Grating Acuity Research Articles

Small and large field grating acuity versus recognition acuity in different groups of amblyopic patients

Grating acuity was tested in 74 subjects with different amblyopia and five patients with pathological morphological findings, reducing visual acuity. Recognition acuity was tested with Landolt C single symbol and close inter-space 2.6 min. of arc. Grating acuity was tested with the Teller Acuity Cards with large (12.5 cm × 12.5 cm) and small (6.25 cm × 6.25 cm) test fields. Reduction of field size did not cause any significant difference. Patients with strabismic or strabismic and anisometropic amblyopia did not show a close correlation of recognition acuity with grating acuity. Stimulus deprivation amblyopia showed a good correlation between grating acuity (large and small field) and Landolt single optotype acuity. In children at risk for amblyopia, a difference in favour of large field gratings was found for both eyes (risk and non-risk eye) in the lower visual acuity group (identical with the lower age group). Better grating acuity for large fields seems to reflect an early stage of the visual development rather than that it is indicative for amblyopia.

Assessment of visual acuity in children with trisomy 18.

Although 90% of children with trisomy 18 (Edwards syndrome) die in the first year of life, a small proportion survive into the second and third decade. Many do not have associated ocular abnormalities that might affect vision. Measurable visual acuity has not been reported in these profoundly developmentally delayed individuals. Five children with trisomy 18, aged six months to eight years, underwent complete eye examination including assessment of binocular grating acuity with Teller acuity cards and assessment of binocular vernier acuity with vernier cards. All children were nonverbal with profound developmental delay. Binocular grating acuity ranged from 0.9 cycles per degree (cpd) to 2.2 cpd. This represents a reduction of 1.9 to 5.1 octaves (mean 3.5 octaves, SD 1.3 octaves) compared to age matched norms. None of the children responded to any of the vernier offsets, including the largest of 64 minutes of arc. All children with trisomy 18 demonstrated a measurable grating acuity that was well below normal for age, consistent with profound developmental delay.

Orientation acuity for sine-wave gratings with random variation of spatial frequency

Orientation discrimination thresholds were estimated for sine-wave grating stimuli using a two-alternative forced-choice procedure on the two principal, and the two main oblique axes. The two discriminanda within any trial were subject to independent random variation of their spatial frequency. The variation of spatial frequency was defined by a Gaussian probability density function. Thresholds were obtained for five different depths of frequency variation within the range 0.0–1.0 octave. We find that orientation acuity on any of the axes tested is unaffected by variation in the spatial frequency of the targets. The data support the hypothesis that orientation and spatial frequency are independent and orthogonal neural codes.

The effects of luminance on FPL and VEP acuity in human infants

Grating acuity was measured in 16-week-old human infants. Three measurement techniques were used: forced-choice preferential-looking (FPL), and two visual-evoked-potential (VEP) techniques. The stimuli were counterphase flickering sinewave gratings with a space-average luminance of -1.0 or 2.0 log cd/m2. Slightly different luminance-dependent changes occur between FPL and VEP acuities, suggesting that some factor influences the two methods differently as stimulus luminance varies. A comparison between FPL acuities and VEP acuities within infants suggests a quantitative relationship between techniques. Infant's acuity for sinewave gratings with a space-average luminance of -2.0, -1.0, 0.0, 1.0 and 2.0 log cd/m2 was also measured using a single VEP paradigm. The results are compared to the same measurements in adults and to infant and adult ideal observers. VEP acuity in this group of infants improves by about 0.5 log units between -2.0 and 0.0 log cd/m2 and remains asymptotic between 0.0 and 2.0 log cd/m2. This result suggests that luminance-dependent changes in infant acuity cannot be fully accounted for by immaturities in the optics and photoreceptor spacing and efficiency.

Visual field and grating acuity development in low-risk preterm infants during the first 2 1/2 years after term

The effect of early visual experience on visual field size and grating acuity development was studied longitudinally in 36 appropriate for gestational age (AGA) and 26 small for gestational age (SGA) low-risk preterm infants. These were selected out of 194 very low birth weight (VLBW) infants (birthweight less than 1500 g) born in 1985 and 1986. Criteria for inclusion as low-risk were the absence of neurological, respiratory, circulatory and alimentary problems in the neonatal period; no retinopathy of prematurity and no evidence of abnormality on the neonatal cranial ultrasound scans. Binocular field sizes were assessed using kinetic arc perimetry. Binocular grating acuity was tested by means of the prototype version of the acuity card procedure. Results were compared with norms obtained in control fullterms in earlier studies. Infants were tested at 6 weeks, 6, 6, 9 and 12 months of age from the expected term date. Twenty-two of these infants were retested at 2 1/2 years of corrected age. Visual field size and visual acuity estimates of (both AGA and SGA) low-risk, VLBW preterms and control fullterms overlapped at all test ages, except for a slight but significantly faster development of the upper and the lower visual field at 6 weeks corrected age in the preterm group. These results indicate that for clinical purposes visual experience before the expected term date has not only no measurable effect on the normal development of behavioural acuity, but also no accelerating effect on the development of peripheral vision.

Visual Development of Infants with Severe Ocular Disorders

Among 11 patients who presented as blind in early infancy, with Leber's congenital amaurosis (5 patients), optic nerve hypoplasia (4 patients), or macular colobomata (2 patients), 8 developed visually guided behavior and measurable grating acuity by age 5 to 46 months. All children with measurable grating acuity demonstrated visually guided mobility. Grating acuity was predictive of later visual performance in 10 of 11 patients by age 12 to 16 months. The best grating acuity attained by 7 months was 1.3 to 3.0 cycles/degree (20/460 to 20/200) and 0.13 cycles/degree (20/4700) by month 8. Two patients with Leber's congenital amaurosis and one with optic nerve hypoplasia remained blind. No clinical features existed to differentiate these three patients from the eight whose visual status improved. Posterior visual pathway maturation may underlie the improvement.

Spatial vision of the cat: Variation with eccentricity

We examined the grating acuity and contrast sensitivity of cats whose eye position was monitored with a scleral search-coil technique. During each trial, the cat was required to maintain fixation on a laser spot and respond to the presence or the absence of a grating by pressing a right or left pedal. With this procedure, acuity was measured along the horizontal and vertical meridian over a range of eccentricities in the nasal, temporal, superior, and inferior retina. In addition, contrast sensitivity for stationary and drifting gratings was measured for the temporal retina along the horizontal meridian. Acuity in area centralis reached about 3.5 cycle/deg and declined by 0.5 octaves at 4 deg and by about 1.3 octaves at 16-deg eccentricity in the nasal retina. The acuity was higher in the nasal than temporal retina. At all eccentricities, spatial resolution exceeded the resolution limit derived from Y (alpha)-cell properties. Contrast sensitivity also decreased as the eccentricity increased when the target size was held constant. The slope of sensitivity-eccentricity function was relatively shallow for a low spatial frequency (0.30 cycle/deg) with sensitivity decreasing by a factor of 1.5-2 at 8-deg eccentricity. The slope of the sensitivity falloff for high spatial-frequency gratings (1.2 cycle/deg) was steeper, with a 5-10-fold difference in sensitivity between 0 and 8 deg. By varying the target size, we determined that the summation area in the cat is about a factor of 3 smaller in area centralis than a 16-deg eccentricity.(ABSTRACT TRUNCATED AT 250 WORDS)

Vernier acuity of normal and visually deprived cats

This study examines the relationship between grating and vernier acuity in cats that were either normally reared, unilaterally amblyopic as a result of a period of monocular deprivation, or bilaterally amblyopic resulting from a period of reverse occlusion followed by binocular visual experience. Vernier acuity was assessed on a jumping stand by use of a vernier-grating stimulus similar to that devised for use with human infants. The vernier thresholds for normal cats were 1.2–1.3 min arc, values that were approx. 6 times better than their grating acuity, and hence may represent a true hyperacuity. By contrast, the vernier acuity of the visually deprived cats were substantially below normal (19–83 min arc). The vernier thresholds for the deprived eye of the monocularly deprived cat and both eyes of the reverse occluded cats had fallen to the point where they were at best equal, and sometimes worse than the corresponding grating acuity. This pattern of results is similar to those oberved in some types of human amblyopia, where vernier acuity also no longer represents a hyperacuity, and where in severe cases the thresholds may be worse than grating acuity.

Effects of Dioptric Blur on Snellen and Grating Acuity

Effects of Dioptric Blur on Snellen and Grating Acuity

Influence of retinal lesions on grating acuity of the cat

Grating acuity was investigated behaviorally in the cat. Retinal lesions of increasing diameter centered on the area centralis were made by photocoagulations in one eye, while the intact eye was used as control. Lesion size evaluated from fundus photographs was precisely correlated with the anatomical lesion size measured in retinal whole mounts. Grating acuity improved with increasing grating area in cats with intact retinae and after small (<3° diameter) retinal lesions but not after large lesions (>4° diameter). Overall, grating acuity clearly decreased when lesions became larger than 4° in diameter. The acuity-eccentricity relationship closely fits the cutoff frequency of brisk sustained cells at the corresponding eccentricities.

Effects of Dioptric Blur on Snellen and Grating Acuity

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of "spurious resolution" in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patient's refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.

Spatial resolution across the macaque retina

Grating acuity was measured as a function of eccentricity from the fovea in two macaques. A vertical-horizontal orientation discrimination was used to determine acuity, and the retinal locus of the test grating was controlled by training them to fixate a spot placed at various distances from the stimulus. Their head was fixed in place and fixation was monitored with a scleral search coil. The acuity of monkeys across the retina was similar to that previously measured in human subjects, reaching a peak of about 38 c/deg at the fovea, and decreasing about 10-fold by 30 deg eccentricity. Acuity was slightly higher in the temporal than in the nasal visual field. The shape of the acuity-eccentricity function suggested a dependence on cone density near the fovea, and on the density of P ganglion cells at eccentricities beyond 10 deg. Existing physiological data suggest the possibility that macaque acuity may also be limited in part by spatial averaging across the receptive field of retinal ganglion cells.

The early development of visual acuity in the binocular and monocular peripheral fields

We used FPL to assess 1-, 2-, and 3-month-olds' visual acuity at 20° in the binocular and monocular peripheral fields. Acuities were very poor at 1 month under both binocular and monocular viewing conditions but improved steadily over the next 2 months. Three-month-olds showed better acuity for gratings located in the temporal than in the nasal visual field.

A preferential looking clinical acuity test: Improvements in implemented microcomputer control

A manually operated preferential looking grating acuity test was modified to include a micro-computer for controlling both stimulus presentation and data acquisition, storage, and analysis. The computerized test requires only one observer-operator and minimizes several testing biases and errors of the manual test. A technique was developed to keep the observer-operator masked during the generation of a transformed staircase. The monocular test time of about 8 min, and the success rate—81% for 224 children aged 2–39 months—compare favorably with our experience with the manual two-operator test. For 39 pairs of age-matched children without visual pathology, grating acuities were 0.6 octaves better with the computerized test. The computerized version was used to measure acuity in 120 children who had, according to clinical examination, poorer vision in one eye than in the other. The suspect eyes showed an average of 0.8 octaves poorer grating acuity. The improvements of computerization were realized without loss of the advantages of the manual test.

Human albinos can discriminate spatial frequency and phase as accurately as normal subjects

Previous experiments testing grating and vernier acuities in albino central vision are consistent with the hypothesis that the deficit in their monocular spatial processing is mainly due to the increased spacing of their foveal cones. This was tested by measuring albino spatial frequency discrimination over the range 0.25–4.0 cpd. The same experiments were performed on three normal subjects both in the fovea and at a peripheral locus at which their grating acuity was identical to that of the albino subjects. Spatial frequency discrimination thresholds averaged 3.71% for albinos, 5.18% for the normal fovea, and 8.81% for the normal periphery, the latter being over 2.3 times greater than albino thresholds. A comparable pattern of results was observed in phase discrimination experiments. These data reject the possibility that albino central vision is similar to normal peripheral vision, but the results are predictable on the hypothesis that the central retina of albinos is a spatially magnified (underdeveloped) version of the normal fovea.

Acuity card testing of retarded children.

Evaluating the vision of retarded children is important, but it is often very difficult because the children cannot cooperate with the usual symbol recognition acuity tests. A total of 33 unselected children with Down's syndrome were tested with a standard symbol recognition test (Osterberg) and with a new test using acuity cards, which contain black and white square-wave gratings. The acuity cards were found to be useful for testing this group, both in terms of the children's ability to cooperate with the test and the test-retest variability of the results. Although eyes with strabismic amblyopia showed better acuity for gratings than for symbols, there was general agreement with symbol recognition acuities of the same patients. Acuity cards were also used to test 19 severely retarded young children with cerebral palsy, who were unable to be tested with symbols. Acuity card tests were almost always possible with this group, too, though test-retest variability was greater for these children. Acuity card testing can provide useful information about the visual acuity of many retarded children in less than 10 minutes.

Positional uncertainty in peripheral and amblyopic vision

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.

Infant VEP acuity measurements: Analysis of individual differences and measurement error

The reliability of visual evoked potential (VEP) measurements of acuity was determined by estimating acuity for sinewave luminance gratings which were counterphase modulated at either 12 or 20 reversals/sec (rps). Gratings were swept in spatial frequency beyond the acuity limit and acuity was estimated from an extrapolation based on the last peak in the VEP amplitude versus spatial frequency function. Twenty-five infants ranging in age from 17 to 25 weeks were studied. Individual 10 sec sweeps resulted in records with a criterion response in 65–75% of trials. The reliability of acuities obtained from individual 10 sec sweeps was ±0.54 octaves at 95% confidence across 12 and 20 rps recording conditions (RMS error of ±0.27 octaves). The best acuity attained by each infant on either a single sweep, or on their vector average, was reliable to ±0.38 octaves at 95% confidence (RMS error of ±0.19 octaves) compared to a range of individual acuities of about 2 octaves. Much of the variability of sweep VEP acuity in cross-sectional samples of infants is therefore attributable to reliable individual differences rather than to measurement error. In testing individual infants our analysis indicates that choice of temporal frequency accounts for only 14% of the variation in acuity estimates within subjects.

Spatial-frequency model for hyperacuity

Humans can detect vernier displacements of two abutted lines that are 30 times smaller than the bar spacings that determine their grating acuity. Since vernier acuity tasks, and hyperacuity tasks in general, reveal such drastically improved sensitivity, it has been traditionally assumed that the detection mechanisms responsible for hyperacuity are fundamentally different from those underlying ordinary spatial acuity. The need for unusual mechanisms is reinforced by the observation that hyperacuity is weakly affected by changes in suprathreshold contrast, whereas ordinary acuity is strongly influenced by contrast. Nevertheless, we argue that many hyperacuity tasks can be understood without resorting to special mechanisms. We have taken a previously developed contrast-detection model, based on spatial-frequency channels, and have applied it directly to a set of hyperacuity experiments. Hyperacuity performance is readily predicted without modification of the model. In addition, the model correctly predicts the insensitivity of hyperacuity to suprathreshold contrast as well as the measured result that moderate low-pass filtering of hyperacuity images does not significantly decrease hyperacuity performance.

Capabilities of monkey cortical cells in spatial-resolution tasks.

The performance of individual neurons in monkey striate cortex has been examined in three spatial-resolution tasks by making microelectrode recordings from single cells in anaesthetized, paralyzed animals. The statistical reliability of responses from cells was used to estimate threshold levels of performance. For each task (resolution acuity for high-contrast gratings, discrimination of gratings varying in spatial frequency, and localization ability, i.e., discrimination of spatial phase), performance approaching psychophysical thresholds was obtained from single cortical cells. The receptive-field organization underlying localization performance was examined in detail by the use of a linear model that relates localization ability to the sensitivity of the receptive field to luminance contrast. Calculations from this model agree well with direct measurements of localization performance and are comparable with psychophysical measurements of hyperacuity. Though it has been suggested that cells with nonoriented receptive fields in cortical layer IVc beta may be responsible for recovering fine-grain spatial information, our calculations indicate that these cells are poorer at localization than many other cells in the cortex.