovisual integration. Future studies must be conducted to confirm these results in a more representative sample.

Optokinetic stimulation induces vertical vergence, possibly through a non-visual pathway

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. However, vertical vergence can also be induced by seemingly unrelated visual conditions, such as optokinetic rotations. This study aims to investigate the effect of vision on this latter form of vertical vergence. Eight subjects (4m/4f) viewed a visual scene in head erect position in two different viewing conditions (monocular and binocular). The scene, containing white lines angled at 45° against a black background, was projected at an eye-screen distance of 2 m, and rotated 28° at an acceleration of 56°/s2. Eye movements were recorded using a Chronos Eye-Tracker, and eye occlusions were carried out by placing an infrared-translucent cover in front of the left eye during monocular viewing. Results revealed vergence amplitudes during binocular viewing to be significantly lower than those seen for monocular conditions (p = 0.003), while torsion remained unaffected. This indicates that vertical vergence to optokinetic stimulation, though visually induced, is visually suppressed during binocular viewing. Considering that vertical vergence is generally viewed as a vestibular signal, the findings may reflect a visually induced activation of a vestibular pathway.

Binocular and Monocular Coincidence-Anticipation Timing Responses

Background Coincidence-anticipation timing (CAT) responses require individuals to determine the time at which an approaching object will arrive at (time to collision) or pass by (time to passage) the observer and to then make a response coincident with this time. Previous studies suggest that under some conditions time to collision estimates are more accurate when binocular and monocular cues are combined. The purpose of this study was to compare binocular and monocular coincidence anticipation timing responses with the Bassin Anticipation Timer, a device for testing and training CAT responses. Methods: Useable data were obtained from 20 participants. Coincidence-anticipation timing responses were determined using a Bassin Anticipation Timer over a range of approaching stimulus linear velocities of 5 to 40mph. Participants stood to the left side of the Bassin Anticipation track. The track was below eye height. The participants’ task was to push a button to coincide with arrival of the approaching stimulus at a location immediately adjacent to the participant. CAT responses were made under three randomized conditions: binocular viewing, monocular dominant eye viewing, and monocular non-dominant eye viewing. Results: Signed (constant), unsigned (absolute), and variable (standard deviation) CAT response errors were determined and compared across viewing conditions at each stimulus velocity. There were no significant differences in CAT errors between the conditions at any stimulus velocity, although the differences in signed and unsigned errors approached significance at 40mph. Conclusions: The addition of binocular cues did not result in a reduction in coincidence anticipation timing response errors compared to the monocular viewing conditions. There were no differences in CAT response errors between the monocular dominant eye viewing and monocular non-dominant eye viewing conditions.

Changes in Fixation Stability with Time during Binocular and Monocular Viewing in Maculopathy

The purpose of this study was to examine changes in fixation stability over time during binocular and monocular viewing in patients with age-related macular degeneration (AMD). Seventeen patients with AMD and 17 controls were enrolled. Using an EyeLink eyetracker (SR Research Ltd., Mississauga, Ontario, Canada), fixation stability was recorded binocularly and monocularly with each eye for a duration of 15 s while the fellow eye was covered. Fixation stability was analyzed over 3 s intervals for each condition using a 68% bivariate contour ellipse area. Fixation stability did not change with time during binocular viewing for both groups, both monocular conditions for the control group, and monocular viewing with the better eye for the AMD group. However, during monocular viewing with the worse eye, the test of within-subject contrasts showed linear improvement in fixation stability with time (p = 0.016). In conclusion, in patients with AMD, monocular fixational control with the worse eye is poor, but improves with time.