Contrast sensitivity, visual acuity and the discrimination of Snellen letters in multiple sclerosis

Contrast Sensitivity and Low Contrast Visual Acuity in Children With Normal Visual Acuity

PURPOSE

To measure and report the distribution of distance and near contrast sensitivity (CS) and low contrast visual acuity (LCVA) at 5% and 2.5% contrast in children aged 5-15 years with normal visual acuity (VA).

DESIGN

Prospective, cross-sectional study.

SETTING

Schools in Southern India.

STUDY POPULATION

One thousand fifty-two children aged 5 to 15 years (mean age 10.61 ± 2.85 years) with a presenting visual acuity of 0.00 logMAR or better in both eyes and a stereo acuity of 40 seconds of arc or better were recruited from nine schools. Repeatability of contrast sensitivity and low contrast visual acuities were tested in 246 children. Pelli-Robson charts were used to measure the distance and near contrast sensitivity at 1 m and 40 cm, respectively. The low contrast visual acuity was recorded at 5% and 2.5% contrast using LEA Symbols at 3 m.

RESULTS

Overall, the mean ± SD, (95% CI) monocular distance and near CS were 1.75 ± 0.11 (1.76-1.75) logCS and 1.72 ± 0.10 (1.73-1.71) logCS, respectively. The mean LCVA at 5% and 2.5% contrasts were 0.20 ± 0.10 (0.21-0.20) logMAR and 0.39 ± 0.11 (0.40-0.39) logMAR, respectively. Distance and near CS gradually improved till the ages of 11 and 13, respectively, and then plateaued. Similarly, LCVA at 5% and 2.5% contrasts gradually improved till age ten before plateauing. The Coefficient of Repeatability (CoR) for CS was ±0.02 logCS for distance, ±0.05 logCS for near, and ±0.01 logMAR for both LCVA contrasts.

CONCLUSIONS

The study provides age-specific normative values for distance and near CS, and LCVA in a cohort of children aged 5-15 years. These results are important to understand the impact of ocular conditions on CS in children and have utility in clinical evaluations.

Low Contrast Visual Evoked Potentials for Early Detection of Optic Neuritis

Optic neuritis (ON) detection is important for the early diagnosis and management of multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD). However, the conventional high-contrast visual evoked potential (VEP) used for ON detection lacks sensitivity for identifying ON presenting as mild or unremarkable visual disturbance, which is common in first-episode ON. Therefore, this study aimed to investigate whether a change in contrast or check size improves the sensitivity of VEP to first-ever ON. In total, 60 patients with the demyelinating disease (29 MS and 31 idiopathic patients with ON) without ON or with first-ever ON at least 6 months prior and 32 healthy controls underwent neuro-ophthalmic evaluations. VEPs were induced using three pattern-reversal checkerboard stimuli having, respectively, 10% contrast with a check size of 32' (LC32 VEP), 100% contrast with a check size of 32' (HC32 VEP; conventional VEP), and 100% contrast with a check size of 16' (HC16 VEP). The receiver operating characteristic (ROC) curve analysis and area under the curve (AUC) were calculated to determine the most appropriate VEP method for detecting optic nerve involvement. The optimal cut-off point was determined using the Youden index (J-index). The McNemar test was used to determine whether dichotomous proportions were equivalent. In comparison with first-ever ON eyes (n = 39) and healthy eyes (n = 64), LC32 VEP showed the highest AUC for discriminating ON (0.750, p < 0.001; 0.730 for HC32 VEP, p < 0.001; 0.702 for HC16 VEP, p = 0.001). In the first-ever ON group, LC32 VEP and conventional HC32 VEP were abnormal in 76.9 and 43.6%, respectively (McNemar, p < 0.001), and combining these tests did not improve sensitivity. These indicate that LC32 VEP is the most sensitive method for detecting first-ever ON. Visual evoked potential with 10% contrast stimuli was superior to conventional VEP for detecting first-ever ON. Thus, adding these LC stimuli might be helpful in identifying optic nerve involvement in ON with mild or unremarkable visual impairment.

Low Contrast Visual Acuity Might Help to Detect Previous Optic Neuritis

Optic neuritis (ON) has been considered to be an important factor in the diagnosis of multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD), making ON detection increasingly critical for early diagnosis. Furthermore, subclinical ONs presenting no distinct decrease in visual acuity can be missed. Low contrast visual acuity (LC-VA) is known to be able to capture visual loss not seen in conventional high-contrast visual acuity (HC-VA) in MS. Therefore, to increase the sensitivity of ON detection, we investigated the advantage of LC-VA over conventional HC-VA. One hundred and eight patients with demyelinating disease (35 MS, 73 NMOSD) with ON at least 3 months prior and 35 controls underwent neuro-ophthalmic evaluation, including best-corrected conventional high contrast visual acuity (HC-VA) and 2.5% and 1.25% low contrast visual acuity (LC-VA). Receiver operating characteristic (ROC) curve analysis and the area under the curve (AUC) of various visual functions were used to determine the most relevant visual function test for the detection of optic nerve involvement. Additionally, the optimal cutoff point was obtained from the Youden index (J-index) as the points with the best sensitivity-specificity balance. When distinguishing ON from non-ON, the area under the ROC curve (AUC) was highest for the 2.5% LC-VA (0.835, P < 0.001; sensitivity 71.5%, specificity 88.6%), while it was 0.710 (P < 0.001) for the HC-VA and 0.770 (P < 0.001) for the 1.25% LC-VA. In discriminating between controls and ON, the AUC was also highest for the 2.5% LC-VA 0.754 (P < 0.001; sensitivity 71.5%, specificity 78.5%), while it was 0.719 (P < 0.001) for HC-VA and 0.688 (P < 0.001) for 1.25% LC-VA. In eyes with a history of ON (n = 137), the HC-VA and 2.5% LC-VA were abnormal in 64.2 and 71.5%, respectively (P < 0.001), with their combination detecting abnormalities in approximately 85.4% (P < 0.001). The 2.5% LC-VA was superior to HC-VA in detecting ON when distinguishing ON from non-ON or control. The 2.5% LC-VA might be a useful, feasible, and rapid method to detect ON. Furthermore, combining 2.5% LC-VA with conventional HC-VA would be better for detecting optic nerve involvements.

Gradiate: A radial sweep approach to measuring detailed contrast sensitivity functions from eye movements

The contrast sensitivity function (CSF) is an informative measure of visual health, but the practical difficulty of measuring it has impeded detailed analyses of its relationship to different visual disorders. Furthermore, most existing tasks cannot be used in populations with cognitive impairment. We analyzed detailed CSFs measured with a nonverbal procedure called "Gradiate," which efficiently infers visibility from eye movements and manipulates stimulus appearance in real time. Sixty observers of varying age (38 with refractive error) were presented with moving stimuli. Stimulus spatial frequency and contrast advanced along 15 radial sweeps through CSF space in response to stimulus-congruent eye movements. A point on the CSF was recorded when tracking ceased. Gradiate CSFs were reliable and in high agreement with independent low-contrast acuity thresholds. Overall CSF variation was largely captured by two orthogonal factors ("radius" and "slope") or two orthogonal shape factors when size was normalized ("aspect ratio" and "curvature"). CSF radius was highly predictive of LogMAR acuity, as were aspect ratio and curvature together, but only radius was predictive of observer age. Our findings suggest that Gradiate holds promise for assessing spatial vision in both verbal and nonverbal populations and indicate that variation between detailed CSFs can reveal useful information about visual health.

Microperimetry as a diagnostic tool for the detection of early, subclinical retinal damage and visual impairment in multiple sclerosis

BACKGROUND

A majority of multiple sclerosis patients experience visual impairment, often as the initial presenting symptom of the disease. While structural changes in the retinal nerve fiber layer and optic nerve have demonstrated correlations with brain atrophy in multiple sclerosis using magnetic resonance imaging, a non-invasive, cost-effective, and clinically efficacious modality to identify early damage and facilitate prompt therapeutic intervention to slow the progression of multiple sclerosis and its ocular manifestations, is still urgently needed. In this study, we sought to determine the role of macular sensitivity measured by microperimetry in the detection of subclinical multiple sclerosis-related retinal damage and visual dysfunction.

METHODS

This cross-sectional observational case-control study involved population-based samples of multiple sclerosis patients and age-, race-, and gender-matched healthy control subjects. Among the key criteria for the multiple sclerosis patients were diagnosis by the McDonald criteria, visual acuity greater than 20/25, and no history of optic neuritis. Macular sensitivity and average macular thickness were measured in all subjects using microperimetry and spectral-domain optical coherence tomography, respectively. Pearson correlation coefficients were measured using bivariate correlations. Sample means, mean differences, and 95% confidence intervals were calculated using independent sample t-tests.

RESULTS

Twenty-eight eyes from 14 MS patients and 18 eyes from 9 control subjects were included. Mean macular sensitivity of control subjects and multiple sclerosis patients in decibels was 18.2 ± 0.4 and 16.5 ± 0.4, respectively, corresponding to a mean difference of 1.7 (95% CI, 1.1-2.4; P < 0.001). Macular sensitivity was positively correlated with macular thickness in multiple sclerosis patients (r = 0.49, P = 0.01) but not control subjects (r = 0.15, P = 0.55).

CONCLUSIONS

Macular sensitivity as measured by microperimetry was decreased in multiple sclerosis patients with normal visual acuity and no history of optic neuritis. Furthermore, macular sensitivity demonstrated a positive correlation with macular thickness as measured by optical coherence tomography. As such, microperimetry may represent a non-invasive and efficient method to identify signs of subclinical visual dysfunction that correspond with early macular architectural changes characteristic of multiple sclerosis.

A critical review: Psychophysical assessments of diabetic retinopathy

Diabetic retinal disease remains a leading cause of vision loss despite currently available screening methods, ocular treatments, and efforts to control metabolic dysfunction. It is now understood that diabetes damages the entire retina and the cellular components of the neurovascular unit. Multiple studies have demonstrated impairment of various aspects of retinal function across the spectrum of retinopathy severity. Here we review these tests, the principles underlying their use, clinical data from multiple publications, the strengths and limitations of the studies, and prospects for their application to understand the pathophysiology of diabetic retinal disease and monitor its response to therapy. We focus on visual acuity, contrast sensitivity, color vision, visual field, and dark adaptation and their use to understand the pathophysiology of diabetic retinopathy and as potential endpoints for clinical trials.

Mapping the Contrast Sensitivity of the Visual Field With Bayesian Adaptive qVFM

Current clinical evaluation, which focuses on central vision, could be improved through characterization of residual vision with peripheral testing of visual acuity, contrast sensitivity, color vision, crowding, and reading speed. Assessing visual functions in addition to light sensitivity, a comprehensive visual field map (VFM) would be valuable for detecting and managing eye diseases. In a previous study, we developed a Bayesian adaptive qVFM method that combines a global module for preliminary assessment of the VFM's shape and a local module for assessment at individual retinal locations. The method was validated in measuring the light sensitivity VFM. In this study, we extended the qVFM method to measure contrast sensitivity across the visual field. In both simulations and psychophysics, we sampled 64 visual field locations (48 x 48 deg) and compared the qVFM method with a procedure that tested each retinal location independently (qFC; Lesmes et al., 2015). In each trial, subjects were required to identify a single optotype (size: 2.5 x 2.5 deg), one of 10 filtered Sloan letters. To compare the accuracy and precision of the two methods, three simulated eyes were tested in 1,280 trials with each method. In addition, data were collected from 10 eyes (5 OS, 5 OD) of five normal observers. For simulations, the average RMSE of the estimated contrast sensitivity with the qVFM and qFC methods were 0.057 and 0.100 after 320 trials, and 0.037 and 0.041 after 1,280 trials [all in log10 units, represent as log(sensitivity)], respectively. The average SD of the qVFM and qFC estimates were 0.054 and 0.096 after 320 trials, and 0.032 and 0.041 after 1,280 trials, respectively. The within-run variability (68.2% HWCIs) were comparable to the cross-run variability (SD). In the psychophysics experiment, the average HWCI of the estimated contrast sensitivity from the qVFM and qFC methods across the visual field decreased from 0.33 on the first trial to 0.072 and 0.16 after 160, and to 0.060 and 0.10 after 320 trials. The RMSE between the qVFM and qFC estimates started at 0.26, decreased to 0.12 after 160 and to 0.11 after 320 qVFM trials. The qVFM provides an accurate, precise, and efficient mapping of contrast sensitivity across the entire visual field. The method might find potential clinical applications in monitoring vision loss, evaluating therapeutic interventions, and developing effective rehabilitation for visual diseases.

Precision and Normative Values of a New Computerized Chart for Contrast Sensitivity Testing

The purpose was to define a normative database for a grating test for contrast sensitivity, based on a chart monitor with high-definition liquid crystal display, and validate its measurements by assessing their repeatability and determining responsiveness to cataract surgery. Three samples were analyzed: (1) healthy volunteers to assess the repeatability of measurements, (2) healthy subjects to develop the normative database, (3) patients undergoing cataract surgery. All subjects were tested with the grating contrast sensitivity test (Vision Chart, CSO) at 1.5, 3, 6, 12 and 18 cycles per degree. The instrument software progressively reduces the contrast of the gratings according to the Quick Estimate by Sequential Testing (QUEST) procedure. In the subjects of the first sample, three consecutive measurements were taken and repeatability was assessed on the basis of the intra-session test-retest variability and the coefficient of variation. The test offered high repeatability, with test-retest variability ranging between 0.05 and 0.23 Log CS and the coefficient of variation between 0.61 and 4.21%. Normative data did not show a normal distribution. The highest median values were observed at 1.5, 3 and 6 cycles per degree frequencies. At these frequencies a ceiling effect was evident. In cataract patients, postoperative values showed an improvement at all spatial frequencies. In conclusion, the new contrast sensitivity test provides repeatable measurements that can be used for clinical purposes. In patients with healthy eyes and good vision, attention has to be paid to the ceiling effect.

Curveball: A tool for rapid measurement of contrast sensitivity based on smooth eye movements

The contrast sensitivity function (CSF) is an informative measure of visual function, but current tools for assessing it are limited by the attentional, motor, and communicative abilities of the participant. Impairments in these abilities can prevent participants from engaging with tasks or following an experimenter's instructions. Here, we describe an efficient new tool for measuring contrast sensitivity, Curveball, and empirically validate it with a sample of healthy adults. The Curveball algorithm continuously infers stimulus visibility through smooth eye tracking instead of perceptual report, and rapidly lowers stimulus contrast in real time until a threshold is found. The procedure requires minimal instruction to administer and takes only five minutes to estimate a full CSF, which is comparable to the best existing methods available for healthy adults. Task repeatability was high: the coefficients of repeatability were 0.275 (in log10 units of RMS contrast) within the same session and 0.227 across different days. We also present evidence that the task is robust across illumination changes, well correlated with results from conventional psychophysical methods, and highly sensitive to improvements in visual acuity from refractive correction. Our findings indicate that Curveball is a promising means of accurately assessing contrast sensitivity in previously neglected populations.

Using the Anterior Visual System to Assess Neuroprotection and Remyelination in Multiple Sclerosis Trials

PURPOSE OF REVIEW

Clinical trials using agents directed at neuroprotection and remyelination in multiple sclerosis (MS) are needed. As optic neuritis (ON) is common in people with MS and the pathology of ON is similar to other MS lesions in the brain, measurements of the anterior visual system are frequently utilized in neuroprotection and remyelination trials. Understanding the strengths and weaknesses of the measurements is vital when interpreting the results of this research.

RECENT FINDINGS

Techniques such as visual evoked potentials (VEP) and optical coherence tomography (OCT) are well established in MS and are thought to measure axonal integrity and myelination. Novel imaging techniques can also be used in conjunction with these measurements to provide better insight into optic nerve structure and function. Magnetization transfer imaging (MTR) together with optic nerve area and volume measures neurodegeneration; diffusion tensor imaging (DTI) measures myelination status and neurodegeneration. However, these techniques require various levels of experience to interpret, and all can be confounded by ocular motion and surrounding fat and bone. This article provides a review of established and novel techniques to measure the anterior visual system in multiple sclerosis with a focus on the evidence to support their use as outcome measures in clinical trials focused on neuroprotection and remyelination therapies.

Visual Function during Acute Hypoglycaemia

Hypoglycaemia symptoms are of particular interest in view of the importance of hypoglycaemia unawareness. Visual symptoms arising during acute hypoglycaemia may be the result of metabolic disturbances in the visual pathways within the central nervous system or impairment of the refractive apparatus of the eye and binocular function. This study investigated the effect of hypoglycaemia upon visual acuity, binocular interaction and contrast sensitivity. Various aspects of visual function were examined in ten normal subjects before, during and after acute insulin-induced hypoglycaemia. A simultaneous study of hypoglycaemic symptoms enabled us to relate the objective findings to the symptoms as reported by the subjects. Snellen visual acuity, fusion and stereopsis were not affected by hypoglycaemia. Five subjects noted visual disturbance. Eight developed significant impairment of contrast sensitivity, which closely matched the lowered blood glucose concentration.

These observations suggest that hypoglycaemic visual symptoms are due to neuroglycopenia of central visual pathways rather than changes within the refractive apparatus or abnormality of binocular function. Although these symptoms are not a constant feature of the hypoglycaemic state, subtle impairment of visual function occurs in most cases. Contrast sensitivity testing can be useful for assessment of fine changes in visual function.

Bayesian Inference of Two-Dimensional Contrast Sensitivity Function from Data Obtained with Classical One-Dimensional Algorithms Is Efficient

The contrast sensitivity function that spans the two dimensions of contrast and spatial frequency is crucial in predicting functional vision both in research and clinical applications. In this study, the use of Bayesian inference was proposed to determine the parameters of the two-dimensional contrast sensitivity function. Two-dimensional Bayesian inference was extensively simulated in comparison to classical one-dimensional measures. Its performance on two-dimensional data gathered with different sampling algorithms was also investigated. The results showed that the two-dimensional Bayesian inference method significantly improved the accuracy and precision of the contrast sensitivity function, as compared to the more common one-dimensional estimates. In addition, applying two-dimensional Bayesian estimation to the final data set showed similar levels of reliability and efficiency across widely disparate and established sampling methods (from classical one-dimensional sampling, such as Ψ or staircase, to more novel multi-dimensional sampling methods, such as quick contrast sensitivity function and Fisher information gain). Furthermore, the improvements observed following the application of Bayesian inference were maintained even when the prior poorly matched the subject's contrast sensitivity function. Simulation results were confirmed in a psychophysical experiment. The results indicated that two-dimensional Bayesian inference of contrast sensitivity function data provides similar estimates across a wide range of sampling methods. The present study likely has implications for the measurement of contrast sensitivity function in various settings (including research and clinical settings) and would facilitate the comparison of existing data from previous studies.

A Methodological Investigation of Three Psychophysical Techniques for Rapid Measurement of Contrast Sensitivity

A general method for assessing visual capacity, contrast sensitivity function (CSF), has emerged in the last decade. The CSF describes visual detection of sine-wave gratings over a range of visual sizes. The purpose of this investigation was to compare the stability and sensitivity of three methods of obtaining rapid CSFs: Method of Adjustment (ADJ), Bekesy Tracking (B), and Method of Increasing Contrast (MIC). For each method, a microcomputer controlled video display was used to generate five different stationary gratings: 1, 2, 4, 8, and 16 cycles per degree (cpd). Individual contrast sensitivity functions were determined for each of 13 observers over five days. Analyses were conducted on reliabilities, means, and variances across frequencies, methods, and days. The MIC demonstrated near-largest or largest reliabilities over all frequencies across days. The MIC, in addition, appeared differentially stable from the onset of measurement (p > .10); while, both ADJ and B methods demonstrated differential instability (p < .002). The Methodof Increasing Contrast (MIC) may be recommended as the method of choice for assessing CSF based on the present findings.

Mobile app Aston contrast sensitivity test

BACKGROUND

Contrast detection is an important aspect of the assessment of visual function; however, clinical tests evaluate limited spatial frequencies and contrasts. This study validates the accuracy and inter-test repeatability of a swept-frequency near and distance mobile app Aston contrast sensitivity test, which overcomes this limitation compared to traditional charts.

METHOD

Twenty subjects wearing their full refractive correction underwent contrast sensitivity testing on the new near application (near app), distance app, CSV-1000 and Pelli-Robson charts with full correction and with vision degraded by 0.8 and 0.2 Bangerter degradation foils. In addition repeated measures using the 0.8 occluding foil were taken.

RESULTS

The mobile apps (near more than distance, p = 0.005) recorded a higher contrast sensitivity than printed tests (p < 0.001); however, all charts showed a reduction in measured contrast sensitivity with degradation (p < 0.001) and a similar decrease with increasing spatial frequency (interaction > 0.05). Although the coefficient of repeatability was lowest for the Pelli-Robson charts (0.14 log units), the mobile app charts measured more spatial frequencies, took less time and were more repeatable (near: 0.26 to 0.37 log units; distance: 0.34 to 0.39 log units) than the CSV-1000 (0.30 to 0.93 log units). The duration to complete the CSV-1000 was 124 ± 37 seconds, Pelli-Robson 78 ± 27 seconds, near app 53 ± 15 seconds and distance app 107 ± 36 seconds.

CONCLUSIONS

While there were differences between charts in contrast levels measured, the new Aston near and distance apps are valid, repeatable and time-efficient method of assessing contrast sensitivity at multiple spatial frequencies.

Discriminating anisometropic amblyopia from myopia based on interocular inhibition

Amblyopia screening during childhood is critical for early detection and successful treatment. In the current study, we develop and evaluate a screening method that exploits the imbalanced interocular inhibition between amblyopic and fellow eyes. In nineteen subjects with anisometropic amblyopia and twenty-two age-matched subjects with myopia, we measured the area under the contrast sensitivity functions (AUCSFs) in eight monocular conditions defined by the tested eye (left, right), patching of the untested eye (translucent, opaque), and refractive status (corrected, uncorrected). For each tested eye, we defined the inhibition index as the ratio between the AUCSF values obtained in the translucent and opaque patching conditions of the untested eye. To evaluate the screening potential of the inhibition index, we compared results from patients with amblyopia and myopia. With and without optical correction, the index was significantly lower in the amblyopic eye than in the fellow eye of the amblyopic subjects and both eyes of the myopic subjects. No significant difference was found among the two eyes of the myopic subjects and the fellow eyes of the amblyopic subjects. With the inhibition index as the predictor, a logistic regression model successfully discriminated amblyopic eyes from myopic eyes with 100% accuracy in the uncorrected condition. In the corrected condition, with the inhibition index and interocular visual acuity difference as predictors, amblyopic eyes were likewise discriminated from myopic eyes with 100% accuracy. This pattern of CSF changes, caused by the different patching modes of the untested eye, provides a potential CSF signature to discriminate anisometropic amblyopia from myopia.

The effect of exposure duration on visual acuity for letter optotypes and gratings

This study compared the effects of exposure duration on letter and grating targets in a visual acuity (VA) task and determined if the broadband nature of letters accounts for their temporal summation characteristics. Log MAR (minimum angle of resolution) VA of five individuals (ages 25-36) was measured with a set of tumbling E optotypes for durations of 24 ms to 1s. The Es were either unfiltered or low-pass filtered to determine the object frequencies (cycles per letter; cplE) mediating VA. The retinal frequencies mediating VA for the unfiltered E (cycles per degree; cpdE) were derived from the ratio of cplE to MAR. Values of cpdE were compared to threshold retinal frequency obtained with band-limited Es and gratings to further evaluate the effects of stimulus bandwidth. Both log MAR and log cplE for the unfiltered E decreased as duration increased up to approximately 260 ms, and were constant thereafter. VA also improved for gratings and band-pass filtered Es, but over a shorter time course (approximately 150 ms). The effect of duration on VA for the broadband E, Gabor, and band-pass filtered E was similar when the object frequencies mediating VA were included in the definition of VA by converting to cpdE. The results indicate that the pattern of temporal integration for the tumbling E is related to its broadband nature. Band-pass filtered letters can simplify the interpretation of VA because the object frequency information mediating VA is known exactly and is independent of duration and letter size.

Effect of luminance noise on the object frequencies mediating letter identification

Purpose: To determine if the same object frequency information mediates letter contrast threshold in the presence and absence of additive luminance noise (i.e., “noise-invariant processing”) for letters of different size.

Methods: Contrast thresholds for Sloan letters ranging in size from 0.9 to 1.8 log MAR were obtained from three visually normal observers under three paradigms: (1) high- and low-pass Gaussian filtered letters were presented against a uniform adapting field; (2) high- and low-pass Gaussian filtered letters were presented in additive white luminance noise; and (3) unfiltered letters were presented in high- and low-pass Gaussian filtered luminance noise. A range of high- and low-pass filter cutoffs were used to limit selectively the object frequency content of the letters (paradigms 1 and 2) or noise (paradigm 3). The object frequencies mediating letter identification under each paradigm were derived from plots of log contrast threshold vs. log filter cutoff frequency.

Results: The object frequency band mediating letter identification systematically shifted to higher frequencies with increasing log MAR letter size under all three paradigms. However, the relationship between object frequency and letter size depended on the paradigm under which the measurements were obtained. The largest difference in object frequency among the paradigms was observed at 1.8 log MAR, where the addition of white noise nearly doubled the center frequency of the band of object frequencies mediating letter identification, compared to measurements made in the absence of noise.

Conclusion: Noise can affect the object frequency band mediating letter contrast threshold, particularly for large letters, an effect that is likely due to strong masking of the low frequency letter components by low frequency noise checks. This finding indicates that noise-invariant processing cannot necessarily be assumed for large letters presented in white noise.

Rapid and reliable assessment of the contrast sensitivity function on an iPad

PURPOSE

Letter acuity, the predominant clinical assessment of vision, is relatively insensitive to slow vision loss caused by eye disease. While the contrast sensitivity function (CSF) has demonstrated the potential to monitor the slow progress of blinding eye diseases, current tests of CSF lack the reliability or ease-of-use to capture changes in vision timely. To improve the current state of home testing for vision, we have developed and validated a computerized adaptive test on a commercial tablet device (iPad) that provides an efficient and easy-to-use assessment of the CSF.

METHODS

We evaluated the reliability, accuracy, and flexibility of tablet-based CSF assessment. Repeated tablet-based assessments of the spatial CSF, obtained from four normally-sighted observers, which each took 3 to 5 minutes, were compared to measures obtained on CRT-based laboratory equipment; additional tablet-based measures were obtained from six subjects under three different luminance conditions.

RESULTS

A Bland-Altman analysis demonstrated that tablet-based assessment was reliable for estimating sensitivities at specific spatial frequencies (coefficient of repeatability 0.14-0.40 log units). The CRT- and tablet-based results demonstrated excellent agreement with absolute mean sensitivity differences <0.05 log units. The tablet-based test also reliably identified changes in contrast sensitivity due to different luminance conditions.

CONCLUSIONS

We demonstrate that CSF assessment on a mobile device is indistinguishable from that obtained with specialized laboratory equipment. We also demonstrate better reliability than tests used currently for clinical trials of ophthalmic therapies, drugs, and devices.

Measuring contrast sensitivity

Contrast sensitivity defines the threshold between the visible and invisible, which has obvious significance for basic and clinical vision science. Fechner's 1860 review reported that threshold contrast is 1% for a remarkably wide range of targets and conditions. While printed charts are still in use, computer testing is becoming more popular because it offers efficient adaptive measurement of threshold for a wide range of stimuli. Both basic and clinical studies usually want to know fundamental visual capability, regardless of the observer's subjective criterion. Criterion effects are minimized by the use of an objective task: multiple-alternative forced-choice detection or identification. Having many alternatives reduces the guessing rate, which makes each trial more informative, so fewer trials are needed. Finally, populations who may experience crowding or target confusion should be tested with one target at a time.

Measures of visual pathway structure and function in MS: Clinical usefulness and role for MS trials

Over the past decade, the visual pathway in multiple sclerosis (MS) has become an important system for assessing both patient function and disease burden. Abnormalities of low-contrast acuity, long recognized as important correlates of driving, facial recognition, and other activities of daily living, are now noted to be common among patients with MS, even among those with no history of acute optic neuritis (ON). Low-contrast letter acuity scores correlate well with brain MRI lesion burden, visual-evoked potential (VEP) amplitudes, health-related quality of life (QOL), and retinal nerve fiber layer (RNFL) axonal and neuronal loss as measured by optical coherence tomography (OCT). Axonal and neuronal degeneration in MS is likely to be an important cause of visual impairment and disability, particularly among patients with progressive MS subtypes. With the advent of OCT and the use of low-contrast letter acuity charts in MS research and clinical trials, the structure-function correlations afforded by the anterior visual pathway can be assessed and potentially harnessed as a model for testing new therapies. Recent advances in OCT, such as high resolution spectral-domain techniques and computerized algorithms for image analysis, have allowed for measurement of specific retinal layers, including the ganglion cell (GCL) neuronal layer and its intimately associated, thin layer of interneurons, the inner plexiform layer (IPL). Longitudinal collaborative studies of GCL+IPL thinning and RNFL axonal loss are providing an in vivo view into neuroretinal pathology, and are providing new insights into how the visual pathway may reflect overall mechanisms of disease in MS.

Vision in multiple sclerosis: the story, structure-function correlations, and models for neuroprotection

Visual dysfunction is one of the most common clinical manifestations of multiple sclerosis (MS). Just over a decade ago, MS clinical trials did not include visual outcomes, but experts recognized the need for more sensitive measures of visual function. Low-contrast letter acuity emerged as the leading candidate to measure visual disability in MS, and subsequent studies found low-contrast acuity testing to correlate well with brain MRI lesion burden, visual-evoked potentials, quality of life (QOL), and retinal nerve fiber layer (RNFL) loss, as measured by optical coherence tomography (OCT). OCT in MS has allowed for assessment of structure-function correlations that make the anterior visual pathway and acute optic neuritis (ON) ideal models for testing novel agents for neuroprotection and repair. New therapies that reduce axonal loss by neuroprotective or myelin repair mechanisms can now be assessed noninvasively by OCT and coupled with visual function data. Based on OCT studies in MS, RNFL thickness is reduced significantly among patients (92 μm) vs controls (105 μm) and is particularly reduced in MS eyes with a history of ON (85 μm). Worsening of visual function by a clinically significant ≥ 7 letters or approximately 1.5 lines for low-contrast acuity is associated with approximately 4.5 μm reductions in RNFL thickness in MS eyes. Longitudinal studies of OCT have also shown RNFL axonal loss over time that occurs even in the absence of acute ON and that correlates with clinically meaningful worsening of vision and QOL, even in patients with benign MS. The latest OCT investigations involve high-resolution spectral-domain (SD) OCT with segmentation and measurement of specific retinal layers using computerized algorithms. These methods allow quantitation of ganglion cell (neuronal) layer loss and axonal degeneration in MS in vivo. In this review, we examine the data from these studies and ongoing trials that highlight the entity of ON as a model to investigate neuroprotection and neurorepair. In doing so, we also present representative group data from studies that have examined visual function, OCT measures, and QOL scales in patients with MS and ON and disease-free controls. These data, and those from recent meta-analyses, may be used to provide reference values for the development of clinical trial protocols.

The effect of decreased visual acuity on control of posture

OBJECTIVES

The goal of this study was to investigate the effect of visual acuity on the anticipatory (APAs) and compensatory (CPAs) components of postural control.

METHODS

Ten individuals participated in the experiments involving perturbations induced by a pendulum while their visual acuity was altered. The different visual acuity conditions were no glasses, blurred vision induced by wearing glasses with positive or negative lenses, and no vision. EMG activity of trunk and leg muscles and ground reaction forces were recorded during the typical anticipatory and compensatory periods.

RESULTS

In the no vision condition the subjects did not generate APAs, which resulted in the largest displacements of the center of pressure (COP) after the perturbation (p<0.01). In all other visual conditions APAs were present showing a distal to proximal order of muscle activation. The subjects wearing positive glasses showed earlier and larger anticipatory EMGs than subjects wearing negative glasses or no glasses at all.

CONCLUSIONS

The study outcome revealed that changes in visual acuity induced by wearing differently powered eye glasses alter the generation APAs and as a consequence, affect the compensatory components of postural control.

SIGNIFICANCE

The observed changes in APAs and CPAs in conditions with blurred vision induced by positive and negative glasses suggest the importance of using glasses with an appropriate power. This outcome should be taken into consideration in balance rehabilitation of individuals wearing glasses.

Images perceived after chromatic or achromatic contrast sensitivity losses

PURPOSE

We simulate how subjects with losses in chromatic and achromatic contrast sensitivity perceive colored images by using the spatiochromatic corresponding pair algorithm.

METHODS

This is a generalized version of the algorithm by Capilla et al. (J Opt Soc Am (A) 2004;21:176-186) for simulating color perception of color deviant subjects, which incorporates a simple spatial vision model, consisting of a linear filtering stage, with a band-pass achromatic filter and two low-pass chromatic ones, for the red-green and blue-yellow mechanisms. These filters, except for the global scaling, are the subject's contrast sensitivity functions measured along the cardinal directions of the color space. In its present form, the algorithm would serve to simulate alterations both in the spectral sensitivities and in the contrast sensitivities of the visual mechanisms.

RESULTS

After a preliminary theoretical study on the effect of frequency selective and overall reductions in the contrast sensitivity function of a single mechanism, we present cases of real subjects with glaucoma and diabetes, suffering alterations of different magnitude in the three mechanisms.

CONCLUSIONS

The simulations allow us to learn about the different types of distortions that can be experienced by a subject with impaired contrast sensitivities (blur, haloes, color shifts, local or global contrast, brightness and colorfulness reductions, etc.) and highlight the difficulties arising when trying to predict the quality of the final image from the losses in the individual mechanisms.

Bayesian adaptive estimation of the contrast sensitivity function: the quick CSF method

The contrast sensitivity function (CSF) predicts functional vision better than acuity, but long testing times prevent its psychophysical assessment in clinical and practical applications. This study presents the quick CSF (qCSF) method, a Bayesian adaptive procedure that applies a strategy developed to estimate multiple parameters of the psychometric function (A. B. Cobo-Lewis, 1996; L. L. Kontsevich & C. W. Tyler, 1999). Before each trial, a one-step-ahead search finds the grating stimulus (defined by frequency and contrast) that maximizes the expected information gain (J. V. Kujala & T. J. Lukka, 2006; L. A. Lesmes et al., 2006), about four CSF parameters. By directly estimating CSF parameters, data collected at one spatial frequency improves sensitivity estimates across all frequencies. A psychophysical study validated that CSFs obtained with 100 qCSF trials ( approximately 10 min) exhibited good precision across spatial frequencies (SD < 2-3 dB) and excellent agreement with CSFs obtained independently (mean RMSE = 0.86 dB). To estimate the broad sensitivity metric provided by the area under the log CSF (AULCSF), only 25 trials were needed to achieve a coefficient of variation of 15-20%. The current study demonstrates the method's value for basic and clinical investigations. Further studies, applying the qCSF to measure wider ranges of normal and abnormal vision, will determine how its efficiency translates to clinical assessment.

Determinants of contrast sensitivity for the tumbling E and Landolt C

PURPOSE

To compare the object spatial frequencies that underlie contrast sensitivity for the tumbling E and Landolt C across a range of optotype sizes and under conditions biased toward the magnocellular (MC) and parvocellular (PC) pathways.

METHODS

Contrast thresholds of two visually normal observers were measured using tumbling E optotypes that were either low-pass filtered or high-pass filtered with a two-dimensional Gaussian filter. Optotypes were presented using steady-pedestal and pulsed-pedestal paradigms to target the MC and PC pathways, respectively. Object frequencies essential for orientation judgments of the tumbling E were derived from plots of log contrast threshold vs. log filter cutoff frequency, and results were compared with those obtained previously for the Landolt C under identical testing conditions.

RESULTS

The object frequency used to judge the orientation of the tumbling E increased systematically with increasing target angular subtense, and the effect of target size differed depending on whether performance was mediated by the inferred MC or PC pathway. The overall pattern of results was similar for the tumbling E and Landolt C, but there was generally less dependence of object frequency on target angular subtense for the tumbling E.

CONCLUSIONS

The tumbling E and Landolt C are not equivalent in terms of the object frequencies that mediate orientation judgments. However, both optotypes show scale-dependent changes in object frequency, particularly under test conditions that favor the PC pathway. The scale dependence of these broadband optotypes can pose a challenge in interpreting test results using these targets. A potential solution is to use spatially filtered optotypes with limited, known object frequency content.

The human magno and parvo systems and selective impairments of their functions

The magno and parvo systems, consisting of large and small cells respectively and showing the greatest differences in temporospatial characteristics, are two of the most interesting channels in the organization of retinotopic input pathways of the human visual system. We tested the hypothesis that selective lesions of fibers of a specified diameter occurring in pathology of the conducting pathways, i.e., multiple sclerosis, could be used as a "key" for studies of the different functional properties of the magno and parvo systems. Measurements were made of spatial-frequency contrast sensitivity in healthy subjects and patients in the early stages of multiple sclerosis. Decreased sensitivity at low and intermediate or at high spatial frequencies provided indirect reflections of changes in the activities of the magno and parvo systems. These psychophysical data are compared with electrophysiological results. The amplitudes of the early components of visual evoked potentials reflect activation of the two systems in healthy people and patients with multiple sclerosis. We identified two groups of patients, with statistically significantly dominant lesions of the magno and parvo systems respectively.

Spatial frequencies used in Landolt C orientation judgments: relation to inferred magnocellular and parvocellular pathways

The purpose of this study was to define the spatial frequencies that underlie judgments of Landolt C orientation under test conditions designed to favor either the magnocellular (MC) or parvocellular (PC) pathway. Contrast thresholds of two observers were measured for briefly presented Landolt Cs of four sizes, using steady- and pulsed-pedestal paradigms to bias performance toward the MC and PC pathways, respectively. Contrast thresholds were derived from a two-alternative forced-choice orientation judgment task using the QUEST procedure. The Landolt Cs were either low-pass or high-pass Gaussian filtered with a range of cutoff object spatial frequencies (cycles per letter) to limit their frequency content. Center object frequencies were derived from plots of log contrast threshold for orientation judgments vs. log filter cutoff object frequency. The function relating center object frequency to Landolt C angular subtense was nonlinear on log-log coordinates for both the steady- and pulsed-pedestal paradigms, indicating that different object frequencies were used to judge Landolt C orientation at different optotype sizes. However, the function was substantially steeper under the pulsed-pedestal than under the steady-pedestal paradigm, such that a large change in optotype size produced a relatively small change in retinal spatial frequency (cycles per degree). The pattern of results is consistent with previously reported differences between the spatial contrast sensitivity functions of the inferred MC and PC pathways.

Convergence of boundary and body information in the visual processing of aspect ratio

We used DOG ellipses and outlined ellipses that contained, respectively, only low-frequency and only high-frequency information about aspect ratio. Cross-adaptation (i.e., adapt DOG/test outline and adapt outline/test DOG) produced aspect ratio aftereffects.

CONCLUSION

Spatial information encoded in terms of the body of the stimulus and in terms of the boundary of the stimulus have substantially converged before the information-processing stage at which aspect ratio is encoded. We also report that when observers were instructed to discriminate horizontally elongated from vertically elongated test ellipses, the "just noticeably different from circular" threshold was closely constant over a 16:1 range of test ellipse areas.

Contrast sensitivity for letter optotypes vs. gratings under conditions biased toward parvocellular and magnocellular pathways

This study examined the extent to which letter optotypes and grating stimuli provide equivalent measures of contrast sensitivity under conditions designed to favor the magnocellular (MC) and parvocellular (PC) pathways. The contrast sensitivity functions (CSFs) of three visually normal observers were measured for Sloan letters and Gabor patches, using steady- and pulsed-pedestal paradigms to bias processing toward MC and PC pathways, respectively. CSFs for Gabor patches were low-pass for the steady-pedestal paradigm and band-pass for the pulsed-pedestal paradigm, in agreement with previous reports. However, CSFs for letters were low-pass for both testing paradigms. CSFs for letters restricted in frequency content by spatial filtering were equivalent to those for Gabor patches for both testing paradigms. Results indicate that conventional letter optotypes can provide a misleading measure of contrast sensitivity, especially under conditions emphasizing the PC pathway. The use of spatially band-pass filtered letters can provide a more appropriate evaluation of spatial contrast sensitivity while maintaining some of the potential advantages of letters.

Functional MRI of the visual cortex and visual testing in patients with previous optic neuritis

The volume of cortical activation as detected by functional magnetic resonance imaging (fMRI) in the visual cortex has previously been shown to be reduced following optic neuritis (ON). In order to understand the cause of this change, we studied the cortical activation, both the size of the activated area and the signal change following ON, and compared the results with results of neuroophthalmological testing. We studied nine patients with previous acute ON and 10 healthy persons served as controls using fMRI with visual stimulation. In addition to a reduced activated volume, patients showed a reduced blood oxygenation level dependent (BOLD) signal increase and a greater asymmetry in the visual cortex, compared with controls. The volume of visual cortical activation was significantly correlated to the result of the contrast sensitivity test. The BOLD signal increase correlated significantly to both the results of the contrast sensitivity test and to the Snellen visual acuity. Our results indicate that fMRI is a useful method for the study of ON, even in cases where the visual acuity is severely impaired. The reduction in activated volume could be explained as a reduced neuronal input; however, the greater asymmetry might point to a cortical reorganization as a consequence of neuronal damage. Future fMRI studies in ON will add to the understanding of the neural adaptive behaviour following ON.

Clinical outcome measures for research in multiple sclerosis

The development of new and more sensitive clinical outcome measures for research in multiple sclerosis (MS) has been fueled by the development of effective therapies. As such, active arm comparison studies that require more sensitive clinical outcome measures are now commonplace. The Kurtzke Expanded Disability Status Scale (EDSS), the most widely used measure of neurologic impairment in MS, is particularly designed for classifying patients with respect to disease severity but has been criticized for its noninterval scaling, emphasis on ambulation status, relatively reduced sensitivity in the mid and upper ranges of scores, and absence of adequate cognitive and visual components. In response to perceived difficulties with the EDSS, the National Multiple Sclerosis Society Clinical Outcomes Assessment Task Force has developed the Multiple Sclerosis Functional Composite (MSFC). The MSFC includes three components that yield objective and quantitative results: 1) the timed 25-ft walk, 2) the nine-hole peg test, and 3) the 3-second paced auditory serial addition test. This scale has the advantages of continuous scoring with a composite Z score, standardized protocols, and high degrees of reliability and validity. Candidate visual function outcome measures for the MSFC, including the low-contrast Sloan letter chart, are currently under investigation. In addition to measures of neurologic impairment, health-related quality of life (HRQOL) measures have gained increasing importance as clinical trial outcome measures. The MS Quality of Life Inventory, a disease-specific HRQOL measure, has been developed to capture self-reported neurologic dysfunction and the impact of MS upon activities of daily living. MS clinical trials of the future, particularly active-arm comparison studies, will require more sensitive clinical outcome measures such as the MSFC. Measures of visual function and HRQOL should also be incorporated to capture the broad scope of neurologic impairment and disability in MS populations.

New low-contrast vision charts: reliability and test characteristics in patients with multiple sclerosis

The quantitative assessment of visual function in multiple sclerosis (MS) clinical trials has been limited to Snellen visual acuity. The purpose of this study was to examine the inter-rater reliability and test characteristics of a new visual outcome measure, the Low-Contrast Sloan Letter Charts, in patients with MS and visually-asymptomatic volunteers. Contrast letter acuity scores (letter scores) were measured at each of four contrast levels (100, 5, 1.25 and 0.6%) by two independent raters. Inter-rater agreement was described with the intraclass correlation coefficient (ICC) and comparison of mean scores. Excellent inter-rater agreement (ICC=0.86 - 0.95) was demonstrated at each contrast level among MS patients (n=100) and visually-asymptomatic volunteers (n=33). Average letter scores at the lowest contrast level (0.6%) were highly variable in the MS group, even among patients with visual acuities of 20/20 or better, and among those who required no assistance for ambulation. Low-Contrast Sloan Letter Chart testing is a highly reliable method of visual assessment, and provides information on an aspect of neurologic impairment in MS which is not captured by Snellen visual acuity or ambulation status. This new method demonstrates excellent potential as a visual function outcome measure for future MS clinical trials.

Visual acuity versus letter contrast sensitivity in early cataract

Large and small letter contrast sensitivity and visual acuity were assessed in 37 elderly eyes (mean VA -0.01 logMAR, Snellen 6/6) and their lens opacities were categorised and graded using the LOCS III system. Large letter contrast sensitivity was often not reduced in cataract from age-matched normal values and provided limited information. Small letter contrast sensitivity was shown to be a more sensitive measure of early cataract than visual acuity and large letter contrast sensitivity. Its usefulness may be limited by its strong correlation with visual acuity (r2 = 0.70), which is the standard and traditional measure of vision in cataract.

Cytidine-5'-diphosphocholine improves visual acuity, contrast sensitivity and visually-evoked potentials of amblyopic subjects

PURPOSE

Cytidine-5'-diphosphocholine (CDP-choline) therapy is currently used to improve the consciousness level in patients with brain lesions and as a complement to levodopa therapy in Parkinson's disease. Recently, the substance has been shown to improve the visual acuity (VA) of both eyes of adults with amblyopia. This study aims at establishing whether Contrast Sensitivity (CS) and visually-evoked potentials (VEPs) also change after CDP-choline treatment.

METHODS

VA, CS, and VEPs were measured in a group of amblyopic volunteers (n = 10, mean age 24.8 years) before treatment with Neuroton (CDP-choline, 1 g/day intramuscularly [IM] for 15 days) and the day after termination of the same. CS was evaluated, using a forced-choice, automatic procedure (QUEST: Watson and Pelli, 1983). Steady-state VEPs were recorded in response to counterphased (8 Hz) sinusoidal gratings (2 c/deg) of different contrasts.

RESULTS

On average, after treatment, VA improved 1.4-1.5 lines in the amblyopic eyes and 0.4 lines in the normal eyes. CSs improved in both dominant and amblyopic eyes by about 3 dB. VEPs increased in amplitude (about 30%) and advanced in phase (about 0.2 pi rad). Amplitude and phase changes were not correlated.

CONCLUSIONS

Treating adult amblyopes with CDP-choline has the effect of improving their VA, CS and VEPs. Changes occur in both eyes, although to different extents, and resemble those previously reported for levodopa treatment.

Residual symptoms and signs after untreated acute optic neuritis. A one-year follow-up

PURPOSE

To study the symptoms and signs one year after optic neuritis.

METHODS

Seventy population-based untreated patients, (44F, 26M; median age 35 years, range 14-48 years) were eligible for study, but the 11 bilateral cases were excluded. Among 59 patients participating in a one-year follow-up of the visual function, 58 answered a questionnaire.

RESULTS

At follow-up 59% of the patients complained of visual impairment, though 49% of these patients had a Snellen visual acuity of 6/9 or better. The affected eyes showed decreased visual acuity in 34%, decreased contrast sensitivity in 63%, an abnormal score in 40% with the Lanthony D-15 desaturated panel, and in 13% with the Ishihara test, prolonged latencies of VEP in 58%, and decreased amplitudes of VEP in 41%.

CONCLUSIONS

All tests except for VEP correlated significantly to the degree of perceived visual impairment. Subjective visual complaints and more sensitive tests than visual acuity should be used to evaluate the visual function following optic neuritis.

Visual acuity and contrast sensitivity for individual Sloan letters

Sloan letter optotypes are used frequently to evaluate visual impairment, and scoring procedures have been developed that are based on the numbers of letters that are identified correctly. However, previous studies have presented conflicting evidence regarding the relative identifiability of the individual Sloan letters. To investigate this issue further, we measured psychometric functions for the identification of each of the 10 Sloan letters, with individual letters presented in random order on the gray-scale display of a Macintosh computer-based testing system. Data were obtained from three visually normal subjects under each of three conditions: (1) as a function of log contrast at a relatively large letter size; (2) as a function of log contrast at a letter size near the acuity limit; and (3) as a function of log MAR (minimum angle of resolution) at maximum letter contrast. Estimates of threshold log contrast and threshold log MAR were derived from best-fitting Weibull functions. Threshold log contrast for small letters showed the greatest interletter variability. There was relatively little interletter variability in either threshold log contrast for large letters or threshold log MAR for high-contrast letters. However, due to the relatively steep psychometric functions under these latter two conditions, the different Sloan letters had considerably different percent correct values near threshold. The overall pattern of results suggests that the contrast sensitivity functions for individual Sloan letters are displaced laterally along a log MAR axis, while their vertical positions are essentially equivalent.

Object perception by visually impaired people at different light levels

We investigated the relationship between the illumination level and the ability of visually impaired subjects to detect and recognize objects in a realistic visual environment. Subjects often continued to show substantial improvement at light levels where normal subjects have reached maximum performance. Integrated contrast sensitivity, a summary measure for the contrast sensitivity function, was better at predicting performance than either visual acuity or peak contrast sensitivity. However, when combined, the latter two predicted performance as well as the former. We conclude that when we try to find the best illumination for orientation and day-to-day activities we should optimize it for both visual acuity and contrast sensitivity.

Spatial-frequency characteristics of letter identification

To investigate the spatial-frequency components that govern letter identification we compared contrast thresholds for three types of visual stimulus (1) standard Sloan letters, (2) Sloan letters that were spatially bandpass filtered by cosine log filters, and (3) D6 patterns (sixth spatial derivatives of Gaussians). Stimuli were presented on a gray-scale display screen of a Macintosh computer-based testing system at temporal frequencies primarily of 2 and 16 Hz. Contrast thresholds were measured in two subjects with normal visual acuity with use of forced-choice staircases. Contrast sensitivity functions for standard Sloan letters and D6 patterns were comparable at a temporal frequency of 16 Hz but differed systematically at a temporal frequency of 2 Hz. The measurement of contrast sensitivity for cosine log filtered letters presented at a temporal frequency of 2 Hz indicated that the object spatial frequency of maximum sensitivity shifted to lower frequencies as letter size decreased, whereas the retinal spatial frequency of maximum sensitivity remained relatively constant. When letters were spatially bandpass filtered at a peak object spatial frequency of 2.5 cycles/letter, then contrast sensitivity functions for letter identification were equivalent to those for D6 patterns at both temporal frequencies. These results suggest that spatially filtered letters may provide a more appropriate test of visual function than do standard letter optotypes.

Measurement of glare sensitivity in cataract patients using low-contrast letter charts

The Committee on Ophthalmic Procedures Assessment of the American Academy of Ophthalmology suggested using low-contrast visual acuity measured before and after adding a glare source as a test for assessing overall visual disability from immature cataracts. We have developed a test that follows the Committee's three principles of design, and we report that the effect of glare on visual acuity is considerably greater for recognizing low-contrast letters than for recognizing high-contrast letters. The effect of glare on visual acuity increases progressively as letter contrast is reduced in the stages 96%, 50%, 25%, 11% and 4%. The 25% chart (and possibly the 11% chart) gives the most suitable sensitivity for eyes with immature cataracts. Age-related brunescence and aging itself do not necessarily produce high sensitivity to glare. Sensitivity to glare was markedly different in eyes with different kinds of cataract.

Motion-defined letter detection and recognition in patients with multiple sclerosis

Two important distinctions in visual perception are (1) between the detection and recognition of shape (e.g., letters), and (2) between the recognition of shapes defined by a difference in brightness and the recognition of shapes defined by a difference in motion. We report that 6 of 10 patients with multiple sclerosis showed impaired recognition for motion-defined (MD) letters, although the detection of MD letters was normal as were both detection and recognition of luminance-defined letters. We have shown that this was not a function of acuity loss or the loss of ability to detect motion or a general failure of recognition per se, but was confined to a loss of ability to recognize MD letters. The neurological implications of these findings are discussed, and it is suggested that the MD letter test be used by others interested in the central pathology of visual disorders.