A new source of variance in visual acuity

In an ocularly healthy population, decimal visual acuities range from 1 to 3. We wondered how this spread can be understood. Using a maximum likelihood adaptive procedure, "visual acuity" was measured in a healthy population with four stimulus sets: unblurred Landolt Cs and Cs blurred with a Gaussian of width sigma = 2.12', 4.24' and 8.48'. A simple model based on scale invariance of the visual system was applied. This model was tested by predicting the outcomes of the 2.12' measurements based on the other measurements. The minimum angle of resolution (MAR) values found are closely proportional to "equivalent blur" of the stimulus defined as the convolution of a value for intrinsic blur of the eye and added blur. The proportionality factor is different between individuals and is an important source for the spread in acuities found in a healthy population. The differences between the proportionality factors are interpreted as differences in the (neuronal) judgment capability between individuals. The total standard deviation of log(MAR) found in our study was 0.11. This value can be subdivided in 0.06 for the (neuronal) judgment capability, 0.08 for the intrinsic retinal blur and a measurement accuracy of 0.04.     

The effect of refractive surgery on blur thresholds

Purpose:The aim of this study was to measure blur thresholds before and after refractive surgery.Methods:In this prospective cohort study conducted in a tertiary eye hospital in South India. Blur thresholds were measured for 30 young adult myopic patients 1 month prior to and after refractive surgery. Patients were asked to report three stages of blur, namely Detectable Blur (DB), Bothersome Blur (BB), and Non-resolvable Blur (NB). Blur was created by adding plus lenses (in steps of 0.12D) over their optimal subjective refraction. The blur judgments were made both monocularly and binocularly when looking through a 3 mm artificial pupil at one line above the best-corrected visual acuity.Results:A total of 30 participants were included in this study (mean age = 25.5 ± 3.8 (20–36) years; 77% female). The mean binocular preoperative blur of this group was: DB = 0.39 ± 0.26D, BB = 0.74 ± 0.28D and NB = 1.04 ± 0.42D. The corresponding mean binocular blur one-month post-operatively was DB = 0.46 ± 0.28D, BB = 0.83 ± 0.35D, and NB = 1.21 ± 0.44D. Although there was a marginal increase in the blur thresholds postoperatively, the difference was not statistically significant (DB: P = 0.320; BB: P = 0.229; NB: P = 0.054).Conclusion:All three blur thresholds showed an insignificant minimal increase at 1 month post-operatively suggesting that patients adapt to the induced blur following refractive surgery. A longer follow up would reveal how the adaptation to blur would change with time.     

Equivalent intrinsic blur in spatial vision

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line “resolution”; (ii) line detection; and (iii) spatial interval discrimination, in both central and peripheral vision. The results of our experiments can be summarized as follows.

(i) 2-Line “resolution”: thresholds for pairs of unblurred, low contrast, stimuli are approx. 0.5min arc in the fovea. When the stimulus blur is small, it has little effect upon 2-line “resolution”; however, when the stimulus blur, σ, exceeds 0.5 min, thresholds are degraded. We operationally define this transition point as the equivalent intrinsic blur or B_i. When the standard deviation of the stimulus blur, σ, is greater than B_i, then the “ resolution” threshold is approximately equal to σ. Both the unblurred “resolution” threshold, and the equivalent intrinsic blur, B_i, vary with eccentricity in a manner consistent with the variation of cone separation within the central 10 deg. When the stimulus blur exceeds the equivalent intrinsic blur, “resolution” in the periphery is the same as in the fovea.

(ii) Line detection: when the standard deviation of the stimulus blur, σ, is less than B_i, then the line detection threshold is approximately inversely proportional to σ (it is≈ T_dB_i/gs) i.e. it obeys Ricco's law. When the standard deviation of the stimulus blur, σ, is greater than B_i, then the “resolution” threshold is approximately equal to σ and the detection threshold is approximately a fixed contrast (to be referred to as T_d).

According to (i) and (ii), the equivalent intrinsic blur, B_i, plays a dual role in determining both the “resolution” threshold and the detection threshold, B_i corresponds to the “Ricco's diameter” for spatial summation in a detection task, and it also corresponds to the “resolution” threshold for thin lines. This connection between detection and “resolution” is somewhat surprising.

(iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, the thresholds represent a “hyperacuity” (i.e. they are smaller than the “resolution” threshold). For unblurred lines, the optimal separation is approximately 2–3 times the “resolution” limit at all eccentricities, so the optimal separation varies with eccentricity at the same rate as the equivalent intrinsic blur, B_i. However, the optimal spatial interval threshold falls off with eccentricity about 3–4 times more rapidly, consistent with the rate of decline of other position acuity tasks. For Gaussian blurred lines, over a wide range of separations and eccentricities, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds between about⅓ and ½ the separation of the lines. The strong elevation of the optimal spatial interval discrimination threshold in the periphery cannot be predicted on the basis of detectability of the lines, “resolution”, or on the basis of the equivalent intrinsic blur. We hypothesize that the increased spatial interval discrimination thresholds are a consequence of position uncertainty, perhaps due to sparse spatial sampling in the periphery.

Aging effects on the identification of digitally blurred text,scenes and faces: Evidence for optical compensation on everyday tasks in the senescent eye

When older observers are defocused optically to the same reduced acuity levels as their younger adult counterparts they are better able to read distant text. This study sought to determine if this ability extended to intrinsically blurred (i.e., image-processed) stimuli of different types. Such an outcome suggests an explanation based on older persons’ greater experience with blurred stimuli; otherwise, one attributable to compensatory changes in the optic media of the older eye would be favored. Twelve young and 12 old healthy community-resident adult observers with excellent acuity were compared on their ability to identify low-pass filtered real words, nonsense words, scenes, and faces arranged in a sequence of decreasingly blurred images. Young observers were able to identify the images correctly earlier in the blur sequence than older adult observers, significantly so for both real and nonsense words. This finding suggests that compensatory changes in the eye’s optical media rather than older observers’ greater experience with blur accounts for their superior legibility performance with optically defocused text. While the image-enhancing effects of the age-related decline in pupil size (senile miosis) may be involved, further research is needed to clarify the mechanism(s) underlying this ability. Paul Bartel received his B.Sc. (Honours) in Psychology (Vision and Aging Lab) and the Bachelor of Social Work degrees from the University of Calgary. He is currently the Director of Community Programs at the North Central Community Resource Centre in Calgary. Donald W. Kline is a professor of psychology and the Director of the Vision and Aging Lab (PACE Program) at the University of Calgary. His research interests include the neural and optical mechanisms of visual aging, visual human factors, and visual health, eye surgery and quality of life. This research was supported by a grant to D. Kline (No. OGP0046593) from the Natural Sciences and Engineering Research Council of Canada (NSERC).     

运动模糊图象中运动方向的自动检测算法

匀速直线运动模糊图象中运动方向的检测是图象恢复过程的重要步骤之一。对运动模糊图象实施方向微分 ,根据方向微分图像灰度值 (绝对值 )分布的特点 ,首先给出了计算任意角度的方向微分的公式 ,然后将微分图象灰度值 (绝对值 )通过灰度线性变换 ,求取和值中的极大值来自动且有效地识别出运动方向 ,并通过实验验证     

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.     

Conceptual model of human blur perception

An empirically based, conceptual model of human blur perception is presented. It incorporates the concepts of blur detection and blur discrimination in depth, and across the central and peripheral retina, in two- and three-dimensional visual space. Key aspects of the model are its dynamic nature, predictability regarding the blur-based depth-ordering of objects, patterns of retinal defocus with far and near viewing, and interactions related to retinal defocus between the central and peripheral retina. Furthermore, a two-dimensional schematic representation of the blur-free region during near viewing is depicted in dioptric space. This model has implications with respect to accommodative control, depth perception, and refractive error development and progression.     

Equivalent intrinsic blur in amblyopia

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line resolution; (ii) line detection; and (iii) spatial interval discrimination, in observers with amblyopia due to anisometropia, strabismus, or both. The results of our experiments can be summarized as follows.

(i) 2-Line resolution: in normal foveal vision, thresholds for unblurred stimuli are approx. 0.5 min arc in the fovea. When the standard deviation (σ) of the stimulus blur is less than 0.5 min, it has little effect upon 2-line resolution; however, thresholds are degraded when the stimulus blur, σ, exceeds 0.5 min. We operationally define this transition point, as the equivalent intrinsic blur, or B_i. When the stimulus blur, σ, is greater than B_i, then the resolution threshold is approximately equal to σ. In all of the amblyopic eyes, 2-line resolution thresholds for unblurred stimuli were elevated, and the equivalent intrinsic blur was much larger. When the stimulus blur exceeds the equivalent intrinsic blur, resolution thresholds were similar in amblyopic and nonamblyopic eyes.

(ii) Line detection: in both normal and amblyopic eyes, when the stimulus blur, σ, is less than B_i, then the line detection threshold is approximately inversely proportional to σ; i.e. (it obeys Ricco's law). When σ is greater than B_i, the equivalent intrinsic blur, then the detection threshold is approximately a fixed contrast. All of the amblyopic eyes showed markedly elevated thresholds for detecting thin lines, but normal or near normal thresholds for detecting very blurred lines. Consquently, Ricco's diameter is larger in amblyopic than in normal eyes.

(iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds represent a “hyperacuity” (i.e. they are smaller than the resolution threshold). For unblurred lines, the optimal separation is approx. 2–3 times B_i. In the normal fovea, and in the amblyopic eyes of anisometropic amblyopes the optimal spatial interval discrimination threshold is about one-fifth of the resolution threshold (i.e. a hyperacuity); and over a wide range of separations, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds about one-third of the separation between the lines as long as the contrast is sufficiently high. In contrast, in strabismic amblyopes, like the normal periphery, the optimal spatial interval discrimination thresholds are worse (higher) than would be expected based upon the resolution limit of the strabismic amblyopic eye.

In anisometropic amblyopes the elevated resolution and spatial interval discrimination thresholds are consistent with a raised level of equivalent intrinsic blur, and a reduced contrast response function. In strabismic amblyopia, there appears to be an additional source of loss, which affects spatial localization to a greater degree than resolution. This extra loss may be modeled in terms of abnormal positional uncertainty due to a sparse cortical spatial sampling grain.     

Detection of the depth order of defocused images

The sign of an accommodative response is provided by differences in chromatic aberration between under- and over-accommodated images. We asked whether these differences enable people to judge the depth order of two stimuli in the absence of other depth cues. Two vertical edges separated by an illuminated gap were presented at random relative distances. Exposure was brief, or prolonged with fixed or changing accommodation. The gap was illuminated with tungsten light or monochromatic light. Subjects could detect image blur with brief exposure for both types of light. But they could detect depth order only in tungsten light with long exposure, with or without changes in accommodation.