Foveal crowding differs in children and adults

We used custom-designed acuity tests to compare the magnitude and extent of crowded letter recognition in children and adults. Visual acuity (logMAR) was measured monocularly in children and adults using five custom-designed letter tests with varying degrees of crowding: single letter, single letter surrounded by four flanking bars, single letter surrounded by four flanking letters, line of five letters surrounded by flanking bars, and line of five letters surrounded by flanking letters. The tests were constructed using Sloan letters and presented on an iPad (Apple Incorporated, Cupertino, CA) at 4 m using a standardized endpoint and instructions. Crowded logMAR was normalized to unflanked logMAR and results were analyzed in three groups: younger children aged 4–6 (n = 32), older children, aged 7–9 (n = 30), and adults (n = 27). Both groups of children showed a greater extent of crowding than the adults. The adult participants showed no difference in performance between single or linear presentation and letter or bar flankers. Letter flankers and linear presentation individually resulted in poorer performance in the younger children p < 0.001 and p = 0.003, respectively (mean normalized logMAR 0.17 in each case) and together had an additive effect (mean 0.24), p < 0.001. Crowding in the older children was adult-like except in the linear presentation with letter flankers, p < 0.001. These results indicate that both target-flanker similarity and linear presentation contribute more to foveal crowding in young children than in adults

Effect of stimulus configuration on crowding in strabismic amblyopia

Foveal vision in strabismic amblyopia can show increased levels of crowding, akin to typical peripheral vision. Target–flanker similarity and visual-acuity test configuration may cause the magnitude of crowding to vary in strabismic amblyopia. We used custom-designed visual acuity tests to investigate crowding in observers with strabismic amblyopia. LogMAR was measured monocularly in both eyes of 11 adults with strabismic or mixed strabismic/anisometropic amblyopia using custom-designed letter tests. The tests used single-letter and linear formats with either bar or letter flankers to introduce crowding. Tests were presented monocularly on a high-resolution display at a test distance of 4 m, using standardized instructions. For each condition, five letters of each size were shown; testing continued until three letters of a given size were named incorrectly. Uncrowded logMAR was subtracted from logMAR in each of the crowded tests to highlight the crowding effect. Repeated-measures ANOVA showed that letter flankers and linear presentation individually resulted in poorer performance in the amblyopic eyes (respectively, mean normalized logMAR = 0.29, SE = 0.07, mean normalized logMAR = 0.27, SE = 0.07; p 0.05). Both linear presentation and letter rather than bar flankers increase crowding in the amblyopic eyes of people with strabismic amblyopia. These results suggest the influence of more than one mechanism contributing to crowding in linear visual-acuity charts with letter flankers

Development of robust methods of assessment of clinical competency in ophthalmic dispensing – results of a pilot trial

Purpose: We evaluated whether the use of Objective Structured Clinical Examinations (OSCEs) combined with established standard setting procedures, could be used to define standards for the achievement of clinical competencies in ophthalmic dispensing. Methods: Ten OSCE stations were created to assess a range of professional competencies for dispensing opticians. For each station, examiners made a global judgment about student performance using a rating scale (bad fail, fail, borderline, pass, good pass). Examiners were instructed not to base their rating on the checklist marks. We also introduced performance based standard setting using a linear regression method. The rating of the student was plotted against marks obtained for the station and a line of best fit was derived from the data. The pass mark for each station was set at the mark for the borderline rating. Results: The average pass mark across stations was 57%. Students with higher marks also tended to be rated more highly by examiners; however, this was not universal. The slopes of the regression lines were significantly greater than zero across stations suggesting that the checklists were appropriate. Feedback from students and examiners was positive. Conclusions: Our study has demonstrated that the OSCE format and use of standard setting procedures is a viable approach to assess clinical competencies in ophthalmic dispensing. More data are required to confirm the reliability of the stations over repeated use

Crowding in childrenʼs visual acuity tests - effect of test design and age

Purpose. The aim of this study was to investigate the effect of test design (crowding) and age on visual acuity in a sample of young children. Methods. Vision was measured in 103 children aged between 4 and 9 years using five different visual acuity tests. The tests included three crowded tests: logMAR Crowded test, the Sonksen logMAR test, and the Crowded Kay Picture test, and two single optotype tests: the Single Kay Picture and Sheridan Gardiner tests. Tests were presented in a random order using standardized instructions and a defined end-point. Results were analyzed in two age groups, younger (4 to 6 years) and older (7 to 9 years). Results. In both groups, there was a significant main effect of test on acuity (younger: F = 63.92, dF = 4, p < 0.001; older: F = 63.59, dF = 4, p < 0.001). In the younger group, an effect of crowding was seen in all three crowded tests, but in the older group, an effect of crowding was seen only in the crowded letter tests. In both groups, mean acuity was lowest with the logMAR Crowded test, which has the closest interoptotype spacing (0.5), slightly higher with the Sonksen test (with interoptotype spacing of 1.0) and highest with the single optotype tests (no crowding). More crowding was seen in the younger children. Conclusions. Our results show that maturation of line acuity is still taking place between the ages of 4 and 9 years. Measured acuity is affected by the amount of contour interaction induced by the type of optotype (letter or picture) and by the interoptotype separation. Another factor, probably a maturation of gaze control or selective attention is responsible for the reduction in crowding with age

Monocular microsaccades are visual-task related

During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks

Contour interaction for foveal acuity targets at different luminances

Single-letter visual acuity is impaired by nearby flanking stimuli, a phenomenon known as contour interaction. We showed previously that when foveal acuity is degraded by a reduction of letter contrast, both the magnitude and angular spatial extent of foveal contour interaction remain unchanged. In this study, we asked whether contour interaction also remains unchanged when foveal visual acuity is degraded by a reduction of the target’s background luminance. Percent correct letter identification was measured for isolated, near-threshold black Sloan letters and for letters surrounded by 4 flanking bars in 10 normal observers, 5 at Anglia Ruskin University, UK (ARU) and 5 at Palacky University, Czech Republic (PU). A stepwise reduction in the background luminance over 3 log units resulted in an approximately threefold increase in the near-threshold letter size. At each background luminance, black flanking bars with a width equal to 1 letter stroke were presented at separations between approximately 0.45 and 4.5 min arc (ARU) or 0.32 and 3.2 min arc (PU). The results indicate that the angular extent of contour interaction remains unchanged at approximately 4 min arc at all background luminances. On the other hand, the magnitude of contour interaction decreases systematically as luminance is reduced, from approximately a 50% reduction to a 30% reduction in percent correct. The constant angular extent and decreasing magnitude of contour interaction with a reduction of background luminance suggest foveal contour interaction is mediated by luminance-dependent lateral inhibition within a fixed angular region

Foveal contour interaction on the edge: Response to 'Letter-to-the-Editor' by Drs. Coates and Levi

Recently, we reported that, when considered as a function of the edge-to-edge target-toflanker separation in min arc, the spatial extent of foveal contour interaction is the same for high and low contrast acuity targets. This result resolved an apparent discrepancy in the literature, which suggested that foveal contour interaction was absent or reduced for low contrast targets. In commenting on our results, Drs. Coates and Levi suggest a two-mechanism model for foveal crowding that depends on the center-to-center separation between the acuity target and flanking stimuli, and is based in part on a reanalysis of data from our recent work and a number of other studies. In our reply, we show that the spatial extent of foveal contour interaction for both high and low contrast targets is essentially unchanged by the width of the flanking targets when the target-to-flanker separation is depicted in terms of edge-to-edge separation, but varies systematically when depicted in terms of center-to-center separation. We therefore conclude that for foveal contour interaction in the range of a few min arc, edge-to-edge target-to-flanker separation is the more appropriate metric

Contour interaction under photopic and scotopic conditions

In the present study, we asked whether contour interaction undergoes significant changes for different luminance levels in the central and peripheral visual field. This study included nine normal observers at two laboratories (five at Palacky University Olomouc, Czech Republic and four at the University of Houston, USA). Observers viewed a randomly selected Sloan letter surrounded by four equally spaced bars for several separations measured edge-to-edge in min arc. Stimuli were viewed foveally under photopic and mesopic luminances and between 58 and 128 peripherally for four different background luminances of the display monitors, corresponding to photopic, mesopic, scotopic, and dim scotopic levels. The extent of the contour interaction in the fovea is approximately 20 times smaller than in the periphery. Whereas the magnitude of foveal contour interaction markedly decreases with decreasing luminance, no consistent luminance-induced change occurs in peripheral contour interaction. The extent of contour interaction does not scale with the size of the target letter, either in the fovea or peripherally. The results support a neural origin of contour interaction consistent with the properties of center-surround antagonism

Foveal contour interaction for low contrast acuity targets

Previous investigators reported the impairment of foveal visual acuity by nearby flanking targets (contour interaction) is reduced or eliminated when acuity is measured using low contrast targets. Unlike earlier studies, we compared contour interaction for high and low contrast acuity targets using flankers at fixed angular separations, rather than at specific multiples of the acuity target’s stroke width. Percent correct letter identification was determined in 4 adult observers for computer generated, high and low contrast dark Sloan letters surrounded by 4 equal contrast flanking bars. Two low contrast targets were selected to reduce each observer’s visual acuity by 0.2 and 0.4 logMAR. The crowding functions measured for high and low contrast letters are very similar when percent correct letter identification is plotted against the flanker separation in min arc. These results indicate that contour interaction of foveal acuity targets occurs within a fixed angular zone of a few min arc, regardless of the size or contrast of the acuity target