Monocular and binocular smooth pursuit in central field loss

Smooth pursuit deficits impact dynamic visual acuity in macular degeneration

SIGNIFICANCE

Prior studies with large, highly visible targets report low smooth pursuit gains in individuals with macular degeneration (MD). We show that lower gains persist even when observers are pursuing a target that requires discrimination at the acuity limit. This low gain causes retinal slip, potentially leading to motion blur and target disappearance in the scotoma, which further compromise the visibility of moving object.

PURPOSE

In this study, we examine whether the characteristics of smooth pursuit (pursuit gain and placement of the fixational locus relative to the target) change when the task requires dynamic visual acuity.

METHODS

Using the scanning laser ophthalmoscope, we recorded smooth pursuit eye movements in 10 eyes of 6 MD participants and 7 eyes of 4 age-matched controls in response to leftward- or rightward-moving annular targets (O) that briefly (300 milliseconds) changed to a Landolt C at one of several time points during the pursuit trial. Participants were asked to pursue the target and indicate the direction of the C opening.

RESULTS

Individuals with MD had lower pursuit gains and fewer saccades during the C presentation than during the O, compared with their age-matched peers. Further, pursuit gain, but not the distance of the retinal pursuit locus from the target, predicted task performance in the MD group.

CONCLUSIONS

Our findings suggest that compromised pursuit gain in MD participants likely further compromises their dynamic visual acuity and thus ability to view moving targets.

Eye Tracking to Assess the Functional Consequences of Vision Impairment: A Systematic Review

BACKGROUND

Eye tracking is a promising method for objectively assessing functional visual capabilities, but its suitability remains unclear when assessing the vision of people with vision impairment. In particular, accurate eye tracking typically relies on a stable and reliable image of the pupil and cornea, which may be compromised by abnormalities associated with vision impairment (e.g., nystagmus, aniridia).

OBJECTIVES

This study aimed to establish the degree to which video-based eye tracking can be used to assess visual function in the presence of vision impairment.

DATA SOURCES

A systematic review was conducted using PubMed, EMBASE, and Web of Science databases, encompassing literature from inception to July 2022.

STUDY ELIGIBILITY CRITERIA, PARTICIPANTS, AND INTERVENTIONS

Studies included in the review used video-based eye tracking, included individuals with vision impairment, and used screen-based tasks unrelated to practiced skills such as reading or driving.

STUDY APPRAISAL AND SYNTHESIS METHODS

The included studies were assessed for quality using the Strengthening the Reporting of Observational Studies in Epidemiology assessment tool. Data extraction and synthesis were performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.

RESULTS

Our analysis revealed that five common tests of visual function were used: (i) fixation stability, (ii) smooth pursuit, (iii) saccades, (iv) free viewing, and (v) visual search. The studies reported considerable success when testing individuals with vision impairment, yielding usable data from 96.5% of participants.

LIMITATIONS

There was an overrepresentation of conditions affecting the optic nerve or macula and an underrepresentation of conditions affecting the anterior segment or peripheral retina.

CONCLUSIONS AND IMPLICATIONS OF KEY FINDINGS

The results offer promise for the use of eye tracking to assess the visual function of a considerable proportion of those with vision impairment. Based on the findings, we outline a framework for how eye tracking can be used to test visual function in the presence of vision impairment.

Eye Tracking in Optometry: A Systematic Review

This systematic review examines the use of eye-tracking devices in optometry, describing their main characteristics, areas of application and metrics used. Using the PRISMA method, a systematic search was performed of three databases. The search strategy identified 141 reports relevant to this topic, indicating the exponential growth over the past ten years of the use of eye trackers in optometry. Eye-tracking technology was applied in at least 12 areas of the field of optometry and rehabilitation, the main ones being optometric device technology, and the assessment, treatment, and analysis of ocular disorders. The main devices reported on were infrared light-based and had an image capture frequency of 60 Hz to 2000 Hz. The main metrics mentioned were fixations, saccadic movements, smooth pursuit, microsaccades, and pupil variables. Study quality was sometimes limited in that incomplete information was provided regarding the devices used, the study design, the methods used, participants' visual function and statistical treatment of data. While there is still a need for more research in this area, eye-tracking devices should be more actively incorporated as a useful tool with both clinical and research applications. This review highlights the robustness this technology offers to obtain objective information about a person's vision in terms of optometry and visual function, with implications for improving visual health services and our understanding of the vision process.

Mapping residual stereopsis in macular degeneration

Individuals with macular degeneration typically lose vision in the central region of one or both eyes. A binocular scotoma occurs when vision loss occurs in overlapping locations in both eyes, but stereopsis is impacted even in the non-overlapping region wherever the visual field in either eye is affected. We used a novel stereoperimetry protocol to measure local stereopsis across the visual field (up to 25° eccentricity) to determine how locations with functional stereopsis relate to the scotomata in the two eyes. Participants included those with monocular or binocular scotomata and age-matched controls with healthy vision. Targets (with or without depth information) were presented on a random dot background. Depth targets had true binocular disparity of 20' (crossed), whereas non-depth targets were defined by monocular cues such as contrast and dot density. Participants reported target location and whether it was in depth or flat. Local depth sensitivity (d') estimates were then combined to generate a stereopsis map. This stereopsis map was compared to the union of the monocular microperimetry estimates that mapped out the functional extent of the scotoma in each eye. The "union" prediction aligned with residual stereopsis, showing impaired stereopsis within this region and residual stereopsis outside this region. Importantly, the stereoblind region was typically more extensive than the binocular scotoma defined by the intersection (overlap) of the scotomata. This explains why individuals may have intact binocular visual fields but be severely compromised in tasks of daily living that benefit from stereopsis, such as eye-hand coordination and navigation.

Saccadic contributions to smooth pursuit in macular degeneration

Saccades during smooth pursuit can help bring the fovea on target, particularly in cases of low pursuit gain. Individuals with macular degeneration often suffer damage to the central retina including the fovea, which impacts oculomotor function such as fixation, saccadic and smooth pursuit eye movements. We hypothesized that these oculomotor changes in macular degeneration (MD) would make saccades less appropriately directed (even if more numerous). To investigate saccades during pursuit in MD, we conducted a quantitative analysis of smooth pursuit eye movement data from a prior study, Vision Research 141 (2017) 181-190. Here we examined saccade frequency, magnitude, and direction across viewing conditions for MD and control participants during pursuit of a target moving in a modified step-ramp paradigm. Individuals with MD had more variability in saccade directions that included directions orthogonal to the target trajectory. PRL eccentricity significantly correlated with increases in saccades in non-target directions during smooth pursuit. These results suggest that a large number of saccades during pursuit in MD participants are unlikely to be catch-up saccades that serve to keep the eye on the target.

Altered eye movements during reading under degraded viewing conditions: Background luminance, text blur, and text contrast

Degraded viewing conditions caused by either natural environments or visual disorders lead to slow reading. Here, we systematically investigated how eye movement patterns during reading are affected by degraded viewing conditions in terms of spatial resolution, contrast, and background luminance. Using a high-speed eye tracker, binocular eye movements were obtained from 14 young normally sighted adults. Images of text passages were manipulated with varying degrees of background luminance (1.3-265 cd/m2), text blur (severe blur to no blur), or text contrast (2.6%-100%). We analyzed changes in key eye movement features, such as saccades, microsaccades, regressive saccades, fixations, and return-sweeps across different viewing conditions. No significant changes were observed for the range of tested background luminance values. However, with increasing text blur and decreasing text contrast, we observed a significant decrease in saccade amplitude and velocity, as well as a significant increase in fixation duration, number of fixations, proportion of regressive saccades, microsaccade rate, and duration of return-sweeps. Among all, saccade amplitude, fixation duration, and proportion of regressive saccades turned out to be the most significant contributors to reading speed, together accounting for 90% of variance in reading speed. Our results together showed that, when presented with degraded viewing conditions, the patterns of eye movements during reading were altered accordingly. These findings may suggest that the seemingly deviated eye movements observed in individuals with visual impairments may be in part resulting from active and optimal information acquisition strategies operated when visual sensory input becomes substantially deprived.

Eye Movements in Macular Degeneration

In healthy vision, the fovea provides high acuity and serves as the locus for fixation achieved through saccadic eye movements. Bilateral loss of the foveal regions in both eyes causes individuals to adopt an eccentric locus for fixation. This review deals with the eye movement consequences of the loss of the foveal oculomotor reference and the ability of individuals to use an eccentric fixation locus as the new oculomotor reference. Eye movements are an integral part of everyday activities, such as reading, searching for an item of interest, eye-hand coordination, navigation, or tracking an approaching car. We consider how these tasks are impacted by the need to use an eccentric locus for fixation and as a reference for eye movements, specifically saccadic and smooth pursuit eye movements. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Eye, head, and gaze contributions to smooth pursuit in macular degeneration

Macular degeneration (MD) often leads to the loss of the fovea and surrounding central visual field. This type of visual loss is very common and can present particular challenges for oculomotor tasks that may rely on the fovea. For certain tasks, individuals develop a new, eccentric fixational locus. Our previous work has shown that smooth pursuit is impaired in MD. However, extent of retinal lesion size and eccentricity of fixation do not directly contribute to changes in smooth pursuit gain. Oculomotor limitations due to eccentric eye position in the orbit may be another culprit. Here we test the hypothesis that deficits in smooth pursuit in MD would be reduced under head-unrestrained conditions. To that end, we examined eye, head, and gaze movements in eight individuals with MD and seven age-matched controls in response to a step-ramp pursuit stimulus. We found that despite variability across participants, both groups had similar smooth pursuit head movements (P = 0.76), while both had significantly higher pursuit gains in the head-restrained condition (P < 0.0001), suggesting that in older populations, head movements may lead to a decrease in pursuit gain. Furthermore, we did not find a correlation between eccentricity of fixation and amount of head displacement during the trial (P = 0.25), suggesting that eccentric eye position does not lead to higher reliance on head movements in smooth pursuit. Our finding that individuals with MD have lower pursuit gains, despite similar head movements as controls, suggests a difference in how MD affects mechanisms underlying eye versus head movements in smooth pursuit.NEW & NOTEWORTHY This article is the first to look at eye and head movements in observers with macular degeneration. It is the first to show that in older individuals, regardless of central field defect, freedom of head movement may reduce pursuit gain. Despite oculomotor limitations due to eccentric fixation, individuals with macular degeneration do not rely on head movements more than age-matched controls, with both groups having a similarly heterogenous eye and head movement strategy for pursuit.

Motion perception in central field loss

Motion information is essential in daily life because it provides cues to depth, timing, object identification, and self-motion, as well as input to the oculomotor system. As the peripheral visual field is exquisitely sensitive to motion, we investigated the periphery of individuals with central visual field loss (CFL) to determine whether speed and direction discrimination are intact in this population. We compared CFL participants' (N = 8), older (N = 6), and young controls' (N = 6) ability to discriminate motion speed and direction in a two-spatial-alternative forced-choice design. Participants viewed moving dots on the left and right of a fixation marker and judged which side had the faster speed or more clockwise direction. For the young control group, we repeated the experiment with the stimulus limited to thin strips of fixed width at eccentricities of 5°, 10°, and 15°. There was no significant difference in mean speed or direction discrimination thresholds of CFL participants and older controls for either velocity. Young controls had significantly lower thresholds than the CFL group for both tasks. We did not find an effect of visual acuity, viewing eccentricity, or scotoma location on individuals' ability to discriminate speed or direction. Our results indicate that for high-visibility stimuli moving at 5°-10°/s, speed and direction discrimination are intact in the periphery of individuals with CFL.

Temporal modulation improves dynamic peripheral acuity

Macular degeneration and related visual disorders greatly limit foveal function, resulting in reliance on the peripheral retina for tasks requiring fine spatial vision. Here we investigate stimulus manipulations intended to maximize peripheral acuity for dynamic targets. Acuity was measured using a single interval orientation discrimination task at 10° eccentricity. Two types of image motion were investigated along with two different forms of temporal manipulation. Smooth object motion was generated by translating targets along an isoeccentric path at a constant speed (0-20°/s). Ocular motion was simulated by jittering target location using previously recorded fixational eye movement data, amplified by a variable gain factor (0-8). In one stimulus manipulation, the sequence was temporally subsampled by displaying the target on an evenly spaced subset of video frames. In the other, the contrast polarity of the stimulus was reversed at a variable rate. We found that threshold under object motion was improved at all speeds by reversing contrast polarity, while temporal subsampling improved resolution at high speeds but impaired performance at low speeds. With simulated ocular motion, thresholds were consistently improved by contrast polarity reversal, but impaired by temporal subsampling. We find that contrast polarity reversal and temporal subsampling produce differential effects on peripheral acuity. Applying contrast polarity reversal may offer a relatively simple image manipulation that could enhance visual performance in individuals with central vision loss.

Binocular summation for reflexive eye movements

Psychophysical studies and our own subjective experience suggest that, in natural viewing conditions (i.e., at medium to high contrasts), monocularly and binocularly viewed scenes appear very similar, with the exception of the improved depth perception provided by stereopsis. This phenomenon is usually described as a lack of binocular summation. We show here that there is an exception to this rule: Ocular following eye movements induced by the sudden motion of a large stimulus, which we recorded from three human subjects, are much larger when both eyes see the moving stimulus, than when only one eye does. We further discovered that this binocular advantage is a function of the interocular correlation between the two monocular images: It is maximal when they are identical, and reduced when the two eyes are presented with different images. This is possible only if the neurons that underlie ocular following are sensitive to binocular disparity.

Image Stabilization in Central Vision Loss: The Horizontal Vestibulo-Ocular Reflex

For patients with central vision loss and controls with normal vision, we examined the horizontal vestibulo-ocular reflex (VOR) in complete darkness and in the light when enhanced by vision (VVOR). We expected that the visual-vestibular interaction during VVOR would produce an asymmetry in the gain due to the location of the preferred retinal locus (PRL) of the patients. In the dark, we hypothesized that the VOR would not be affected by the loss of central vision. Nine patients (ages 67 to 92 years) and 17 controls (ages 16 to 81 years) were tested in 10-s active VVOR and VOR procedures at a constant frequency of 0.5 Hz while their eyes and head movements were recorded with a video-based binocular eye tracker. We computed the gain by analyzing the eye and head peak velocities produced during the intervals between saccades. In the light and in darkness, a significant proportion of patients showed larger leftward than rightward peak velocities, consistent with a PRL to the left of the scotoma. No asymmetries were found for the controls. These data support the notion that, after central vision loss, the preferred retinal locus (PRL) in eccentric vision becomes the centre of visual direction, even in the dark.