Ian E. Pacey

Centroid Analysis Predicts Visual Localization of First- and Second-order Stimuli

Perceived alignment of asymmetric Gaussian-windowed stimuli was measured in an attempt to differentiate between stimulus characteristics which might underlie visual localization. These asymmetric stimuli have the advantage of being continuous in the spatial domain and of possessing well-defined spatial characteristics in which centroid, points of inflexion and peak can be separated from each other. Results for both luminance- and contrast-defined stimuli are reasonably well described on the basis that the centroid of the stimulus envelope represents the primitive which determines perceived visual location. Centroid location is inherent in the output of filters which are large enough to cover the object of interest.

Changes to Control of Adaptive Gait in Individuals with Long-standing Reduced Stereoacuity

PURPOSE Gait during obstacle negotiation is adapted in visually normal subjects whose vision is temporarily and unilaterally blurred or occluded. This study was conducted to examine whether gait parameters in individuals with long-standing deficient stereopsis are similarly adapted. METHODS Twelve visually normal subjects and 16 individuals with deficient stereopsis due to amblyopia and/or its associated conditions negotiated floor-based obstacles of different heights (7-22 cm). Trials were conducted during binocular viewing and monocular occlusion. Analyses focused on foot placement before the obstacle and toe clearance over it. RESULTS Across all viewing conditions, there were significant group-by-obstacle height interactions for toe clearance (P < 0.001), walking velocity (P = 0.003), and penultimate step length (P = 0.022). Toe clearance decreased (approximately 0.7 cm) with increasing obstacle height in visually normal subjects, but it increased (approximately 1.5 cm) with increasing obstacle height in the stereo-deficient group. Walking velocity and penultimate step length decreased with increasing obstacle height in both groups, but the reduction was more pronounced in stereo-deficient individuals. Post hoc analyses indicated group differences in toe clearance and penultimate step length when negotiating the highest obstacle (P < 0.05). CONCLUSIONS Occlusion of either eye caused significant and similar gait changes in both groups, suggesting that in stereo-deficient individuals, as in visually normal subjects, both eyes contribute usefully to the execution of adaptive gait. Under monocular and binocular viewing, obstacle-crossing performance in stereo-deficient individuals was more cautious when compared with that of visually normal subjects, but this difference became evident only when the subjects were negotiating higher obstacles; suggesting that such individuals may be at greater risk of tripping or falling during everyday locomotion.

Cross-sectional sample of peripheral refraction in four meridians in myopes and emmetropes.

PURPOSE The study of peripheral refractive error is of growing interest as degradation of the retinal image quality in the periphery is known to affect central refractive error development and ocular shape in animal models. The purpose of this study was to measure peripheral refraction across the horizontal, vertical, and two oblique meridians in a group of myopic and emmetropic adults and to investigate retinal asymmetry in the human retina. METHODS Thirty-one myopes (spherical equivalent between -2.00 and -9.62 D) and 20 emmetropes (spherical equivalent between -0.50 and +0.50 D) with astigmatism less than -0.75 D, participated in the project. Noncycloplegic peripheral refraction measurements were captured with an autorefractor (NVision K-5001; Shin-Nippon, Tokyo, Japan) at the fovea and up to 30° eccentricity in the horizontal, vertical, and two oblique meridians in 10° steps. RPR was calculated by subtracting the foveal spherical equivalent refraction from that obtained at each eccentric location. RESULTS Along all measured meridians, myopic eyes showed a relative hyperopic shift in the periphery, with the superior-temporal portion of the retina exhibiting the smallest shift. Emmetropic eyes, however, exhibited a relatively consistent refractive profile across all meridians and eccentricities, confirming a spherical retinal shape for this group. In addition, off-axis astigmatism increased with eccentricity in all meridians. CONCLUSIONS These results suggest that the myopic eye tends toward an ellipsoid shape, rather than the globular shape of an emmetropic eye.

Bilateral Changes in Foveal Structure in Individuals with Amblyopia

PURPOSE To examine foveal structure in amblyopia using spectral-domain optical coherence tomography (SD-OCT). DESIGN Prospective, cross-sectional study. PARTICIPANTS AND CONTROLS Two subject groups were recruited to the study: 85 amblyopes (34 adults, 51 children) and 110 visually normal controls (44 adults, 66 children). METHODS A detailed eye examination, including an SD-OCT scan, was performed in all participants. A total of 390 eyes of 195 subjects were imaged using a 3-dimensional (3D) macula scan covering a nominal 6 × 6-mm area with a resolution of 256 × 256 (65,536 axial scans). Data from the B-scans bisecting the fovea both horizontally and vertically were fitted with a mathematical model of the fovea to determine a range of foveal parameters. MAIN OUTCOME MEASURES Foveal thickness, foveal pit depth, and foveal pit slope. RESULTS Bilateral differences between the eyes of amblyopes compared with visually normal controls were found. The difference between foveal structure in amblyopic participants relative to structure in subjects with normal vision persisted even when variables such as age, ethnicity, axial length, and sex were taken into account. Amblyopes showed increased foveal thickness (+8.31 μm; P = 0.006) and a reduction in pit depth in the horizontal meridian (-10.06 μm; P = 0.005) but not in the vertical meridian (P = 0.082) when compared with subjects with normal vision. Foveal pit slopes were found to be approximately 1 degree flatter in the nasal (P = 0.033) and temporal (P = 0.014) meridians in amblyopes, but differences between amblyopes and controls in the superior (P = 0.061) and inferior (P = 0.087) meridians did not reach statistical significance. No statistically significant interocular differences were found in the foveal structure between amblyopic and fellow eyes. CONCLUSIONS Differences were found in the foveal structure in both eyes of amblyopes compared with subjects with normal vision. These differences consisted of increased foveal thickness, reduced pit depth when measured along the horizontal meridian, and flattening of the nasal and temporal sides of the foveal pit.

A limited role for suppression in the central field of individuals with strabismic amblyopia.

Background Although their eyes are pointing in different directions, people with long-standing strabismic amblyopia typically do not experience double-vision or indeed any visual symptoms arising from their condition. It is generally believed that the phenomenon of suppression plays a major role in dealing with the consequences of amblyopia and strabismus, by preventing images from the weaker/deviating eye from reaching conscious awareness. Suppression is thus a highly sophisticated coping mechanism. Although suppression has been studied for over 100 years the literature is equivocal in relation to the extent of the retina that is suppressed, though the method used to investigate suppression is crucial to the outcome. There is growing evidence that some measurement methods lead to artefactual claims that suppression exists when it does not. Methodology/Results Here we present the results of an experiment conducted with a new method to examine the prevalence, depth and extent of suppression in ten individuals with strabismic amblyopia. Seven subjects (70%) showed no evidence whatsoever for suppression and in the three individuals who did (30%), the depth and extent of suppression was small. Conclusions Suppression may play a much smaller role in dealing with the negative consequences of strabismic amblyopia than previously thought. Whereas recent claims of this nature have been made only in those with micro-strabismus our results show extremely limited evidence for suppression across the central visual field in strabismic amblyopes more generally. Instead of suppressing the image from the weaker/deviating eye, we suggest the visual system of individuals with strabismic amblyopia may act to maximise the possibilities for binocular co-operation. This is consistent with recent evidence from strabismic and amblyopic individuals that their binocular mechanisms are intact, and that, just as in visual normals, performance with two eyes is better than with the better eye alone in these individuals.

Repeatability and reproducibility of macular thickness measurements using fourier domain optical coherence tomography.

Aim: To evaluate repeatability and reproducibility of macular thickness measurements in visually normal eyes using the Topcon 3D OCT-1000. Methods: Phase 1 investigated scan repeatability, the effect of age and pupil dilation. Two groups (6 younger and 6 older participants) had one eye scanned 5 times pre and post- dilation by 1 operator. Phase 2 investigated between-operator, within and between-visit reproducibility. 10 participants had 1 un-dilated eye scanned 3 times on 2 separate visits by 2 operators. Results: Phase 1: No significant difference existed between repeat scans (p=0.75) and no significant difference was found pre- and post-dilation (p=0.54). In the younger group variation was low (95% limits ± 3.62µm) and comparable across all retinal regions. The older group demonstrated greater variation (95% limits ± 7.6µm). Phase 2: For a given retinal location, 95% confidence limits for within-operator, within-visit reproducibility was 5.16µm. This value increased to 5.56µm for the same operator over two visits and to 6.18µm for two operators over two visits. Conclusion: A high level repeatability, close to 6µm, of macular thickness measurement is possible using the 3D OCT- 1000. Measured differences in macular thickness between successive visits that exceed 6µm in pre-presbyopic individuals are therefore likely to reflect actual structural change. OCT measures are more variable in older individuals and it is advisable to take a series of scans so that outliers can be more easily identified.

Off-axis partial coherence interferometry in myopes and emmetropes

This study describes the axial and peripheral dimensions of myopic and emmetropic eyes in a wide range of retinal locations using a non‐contact optical biometer (Zeiss IOLMaster) based upon the principle of partial coherence interferometry. Understanding the optical properties of the peripheral eye may provide insight into myopia development and the possible effects on off‐axis visual performance.

A comparison of the effects of ageing upon vernier and bisection acuity.

While most positional acuity tasks exhibit an age-related decline in performance, the effect of ageing upon vernier acuity continues to be the subject of some debate. In the present study we employed a stimulus design that enabled the simultaneous determination of bisection and vernier acuities in 36 subjects, aged between 22 and 84 years. This approach provided a means for directly testing the hypothesis that ageing affects bisection acuity but not vernier acuity by ensuring that differences in stimulus configuration and in the subjects task were kept to an absolute minimum. Optimum thresholds increased as a function of age for both bisection and vernier tasks. Inter-subject threshold variability also increased with age. Issues surrounding the comparison of absolute vernier thresholds across different studies are discussed and two important methodological factors are identified: the precise statistical method used to estimate thresholds, and the magnitude, in angular terms, of the smallest spatial offset of the elements of the vernier stimulus which can be displayed. Comparison with previously published data indicates that the discrepancy between this study and most previous investigations with respect to the effect of age upon vernier performance can be at least partly accounted for by differences in the minimum displayable vernier offset. Vernier thresholds do increase with age. The increased variability of vernier thresholds in older subjects would appear to limit the diagnostic value of the test as a means of enabling normal ageing to be distinguished from visual loss due to pathology of the eye or visual system.

The effect of instrument alignment on peripheral refraction measurements by automated optometer

Citation information: Ehsaei A, Chisholm CM, Mallen EAH & Pacey IE. The effect of instrument alignment on peripheral refraction measurements by automated optometer. Ophthalmic Physiol Opt 2011, 31, 413–420. doi: 10.1111/j.1475‐1313.2011.00838.x

Central and peripheral visual performance in myopes: Contact lenses versus spectacles

PURPOSE Myopia is known to degrade visual performance with both optical and retinal changes implicated. Whether contact lenses or spectacles provide better visual performance for myopes is still under debate. The purpose of this study was to examine central and peripheral visual function in myopic subjects corrected with contact lenses versus spectacles. METHODS Size thresholds were measured at 13 locations for 20 myopic subjects (mean spherical equivalent refractive error (SE): -6.43±1.22 D and cylinder power: -0.23±0.22 D) corrected with contact lenses (new etafilcon A contact lens, fitted 15 min prior to measurements) versus spectacles. Measurements were taken at both low (δl/l=14%) and high (δl/l=100%) contrast levels. The data were analysed using one way repeated-measures ANOVA. RESULTS Size thresholds increased with eccentricity in a similar manner for both forms of optical correction. Repeated-measures ANOVA showed no statistically significant difference in central and peripheral visual performance between the two forms of correction for both low and high contrast tasks. The outcome remained the same following correction for spectacle magnification. CONCLUSION Eye care practitioners can be confident that modern soft contact lenses do not impair visual performance compared to spectacle lenses for the majority of myopes.