Nonveridical visual perception in human amblyopia

Structural and functional alterations in the brains of patients with anisometropic and strabismic amblyopia: a systematic review of magnetic resonance imaging studies

Amblyopia is the most common cause of vision loss in children and can persist into adulthood in the absence of effective intervention. Previous clinical and neuroimaging studies have suggested that the neural mechanisms underlying strabismic amblyopia and anisometropic amblyopia may be different. Therefore, we performed a systematic review of magnetic resonance imaging studies investigating brain alterations in patients with these two subtypes of amblyopia; this study is registered with PROSPERO (registration ID: CRD42022349191). We searched three online databases (PubMed, EMBASE, and Web of Science) from inception to April 1, 2022; 39 studies with 633 patients (324 patients with anisometropic amblyopia and 309 patients with strabismic amblyopia) and 580 healthy controls met the inclusion criteria (e.g., case-control designed, peer-reviewed articles) and were included in this review. These studies highlighted that both strabismic amblyopia and anisometropic amblyopia patients showed reduced activation and distorted topological cortical activated maps in the striate and extrastriate cortices during task-based functional magnetic resonance imaging with spatial-frequency stimulus and retinotopic representations, respectively; these may have arisen from abnormal visual experiences. Compensations for amblyopia that are reflected in enhanced spontaneous brain function have been reported in the early visual cortices in the resting state, as well as reduced functional connectivity in the dorsal pathway and structural connections in the ventral pathway in both anisometropic amblyopia and strabismic amblyopia patients. The shared dysfunction of anisometropic amblyopia and strabismic amblyopia patients, relative to controls, is also characterized by reduced spontaneous brain activity in the oculomotor cortex, mainly involving the frontal and parietal eye fields and the cerebellum; this may underlie the neural mechanisms of fixation instability and anomalous saccades in amblyopia. With regards to specific alterations of the two forms of amblyopia, anisometropic amblyopia patients suffer more microstructural impairments in the precortical pathway than strabismic amblyopia patients, as reflected by diffusion tensor imaging, and more significant dysfunction and structural loss in the ventral pathway. Strabismic amblyopia patients experience more attenuation of activation in the extrastriate cortex than in the striate cortex when compared to anisometropic amblyopia patients. Finally, brain structural magnetic resonance imaging alterations tend to be lateralized in the adult anisometropic amblyopia patients, and the patterns of brain alterations are more limited in amblyopic adults than in children. In conclusion, magnetic resonance imaging studies provide important insights into the brain alterations underlying the pathophysiology of amblyopia and demonstrate common and specific alterations in anisometropic amblyopia and strabismic amblyopia patients; these alterations may improve our understanding of the neural mechanisms underlying amblyopia.

Impact of strabismus and binocular dysfunctions in the developmental eye movement test and test of visual perception skills: A multicentric and retrospective study

PURPOSE

To compare the performance in the Developmental Eye Movement test (DEM) and the Test of Visual Perceptual Skills (TVPS) between three groups: individuals with strabismus and amblyopia, patients with binocular and accommodative dysfunctions, and subjects with normal binocular and accommodative function.

METHODS

A multicentric, retrospective study including 110 children aged 6-14 years old was conducted to investigate the potential impact of strabismus, amblyopia, and different binocular conditions in DEM results (adjusted time in vertical and horizontal parts) and TVPS (percentiles in the seven sub-skills).

RESULTS

No significant differences were found in the different subtests of the vertical and horizontal DEM and all the sub-skills in the TVPS between the three groups of the study. We found high variability of performance in the DEM test between participants with strabismus and amblyopia compared with binocular and accommodative problems.

CONCLUSION

DEM and TVPS scores have not been found to be influenced by the presence of strabismus with or without amblyopia, nor by binocular and accommodative dysfunctions. A slightly correlation tendency with horizontal DEM and degree of exotropia deviation was observed.

Refractive prescribing for preschool children by optometrists in England

PURPOSE

Correction of refractive error in children is important for visual and educational development. The aim of this questionnaire-based study was to explore paediatric refractive correction by optometrists in England.

METHODS

An online questionnaire was piloted and distributed to optometrists in England. The questionnaire asked about respondents' characteristics (such as type of practice), management of refractive error in 1- and 3-year-old children and sources of information used as a basis for decisions on prescribing refractive error in children.

RESULTS

Two hundred and ninety-three questionnaires were returned, although only 139 (47%) were fully completed. In an average month, about half of respondents examined no children between 0 and 2 years of age, and about half examined no more than five children aged 3-4 years. A significant proportion indicated they would refer children aged 1 or 3 years with refractive error and no other signs or symptoms into the hospital eye service. Almost a quarter would prescribe in full or in part an isometropic refractive correction of +2.00 D for a 3-year-old (within the normal range) with no other signs or symptoms, suggesting a degree of unnecessary prescribing. Almost all would act in cases of clinically significant refractive error. Respondents made similar use of their colleagues, optometric or postgraduate/continuing education, professional guidance and peer-reviewed research as sources of evidence on which to base decisions about prescribing for paediatric refractive errors. Most reported 'never' or 'rarely' using Cochrane reviews.

CONCLUSIONS

These results suggest optometrists often defer management of paediatric refractive error to the hospital eye service, with implications in terms of underutilisation of community optometric expertise and burden on the National Health Service. In some cases, the results indicate a mismatch between respondents' reported management and existing guidance/guidelines on paediatric prescribing.

Issues Revisited: Shifts in Binocular Balance Depend on the Deprivation Duration in Normal and Amblyopic Adults

INTRODUCTION

Recent studies indicate that short-term monocular deprivation increases the deprived eye's contribution to binocular fusion in both adults with normal vision and amblyopia. In this study, we investigated whether the changes in visual plasticity depended on the duration of deprivation in normal and amblyopic adults.

METHODS

Twelve anisometropia amblyopic observers (aged 24.8 ± 2.3 years) and 12 age-matched normal observers (aged 23.9 ± 1.2 years) participated in the study. The non-dominant eye of normal observers or amblyopic eye of amblyopic observers was deprived for 30, 120, and 300 min in a randomized order. Their eye balance was measured with a phase combination task, which is a psychophysical test, before and after the deprivation. This design enabled us to measure changes induced in binocular balance as an index visual plasticity due to monocular deprivations.

RESULTS

By comparing the ocular dominance changes as a result of monocular deprivation with different deprivation durations, we found evidence that the ocular dominance changes are slightly larger after longer deprivations in both normal and amblyopic observers, albeit with a statistical significance. The changes from 120-min were significantly greater than those from 30-min deprivation in both groups. The magnitude of changes in sensory eye balance was significantly larger in normal observers than that in the amblyopic observers; however, the longevity of changes in visual plasticity was found to be more long-lasting in amblyopic observers than the normal counterparts.

CONCLUSIONS

The duration of deprivation matters in both normal and amblyopic observers. Ocular dominance imbalance that is typically observed in amblyopia can be more ameliorated with a longer duration of deprivation.

Crowding changes appearance systematically in peripheral, amblyopic, and developing vision

Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: Target objects appear either averaged with the flankers (assimilation) or replaced by them (substitution). If amblyopic and developmental crowding share the same mechanism, then their errors should be similarly systematic. We tested foveal vision in children aged 3 to 8 years with typical vision or strabismic amblyopia and peripheral vision in typical adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target “Vac-Man” element. When the target was surrounded by flankers that differed by ± 30°, all three groups (adults and children with typical or amblyopic vision) reported orientations between the target and flankers (assimilation). Errors were reduced with ± 90° differences but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism.

Binocular versus standard occlusion or blurring treatment for unilateral amblyopia in children aged three to eight years

BACKGROUND

Current treatments for amblyopia, typically patching or pharmacological blurring, have limited success. Less than two-thirds of children achieve good acuity of 0.20 logMAR in the amblyopic eye, with limited improvement of stereopsis, and poor adherence to treatment. A new approach, based on presentation of movies or computer games separately to each eye, may yield better results and improve adherence. These treatments aim to balance the input of visual information from each eye to the brain.  OBJECTIVES: To determine whether binocular treatments in children, aged three to eight years, with unilateral amblyopia result in better visual outcomes than conventional patching or pharmacological blurring treatment.

SEARCH METHODS

We searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register), MEDLINE, Embase, ISRCTN, ClinicalTrials.gov, and the WHO ICTRP to 19 November 2020, with no language restrictions.

SELECTION CRITERIA

Two review authors independently screened the results of the search for relevant studies. We included randomised controlled trials (RCTs) that enrolled children between the ages of three and eight years old with unilateral amblyopia. Amblyopia was classed as present when the best-corrected visual acuity (BCVA) was worse than 0.200 logMAR in the amblyopic eye, with BCVA 0.200 logMAR or better in the fellow eye, in the presence of an amblyogenic risk factor, such as anisometropia, strabismus, or both. To be eligible, children needed to have undergone cycloplegic refraction and  ophthalmic examination, including fundal examination and optical treatment, if indicated, with stable BCVA in the amblyopic eye despite good adherence with wearing glasses. We included any type of binocular viewing intervention, on any device (e.g. computer monitors viewed with liquid-crystal display shutter glasses; hand-held screens, including mobile phones with lenticular prism overlay; or virtual reality displays). Control groups received standard amblyopia treatment, which could include patching or pharmacological blurring of the better-seeing eye. We included full-time (all waking hours) and part-time (between 1 and 12 hours a day) patching regimens. We excluded children who had received any treatment other than optical treatment; and studies with less than 8-week follow-up.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures expected by Cochrane. The primary outcome of the review was the change from baseline of distance BCVA in the amblyopic eye after 16 (± 2) weeks of treatment, measured in logMAR units on an age-appropriate acuity test.

MAIN RESULTS

We identified one eligible RCT of conventional patching treatment versus novel binocular treatment, and analysed a subset of 68 children who fulfilled the age criterion of this review. We obtained data for the mean change in amblyopic eye visual acuity, adverse events (diplopia), and adherence to prescribed treatment at 8- and 16-week follow-up intervals, though no data were available for change in BCVA after 52 weeks. Risk of bias for the included study was considered to be low. The certainty of evidence for the visual acuity outcomes at 8 and 16 weeks of treatment and adherence to the study intervention was rated moderate using the GRADE criteria, downgrading by one level due to imprecision. The certainty of evidence was downgraded by two levels and rated low for the proportion of participants reporting adverse events due to the sample size.  Acuity improved in the amblyopic eye in both the binocular and patching groups following 16 weeks of treatment (improvement of -0.21 logMAR in the binocular group and -0.24 logMAR in the patching group, mean difference (MD) 0.03 logMAR (95% confidence interval (CI) -0.10 to 0.04; 63 children). This difference was non-significant and the improvements in both the binocular and patching groups are also considered clinically similar. Following 8 weeks of treatment, acuity improved in both the binocular and patching groups (improvement of -0.18 logMAR in the patching group compared to -0.16 logMAR improvement in the binocular-treatment group) (MD 0.02, 95% CI -0.04 to 0.08). Again this difference was statistically non-significant, and the differences observed between the patching and binocular groups are also clinically non-significant. No adverse event of permanent diplopia was reported. Adherence was higher in the patching group (47% of participants in the iPad group achieved over 75% compliance compared with 90% of the patching group).  Data were not available for changes in stereopsis nor for contrast sensitivity following treatment.

AUTHORS' CONCLUSIONS

Currently, there is only one RCT that offers evidence of the safety and effectiveness of binocular treatment. The authors are moderately confident that after 16 weeks of treatment, the gain in amblyopic eye acuity with binocular treatment is likely comparable to that of conventional patching treatment. However, due to the limited sample size and lack of long term (52 week) follow-up data, it is not yet possible to draw robust conclusions regarding the overall safety and sustained effectiveness of binocular treatment. Further research, using acknowledged methods of visual acuity and stereoacuity assessment with known reproducibility, is required to inform decisions about the implementation of binocular treatments for amblyopia in clinical practice, and should incorporate longer term follow-up to establish the effectiveness of binocular treatment. Randomised controlled trials should also include outcomes reported by users, adherence to prescribed treatment, and recurrence of amblyopia after cessation of treatment.

Geometrically restricted image descriptors: A method to capture the appearance of shape

Shape perception varies depending on many factors. For example, presenting a stimulus in the periphery often yields a different appearance compared with its foveal presentation. However, how exactly shape appearance is altered under different conditions remains elusive. One reason for this is that studies typically measure identification performance, leaving details about target appearance unknown. The lack of appearance-based methods and general challenges to quantify appearance complicate the investigation of shape appearance. Here, we introduce Geometrically Restricted Image Descriptors (GRIDs), a method to investigate the appearance of shapes. Stimuli in the GRID paradigm are shapes consisting of distinct line elements placed on a grid by connecting grid nodes. Each line is treated as a discrete target. Observers are asked to capture target appearance by placing lines on a freely viewed response grid. We used GRIDs to investigate the appearance of letters and letter-like shapes. Targets were presented at 10° eccentricity in the right visual field. Gaze-contingent stimulus presentation was used to prevent eye movements to the target. The data were analyzed by quantifying the differences between targets and response in regard to overall accuracy, element discriminability, and several distinct error types. Our results show how shape appearance can be captured by GRIDs, and how a fine-grained analysis of stimulus parts provides quantifications of appearance typically not available in standard measures of performance. We propose that GRIDs are an effective tool to investigate the appearance of shapes.

Temporal Characteristics of Visual Processing in Amblyopia

PURPOSE

Amblyopia affects not only spatial vision but also temporal vision. In this study, we aim to investigate temporal processing deficits in amblyopia.

METHODS

Twenty amblyopic patients (age: 27.0 ± 5.53 years, 15 males), and 25 normal observers (age: 25.6 ± 4.03 years, 15 males) were recruited in this study. Contrast thresholds in an orientation discrimination task in five target-mask stimulus onset asynchronies (SOA) conditions (16.7 ms, 33.4 ms, 50.0 ms, 83.4 ms, and ∞/no noise) were measured. An elaborated perceptual template model (ePTM) was fit to the behavioral data to derive the temporal profile of visual processing for each participant.

RESULTS

There were significant threshold differences between the amblyopic and normal eyes [F(1,43) = 10.6, p = 0.002] and a significant group × SOA interaction [F(2.75,118) = 4.98, p = 0.004], suggesting different temporal processing between the two groups. The ePTM fitted the data well (χ 2 test, all ps > 0.50). Compared to the normal eye, the amblyopic eye had a lower template gain (p = 0.046), and a temporal window with lower peak and broader width (all ps < 0.05). No significant correlation was found between the observed temporal deficits and visual acuity in amblyopia (ps > 0.50). Similar results were found in the anisometropic amblyopia subgroup. No significant difference was found between the fellow eyes of the monocular amblyopia and the normal eyes.

CONCLUSION

Amblyopia is less efficient in processing dynamic visual stimuli. The temporal deficits in amblyopia, represented by a flattened temporal window, are likely independent of spatial vision deficits.

Motor Competence in Children With and Without Ambliopia

The purpose of this study was to assess the motor competence of children with and without amblyopia. Study participants were 165 primary school children, aged 6-9 years, divided into three groups based on their visual acuity with the Snellen chart: (a) non-amblyopia, (b) corrected amblyopia, and (c) non-corrected amblyopia. We assessed the children's motor competence with the Motor Competence Assessment battery (MCA) and their physical activity with the Physical Activity Questionnaire for Older Children (PAQ-C). The non-amblyopia group presented significantly better motor competence on the MCA than either the corrected amblyopia group or the non-corrected amblyopia group; there were no statistically significant motor differences between the two amblyopia subgroups. Amblyopia versus non-amblyopia differences on the MCA were mainly in stability and locomotor components, involving dynamic balance and the change of spatial position and direction of movement, but not in the manipulative component (ball throwing velocity and ball kicking velocity). Predictably, from within an integrated visual motor perspective of child development, our findings suggest that intact vision played an important role in children's motor competence. The development of fundamental motor skills, especially of stability and locomotor skills, may be affected by poor visual processing in that participants with uncorrected amblyopia showed poor movement accuracy, uncoordinated movement, and impaired balance.

Characterising the orientation-specific pattern-onset visual evoked potentials in children with bilateral refractive amblyopia and non-amblyopic controls

PURPOSE

An orientation-specific visual evoked potential (osVEP) protocol was developed to probe meridional anisotropies in children with refractive amblyopia. The aim was to characterise the osVEP response in children with bilateral refractive amblyopia, evaluate the intra-session repeatability of the main osVEP components (C1, C2 and C3), coefficient of repeatability (CoR) of the response to gratings in different meridians and determine if refractive amblyopes have poorer repeatability as compared with non-amblyopic controls.

METHODS

Children aged 4-7 years with newly diagnosed and untreated bilateral refractive amblyopia and non-amblyopic controls were recruited. Orientation-specific pattern-onset VEPs were recorded in response to an achromatic sinewave grating stimulus of 4 cycles per degree under monocular and binocular stimulation. The grating lines used for monocular stimulation were parallel with the subjects' most positive and negative astigmatic meridians when considered in sphero-minus cylinder form (Meridians 1 and 2, respectively). In subjects without astigmatism, meridians 1 and 2 were designated horizontal and vertical gratings, respectively. Binocular stimuli were presented with grating lines parallel to meridians 45, 90, 135 and 180°. The repeatability of latencies of the main osVEP components (C1, C2 and C3) were investigated using two successive osVEPs recordings for each stimulus meridian and the CoR for each component's latencies were assessed.

RESULTS

Seven amblyopic children (Visual acuity (VA) ranging from 0.08 to 0.40 LogMAR in the less amblyopic eye and 0.26-0.52 LogMAR in the more amblyopic eye) and 7 non-amblyopic controls (VA ranging from 0.00 to 0.02 LogMAR in either eye), with a median age of 4.6 and 7.0 years, respectively, completed the study. C1 had the highest CoR for most conditions assessed. Ratio of CoRs C1:C2 was > 2 for all binocular meridians in controls and the 90 and 180 meridians in the amblyopes; C1:C3 was > 2 for the binocularly assessed 45, 90 and 135 meridians in the controls and the 90 and 180 meridians in the amblyopes; C2:C3 were all < 2 for all meridians assessed in both groups.

CONCLUSIONS

The osVEP waveforms are reliable and useful for future investigations into the meridional anisotropies in children with refractive amblyopia, particularly the C3 component. Component C1 had the poorest repeatability, which consequentially affected C2 amplitude estimation. Only C3 amplitude and latency could be consistently estimated as C2 and C3 latencies were similarly repeatable. Coefficients of repeatability of osVEP latencies did not appear to systematically differ between non-amblyopic and amblyopic children.

Attention in visually typical and amblyopic children

Amblyopia is a cortical visual disorder caused by unequal visual input to the brain from the two eyes during development. Amblyopes show reduced visual acuity and contrast sensitivity and abnormal binocularity, as well as more “global” perceptual losses, such as figure-ground segregation and global form integration. Currently, there is no consensus on the neural basis for these higher-order perceptual losses. One contributing factor could be that amblyopes have deficiencies in attention, such that the attentional processes that control the selection of information favor the better eye. Previous studies in amblyopic adults are conflicting as to whether attentional deficits exist. However, studies where intact attentional ability has been shown to exist were conducted in adults; it is possible that it was acquired through experience. To test this hypothesis, we studied attentional processing in amblyopic children. We examined covert endogenous attention using a classical spatial cueing paradigm in amblyopic and visually typical 5- to 10-year old children. We found that all children, like adults, independently of visual condition, benefited from attentional cueing: They performed significantly better on trials with an informative (valid) cue than with the uninformative (neutral) cue. Response latencies were also significantly shorter for the valid cue condition. No statistically significant difference was found between the performance of the amblyopic and the visually typical children or between dominant and nondominant eyes of all children. The results showed that covert spatial attention is intact in amblyopic and visually typical children and is therefore not likely to account for higher-order perceptual losses in amblyopic children.

Interocular suppressive interactions in amblyopia depend on spatial frequency

In amblyopia, there is an interocular suppressive imbalance that results in the fixing eye dominating perception. In this study, we aimed to determine whether these suppressive interactions were narrowband and tuned for spatial frequency or broadband and independent of spatial frequency. We measured the contrast sensitivity and masking functions of fifteen amblyopic subjects and seventeen control subjects using the quick Contrast Sensitivity Function (qCSF) approach (Lesmes, Lu, Baek, & Albright, 2010). We first measured the monocular sensitivity functions of each participant and thereafter corrected for it. We then measured masking sensitivity functions for low, mid and high spatial frequency masks, normalized to their visibility. In the control group, we observed that the strength of dichoptic masking is equivalent between the two eyes. It is also tuned such that masking by low spatial frequencies in one eye mainly affects low spatial frequencies in the other eye and masking by high spatial frequencies mainly affects high spatial frequencies. In amblyopes, although the interocular masking is also tuned for spatial frequency, it is not equivalent between the two eyes: the masking effect from the amblyopic to fixing eye is weaker than the other way around. The asymmetry observed in the strength of masking between the two eyes in amblyopia is tuned for spatial frequency. It is not the consequence of the contrast sensitivity deficit of the amblyopic eye nor is it the consequence of abnormally strong masking from the fixing eye. Rather it is due to an abnormally weak masking strength by the amblyopic eye per se.

Effects of temporal frequency on binocular deficits in amblyopia

Amblyopia is associated with a range of well-known visual spatial deficits, which include reduced contrast sensitivity, spatial distortions, interocular suppression, and impaired stereopsis. Previous work has also pointed to deficits in processing dynamic visual information, but it is unknown whether these deficits influence performance under binocular conditions. We examined the effects of temporal modulation on contrast sensitivity and binocular interactions in a preliminary study of 8 adults with amblyopia and 14 normally-sighted control subjects. For each observer, we measured interocular balance and stereopsis thresholds with binocular flicker across a range of four temporal (0, 4, 7.5, and 12 Hz) and spatial (1, 2, 4, and 8 cpd) frequencies. Interocular balance was estimated by varying the relative contrast of dichoptic letter pairs to produce perceptual reports of each letter with equal frequency, and stereopsis thresholds were measured by determining the minimum disparity at which subjects identified a front-depth target with 75% accuracy. Consistent with previous findings, we observed greater interocular imbalance and impaired stereoacuity at high spatial frequencies in amblyopes. In contrast, the effects of temporal frequency on performance were smaller: across both groups, interocular imbalance was largest at mid-to-low temporal frequencies, and stereopsis thresholds were unaffected by temporal frequency. Our results suggest that there may be a previously unreported effect of temporal frequency on interocular balance, as well as a possible dissociation between the effects of flicker on interocular balance and stereopsis.

Use of diffusing filters for artificially reducing visual acuity when testing equipment and procedures

PURPOSE

When evaluating ophthalmological devices and procedures, for instance those for visual electrophysiology, it is often desirable to perform tests with reduced acuity. Doing this with individuals with actual visual impairments has a number of disadvantages, such as considerable recruitment efforts, especially when a specific acuity range is targeted, and little control about the actual perceptual characteristics of the impairment, which are normally not fully known. Lenses with positive diopters or blurring filters that are placed in front of the eyes of visually normal observers promise a simple solution to the problem. However, defocus results in considerable spurious resolution, and previous studies suggest that the frequently used Bangerter occluders are not optimal for the purpose. The present study therefore reviews a number of other options and tests a selection of filters with respect to their effect on acuity and contrast sensitivity with the aim of identifying filters that primarily degrade acuity while mostly sparing contrast sensitivity.

METHODS

First, we screened several filters for potential usefulness. The Freiburg Acuity and Contrast Test was then used to measure visual acuity and contrast sensitivity with a subset of three filters (Luminit LSD 0.5° and 1°, and LEE 420) and, for comparison, with a Bangerter occluder with a nominal acuity grade of 0.1. A qualitative comparison of the filters' effect on the checkerboard-reversal VEP was also performed.

RESULTS

With both Luminit filters, variability in acuity across participants was relatively small, and at least with the 0.5° version, contrast sensitivity was relativity little affected. The LEE filter and the Bangerter occluder resulted in more variability and, compared to the effect on acuity, a relatively strong reduction in contrast sensitivity. Comparing the Luminit 0.5° and 1° filters, the reduction of acuity was not proportional to physical stimulus degradation. The effect on VEP responses was consistent with the psychophysical data.

CONCLUSIONS

The Luminit filters, which have a Gaussian light diffusion profile, appear to be a good choice for artificial reduction of acuity.

Visuomotor Behaviour in Amblyopia: Deficits and Compensatory Adaptations

Amblyopia is a neurodevelopmental visual disorder arising from decorrelated binocular experience during the critical periods of development. The hallmark of amblyopia is reduced visual acuity and impairment in binocular vision. The consequences of amblyopia on various sensory and perceptual functions have been studied extensively over the past 50 years. Historically, relatively fewer studies examined the impact of amblyopia on visuomotor behaviours; however, research in this area has flourished over the past 10 years. Therefore, the aim of this review paper is to provide a comprehensive review of current knowledge about the effects of amblyopia on eye movements, upper limb reaching and grasping movements, as well as balance and gait. Accumulating evidence indicates that amblyopia is associated with considerable deficits in visuomotor behaviour during amblyopic eye viewing, as well as adaptations in behaviour during binocular and fellow eye viewing in adults and children. Importantly, due to amblyopia heterogeneity, visuomotor development in children and motor skill performance in adults may be significantly influenced by the etiology and clinical features, such as visual acuity and stereoacuity. Studies with larger cohorts of children and adults are needed to disentangle the unique contribution of these clinical characteristics to the development and performance of visuomotor behaviours.

Modified images reflecting effects of age-related macular degeneration on perception of everyday scenes

BACKGROUND

Depictions of vision with age-related macular degeneration (AMD) in public information material typically show a central region of absolute vision loss. Patients with early and moderate disease frequently do not report this. We aimed to measure how a group of people with AMD perceive everyday scenes in order to produce accurate depictions.

METHODS

We report on six people aged 65-82 years with monocular AMD (visual acuity +0.04 to +1.64 logMAR) and normal vision in the fellow eye. Participants viewed four images monocularly, alternating between eyes. The image was digitally altered to approximate participants' descriptions of their perception with the affected eye. The altered image was viewed with the unaffected eye, and compared with the original image viewed with the affected eye. This was repeated iteratively until a perceptual match was achieved between the modified image/unaffected eye and the original image/affected eye.

RESULTS

For five AMD participants with visual acuity +0.04 to +0.50 logMAR the modified images did not resemble those in current public information material. Image modifications required to achieve perceptual similarity with the affected eyes included localised distortion, contrast reduction and blur. Widespread colour desaturation was also required in some cases. One participant with advanced geographic atrophy reported an absolute positive scotoma, similar to existing depictions.

CONCLUSIONS

Vision in people with AMD may not conform to the common depiction of a central region of absolute vision loss. The accurate representations of AMD patients' vision produced in this study will enable better understanding of the visual consequences of AMD.

The functional impact of amblyopia

Amblyopia is the most common disorder managed in paediatric ophthalmic practice in industrialised countries. Reports on the impact of amblyopia on tasks relevant to the activities of children, or on skills pertinent to their education and quality of life, is leading to greater understanding of the functional disabilities associated with the condition. This review considers the extent to which amblyopia affects the ability to carry out everyday tasks, with particular attention to studies of motor skills and reading proficiency in children. Collectively, these studies show that amblyopia results in poorer outcomes on tests of skills required for proficiency in everyday tasks and which relate to childhood academic performance. However, the relative contributions that the documented vision anomalies inherent in amblyopia contribute to various functional disabilities is not fully determined. Recent reports have demonstrated improvement following treatment in standardised measures of fine motor skills involved in practical, everyday tasks. Including measurement of functional performance skills in amblyopia treatment trials is desirable to show treatment effect on crucial, real-world activities.

P300-based acuity estimation in imitated amblyopia

PURPOSE

A frequent approach to estimating visual acuity objectively is the recording of visual evoked potentials (VEPs) to patterns of different coarseness. This, however, overestimates acuity in patients with fragmented and distorted vision such as in amblyopia. This is likely due to VEP-based techniques using checkerboard or grating stimuli. We hypothesized that no overestimation would occur when the event-related potential P300 in response to optotype stimuli is used for acuity estimation.

METHODS

In 29 visually normal participants, we recorded P300 responses to Landolt C optotypes of different sizes. Vision was artificially degraded by placing a patterned polymethyl methacrylate pane in front of the monitor, which resulted in fragmentation and distortion of the stimulus. As control, a frosted pane was used. Both panes were adjusted to yield the same reduction of acuity in a standard subjective acuity test. A difference of less than 0.1 log MAR was defined as criterion to judge the outcomes of the objective tests as equivalent for both types of artificial visual impairment.

RESULTS

The average difference of P300-based objective acuity estimates between types of visual degradation was significantly smaller than 0.1 log MAR, indicating that the performance of the objective acuity test was equivalent for both types of visual degradation.

CONCLUSION

Our data suggest that P300-based objective acuity testing with optotype stimuli is more akin to standard psychophysical acuity testing and thus a suitable approach in cases of visual impairment where VEP-based methods fail to yield reliable results.

[Improvement of vision through perceptual learning in the case of refractive errors and presbyopia : A critical valuation]

The idea of compensating or even rectifying refractive errors and presbyopia with the help of vision training is not new. For most approaches, however, scientific evidence is insufficient. A currently promoted method is "perceptual learning", which is assumed to improve stimulus processing in the brain. The basic phenomena of perceptual learning have been demonstrated by a multitude of studies. Some of these specifically address the case of refractive errors and presbyopia. However, many open questions remain, in particular with respect to the transfer of practice effects to every-day vision. At present, the method should therefore be judged with caution.

Neural architectures for stereo vision

Stereoscopic vision delivers a sense of depth based on binocular information but additionally acts as a mechanism for achieving correspondence between patterns arriving at the left and right eyes. We analyse quantitatively the cortical architecture for stereoscopic vision in two areas of macaque visual cortex. For primary visual cortex V1, the result is consistent with a module that is isotropic in cortical space with a diameter of at least 3 mm in surface extent. This implies that the module for stereo is larger than the repeat distance between ocular dominance columns in V1. By contrast, in the extrastriate cortical area V5/MT, which has a specialized architecture for stereo depth, the module for representation of stereo is about 1 mm in surface extent, so the representation of stereo in V5/MT is more compressed than V1 in terms of neural wiring of the neocortex. The surface extent estimated for stereo in V5/MT is consistent with measurements of its specialized domains for binocular disparity. Within V1, we suggest that long-range horizontal, anatomical connections form functional modules that serve both binocular and monocular pattern recognition: this common function may explain the distortion and disruption of monocular pattern vision observed in amblyopia.

This article is part of the themed issue ‘Vision in our three-dimensional world’.

Noisy Spiking in Visual Area V2 of Amblyopic Monkeys

Interocular decorrelation of input signals in developing visual cortex can cause impaired binocular vision and amblyopia. Although increased intrinsic noise is thought to be responsible for a range of perceptual deficits in amblyopic humans, the neural basis for the elevated perceptual noise in amblyopic primates is not known. Here, we tested the idea that perceptual noise is linked to the neuronal spiking noise (variability) resulting from developmental alterations in cortical circuitry. To assess spiking noise, we analyzed the contrast-dependent dynamics of spike counts and spiking irregularity by calculating the square of the coefficient of variation in interspike intervals (CV²) and the trial-to-trial fluctuations in spiking, or mean matched Fano factor (m-FF) in visual area V2 of monkeys reared with chronic monocular defocus. In amblyopic neurons, the contrast versus response functions and the spike count dynamics exhibited significant deviations from comparable data for normal monkeys. The CV² was pronounced in amblyopic neurons for high-contrast stimuli and the m-FF was abnormally high in amblyopic neurons for low-contrast gratings. The spike count, CV², and m-FF of spontaneous activity were also elevated in amblyopic neurons. These contrast-dependent spiking irregularities were correlated with the level of binocular suppression in these V2 neurons and with the severity of perceptual loss for individual monkeys. Our results suggest that the developmental alterations in normalization mechanisms resulting from early binocular suppression can explain much of these contrast-dependent spiking abnormalities in V2 neurons and the perceptual performance of our amblyopic monkeys.

SIGNIFICANCE STATEMENT

Amblyopia is a common developmental vision disorder in humans. Despite the extensive animal studies on how amblyopia emerges, we know surprisingly little about the neural basis of amblyopia in humans and nonhuman primates. Although the vision of amblyopic humans is often described as being noisy by perceptual and modeling studies, the exact nature or origin of this elevated perceptual noise is not known. We show that elevated and noisy spontaneous activity and contrast-dependent noisy spiking (spiking irregularity and trial-to-trial fluctuations in spiking) in neurons of visual area V2 could limit the visual performance of amblyopic primates. Moreover, we discovered that the noisy spiking is linked to a high level of binocular suppression in visual cortex during development.

Diagnosing the Periphery: Using the Rey-Osterrieth Complex Figure Drawing Test to Characterize Peripheral Visual Function

Peripheral vision is strongly limited by crowding, the deleterious influence of neighboring stimuli on target perception. Many quantitative aspects of this phenomenon have been characterized, but the specific nature of the perceptual degradation remains elusive. We utilized a drawing technique to probe the phenomenology of peripheral vision, using the Rey-Osterrieth Complex Figure, a standard neuropsychological clinical instrument. The figure was presented at 12° or 6° in the right visual field, with eye tracking to ensure that the figure was only presented when observers maintained stable fixation. Participants were asked to draw the figure with free viewing, capturing its peripheral appearance. A foveal condition was used to measure copying performance in direct view. To assess the drawings, two raters used standard scoring systems that evaluated feature positions, spatial distortions, and omission errors. Feature scores tended to decrease with increasing eccentricity, both within and between conditions, reflecting reduced resolution and increased crowding in peripheral vision. Based on evaluation of the drawings, we also identified new error classes unique to peripheral presentation, including number errors for adjacent similar features and distinctive spatial distortions. The multifaceted nature of the Rey-Osterrieth Complex Figure-containing configural elements, detached compound features, and texture-like components-coupled with the flexibility of the free-response drawing paradigm and the availability of standardized scoring systems, provides a promising method to probe peripheral perception and crowding.

Imitating the effect of amblyopia on VEP-based acuity estimates

PURPOSE

Acuity testing based on visual evoked potentials (VEPs) overestimates acuity in patients with amblyopia. We hypothesized that distortion and fragmentation of the stimulus in amblyopia impede recognition of optotypes, while it leaves the pattern onset response in the VEP mostly unaffected, resulting in overestimation of acuity.

METHODS

Acuity VEPs were recorded in visually normal participants with the stimulus degraded by patterned polymethyl methacrylate panes, which induce distortion and fragmentation. For comparison, frosted panes were used to induce blur through wide-angle scattering. Standard psychophysical optotype acuity was recorded under the same conditions.

RESULTS

With the distorted and fragmented stimuli, the VEP consistently overestimated acuity relative to psychophysical optotype acuity. With blurred stimuli, both measures were in good agreement.

CONCLUSIONS

The data support the assumption that stimulus distortion and fragmentation leave VEP-based measures of acuity relatively unaffected, resulting in a discrepancy between measures of acuity that are based on checkerboard VEPs on one hand and psychophysical optotype recognition on the other hand. The technique of stimulus degradation described here provides a simple and efficient way of imitating effects that are known from amblyopia and may thus serve as a tool in the evaluation of vision tests.

Understanding the Neural Basis of Amblyopia

Amblyopia is the condition in which reduced visual function exists despite full optical correction and an absence of observable ocular pathology. Investigation of the underlying neurology of this condition began in earnest around 40 years ago with the pioneering studies conducted by Hubel and Wiesel. Their early work on the impact of monocular deprivation and strabismus initiated what is now a rapidly developing field of cortical plasticity research. Although the monocular deprivation paradigm originated by Hubel and Wiesel remains a key experimental manipulation in studies of cortical plasticity, somewhat ironically, the neurology underlying the human conditions of strabismus and amblyopia that motivated this early work remains elusive. In this review, the authors combine contemporary research on plasticity and development with data from human and animal investigations of amblyopic populations to assess what is known and to reexamine some of the key assumptions about human amblyopia.

Abnormal visual experience during development alters the early stages of visual-tactile integration

Visual experience during the critical periods in early postnatal life is necessary for the normal development of the visual system. Disruption of visual input during this period results in amblyopia, which is associated with reduced activation of the striate and extrastriate cortices. It is well known that visual input converges with other sensory signals and exerts a significant influence on cortical processing in multiple association areas. Recent work in healthy adults has also shown that task-relevant visual input can modulate neural excitability at very early stages of information processing in the primary somatosensory cortex. Here we used electroencephalography to investigate visual-tactile interactions in adults with abnormal binocular vision due to amblyopia and strabismus. Results showed three main findings. First, in comparison to a visually normal control group, participants with abnormal vision had a significantly lower amplitude of the P50 somatosensory event related potential (ERP) when visual and tactile stimuli were presented concurrently. Second, the amplitude of the P100 somatosensory ERP was significantly greater in participants with abnormal vision. These results indicate that task relevant visual input does not significantly influence the excitability of the primary somatosensory cortex, instead, the excitability of the secondary somatosensory cortex is increased. Third, participants with abnormal vision had a higher amplitude of the P1 visual ERP when a tactile stimulus was presented concurrently. Importantly, these results were not modulated by viewing condition, which indicates that the impact of amblyopia on crossmodal interactions is not simply related to the reduced visual acuity as it was evident when viewing with the unaffected eye and binocularly. These results indicate that the consequences of abnormal visual experience on neurophysiological processing extend beyond the primary and secondary visual areas to other modality-specific areas.

Temporal Instabilities in Amblyopic Perception: A Quantitative Approach

The purpose of this study is to quantify the temporal characteristics of spatial misperceptions in human amblyopia. Twenty-two adult participants with strabismus, strabismic, anisometropic, or mixed amblyopia were asked to describe their subjective percept of static geometrical patterns with different spatial frequencies and shapes, as seen with their non-dominant eye. We generated digital reconstructions of their perception (static images or movies) that were subsequently validated by the subjects using consecutive matching sessions. We calculated the Shannon entropy variation in time for each recorded movie, as a measure of temporal instability. Nineteen of the 22 subjects perceived temporal instabilities that can be broadly classified in two categories. We found that the average frequency of the perceived temporal instabilities is ∼1 Hz. The stimuli with higher spatial frequencies yielded more often temporally unstable perceptions with higher frequencies. We suggest that type and amount of temporal instabilities in amblyopic vision are correlated with the etiology and spatial frequency of the stimulus.

Spatial-frequency dependent binocular imbalance in amblyopia

While amblyopia involves both binocular imbalance and deficits in processing high spatial frequency information, little is known about the spatial-frequency dependence of binocular imbalance. Here we examined binocular imbalance as a function of spatial frequency in amblyopia using a novel computer-based method. Binocular imbalance at four spatial frequencies was measured with a novel dichoptic letter chart in individuals with amblyopia, or normal vision. Our dichoptic letter chart was composed of band-pass filtered letters arranged in a layout similar to the ETDRS acuity chart. A different chart was presented to each eye of the observer via stereo-shutter glasses. The relative contrast of the corresponding letter in each eye was adjusted by a computer staircase to determine a binocular Balance Point at which the observer reports the letter presented to either eye with equal probability. Amblyopes showed pronounced binocular imbalance across all spatial frequencies, with greater imbalance at high compared to low spatial frequencies (an average increase of 19%, p < 0.01). Good test-retest reliability of the method was demonstrated by the Bland-Altman plot. Our findings suggest that spatial-frequency dependent binocular imbalance may be useful for diagnosing amblyopia and as an outcome measure for recovery of binocular vision following therapy.

Perceptual Visual Distortions in Adult Amblyopia and Their Relationship to Clinical Features

PURPOSE

Develop a paradigm to map binocular perceptual visual distortions in adult amblyopes and visually normal controls, measure their stability over time, and determine the relationship between strength of binocular single vision and distortion magnitude.

METHODS

Perceptual visual distortions were measured in 24 strabismic, anisometropic, or microtropic amblyopes (interocular acuity difference ≥ 0.200 logMAR or history of amblyopia treatment) and 10 controls (mean age 27.13 ± 10.20 years). The task was mouse-based target alignment on a stereoscopic liquid crystal display monitor, measured binocularly five times during viewing dichoptically through active shutter glasses, amblyopic eye viewing cross-hairs, fellow eye viewing single target dots (16 locations within central 5°), and five times nondichoptically, with all stimuli visible to either eye. Measurements were repeated over time (1 week, 1 month) in eight amblyopic subjects, evaluating test-retest reliability. Measurements were also correlated against logMAR visual acuity, horizontal prism motor fusion range, Frisby/Preschool Randot stereoacuity, and heterophoria/heterotropia prism cover test measurement.

RESULTS

Sixty-seven percent (16/24) of amblyopes had significant perceptual visual distortions under dichoptic viewing conditions compared to nondichoptic viewing conditions and dichoptic control group performance. Distortions correlated with the strength of motor fusion (r = -0.417, P = 0.043) and log stereoacuity (r = 0.492, P = 0.015), as well as near angle of heterotropic/heterophoric deviation (r = 0.740, P < 0.001), and, marginally, amblyopia depth (r = 0.405, P = 0.049). Global distortion index (GDI, mean displacement) remained, overall, consistent over time (median change in GDI between baseline and 1 week = -0.03°, 1 month = -0.08°; x-axis Z = 4.4256, P < 0.001; y-axis Z = 5.0547, P < 0.001).

CONCLUSIONS

Perceptual visual distortions are stable over time and associated with poorer binocular function, greater amblyopia depth, and larger angles of ocular deviation. Assessment of distortions may be relevant for recent perceptual learning paradigms specifically targeting binocular vision.

Binocular versus standard occlusion or blurring treatment for unilateral amblyopia in children aged three to eight years

BACKGROUND

Current treatments for amblyopia in children, occlusion and pharmacological blurring, have had limited success, with less than two-thirds of children achieving good visual acuity of at least 0.20 logMAR in the amblyopic eye, limited improvement of stereopsis, and poor compliance. A new treatment approach, based on the dichoptic presentation of movies or computer games (images presented separately to each eye), may yield better results, as it aims to balance the input of visual information from each eye to the brain. Compliance may also improve with these more child-friendly treatment procedures.

OBJECTIVES

To determine whether binocular treatments in children aged three to eight years with unilateral amblyopia result in better visual outcomes than conventional occlusion or pharmacological blurring treatment.

SEARCH METHODS

We searched the Cochrane Eyes and Vision Group Trials Register (last date of searches: 14 April 2015), the Cochrane Central Register of Controlled Trials (CENTRAL; 2015, Issue 3), Ovid MEDLINE, Ovid MEDLINE In-Process and Other Non-Indexed Citations, Ovid MEDLINE Daily, Ovid OLDMEDLINE (January 1946 to April 2015), EMBASE (January 1980 to April 2015), the ISRCTN registry (www.isrctn.com/editAdvancedSearch), ClinicalTrials.gov (www.clinicaltrials.gov), and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) (www.who.int/ictrp/search/en). We did not use any date or language restrictions in the electronic searches for trials.

SELECTION CRITERIA

Two review authors independently screened the results of the search in order to identify studies that met the inclusion criteria of the review: randomised controlled trials (RCTs) that enrolled participants between the ages of three and eight years old with unilateral amblyopia, defined as best-corrected visual acuity (BCVA) worse than 0.200 logMAR in the amblyopic eye, and BCVA 0.200 logMAR or better in the fellow eye, in the presence of an amblyogenic risk factor such as anisometropia, strabismus, or both. Prior to enrolment, participants were to have undergone a cycloplegic refraction and comprehensive ophthalmic examination including fundal examination. In addition, participants had to have completed a period of optical treatment, if indicated, and BCVA in the amblyopic eye had to remain unchanged on two consecutive assessments despite reportedly good compliance with glasses wearing. Participants were not to have received any treatment other than optical treatment prior to enrolment. We planned to include any type of binocular viewing intervention; these could be delivered on different devices including computer monitors viewed with LCD shutter glasses or hand-held screens including mobile phone screens with lenticular prism overlay. Control groups were to have received standard amblyopia treatment; this could include occlusion or pharmacological blurring of the better-seeing eye. We planned to include full-time (all waking hours) and part-time (between 1 and 12 hours a day) occlusion regimens.

DATA COLLECTION AND ANALYSIS

We planned to use standard methodological procedures expected by The Cochrane Collaboration. We had planned to meta-analyse the primary outcome, that is mean distance BCVA in the amblyopic eye at 12 months after the cessation of treatment.

MAIN RESULTS

We could identify no RCTs in this subject area.

AUTHORS' CONCLUSIONS

Further research is required to allow decisions about implementation of binocular treatments for amblyopia in clinical practice. Currently there are no clinical trials offering standardised evidence of the safety and effectiveness of binocular treatments, but results from non-controlled cohort studies are encouraging. Future research should be conducted in the form of RCTs, using acknowledged methods of visual acuity and stereoacuity assessment with known reproducibility. Other important outcome measures include outcomes reported by users, compliance with treatment, and recurrence of amblyopia after cessation of treatment.

Early monocular defocus disrupts the normal development of receptive-field structure in V2 neurons of macaque monkeys

Experiencing different quality images in the two eyes soon after birth can cause amblyopia, a developmental vision disorder. Amblyopic humans show the reduced capacity for judging the relative position of a visual target in reference to nearby stimulus elements (position uncertainty) and often experience visual image distortion. Although abnormal pooling of local stimulus information by neurons beyond striate cortex (V1) is often suggested as a neural basis of these deficits, extrastriate neurons in the amblyopic brain have rarely been studied using microelectrode recording methods. The receptive field (RF) of neurons in visual area V2 in normal monkeys is made up of multiple subfields that are thought to reflect V1 inputs and are capable of encoding the spatial relationship between local stimulus features. We created primate models of anisometropic amblyopia and analyzed the RF subfield maps for multiple nearby V2 neurons of anesthetized monkeys by using dynamic two-dimensional noise stimuli and reverse correlation methods. Unlike in normal monkeys, the subfield maps of V2 neurons in amblyopic monkeys were severely disorganized: subfield maps showed higher heterogeneity within each neuron as well as across nearby neurons. Amblyopic V2 neurons exhibited robust binocular suppression and the strength of the suppression was positively correlated with the degree of hereogeneity and the severity of amblyopia in individual monkeys. Our results suggest that the disorganized subfield maps and robust binocular suppression of amblyopic V2 neurons are likely to adversely affect the higher stages of cortical processing resulting in position uncertainty and image distortion.

Estimation of cortical magnification from positional error in normally sighted and amblyopic subjects

We describe a method for deriving the linear cortical magnification factor from positional error across the visual field. We compared magnification obtained from this method between normally sighted individuals and amblyopic individuals, who receive atypical visual input during development. The cortical magnification factor was derived for each subject from positional error at 32 locations in the visual field, using an established model of conformal mapping between retinal and cortical coordinates. Magnification of the normally sighted group matched estimates from previous physiological and neuroimaging studies in humans, confirming the validity of the approach. The estimate of magnification for the amblyopic group was significantly lower than the normal group: by 4.4 mm deg(-1) at 1° eccentricity, assuming a constant scaling factor for both groups. These estimates, if correct, suggest a role for early visual experience in establishing retinotopic mapping in cortex. We discuss the implications of altered cortical magnification for cortical size, and consider other neural changes that may account for the amblyopic results.

Infants' visual system nonretinotopically integrates color signals along a motion trajectory

Whereas early visual processing has been considered primarily retinotopic, recent studies have revealed significant contributions of nonretinotopic processing to the human perception of fundamental visual features. For adult vision, it has been shown that information about color, shape, and size is nonretinotipically integrated along the motion trajectory, which could bring about clear and unblurred perception of a moving object. Since this nonretinotopic processing presumably includes tight and elaborated cooperation among functional cortical modules for different visual attributes, how this processing matures in the course of brain development is an important unexplored question. Here we show that the nonretinotopic integration of color signals is fully developed in infants at five months of age. Using preferential looking, we found significantly better temporal segregation of colors for moving patterns than for flickering patterns, even when the retinal color alternation rate was the same. This effect could be ascribed to the integration of color signals along a motion trajectory. Furthermore, the infants' color segmentation performance was comparable to that of human adults. Given that both the motion processing and color vision of 5-month-old infants are still under development, our findings suggest that nonretinotopic color processing develops concurrently with basic color and motion processing. Our findings not only support the notion of an early presence of cross-modal interactions in the brain, but also indicate the early development of a purposive cross-module interaction for elegant visual computation.

Age- and stereovision-dependent eye-hand coordination deficits in children with amblyopia and abnormal binocularity

PURPOSE

To examine factors contributing to eye-hand coordination deficits in children with amblyopia and impaired stereovision.

METHODS

Participants were 55 anisometropic or strabismic children aged 5.0 to 9.25 years with different degrees of amblyopia and abnormal binocularity, along with 28 age-matched visually-normal controls. Pilot data were obtained from four additional patients studied longitudinally at different treatment stages. Movements of the preferred hand were recorded using a 3D motion-capture system while subjects reached-to-precision grasp objects (two sizes, three locations) under binocular, dominant eye, and amblyopic/nonsighting eye conditions. Kinematic and "error" performance measures were quantified and compared by viewing condition and subject group using ANOVA, stepwise regression, and correlation analyses.

RESULTS

Movements of the younger amblyopes (age 5-6 years; n = 30) were much slower, particularly in the final approach to the objects, and contained more spatial errors in reaching (∼×1.25-1.75) and grasping (∼×1.75-2.25) under all three views (P < 0.05) than their age-matched controls (n = 13). Amblyopia severity was the main contributor to their slower movements with absent stereovision a secondary factor and the unique determinant of their increased error-rates. Older amblyopes (age 7-9 years; n = 25) spent longer contacting the objects before lifting them (P = 0.015) compared with their matched controls (n = 15), with absence of stereovision still solely related to increases in reach and grasp errors, although these occurred less frequently than in younger patients. Pilot prospective data supported these findings by showing positive treatment-related associations between improved stereovision and reach-to-grasp performance.

CONCLUSIONS

Strategies that children with amblyopia and abnormal binocularity use for reach-to-precision grasping change with age, from emphasis on visual feedback during the "in-flight" approach at ages 5 to 6 years to more reliance on tactile/kinesthetic feedback from object contact at ages 7 to 9 years. However, recovery of binocularity confers increasing benefits for eye-hand coordination speed and accuracy with age, and is a better predictor of these fundamental performance measures than the degree of visual acuity loss.

Assessing binocular interaction in amblyopia and its clinical feasibility

PURPOSE

To measure binocular interaction in amblyopes using a rapid and patient-friendly computer-based method, and to test the feasibility of the assessment in the clinic.

METHODS

Binocular interaction was assessed in subjects with strabismic amblyopia (n = 7), anisometropic amblyopia (n = 6), strabismus without amblyopia (n = 15) and normal vision (n = 40). Binocular interaction was measured with a dichoptic phase matching task in which subjects matched the position of a binocular probe to the cyclopean perceived phase of a dichoptic pair of gratings whose contrast ratios were systematically varied. The resulting effective contrast ratio of the weak eye was taken as an indicator of interocular imbalance. Testing was performed in an ophthalmology clinic under 8 mins. We examined the relationships between our binocular interaction measure and standard clinical measures indicating abnormal binocularity such as interocular acuity difference and stereoacuity. The test-retest reliability of the testing method was also evaluated.

RESULTS

Compared to normally-sighted controls, amblyopes exhibited significantly reduced effective contrast (∼20%) of the weak eye, suggesting a higher contrast requirement for the amblyopic eye compared to the fellow eye. We found that the effective contrast ratio of the weak eye covaried with standard clincal measures of binocular vision. Our results showed that there was a high correlation between the 1st and 2nd measurements (r = 0.94, p<0.001) but without any significant bias between the two.

CONCLUSIONS

Our findings demonstrate that abnormal binocular interaction can be reliably captured by measuring the effective contrast ratio of the weak eye and quantitative assessment of binocular interaction is a quick and simple test that can be performed in the clinic. We believe that reliable and timely assessment of deficits in a binocular interaction may improve detection and treatment of amblyopia.

Linking assumptions in amblyopia

Over the last 35 years or so, there has been substantial progress in revealing and characterizing the many interesting and sometimes mysterious sensory abnormalities that accompany amblyopia. A goal of many of the studies has been to try to make the link between the sensory losses and the underlying neural losses, resulting in several hypotheses about the site, nature, and cause of amblyopia. This article reviews some of these hypotheses, and the assumptions that link the sensory losses to specific physiological alterations in the brain. Despite intensive study, it turns out to be quite difficult to make a simple linking hypothesis, at least at the level of single neurons, and the locus of the sensory loss remains elusive. It is now clear that the simplest notion-that reduced contrast sensitivity of neurons in cortical area V1 explains the reduction in contrast sensitivity-is too simplistic. Considerations of noise, noise correlations, pooling, and the weighting of information also play a critically important role in making perceptual decisions, and our current models of amblyopia do not adequately take these into account. Indeed, although the reduction of contrast sensitivity is generally considered to reflect "early" neural changes, it seems plausible that it reflects changes at many stages of visual processing.

The relationship between anisometropia and amblyopia

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child's first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after the emergence of amblyopia secondary to either deprivation or strabismus, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of 'pure' anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological and ethical challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.

The effects of spatial offset, temporal offset and image speed on sensitivity to global motion in human amblyopia

The presence of a general global motion processing deficit in amblyopia is now well established, although its severity may depend on image speed and amblyopia type, but its underlying cause(s) is still largely indeterminate. To address this issue and to characterize further the nature of the global motion perception deficit in human amblyopia, the effects of varying spatial offset (jump size-Δs) and temporal offset (delay between positional updates-Δt) in discriminating global motion for a range of speeds (1.5, 3 and 9°/s) in both amblyopic and normal vision were evaluated. For normal adult observers (NE) and the non-amblyopic eye (FE) motion coherence thresholds measured when Δt was varied were significantly higher than those when Δs was varied. Furthermore when Δt was varied, thresholds rose significantly as the speed of image motion decreased for both NEs and FEs. AE thresholds were higher overall than the other eyes and appeared independent of both the method used to create movement and speed. These results suggest that the spatial and temporal limits underlying the perception of global motion are different. In addition degrading the smoothness of motion has comparatively little effect on the motion mechanisms driven by the AE, suggesting that the internal noise associated with encoding motion direction is relatively high.

The effect of sensory uncertainty due to amblyopia (lazy eye) on the planning and execution of visually-guided 3D reaching movements

BACKGROUND

Impairment of spatiotemporal visual processing in amblyopia has been studied extensively, but its effects on visuomotor tasks have rarely been examined. Here, we investigate how visual deficits in amblyopia affect motor planning and online control of visually-guided, unconstrained reaching movements.

METHODS

Thirteen patients with mild amblyopia, 13 with severe amblyopia and 13 visually-normal participants were recruited. Participants reached and touched a visual target during binocular and monocular viewing. Motor planning was assessed by examining spatial variability of the trajectory at 50-100 ms after movement onset. Online control was assessed by examining the endpoint variability and by calculating the coefficient of determination (R(2)) which correlates the spatial position of the limb during the movement to endpoint position.

RESULTS

Patients with amblyopia had reduced precision of the motor plan in all viewing conditions as evidenced by increased variability of the reach early in the trajectory. Endpoint precision was comparable between patients with mild amblyopia and control participants. Patients with severe amblyopia had reduced endpoint precision along azimuth and elevation during amblyopic eye viewing only, and along the depth axis in all viewing conditions. In addition, they had significantly higher R(2) values at 70% of movement time along the elevation and depth axes during amblyopic eye viewing.

CONCLUSION

Sensory uncertainty due to amblyopia leads to reduced precision of the motor plan. The ability to implement online corrections depends on the severity of the visual deficit, viewing condition, and the axis of the reaching movement. Patients with mild amblyopia used online control effectively to compensate for the reduced precision of the motor plan. In contrast, patients with severe amblyopia were not able to use online control as effectively to amend the limb trajectory especially along the depth axis, which could be due to their abnormal stereopsis.

Effects of speed, age, and amblyopia on the perception of motion-defined form

We determined the effect of dot speed on the typical and atypical development of motion-defined form perception. Monocular motion coherence thresholds for orientation discrimination of motion-defined rectangles were determined at slow (0.1 deg/s), medium (0.9 deg/s) and fast (5.0 deg/s) dot speeds. First we examined typical development from age 4 to 31 years. We found that performance was most immature at the slow speed and in the youngest group of children (4-6 years). Next we measured motion-defined form perception in the amblyopic and fellow eyes of patients with amblyopia. Deficits were found in both eyes and were most pronounced at the slow speed. These results demonstrate the importance of dot speed to the development of motion-defined form perception. Implications regarding sensitive periods and the neural correlates of motion-defined form perception are discussed.

Amblyopia: a mini review of the literature

Amblyopia is a form of cerebral visual impairment in the absence of an organic cause. It is considered to derive from the degradation of the retinal image associated with abnormal visual experience during the developmental period of the visual system in infancy and early childhood. Amblyopia is a significant cause of unilateral visual deficit in childhood and is still considered as one of the most common causes of persistent unilateral visual impairment in adulthood. The following review aims at presenting the contemporary literature regarding the prevalence, the aetiology, the neural correlates, the period of critical development, the treatment, the prognosis and the disability associated with this visual deficit.

Impaired spatial and binocular summation for motion direction discrimination in strabismic amblyopia

Amblyopia is characterised by visual deficits in both spatial vision and motion perception. While the spatial deficits are thought to result from deficient processing at both low and higher level stages of visual processing, the deficits in motion perception appear to result primarily from deficits involving higher level processing. Specifically, it has been argued that the motion deficit in amblyopia occurs when local motion information is pooled spatially and that this process is abnormally susceptible to the presence of noise elements in the stimulus. Here we investigated motion direction discrimination for abruptly presented two-frame Gabor stimuli in a group of five strabismic amblyopes and five control observers. Motion direction discrimination for this stimulus is inherently noisy and relies on the signal/noise processing of motion detectors. We varied viewing condition (monocular vs. binocular), stimulus size (5.3-18.5°) and stimulus contrast (high vs. low) in order to assess the effects of binocular summation, spatial summation and contrast on task performance. No differences were found for the high contrast stimuli; however the low contrast stimuli revealed differences between the control and amblyopic groups and between fellow fixing and amblyopic eyes. Control participants exhibited pronounced binocular summation for this task (on average a factor of 3.7), whereas amblyopes showed no such effect. In addition, the spatial summation that occurred for control eyes and the fellow eye of amblyopes was significantly attenuated for the amblyopic eyes relative to fellow eyes. Our results support the hypothesis that pooling of local motion information from amblyopic eyes is abnormal and highly sensitive to noise.

Effects of anisometropic amblyopia on visuomotor behavior, part 2: visually guided reaching

PURPOSE

The effects of impaired spatiotemporal vision in amblyopia on visuomotor skills have rarely been explored in detail. The goal of this study was to examine the influences of amblyopia on visually guided reaching.

METHODS

Fourteen patients with anisometropic amblyopia and 14 control subjects were recruited. Participants executed reach-to-touch movements toward targets presented randomly 5° or 10° to the left or right of central fixation in three viewing conditions: binocular, monocular amblyopic eye, and monocular fellow eye viewing (left and right monocular viewing for control subjects). Visual feedback of the target was removed on 50% of the trials at the initiation of reaching.

RESULTS

Reaching accuracy was comparable between patients and control subjects during all three viewing conditions. Patients' reaching responses were slightly less precise during amblyopic eye viewing, but their precision was normal during binocular or fellow eye viewing. Reaching reaction time was not affected by amblyopia. The duration of the acceleration phase was longer in patients than in control subjects under all viewing conditions, whereas the duration of the deceleration phase was unaffected. Peak acceleration and peak velocity were also reduced in patients.

CONCLUSIONS

Amblyopia affects both the programming and the execution of visually guided reaching. The increased duration of the acceleration phase, as well as the reduced peak acceleration and peak velocity, might reflect a strategy or adaptation of feedforward/feedback control of the visuomotor system to compensate for degraded spatiotemporal vision in amblyopia, allowing patients to optimize their reaching performance.

Effects of anisometropic amblyopia on visuomotor behavior, I: saccadic eye movements

PURPOSE

Impairment of spatiotemporal visual processing is the hallmark of amblyopia, but its effects on eye movements during visuomotor tasks have rarely been studied. Here the authors investigate how visual deficits in anisometropic amblyopia affect saccadic eye movements.

METHODS

Thirteen patients with anisometropic amblyopia and 13 control subjects participated. Participants executed saccades and manual reaching movements to a target presented randomly 5° or 10° to the left or right of fixation in three viewing conditions: binocular, amblyopic, and fellow eye viewing. Latency, amplitude, and peak velocity of primary and corrective saccades were measured.

RESULTS

Initiation of primary saccades was delayed and more variable when patients viewed monocularly with their amblyopic eye. During binocular viewing, saccadic latency exhibited increased variability and no binocular advantage in patients (i.e., mean latency was similar to that during fellow eye viewing). Mean amplitude and peak velocity of primary saccades were comparable between patients and control subjects; however, patients exhibited greater variability in saccade amplitude. The frequency of corrective saccades was greater when patients viewed with their fellow eye than it was with binocular or amblyopic eye viewing. Latency, amplitude, and peak velocity of corrective saccades in patients were normal in all viewing conditions.

CONCLUSIONS

Saccades had longer latency and decreased precision in amblyopia. Once saccades were initiated, however, the dynamics of saccades were not altered. These findings suggest that amblyopia is associated with slower visual processing in the afferent (sensory) pathway rather than a deficit in the efferent (motor) pathway of the saccadic system.

(In) sensitivity to spatial distortion in natural scenes

The perception of object structure in the natural environment is remarkably stable under large variation in image size and projection, especially given our insensitivity to spatial position outside the fovea. Sensitivity to periodic spatial distortions that were introduced into one quadrant of gray-scale natural images was measured in a 4AFC task. Observers were able to detect the presence of distortions in unfamiliar images even though they did not significantly affect the amplitude spectrum. Sensitivity depended on the spatial period of the distortion and on the image structure at the location of the distortion. The results suggest that the detection of distortion involves decisions made in the late stages of image perception and is based on an expectation of the typical structure of natural scenes.

Comparison of normal and amblyopic retinas by optical coherence tomography in children

PURPOSE

To measure retinal thickness in eyes of children with regular visual development and in amblyopic eyes (distinguishing between mild and severe) by optical coherence tomography (OCT), establishing significant differences between them.

METHODS

Following our previous study, 192 eyes of children from 4 to 10 years old were examined: 68 with regular vision and 124 with ametropic amblyopia at the time of diagnosis (66 mild and 58 severe). The outside macular ring was analyzed by OCT, divided into 4 areas (superior, inferior, temporal and nasal). Effect of sex, age and refraction in retinal thickness obtained were studied.

RESULTS

All retinal areas in the various groups showed a high correlation between them, with rates ranging between 0.652 and 0.718 (all p <0.001). According to results, all retinal areas are thicker in amblyopic eyes than in normal ones (p <0.05 in the upper and nasal), especially in slight amblyopia, which is up to 4.5% thicker than in normal eyes (maximum 11 mum in the nasal area, p <0.001). Differences were greater in females (up to 5.9% thicker). The inferior area in hyperopic eyes proved to be thicker, with no differences according to age.

CONCLUSIONS

Retinas of eyes with ametropic amblyopia have a more considerable retinal thickness than in a normal eye, at least in some areas. The severity of amblyopia and gender are factors of interaction, not of confounding.