Performance on three-point vernier acuity targets as a function of age

Visuospatial interpolation in typically developing children and in people with Williams Syndrome

Visuospatial interpolation is the estimation of object position or contour shape computed from known "anchor" positions. We characterized the developmental profile of interpolation by measuring positional thresholds as a function of inter-element separation without (Experiment 1) and with (Experiment 2) the context of illusory contours in typically developing children, typical adults and individuals with Williams Syndrome (WS), a genetic disorder that causes impaired global visuospatial abilities. We found that typically developing children and WS individuals had more difficulty integrating information across distant elements than typical adults. However, illusory contours improved thresholds in all participant groups in a similar way. Our results suggest that in WS individuals, and in typically developing children, the grouping mechanisms that enable long-range spatial integration are immature. We hypothesize that WS individuals and young children can use stimulus-driven grouping cues for bottom-up integration, but have immature mechanisms for top-down integration of spatial information.

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.

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.

Aging effects on vernier hyperacuity: a function of oscillation rate but not target contrast

PURPOSE

Most previous studies have shown static vernier acuity for high-contrast targets to be largely unaffected by aging; those examining adult age differences on oscillatory displacement thresholds have consistently reported marked age deficits. The goals of this study were to (1) measure the age deficit on oscillatory discrimination beyond that attributable to any change in spatial discrimination by using the same target configuration for both task types and (2) determine whether an age-related change in the contrast response of the visual system contributes to age differences on static or oscillatory discrimination.

METHODS

The displacement thresholds of young and old observers for a vernier task configuration were determined at two target contrast levels (0.08 and 0.64) for static and oscillating targets (2 and 6 Hz)

RESULTS

No age differences were seen on static displacement thresholds at either high or low contrast. A marked age deficit that emerged when oscillation was increased was unrelated to target contrast or observer contrast sensitivity.

CONCLUSIONS

Age-related declines in oscillatory discrimination beyond those attributable to spatial discrimination do not appear to be attributable to optical factors nor to a decline in the contrast response of the senescent visual system. These findings are discussed in terms of a functional decline in the magnocellular pathway, or "neural entropy," possibly due to random cell loss.

Variation in vernier acuity with age

Many visual functions deteriorate with increasing age, vernier acuity apparently being an exception. We used an improved experimental protocol to investigate vernier thresholds in subjects in four age groups. Vernier threshold was found to increase with increasing age, and the difference in this, compared with previous findings is probably due to the smaller inter-pixel angle used. Vernier acuities in young subjects in other studies seem to have been underestimated. As vernier acuity did not vary with retinal illuminance and the sensitivity loss does not seem to be a learning effect, neural rather than optical changes are likely to account for the deterioration. Measurement of vernier acuity may have clinical application in the determination of visual function behind cataract, and it will be necessary to consider the effect of age on vernier acuity.

Performance on the three-point Vernier alignment or acuity test as a function of age: measurement extended to ages 5 to 9 years

The three-point Vernier alignment (or acuity) test was conducted on children from 5 to 9 years old. There is a significant difference between the 5-9 year old subjects and those in age groups 10 to 19 and 20 to 29 years in Vernier performance. These data were also compared with previously published data from older subjects (up to age 70). We conclude that Vernier function has not fully matured within the age range of 5 to 9 years; this finding is consistent with previous results reported in the literature.

Vernier acuity with compound gratings: the whole is equal to the better of its parts

To evaluate the relative importance of local feature and spatial filter models for hyperacuity, vernier thresholds were determined for gratings consisting of a fundamental (Fo) and third harmonic (3F) presented alone, and added together in compound gratings where the relative phase offsets of 3F to Fo was 0, 90, 180 and 270 degrees. Thresholds were determined for a range of spatial frequencies of Fo (0.5-16 c deg-1) for abutting and non-abutting stimuli. Compound grating vernier performance was found to be: (i) invariant with relative phase offset for the abutting and non-abutting conditions; and (ii) predictable from the vernier thresholds for the individual grating components making up the compound stimulus. The results support a view that supra-threshold components in a multi-frequency stimulus act independently and it is the spatial frequency content, not the local feature characteristics, which limit vernier performance.

Fixation disparity and nonius bias

Fixation disparity, i.e. the vergence error within Panum's area, can be measured psychophysically with two nonius (vernier) lines that are presented dichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates the amount of fixation disparity. The present experiments investigate the relation between fixation disparity and the nonius bias, which is the physical offset of the nonius lines that is adjusted by the observer in order to perceive them as aligned when both nonius lines are presented to both eyes (binocular nonius bias) or both to the left or both to the right eye (monocular nonius bias). It was found that (1) the fixation disparity is correlated with the binocular nonius bias in the horizontal and vertical meridian and (2) the binocular nonius bias can be predicted from the average of the right eye and left eye monocular nonius bias. To remove the influence of the nonius bias on measured fixation disparity it is possible to calculate the fixation disparity relative to the individual binocular nonius bias, rather than to the physical coincidence of the nonius lines. This procedure tends to increase the correlation between fixation disparity and the tonic resting position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compared to physical recordings of eye position.

No effect of age on spatial interval discrimination as a function of eccentricity or separation

PURPOSE

Performance on positional acuity tasks exhibits marked resistance to the effects of optical image degradation. This places positional acuity tasks in a unique position for psychophysical examination of the effects of age upon visual performance because any observed age-related changes reflect losses in retinal/neural function. One positional acuity task with important consequences for "real-world" vision is spatial interval discrimination, a task in which the subject is required to detect changes in the size of the gap between two objects. In the present study we examine spatial interval discrimination in young and elderly observers as a function of separation and eccentricity.

METHODS

Stimuli were two Gaussian-modulated luminance patches placed side by side around an imaginary iso-eccentric arc in the upper visual field, allowing eccentricity and separation to be varied independently. Changes in separation were achieved by moving the stimuli around the arc, while eccentricity was varied by changing the radius of the arc. Thresholds were obtained for healthy young and elderly observers using a forced-choice method of constants at two eccentricities (1.25 degrees and 10 degrees) and five separations.

RESULTS

When thresholds, expressed as Weber fractions, are plotted as a function of the geometric ratio of the stimuli (separation/eccentricity) the data from the young and elderly groups collapse to a single function. Performance is independent of age and eccentricity, and depends only on the geometric ratio of the stimuli.

CONCLUSIONS

No effect of age was found for spatial interval discrimination. Our results suggest that spatial interval discrimination belongs to the wider group of positional acuities whose neural substrates are unaffected by ageing.

Vernier acuity and aging

PURPOSE/BACKGROUND

Many visual functions change with age. Hyperacuity vernier thresholds, however, seem to remain practically invariant with age.

METHODS

We critically review reports on age-related changes in hyperacuity thresholds and present new data using two and three-point vernier hyperacuity thresholds.

RESULTS

It appears as though the vernier (two dot or two line) alignment hyperacuity thresholds are not sensitive to age.

SIGNIFICANCE

Based on these studies we propose that hyperacuity (vernier) thresholds be used as a benchmark, or a gold standard for aging studies. This measurement can serve as a valuable control and reference for other clinical studies of aging.