Letter Acuity Research Articles

Visual function in young adults following intrauterine growth retardation.

Intrauterine growth retardation (IUGR) resulting in infants born small for gestational age is a known risk factor for neurologic deficits and may predispose to poor cognitive development later in life. We recently found an association between IUGR and a reduced neuroretinal rim area at 18 years of age. We evaluated the possible association between IUGR and visual function. We studied 26 subjects who had been born small for gestational age and 20 subjects whose birth weights were appropriate for gestational age (controls) using letter acuity thresholds, color vision testing, full-threshold frequency doubling technology perimetry, and rarebit perimetry at 18 years of age. gestational age had a rarebit hit rate below the normal range as compared with none of the controls (P = .006). These 8 subjects had a significantly smaller rim-disc ratio compared with the subjects who were small for gestational age who had a normal rarebit hit rate (P = .047). The frequency doubling technology indices did not differ significantly between the control group and the group that was small for gestational age, nor did the visual acuity, refraction, and color vision test results. These data indicate that IUGR is associated with an increased rate of impaired visual function, which can be detected by using rarebit perimetry but not frequency doubling technology perimetry, visual acuity, or color vision tests.

Predicting prescribed magnification.

To determine the best method of estimating the optimum magnification needed by visually impaired patients. The magnification of low vision aids prescribed to 187 presbyopic visually impaired patients for reading newspapers or books was compared with logMAR distance and near acuity (at 25 cm) and magnification predicted by +4 D step near additions. Distance letter (r = 0.58) and near word visual acuity (r = 0.67) were strongly correlated to the prescribed magnification as were predictive formulae based on these measures. Prediction using the effect of proximal magnification resulted in a similar correlation (r = 0.67) and prediction was poorer in those who did not benefit from proximal magnification. The difference between prescribed and predicted magnification was found to be unrelated to the condition causing visual impairment (F = 2.57, p = 0.08), the central visual field status (F = 0.57, p = 0.57) and patient psychology (F = 0.44, p = 0.51), but was higher in those prescribed stand magnifiers than high near additions (F = 5.99, p < 0.01). The magnification necessary to perform normal visual tasks can be predicted in the majority of cases using visual acuity measures, although measuring the effect of proximal magnification demonstrates the effect of stronger glasses and identifies those in whom prescribed magnification is more difficult to predict.

Perceptual salience of global structures and the crowding effect in amblyopia.

The crowding effect refers to stronger deficits in linear acuity (e.g., letters in a line) than in single letter acuity in amblyopia. The current work investigated whether the salience of a global structure in which the target for identification is embedded influences the crowding effect in amblyopia. Compound shapes were presented to the amblyopic and fellow eyes respectively of 12 anisometropic amblyopes. The compound stimuli were presented on either a blank or a cross background so that the salience of global structures were manipulated. Reaction times (RTs) and response error rates were recorded when subjects identified global or local shapes, respectively. RTs were shorter to global than local shapes for both the amblyopic and the fellow eyes. The global RT advantage was larger for the amblyopic than the fellow eye. Interestingly, when viewing the stimuli with the amblyopic eye, subjects made more errors to local targets when the compound stimuli were presented against the blank than the cross background. The results suggest that the salience of global structures of visual stimuli contributes to the crowding effect in amblyopia.

Pattern reversal visual evoked potentials as a measure of visual pathway pathology in multiple sclerosis.

Pattern reversal visual evoked potentials (PRVEPs) have a well-documented role in diagnosis of multiple sclerosis (MS), but their value as a visual function surrogate remains controversial. We evaluated PRVEP in 37 patients with MS who were participating in a long-term follow-up study following a phase III trial of interferon beta-1a (Avonex). Patients were examined to determine the Kurtzke Extended Disability Status Score (EDSS), multiple sclerosis functional composite (MSFC), contrast letter acuity (CLA), and had cranial MRI scans to determine whole brain atrophy (BPF). PRVEP was evaluated for P100 latency, amplitude, and waveform morphology. Two summary scores were created: for Score A, abnormal latencies, morphologies, and amplitudes of each individual eye were added; for Score B, abnormal latencies, morphologies, and amplitude ratio between eyes was determined. Sixteen patients in this group also had PRVEP at the time they enrolled in the clinical trial, eight years previously. At the follow-up exam, over 75% of patients had abnormal PVEP parameters while visual acuity (VA) was abnormal only in 59%. Increased PRVEP latency over the eight-year period correlated with deterioration assessed by EDSS (P = 0.006), BPF (P = 0.0001), and MSFC (P = 0.0041). Score A was significantly correlated with EDSS, BPF, CLA, cognitive function, and quality of life assessed with the Sickness Impact profile. No correlation was seen with the MSFC. The results indicate that PRVEP measures MS-related pathology, and can provide not only diagnostic but also prognostic information during evaluation of MS patients.

Hyper-vision in a patient with central and paracentral vision loss reflects cortical reorganization

SM, a 21-year-old female, presents an extensive central scotoma (30 deg) with dense absolute scotoma (visual acuity = 10/100) in the macular area (10 deg) due to Stargardt's disease. We provide behavioral evidence of cortical plastic reorganization since the patient could perform several visual tasks with her poor-vision eyes better than controls, although high spatial frequency sensitivity and visual acuity are severely impaired. Between 2.5-deg and 12-deg eccentricities, SM presented (1) normal acuity for crowded letters, provided stimulus size is above acuity thresholds for single letters; (2) a two-fold sensitivity increase (d-prime) with respect to controls in a simple search task; and (3) largely above-threshold performance in a lexical decision task carried out randomly by controls. SM's hyper-vision may reflect a long-term sensory gain specific for unimpaired low spatial-frequency mechanisms, which may result from modifications in response properties due to practice-dependent changes in excitatory/inhibitory intracortical connections.

Letter contrast sensitivity changes in early diabetic retinopathy

Purpose: To investigate the discriminative ability of letter contrast sensitivity (CS) and visual acuity (VA) in detecting functional losses in participants with type 2 diabetes. Methods: LogMAR VA and letter CS were measured on 20 type 2 diabetic and 24 agematched control participants. The diabetic participants were sub‐grouped according to the level of retinopathy present. Resulte: Letter CS was able to distinguish those participants with and without macular oedema and those with no or minimal diabetic retinopathy from the control group, whereas VA was not. Conclusion: Letter CS may be used as an effective screening tool to assess damage to the visual pathway of diabetic participants both with and without clinically detectable signs.

Velocity dependence of Vernier and letter acuity for band-pass filtered moving stimuli

The ability to see fine detail diminishes when the target of interest moves at a speed greater than a few deg/s. The purpose of this study was to identify fundamental limitations on spatial acuity that result from image motion. Discrimination of Vernier offset was measured for a pair of vertical abutting lines and letter resolution was measured using a four-orientation letter ‘T’. These stimuli were digitally filtered using one of five band-pass (bandwidth=1.5 octaves) filters with a center frequency between 0.83 and 13.2 c/deg, and presented at velocities that ranged from 0 to 12 deg/s. Filtered and unfiltered stimuli were presented for 150 ms at a constant multiple (4× or 2×) of the contrast-detection threshold at each velocity. For stimuli of low to middle spatial frequency (up to 3.3 c/deg), Vernier and letter acuity for equally detectable targets are essentially unaffected by velocity up to 12 deg/s, i.e., for temporal frequencies of motion (velocity × spatial frequency) up to approximately 50 Hz. For stimuli of higher spatial frequency, acuity remains essentially constant until the velocity corresponds to a temporal frequency of about 30 Hz, and increases thereafter. Both Vernier and letter acuities worsen by approximately a factor of two for each one-octave decrease in filter spatial frequency. Both types of acuities worsen also as the contrast of the stimulus is reduced, but Vernier discrimination exhibits a stronger contrast-dependence than letter resolution. Our results support previous suggestions that a shift in the spatial scale used by the visual system to analyze spatial stimuli is principally responsible for the degradation of acuity in the presence of image motion. The results are consistent with a spatio-temporal-frequency limitation on spatial thresholds for moving stimuli, and not with a temporal-frequency limitation per se.

Contrast letter acuity as a measure of visual dysfunction in patients with Friedreich ataxia.

Friedreich ataxia is a progressive neurodegenerative disorder affecting afferent cerebellar pathways and other neuronal systems, including afferent visual pathways. A systematic clinical outcome measure for examination of visual dysfunction in Friedreich ataxia has not been identified. We sought to identify a simple, reliable method for assessing clinical and subclinical visual dysfunction in patients with Friedreich ataxia. Contrast letter acuity was measured binocularly in Friedreich ataxia patients and age-matched visually asymptomatic volunteers (control group) using the Low-contrast Sloan Letter Charts at three different low-contrast levels (5.0%, 1.25%, and 0.6%). Binocular high-contrast visual acuity (100% level) was also determined for each participant. Despite equal median binocular high-contrast visual acuities between the two groups, patients with Friedreich ataxia had significantly lower (worse) Low-contrast Sloan Letter Chart scores compared with controls, particularly at the lowest contrast levels (1.25% and 0.6%). Ambulation status significantly predicted Low-contrast Sloan Letter Charts scores in linear regression models accounting for patient age, suggesting a potential complementary role for Low-contrast Sloan Letter Chart testing in the assessment of disease status as well as visual function in Friedreich ataxia. This study demonstrates that Low-contrast Sloan Letter Chart testing may provide a useful clinical outcome measure for Friedreich ataxia and other neuro-ophthalmologic disorders.

Are all aberrations equal?

To determine for a fixed RMS error (25 microm, over a 6-mm pupil) how each mode of the normalized Zernike polynomial (second through the fourth radial order) affects high and low contrast logMAR visual acuity. Three healthy volunteers served as subjects. CTView was used to generate optically aberrated logMAR charts. Accommodation was paralyzed and pupils dilated. The foveal achromatic axis of the eye was aligned to a 3-mm pupil and the eye was optimally refracted. Aberrated acuity charts were read until five letters were missed. Data were normalized for each subject to the acuity obtained by reading unaberrated charts and plotted as letters lost as a function of Zernike mode. Defocus (Z2(0)) decreased letter acuity more than astigmatism (Z2(2), Z2(-2)). Coma (Z3(1), Z3(-1)) decreased acuity more than trefoil (Z3(3), Z3(-3)). Spherical aberration (Z4(0)) and secondary astigmatism (Z2(2), Z4(-2)) decreased acuity much more than quadrafoil (Z4(4), Z4(-4)). 1. For an equal amount of RMS error not all coefficients of the Zernike polynomial induce equivalent losses in high and low contrast logMAR acuity. 2. Wavefront error concentrated near the center of the pyramid adversely affects visual acuity more than modes near the edge of the pyramid. 3. Large changes in chart appearance are not reflected in equally large decreases in visual performance (ie, subjects could correctly identify highly aberrated letters). 4. Interactions between modes complicate weighting each Zernike mode for visual impact.

Photodynamic therapy of subfoveal choroidal neovascularization with verteporfin in the ocular histoplasmosis syndrome: One-year results of an uncontrolled, prospective case series

ObjectiveTo evaluate the safety and effect on visual acuity of photodynamic therapy with verteporfin (Visudyne, Novartis AG) in patients with subfoveal choroidal neovascularization (CNV) secondary to the ocular histoplasmosis syndrome (OHS). DesignOpen-label, three-center, noncomparative prospective case series. ParticipantsOHS patients with subfoveal CNV lesions no larger than 5400 μm in greatest linear dimension (GLD) with classic or occult CNV extending under the geometric center of the foveal avascular zone and best-corrected visual acuity letter score of 73 to 34 (approximate Snellen equivalent 20/40–20/200). MethodsTwenty-six patients received verteporfin (6 mg/m2) infused IV over 10 minutes. Fifteen minutes after the start of infusion, a laser light at 689 nm delivered 50 J/cm2 at an intensity of 600 mW/cm2 over 83 seconds using a spot size with a diameter 1000 μm larger than the GLD of the lesion. At 3-month follow-up examinations, retreatment with the same regimen was applied if angiography showed fluorescein leakage. Safety assessments were also made. Main outcome measuresVisual function measurements were the changes from baseline in visual acuity and contrast sensitivity scores and the proportion of patients who, based on best-corrected visual acuity scores, (1) gained 7 or more letters, (2) lost 8 or more letters, and (3) lost 15 or more letters. ResultsOne patient was omitted from the study at the month 3 examination for not meeting the visual acuity eligibility requirements at baseline. By the month 12 examination, but excluding any retreatment at that visit, patients had received an average of 2.9 treatments of a maximum of 4 possible treatments. The month 12 median improvement from baseline in visual acuity of the remaining 25 patients was 7 letters, and median contrast sensitivity improved by 2 letters. Median visual acuity improvement was also 7 letters when three patients, who failed to meet all photographic eligibility requirements at baseline, were excluded. At the month 12 examination, 14 (56%) patients gained 7 or more letters of visual acuity from baseline, whereas 4 (16%) patients lost 8 or more letters, of which 2 (8%) lost 15 or more letters. No serious systemic or ocular adverse events were reported. ConclusionsMedian visual acuity improved after verteporfin therapy for at least 1 year in a small uncontrolled prospective case series of patients with subfoveal CNV caused by OHS. Verteporfin therapy seemed to be safe and well tolerated in these patients. Two-year data from this study will provide important, additional information on the safety and effect of verteporfin therapy for the treatment of subfoveal CNV secondary to OHS.

Spatial-frequency properties of letter identification in amblyopia

Amblyopia is characterized by a deficit in identifying small letters (acuity deficit) in the absence of identifiable ocular pathology. One explanation for this deficit is that the amblyopic visual system lacks appropriate channels tuned to high spatial frequencies for identifying small letters. The purpose of this study was to examine the spatial-frequency properties of letter identification in the amblyopic visual system. To do so, we measured contrast thresholds for identifying letters that were band-pass filtered to different bands of spatial frequencies, for letter sizes ranging from 2× to 19.2× larger than acuity letters. Letters were digitally filtered using a set of band-pass filters, with peak object spatial frequencies ranging from 0.88 to 10 c/letter. The bandwidth of the filters was 1 octave. For any given letter size, contrast sensitivity for identifying letters exhibits a spatial-tuning function. The shape of these tuning functions was found to be similar between amblyopic and non-amblyopic eyes, and across all letter sizes. The peak of these functions shifted progressively toward lower object spatial frequency when the letter size became smaller. When compared with the non-amblyopic eyes, the amblyopic eyes have a limited range of tuning functions sensitive to letters. However, when scaled with respect to acuity, the relationship between the peak frequency of the tuning functions and letter size becomes essentially identical in the amblyopic and non-amblyopic eyes. An ideal-observer analysis that takes into account spectral information about letter identity and the contrast-sensitivity function of the observer, but does not invoke narrow-band channels, also shows that the properties of the tuning functions for letter identification are similar between the amblyopic and non-amblyopic eyes. We conclude that the deficit in identifying small letters in amblyopes is not attributable to differences in the shape or selection of “channels”, when compared with the normal visual system. Rather, it is a consequence of the difference in the resolution limit between the amblyopic and the non-amblyopic eyes.

RELATIONSHIPS BETWEEN NEAR LETTER AND WORD VISUAL ACUITIES AND AGE IN NORMAL AND LOW VISION SUBJECTS.

RELATIONSHIPS BETWEEN NEAR LETTER AND WORD VISUAL ACUITIES AND AGE IN NORMAL AND LOW VISION SUBJECTS.

The effect of conventional CR39 and Fresnel prisms on high and low contrast acuity

The effect of conventional CR39 and Fresnel prisms on high and low contrast letter acuity was studied. Visual acuity of the fully corrected better eyes of 15 subjects was measured with the high (90%) and low (10%) contrast logMAR letter charts, while they wore prisms of varying power. The results showed that when the power of the conventional CR39 prism and the Fresnel prism reached 10 prism dioptres and 5prism dioptres, respectively, significant reduction (1 tail t-test, p<0.05) of the high and low contrast acuity occurred. The Fresnel prism caused a significantly greater acuity reduction than the conventional CR39 prism for powers ranging from 5 to 30prism dioptres for both contrasts. The rate of acuity reduction with increasing prism power was greater with the low contrast targets than with the high contrast targets for both prisms. In addition, the rate of acuity reduction with increasing prism power was greater with the Fresnel prism than with the conventional CR39 prism for both contrasts. The conventional CR39 prism reduced acuity by a ratio of about 0.8–0.9 of that of the Fresnel prism for powers ranging from 5 to 20prism dioptres and by about 0.7 for 30prism dioptres. These ratios applied for both high and low contrast acuity, and therefore were independent of the level of contrast used.

Stereopsis in patients with albinism: Clinical correlates

Purpose: A hallmark of albinism is excessive decussation of retinostriate projections at the optic chiasm. This misprojection might lead to abnormalities in the retinal correspondence and may account for the usual absence of stereovision. We report on 2 groups of patients with albinism who have either fine or gross stereopsis and compare the clinical findings of these groups to other patients with albinism with similar visual acuities but no stereopsis. Methods: A retrospective chart review of patients with albinism was used to segregate those with a letter visual acuity of 20/100 or better in one eye, assessment of stereopsis, and strabismus • 10 PD. Forty-five patients were identified. Albinism type, best-corrected visual acuity, motility, Titmus vectograph stereoacuity, iris and macula transparency grades, and the presence or absence of both melanin and an annular reflex in the macula were tabulated. A comparison of the clinical characteristics of the groups with and without stereopsis was made. Results: Those albino subjects who demonstrated stereopsis had better visual acuity, less iris transillumination, more frequent presence of melanin in the macula, less nystagmus, and less marked foveal hypoplasia than the albino subjects without stereopsis. No nystagmus was clinically detected in 5 patients with fine stereopsis. All these differences were statistically significant. Macular transparency grade was not significantly different between the groups. Conclusions: Patients with albinism who demonstrate stereopsis tend to have better visual acuity, more iris pigment, and more melanin pigment in the macula than their counterparts without stereopsis. (J AAPOS 2001;5:98-104)

Retinal illuminance and the dissociation of letter and grating acuity in age-related macular degeneration.

To report on the differential effect of retinal illuminance on letter and grating acuity. For 13 subjects with age-related macular degeneration (ARMD) and four subjects with normal vision, standard distance and near letter acuity, as well as grating acuity data were obtained at mesopic to high-photopic light levels. In general, both acuity forms improved with increasing light level, but not in proportion with one another. The ratio of letter/grating acuity separated the ARMD subjects into two subgroups, one with relatively low letter acuity scores for which the dissociation of the two acuity forms increased with retinal illuminance, and another with higher letter acuity values for which the ratio converged toward unity. These findings demonstrate that both letter and grating acuity increase with retinal illuminance and that equating grating acuity with optotype acuity is untenable in subjects with ARMD, irrespective of light levels. The latter conclusion is of importance whenever acuity is used as a criterion to classify the visually impaired with regard to their legal and social status.

Contrast sensitivity and reading in subjects with age-related macular degeneration

Thirteen subjects with age-related macular degeneration (AMD) participated in a study designed to investigate the predictive value of acuity and contrast sensitivity for reading. Compared with the normally sighted individuals, the spatial contrast sensitivity of all these AMD subjects was markedly reduced, particularly at the higher spatial frequencies. The difference was smaller at frequencies below 1 c/deg. For about half the subjects, maximum monocular contrast sensitivity was better than or equal to binocular sensitivity, indicating binocular inhibition or failure of summation. For the 11 subjects who were able to read at a luminance of 40cd/m², reading speed was correlated with various measures of contrast sensitivity and acuity. Of all the visual functions tested, maximum monocular contrast sensitivity and integrated monocular contrast sensitivity showed the best correlation with monocular reading speed. Binocular contrast sensitivity functions, monocular grating acuity, and monocular letter acuity ...

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.

Improving the reliability of visual acuity measures in young children

SummaryWhilst the methodology of adult letter acuity measurement has been substantially refined over the last two decades, relatively little development has occurred in methods for quantifying letter acuity in young children. This study compares a recently developed visual acuity test (Glasgow Acuity Cards), which incorporates several key design features used in adult test charts to improve the sensitivity and reliability of visual acuity measurements. The equivalence of acuity measurements made with Glasgow Acuity Cards were compared with the Bailey–Lovie logMAR chart and Snellen chart in adults, and with traditional Single Letter Acuity and a modified Single Letter Acuity test in children. The test–retest reliability of acuity measurements made with Glasgow Acuity Cards and the Single Letter Acuity tests were also assessed in a large group of visually normal children. In addition, the ability of the pre‐school letter acuity tests to detect differences in acuity between the two eyes, and to detect amblyopia were examined. Ninety‐five percent of vision measurements made with the Bailey–Lovie chart and Glasgow Acuity Cards differ by less than 0.07 log unit. Furthermore, the sensitivity of Glasgow Acuity Cards to detecting changes in acuity longitudinally and inter‐ocular differences in acuity is considerably greater as compared with traditional Single Letter Acuity tests. Improvements in paediatric acuity chart design are important for the effective detection and management of children with amblyopia.

Relative legibility and confusions of letter acuity targets in the peripheral and central retina.

Patients with macular disease may image optotypes at a peripheral retinal locus during visual acuity testing. In this study, we asked if the relative legibility and confusions between letters are similar in the fovea and at 10 degrees in the peripheral retina. Twenty five upper-case alphabet letters (all except "I"), constructed to the same specifications as the Sloan letters, were presented one at a time on a computer monitor at the fovea and at 10 degrees in the temporal, superior and superior-temporal retina. The range of relative legibility at peripheral loci was generally larger than in the fovea, for all letters and for the subset of 10 Sloan letters. Although many confusion pairs were similar in the fovea and periphery, additional confusion pairs, preferentially involving curved letters, occurred uniquely in the periphery. The increased range of relative legibility for letter targets in the peripheral retina underscores the importance of using letter-by-letter scoring to obtain precise measures of visual acuity in patients without central vision.

Improving the reliability of visual acuity measures in young children

Whilst the methodology of adult letter acuity measurement has been substantially refined over the last two decades, relatively little development has occurred in methods for quantifying letter acuity in young children. This study compares a recently developed visual acuity test (Glasgow Acuity Cards), which incorporates several key design features used in adult test charts to improve the sensitivity and reliability of visual acuity measurements. The equivalence of acuity measurements made with Glasgow Acuity Cards were compared with the Bailey–Lovie logMAR chart and Snellen chart in adults, and with traditional Single Letter Acuity and a modified Single Letter Acuity test in children. The test–retest reliability of acuity measurements made with Glasgow Acuity Cards and the Single Letter Acuity tests were also assessed in a large group of visually normal children. In addition, the ability of the pre-school letter acuity tests to detect differences in acuity between the two eyes, and to detect amblyopia were examined. Ninety-five percent of vision measurements made with the Bailey–Lovie chart and Glasgow Acuity Cards differ by less than 0.07 log unit. Furthermore, the sensitivity of Glasgow Acuity Cards to detecting changes in acuity longitudinally and inter-ocular differences in acuity is considerably greater as compared with traditional Single Letter Acuity tests. Improvements in paediatric acuity chart design are important for the effective detection and management of children with amblyopia.