Modeling logMAR visual acuity scores: effects of termination rules and alternative forced-choice options.

BACKGROUND: Logarithm of the minimum angle of resolution (logMAR) charts come in a variety of alternative forced-choice (AFC) formats and can be used with a variety of different rules to determine when to stop a subject reading down a chart (termination rules). METHODS: Exact calculation and Monte Carlo simulation techniques were used to compare logMAR scores for different termination rules and for infinite-, 26-, 10-, 8-, 4-, and 2-AFC logMAR chart formats. Slope-corrected standard deviation, an index of variability of the underlying ideal threshold, was used as a common metric for the different test conditions. RESULTS: The mean and standard deviation of logMAR scores can be significantly affected by termination rules and AFC format. For different AFC formats, different termination criteria were found to give optimal slope-corrected standard deviations. CONCLUSION: A number of clinically suitable termination rules are proposed for different AFC format logMAR charts. For letter-by-letter scoring of Bailey-Lovie and EDTRS charts, it is recommended that researchers and practitioners use a termination rule of stopping after four or more mistakes on a line.

The slope of the psychometric function for Bailey-Lovie letter charts: defocus effects and implications for modeling letter-by-letter scores.

BACKGROUND: Visual acuity measurement often results in an imprecise endpoint because subjects correctly identify some but not all of the letters on one or more size levels on a letter chart. The extent of this transition zone from seeing to nonseeing can be described by probit size, which is calculated by performing Probit Analysis on letter chart data. There has been no previous research into the effects of optical defocus on letter chart probit size. METHODS: We tested 18 young visually normal subjects monocularly during three different defocus conditions: best spectacle correction (zero defocus) and +1.00 D and +2.00 D additions. Stimuli were Bailey-Lovie-style logarithm of the minimum angle of resolution (log MAR) letter charts constructed with a 0.05 logMAR size progression between size levels. Frequency of seeing data from these charts were used to calculate probit size. RESULTS: There were statistically significant effects of optical defocus on mean probit size. After Monte Carlo correction for bias, we believe that true mean values for probit size are about 0.07 logMAR for well-corrected subjects and up to 0.12 logMAR with optical defocus. CONCLUSION: The smaller probit size for well-corrected subjects should correspond to a sharper logMAR visual acuity endpoint and less intrasubject variability in logMAR acuity than for subjects with a larger probit size (optical defocus). Our modeling shows that these different probit sizes can also significantly affect letter-by-letter visual acuity scoring.

Corneal and refractive error astigmatism in Singaporean schoolchildren: a vector-based Javal's rule.

BACKGROUND: Traditional approaches to Javal's rule do not use data from subjects with oblique astigmatism and have not been used to make predictions about subjects with oblique astigmatism. Vector approaches to analyzing refractive error can circumvent these problems. METHODS: Subjects were 993 Singaporean schoolchildren. We performed linear regression of refractive error astigmatism on corneal astigmatism, using J0 vectors to describe with-the-rule and against-the-rule astigmatism and J45 vectors to describe oblique astigmatism. RESULTS: We obtained the following statistically significant regression relationships: RJ0 = 0.931 x CJ0 - 0.276 and RJ45 = 0.638 x CJ45 + 0.010, where R and C denote refractive error astigmatism and corneal astigmatism, respectively. CONCLUSION: Our vector-based Javal's rule gives closer predictions of refractive astigmatism than the original Javal's rule and the simplified Javal's rule and can be applied in cases of corneal oblique astigmatism.

Development of Vernier acuity in childhood.

PURPOSE: To measure Vernier acuity and resolution development after 3 years of age. METHODS: Observers were 39 children with normal vision (aged 3 to 12 years), 10 adult observers with normal vision (aged 19 to 24 years), and 7 adults with amblyopia. Vernier acuity and resolution were measured using uncrowded static stimuli and a 3AFC psychophysical paradigm. Curve fitting was used to estimate A2, the age at which thresholds are twice asymptotic levels. RESULTS: Vernier acuity was hyperacute (i.e., finger than predicted from foveal cone size or spacing) in 3- to 4-year-old observers, but developed later (A2 = 5.6 +/- 1.5 years) than resolution acuity (A2 = 2.2 +/- 0.9 years). CONCLUSIONS: Children's Vernier thresholds are poorer than would be predicted solely from their decreased foveal photon capture. Therefore cortical immaturity may play a role in children's relative position acuity deficit. R/V ratios (resolution/Vernier thresholds) for the youngest age group are similar to those for adult nonstrabismic amblyopes, but better than for strabismic amblyopes.

Inter-ocular temporal asynchrony (IOTA): psychophysical measurement of inter-ocular asymmetry of visual latency.

Ocular pathology can be associated with inter-ocular asymmetry of conduction latency. We describe a relatively low-cost psychophysical method of quantifying this asymmetry of delay, an 'inter-ocular temporal asynchrony' (IOTA) system. Testing of 28 visually normal young adults gave IOTA results with a mean of 0.11 ms (SD +/- 2.4 ms). These results correspond to 95% confidence limits of -4.6 ms and +4.8 ms. Inter-ocular differences in retinal illumination, induced by using monocular neutral density filters in visually normal subjects, significantly affected IOTA in accord with an imposed perceptual delay in the 'filtered eye'. This demonstration that the IOTA technique is capable of detecting induced asymmetry of perceptual delay in visually normal subjects, along with the small measured confidence limits reported above, suggests that the apparatus should be capable of detecting inter-ocular asymmetries in response latencies with ocular disease, which have been previously measured by other techniques.

Rigid lens dynamics: lid effects.

PURPOSE: Lid architecture, lid tension, blink action, and blink rate may all influence rigid lens centration and stability. The aims of this study were to assess the nature of the relationship between lid geometry and lens position and to examine the influence of lens-lid interactions on the association between lens position and lens center of gravity and mass. METHODS: Eight subjects (four with high riding lenses and four with low riding lenses) participated in the study. Each subject was fit with 12 lenses-six designs in each of two materials. Lens center of gravity was calculated and lens mass was measured in every case. For each lens, the following four lens dynamics variables were assessed over a five blink cycle for both the vertical and horizontal meridians: 1) initial lens position; 2) settled lens position; 3) amount of lens movement; and 4) rate of lens movement. Lower lid position and palpebral aperture height were measured for each subject. RESULTS: Graphical analysis showed that a low upper lid position and small palpebral aperture promoted superior lens decentration, while a high upper lid position and wide palpebral aperature predisposed an individual to inferior lens decentration. Significant lid-lens overlap was a common finding with high riding lenses in both the initial and settled lens positions. The results also suggested that while a rigid lens tended to become more stable as the lens center of gravity shifted further behind the corneal apex, this gravitational effect was reduced for lenses that interacted with the upper lid. CONCLUSIONS: Lid geometry can influence rigid lens centration and stability by modifying the effects of lens design and lens mass. We recommend that consideration be given to the nature of potential lens-lid interactions prior to lens fitting.

The influence of center of gravity and lens mass on rigid lens dynamics.

PURPOSE: Center of gravity and lens mass have both been shown to influence rigid lens centration and stability. The purpose of this study was to investigate the relative contributions of these two factors to rigid lens dynamics. METHODS: Eight subjects (four with high-riding lenses and four with low-riding lenses) participated in the study. Each subject was fit with 12 lenses-six designs in each of two materials. Center of gravity and lens mass were recorded in every case. For each lens, four lens dynamics variables were assessed over a five-blink cycle for both the vertical and the horizontal meridians: 1) initial lens position; 2) settled lens position; 3) amount of lens movement; and 4) rate of lens movement. RESULTS: Multiple regression and correlation analysis showed that center of gravity influences initial and settled lens position, in both the horizontal and the vertical meridians. Mass alone was found not to be a significant predictor of rigid lens dynamics. The effect of center of gravity on settled vertical lens position was only apparent when high rider and low rider subgroups were analyzed separately, with a posterior movement of the center of gravity being associated with a lowering of the settled lens position for high rider subjects, and a raising of the settled lens position for low rider subjects. The results suggest that a rigid lens will become more stable as the center of gravity is shifted further behind the lens vertex, but this effect is reduced as lens mass is increased. CONCLUSIONS: The location of the center of gravity of a rigid lens influences its on-eye centration and stability more so than does lens mass.

Visibility of motion in infant vernier displays, using adult subjects.

Motion is frequently incorporated in stimuli used for psychophysical testing of vernier acuity in infants and young children. In such stimuli, detection of the vernier offset is necessary in order to perceive the motion. Research described in this report tested whether the perception of a vernier offset is sufficient to signal the stimulus motion in adults. We measured how motion detectability changed as a function of vernier offset for two adult subjects, using a stimulus similar to that employed by other authors to measure vernier acuity in infants and children. Motion visibility varied with offset size, achieving a detectability of motion (d') of 0.95 (comparable to two-alternative forced-choice thresholds) at stimulus offsets of 16-19 s arc. In comparison to the motion, the stimulus offset itself was much easier to see, being detectable on 95-100% of trials with the smallest offset, 6.6 s arc. This distinction, between the visibility of motion and the visibility of the vernier offset itself, should be considered when interpreting vernier results using such displays, especially in infants and children for whom motion may be the attractive cue.

Spatial interference with vertical pistol sight alignment.

When competition pistol shooters aim, they perform four basic visual alignment tasks. With optimal stimuli such tasks can be performed with exquisite sensitivity. However, pistol sights and the target may not constitute the optimal stimuli, and this may impose limits on how well shooters perform. By simulating pistol sights and targets on a computer monitor, we investigated how the vernier task of aligning the top edges of the front and rear pistol sights was affected by the proximity of the target aiming mark. Alignment random error in this task, an analogous measure to vernier acuity, was unaffected by the proximity of the aiming mark; however, alignment systematic error did change significantly as the proximity of the aiming mark changed. This effect was unlikely to significantly change pistol shooting performance as shooters can adjust their sights to compensate for systematic errors. Likewise, compared to the vertical spread of shots on the pistol target, alignment random error was extremely small (only 10 s arc), which implies that other sources of variation limit pistol shooting performance.

Regional variations in binocular summation across the visual field.

Binocular summation for a contrast detection task was measured as a function of eccentricity and target size along the horizontal and vertical meridians for ten young normal subjects. Binocular summation at the fovea was of the order of 1.4 for all target sizes, although there was some intersubject variation. Binocular summation was highest along the vertical meridian. With increasing eccentricity from the fovea, binocular summation for target size I (0.108 degrees projected diameter) decreased, remained relatively constant for target size III (0.431 degrees projected diameter) and increased with increasing eccentricity from the fovea for target size V (1.724 degrees projected diameter). For target sizes I and III, binocular summation was present only when interocular differences in sensitivity were under 5 dB, for target size V this relationship did not hold. Influences such as stimulation of corresponding retinal points and cortical representation are considered.

Workstation variables and visual discomfort associated with VDTs.

We have investigated the effects of a range of workstation factors upon the visual symptoms experienced by a group of 92 visual display terminal (VDT) users. Subjects in the study kept a diary over five consecutive working days in which they recorded the types of visual and postural symptoms which occurred and the types of work tasks being performed. Each subject's workstation was analysed for screen legibility and stability, discomfort and disability glare, and required head postures. By the use of multiple regression analysis techniques we have considered the relative contribution of these factors to the symptoms reported by the users of these workstations. Screen legibility significantly influenced the occurrence of symptoms of ocular discomfort and vertical head movements significantly affected the incidence of postural/headache symptoms.