A comparison of the effects of ageing upon vernier and bisection acuity

While most positional acuity tasks exhibit an age-related decline in performance, the effect of ageing upon vernier acuity continues to be the subject of some debate. In the present study we employed a stimulus design that enabled the simultaneous determination of bisection and vernier acuities in 36 subjects, aged between 22 and 84 years. This approach provided a means for directly testing the hypothesis that ageing affects bisection acuity but not vernier acuity by ensuring that differences in stimulus configuration and in the subject's task were kept to an absolute minimum. Optimum thresholds increased as a function of age for both bisection and vernier tasks. Inter-subject threshold variability also increased with age. Issues surrounding the comparison of absolute vernier thresholds across different studies are discussed and two important methodological factors are identified: the precise statistical method used to estimate thresholds, and the magnitude, in angular terms, of the smallest spatial offset of the elements of the vernier stimulus which can be displayed. Comparison with previously published data indicates that the discrepancy between this study and most previous investigations with respect to the effect of age upon vernier performance can be at least partly accounted for by differences in the minimum displayable vernier offset. Vernier thresholds do increase with age. The increased variability of vernier thresholds in older subjects would appear to limit the diagnostic value of the test as a means of enabling normal ageing to be distinguished from visual loss due to pathology of the eye or visual system.     

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 observed 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.     

Development of vernier acuity in infants

The development of vernier acuity in human infants aged 2-9 months was assessed by a preferential looking procedure using a vernier-motion display. The displacement of vernier offsets gives the impression of motion only when the vernier offsets are detected. Grating acuity of the same group of infants was also measured by a preferential looking procedure. Vernier acuity was found to be superior to grating acuity only after 3 months of age. This superiority of vernier acuity was compared with the superiority of stereoscopic acuity to grating acuity. The two classes of hyperacuity proved to be almost equivalent in terms of their developmental time-courses. A common physiological basis for the development of hyperacuities is suggested.     

Exposure duration affects the sensitivity of vernier acuity to target motion

Threshold vernier acuity was measured under different conditions of target movement and exposure duration. In the case of a simple two-line vernier target, image motion up to about 3 deg/sec had little effect upon threshold for a briefly exposed (150 msec) target, which is relatively poor even for a stationary stimulus, but produced a decrement in acuity for a continuously exposed stimulus. This finding was repeated in a second experiment, which used a centroid cue to vernier offset, and which compared the effects of horizontal and vertical target orientation. It is suggested that image motion and reduced exposure duration restrict the proportion of the light spread function that can be usefully sampled by the neural networks responsible for hyperacuity.     

Normal vernier acuity in infants with delayed visual maturation

PURPOSE: Measures of vernier acuity may correlate accurately with measures of Snellen acuity. We used the sweep visual evoked potential to determine whether vernier acuity is normal in children with delayed visual maturation (DVM). DESIGN: Case series of two infants. METHODS: Thresholds for grating and vernier acuity were measured electrophysiologically in two infants with DVM using the sweep visual evoked potential. Acuity thresholds were calculated for each of these visual functions. RESULTS: Two infants with DVM had normal thresholds for grating and vernier acuity, even though their visual behavior as determined clinically was abnormal. CONCLUSIONS: Some children with DVM have normal acuity thresholds, even though the behaviors associated with vision are delayed or absent.     

Preoperative visual acuity of people undergoing cataract surgery in rural and urban Nepal

Background: Preoperative presenting visual acuity (PPVA) is not a commonly reported indicator for the delivery of cataract services. This study aimed to evaluate the PPVA of patients undergoing cataract surgery in rural and urban areas of Nepal. Design: A prospective, non‐interventional study conducted from October 2007 to March 2008 in a tertiary hospital and outreach clinics of Nepal. Participants: A total of 3023 consecutive subjects from urban and rural settings (1323 from the tertiary hospital and1700 from outreach clinics) with cataract were included. Methods: Visual acuity was tested with a Snellen E‐chart at 6 m by ophthalmic assistants. All Outreach Microsurgical Eye Clinic patients were examined by either ophthalmologists or senior ophthalmic assistants. Patients at the Tertiary Eye Care Centre were examined only by ophthalmologists. Main Outcome Measures: Preoperative presenting visual acuity of patients undergoing cataract surgery was measured in both the settings. Results: The sex ratio was 0.96 (male/female). Based on PPVA, 11.5% of patients operated were blind (PPVA < 3/60 in the better eye). The proportion of blind people operated was similar in rural (11.4%) to urban (11.6%) areas. The proportion of blind eyes operated was higher in rural (50.7%) compared to urban (38.9%) areas. Conclusion: Despite a comparatively high volume of cataract surgery carried out in Nepal every year, only about one in eight patients operated for cataract in the sample population was blind. PPVA is an important indicator for the progress towards eliminating cataract blindness.     

The development of vernier acuity in the cat

The development of vernier acuity was assessed in nine kittens using offsets in a series of square-wave gratings as stimulus targets. The kittens were tested on a jumping stand in a two-choice simultaneous discrimination paradigm, and daily thresholds were tracked with a modified staircase procedure. Vernier acuity improved gradually from initial values of 13-57' arc and reached asymptote at 1.5-5.4' arc by 70-90 days of age. The developmental function for vernier acuity is discussed in relation to physiologic development of the kitten visual system and is related to published data on the development of stereoacuity and spatial resolution in the same species.     

Effects of contrast and length on vernier acuity explained with noisy templates

Vernier acuity depends on the integration of information from multiple photoreceptors. For this reason, vernier acuity thresholds ought to exhibit effects of stimulus size and contrast analogous to those that occur in area summation experiments. In this paper, we consider some area and contrast effects found in vernier acuity experiments, and explain them with a model of detection and discrimination which we call the Noisy Template model. The Noisy Template model assumes that psychophysical tasks are performed (or can be approximated) by cross-correlation of the stimulus with a decision template which is optimal for the task at hand. The Noisy Template model crucially adds the assumption that the template contains noise. This yields inefficiency in the decision process which increases with stimulus size and contrast. Predictions of the Noisy Template model are derived for the case of vernier acuity, and compared with existing experiments.     

Is reduced vernier acuity in amblyopia due to position,contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.     

Effects of spatial separation and retinal eccentricity on two-dot vernier acuity

Experiments investigated how vernier acuity for dot targets is affected by dot spatial separation and retinal eccentricity. Beck and Schwartz [Vision Res. 19, 313-319, 1979] found that the vernier thresholds increased linearly with interdot separation from 7.5 to 30 min arc. Experiment 1 showed that the vernier thresholds increased linearly with interdot separation from 0.5 to 8 deg arc. Experiments 2 and 3 showed that moving the dots into the periphery while keeping the interdot separation constant increased the threshold little if at all. The results are interpreted as supporting the hypothesis that there is (1) an encoding of dot positions in retinal coordinates, and (2) that the relative positions of the dots are made explicit in terms of the slope of the virtual line joining them. The increase in threshold with dot separation indicates that the visual system is unable to directly access and compare the retinal positions of the dots. The constancy of the threshold with eccentricity indicates that the visual system encodes the retinal positions of two well-separated dots as accurately in the periphery as in the fovea.     

Eccentric visual acuity in patients with macular disease

A series of cards each containing a two dimensional array of identical Snellen "E's" was used to determine best eccentric visual acuity in patients with macular disease having Snellen visual acuity of 20/70 or worse. Each "full field E" card simultaneously presents the same letter to foveal and parafoveal areas. This test can therefore determine quickly if potentially useful vision is present in any area of the central visual field. In our study of 37 eyes, 70% demonstrated potential visual acuity at least two times better than visual acuity measured by conventional methods, and 20% demonstrated at least a fourfold improvement. This suggests that most patients with macular disease do not spontaneously employ their best remaining area of retina for fixation.     

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.     

Potential vision testing – the relationship between visual acuity and Vernier acuity in the presence of simulated cataract

To evaluate the utility of a computer controlled two-bar Vernier acuity measurement as a predictor of visual function in the presence of cataract we measured logMAR visual acuity and Vernier acuity in a group of 40 young normal observers under various levels of dioptric blur (0-3 D in dioptre steps). The Vernier thresholds were resistant to dioptric blur up to 2 D, but performance degraded with blur of 3 D for non-optimised Vernier stimulus parameters. The stimulus parameters, bar length and bar separation, were further investigated in two subjects under conditions of blur. By extending the Vernier bar length and increasing the bar separation the effect of blur could be further reduced even under the most blurred condition. The relationship between visual acuity and Vernier acuity was determined. Vernier acuity was measured in the presence of Vistech cataract simulating lenses and a prediction of visual acuity was made for three observers, two with no ocular abnormality and one with age-related maculopathy. The cataract simulating lenses affected the measured visual acuity in all three subjects, but had less effect on Vernier acuity. Predicted visual acuities were all within six letters (0.12 log units) of the visual acuity without the simulated cataract. As expected, the subject sufferng from age-related maculopathy, whilst showing similar levels of Vernier acuity to the two ocularly healthy subjects at 1.5 degrees of retinal eccentricity, showed much poorer Vernier acuity for stimuli presented at fixation.     

A comparison of stereo and vernier acuity within spatial channels as a function of distance from fixation

We have used DOG stimuli to selectively stimulate size tuned channels within the visual system in order to investigate the relationship between stereo and vernier acuity. We measured these acuities as a function of spatial frequency, retinal eccentricity and distance from the fixation point in depth. Both hyperacuities are poorer with low spatial frequencies but vernier was effected to a smaller extent. Vernier acuity deteriorated to a much greater degree than stereo acuity as retinal eccentricity increased up to 40 arc min. Stereo acuity was more dependent upon distance from the fixation point in depth than would be expected from the dependence of vernier acuity on retinal eccentricity. We conclude that there must be different limiting factors for the two hyperacuity tasks.     

Visual acuity following treatment of bilateral congenital cataracts

Several studies have indicated that operation during the first months of life in children with dense congenital cataract improves the final visual acuity. In the current study seven otherwise healthy children operated on before the age of fifty-six days are compared with seven children operated on after the age of three months. The patients were followed by a team consisting of a paediatric ophthalmologist, a contact lens optician and an orthoptist. They were treated with contact lenses, spectacles with near addition and occlusion therapy when needed. Visual acuity was initially tested with preferential looking technique and later with Snellen optotypes. In the early treated group the visual development was almost normal with a final visual acuity of 20/20, while in the late treated group no patient obtained better visual acuity than 20/100. The findings indicate that dense congenital cataract should be treated before the age of three months.     

正常人眼黄斑色素密度与游标视力的关联研究

目的研究黄斑色素密度与游标视力之间的关系,探讨黄斑色素对视功能的影响。方法采用异色闪烁光度仪对116例正常成年人右眼进行黄斑色素密度测量。从中选出黄斑色素密度值大于0．90和小于0．40的受试者,将他们分为高黄斑色素密度组和低黄斑色素密度组。采用二项迫选法测量游标视力。采用t检验分析两组的游标视力,采用pearson correlation分析黄斑色素密度与游标视力的相关性。结果116例受试者右眼黄斑色素密度值为（0．64±0．30）logunit。高、低黄斑色素密度组的游标视力值分别为（8．35±2．45）分弧和（11．53±3．56）分弧,两组游标视力的差异具有统计学意义（t=-2．884,P=-0．008）。经相关性分析,黄斑色素密度与游标视力呈负相关（r=-0．401,P=0．028）。结论高黄癍色素密度组的游标视力优于低密度组．且黄斑色素密度与游标视力值呈负相关,黄斑色素密度值越高,游标视力值越低,游标视力越好。欲进一步证实黄斑色素密度与视功能之间的关系．则需要更多的相关研究。     

Vernier acuity in barn owls

Vernier acuity thresholds were obtained psychophysically in three adult barn owls with vertical bars and sinusoidal gratings. A minimal displacement threshold of 0.58 arcmin was observed with the bar stimulus under binocular viewing conditions. The mean binocular bar threshold was 2.51 arcmin. Bar thresholds were lower than grating thresholds. Monocular thresholds, obtained in one bird only, were typically higher than binocular thresholds. With grating acuity being about 3.75 arcmin in this species, we conclude that the findings reported here indicate that vernier acuity is hyperacute in the barn owl. The data presented here are the first demonstration of vernier acuity thresholds in birds.     

Elevated vernier acuity thresholds in glaucoma

PURPOSE: In 1993, Piltz et al. observed that foveal vernier acuity thresholds for achromatic targets are elevated in patients with glaucoma. This study was undertaken to explore whether such elevated thresholds are present when subject groups are measured with targets of effectively equivalent contrast. Vernier acuity measures were also obtained with short-wavelength and frequency-doubled stimuli, to assess spatial hyperacuity performance in the short-wavelength-sensitive and magnocellular pathways, respectively. METHODS: Twenty patients with glaucoma and 19 subjects with normal vision participated. All subjects had visual acuity of 20/25 or better. Achromatic two-dot vernier thresholds were obtained for 90% contrast dots. In addition, individual contrast thresholds to the achromatic dots were measured for each subject, and vernier thresholds were measured at 4, 8, 12, and 16 times contrast threshold. Short-wavelength vernier acuity thresholds were measured for blue dots presented on a bright yellow background. The stimulus for the frequency-doubling grating vernier acuity task was a 90% contrast, 1-cyc/deg, 25-Hz sinusoidal grating. RESULTS: The glaucoma group demonstrated significantly higher foveal vernier acuity thresholds than control subjects for the blue-on-yellow stimulus (P = 0.002) and frequency-doubling grating stimulus (P < 0.001). No significant difference in vernier acuity between groups was found for the 90% contrast achromatic dots (P = 0.09), however a significant difference was found for the normalized contrast targets (P = 0.04). CONCLUSIONS: Vernier acuity tasks can be used to demonstrate abnormal foveal function in glaucoma. Testing with visual-function-specific stimuli may be effective in identifying such dysfunction. Vernier acuity, or other similar hyperacuity tasks that assess spatial sampling, may be useful in the detection of early glaucomatous damage, before it is detected with traditional perimetric tests.     

Retinal acuity evaluation with the potential acuity meter in glaucoma patients

The potential acuity meter (PAM) is designed to evaluate retinal acuity in the presence of media opacities. We looked at patients with glaucoma but with clear media, and compared best corrected visual acuity with PAM results to see if they produced comparable results. Sixty eyes in 38 glaucoma patients and 20 eyes in 10 normal ocular patients were evaluated. Our results indicate that PAM visual acuity is a reliable indicator of Snellen visual acuity in normal eyes, in eyes with mild to moderate glaucomatous damage, and when PAM visual acuity measurements were better than 20/60. However, when visual field loss is severe and when PAM visual acuity readings were worse than 20/60, the correspondence between these and Snellen visual acuity was erratic. Poor PAM results will not correlate with postoperative visual acuity in patients with advanced glaucoma and cataracts.