The Perception of Blur

ABSTRACT The perception of blur is described and discussed in detail. It is noted that an out-of-focus border does not present a smooth continuum from bright to dark but consists of a series of lines and bands. This characteristic of perceived blur is explained on the basis of two factors. Firstly, aberrations will produce discontinuities in the out-of-focus retinal image correlate of a border. Secondly, Mach effects will amplify these discontinuities so that a series of dark and bright bands are seen in lieu of the border.     

屈光不正眼弥散斑变化的实验研究

提出一种应用激光技术测定屈光不正眼弥散斑变化的方法,并将其初步应用结果与理论计算值进行了比较对照。结果表明:屈光不正眼弥散斑随眼离焦量的增加而增大。比较理论值和实测值,两者间存有一定差异。认为实测值比理论值更能准确地评价屈光不正眼的成象质量。     

不同屈光状态的模糊敏感度比较

目的分析不同屈光组模糊阈值之间的差异,探讨模糊阈值在近视发生发展中的可能作用机制。方法38名志愿者参加本实验,正视组15位,等效球镜度为0.50～-0.50D;近视组23位,其中稳定性近视组17位,屈光不正平均为(-4.76±1.40)D,进展性近视组6位,屈光不正平均为(-3.33±1.43)D。受试者在屈光全矫基础上,采用JND标准测量模糊阈值。结果正视组模糊阈值为(0.21±0.06)D,稳定性近视组为(0.27±0.05)D,进展性近视组为(0.33±0.06)D。近视组的模糊阈值高于正视组(P=0.001),近视组中,进展性近视组的模糊阈值高于稳定性近视组(P=0.028)。结论近视对模糊的敏感度下降,其中进展性近视的敏感度下降更显著,这可能是近视调节滞后增加的部分原因。     

Retinal Responses to Simulated Optical Blur Using a Novel Dead Leaves ERG Stimulus

PurposeThe purpose of this study was to evaluate retinal responses to different types and magnitudes of simulated optical blur presented at specific retinal eccentricities using naturalistic images.MethodsElectroretinograms (ERGs) were recorded from 27 adults using 30-degree dead leaves naturalistic images, digitally blurred with one of three types of optical blur (defocus, astigmatism, and spherical aberrations), and one of three magnitudes (0.1, 0.3, or 0.5 µm) of blur. Digitally computed blur was applied to the entire image, or on an area outside the central 6 degrees or 12 degrees of retinal eccentricity.ResultsERGs were significantly affected by blur type, magnitude, and retinal eccentricity. ERGs were differentially affected by defocus and spherical aberrations; however, astigmatism had no effect on the ERGs. When blur was applied only beyond the central 12 degrees eccentricity, the ERGs were unaffected. However, when blur was applied outside the central 6 degrees, the ERG responses were significantly reduced and were no different from the ERGs recorded with entirely blurred images.ConclusionsBlur type, magnitude, and location all affect the retinal responses. Our data indicate that the retinal area between 6 and 12 degrees eccentricity has the largest effect on the retinal responses to blur. In addition, certain optical blur types appear to have a more detrimental effect on the ERGs than others. These results cannot be solely explained by changes to image contrast and spatial frequency content, suggesting that retinal neurons might be sensitive to spatial cues in order to differentiate between different blur types.     

Depth-dependent blur adaptation

Variations in blur are present in retinal images of scenes containing objects at multiple depth planes. Here we examine whether neural representations of image blur can be recalibrated as a function of depth. Participants were exposed to textured images whose blur changed with depth in a novel manner. For one group of participants, image blur increased as the images moved closer; for the other group, blur increased as the images moved away. A comparison of post-test versus pre-test performances on a blur-matching task at near and far test positions revealed that both groups of participants showed significant experience-dependent recalibration of the relationship between depth and blur. These results demonstrate that blur adaptation is conditioned by 3D viewing contexts.     

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.     

Peripheral Defocus and Myopia Management: A Mini-Review

Myopia is the most common refractive error in the world, and its’ prevalence continually increases. The potential pathological and visual complications of progressive myopia have inspired researchers to study the sources of myopia, axial elongation, and explore modalities to arrest progression. Considerable attention has been given over the past few years to the myopia risk factor known as hyperopic peripheral blur, the focus of this review. The primary theories currently believed to be the cause of myopia, the parameters considered to contribute and influence the effect of peripheral blur, such as the surface retinal area or depth of blur will be discussed. The currently available optical devices designed to provide peripheral myopic defocus will be discussed, including bifocal and progressive addition ophthalmic lenses, peripheral defocus single vision ophthalmic lenses, orthokeratology lenses, and bifocal or multifocal center distance soft lenses, as well as their effectivity as mentioned in the literature to date.     

Static accommodative responses following adaptation to differential levels of blur

PURPOSE: To investigate the effects of two levels of blur adaptation on visual resolution and steady-state accommodation responses in emmetropes and myopes. METHODS: Eleven emmetropes (mean refractive error +0.01 +/- 0.31 DS) and 11 early-onset myopes (EOM, mean refractive error -4.44 +/- 1.64 DS) fixated monocularly at 4 m in three trials of 45 min duration with either: optimal refractive correction, +1 DS defocus, or +3 DS defocus. Monocular logMAR visual acuity (VA) was measured at 10 min intervals during each trial, and immediately following completion of the trial. Accommodative stimulus-response function (ASRF), refractive error and pupil size were measured before and after each trial. RESULTS: Blur adaptation was found to have no effect on pupil size or baseline refraction, irrespective of the power of the blurring lens. Adaptation to +1 DS of defocus yielded an improvement in VA of -0.16 +/- 0.07 logMAR and -0.17 +/- 0.11 logMAR in the emmetropes and myopes respectively. An improvement in VA of -0.20 +/- 0.18 logMAR in the emmetropes and -0.26 +/- 0.17 logMAR in the myopes was observed following adaptation to +3 DS of defocus. The changes in acuity became significant following 30 min of exposure to defocus. Blur adaptation was found to have no effect on the ASRF gradient or individual steady-state accommodative responses. CONCLUSIONS: Following blur adaptation, visual resolution was found to increase in both emmetropes and myopes. The magnitude of the blur level did not produce significantly different increases in resolution. Blur adaptation failed to affect either the steady-state responses to an accommodative stimulus or ASRF gradient.     

Blur adaptation and myopia.

Previous studies have demonstrated a significant improvement in visual resolution during sustained periods of retinal defocus. This appears to result from perceptual adaptation designed to restore the perceived contrast of the degraded image. However, it is unclear whether perceptual adaptation to sustained blur is present in all individuals or only in certain subgroups, such as those who have been chronically exposed to sustained periods of blur due to uncorrected ametropia. Accordingly, the present study examined the effects of sustained retinal defocus on both high-and low-contrast visual acuity in emmetropes (n = 13) and myopes (n = 18). Subjects were required to view through +2.50-D spherical lenses worn over their distance refractive correction for a continuous 2-hour period. A significant improvement in both Landolt C and grating visual acuity measured through the fogging lenses was observed in both refractive groups. Although the mean change in grating visual acuity was significantly greater for the myopic subjects, the improvements in Landolt C acuity observed in the emmetropes and myopes were statistically equivalent. We hypothesize that the improvement in visual acuity results from perceptual adaptation to the blurred images, which may occur at central sites within the visual cortex.     

Effect of blur adaptation on blur sensitivity in myopes

Although blur adaptation in myopia has been investigated, knowledge regarding its effect on blur sensitivity remains unknown. In the present study, changes in three blur thresholds (i.e., noticeable, bothersome, and non-resolvable blur) were assessed monocularly after 1h of blur adaptation in myopes. A Badal optical system was used to present either an isolated 20/50 Snellen E or 20/50 lines of text, with the full text field used in the latter condition for all blur judgments. Eight visually normal adult myopes were tested with paralyzed accommodation. All subjects exhibited blur adaptation, with a significant improvement in group mean visual acuity of -0.16 LogMAR. There was a consistent and concurrent significant decrease of 0.15-0.19 D in all blur thresholds for the isolated 20/50 E. However, there was no significant effect of blur adaptation on blur thresholds for the 20/50 text, with large intersubject variability evident. The enhanced blur sensitivity for the isolated E target may in part be attributed to the increased visual resolution following blur adaptation. Differences found in the blur thresholds for the two targets may be related to a variety of neuroperceptual phenomena, in particular lateral masking.     

The effect of retinal defocus on golf putting.

The purpose of this experiment was to determine the effect of type and magnitude of retinal defocus on golf putting accuracy, and on the related eye, head, and putter movements. Eye, head, and putter movements were assessed objectively along with putting accuracy in 16 young adult, visually normal inexperienced golfers during a fixed 9-foot golf putt. Convex spherical (+0.50 D, +1.00 D, +1.50 D, +2.00 D, +10.00 D) and cylindrical (+1.00 D x 90, +2.00 D x 90) lenses were added binocularly to create various types and magnitudes of retinal defocus. Putting accuracy was significantly reduced only under the highest spherical blur lens condition (+10.00 D). No significant differences were found between any other lens conditions for eye, head or putter movements. Small amounts of spherical and astigmatic retinal defocus had a minimal impact on overall golf putting performance, except for putting accuracy under the highest blur condition. This is consistent with the findings of related studies. For a fixed putting distance, factors other than quality of the retinal image, such as blur adaptation and motor learning, appeared to be sufficient to maintain a high level of motor performance.     

Blur Representation in the Amblyopic Visual System Using Natural and Synthetic Images

PurposeAmblyopia is diagnosed as a reduced acuity in an otherwise healthy eye, which indicates that the deficit is not happening in the eye, but in the brain. One suspected mechanism explaining these deficits is an elevated amount of intrinsic blur in the amblyopic visual system compared to healthy observers. This “internally produced blur” can be estimated by the “equivalent intrinsic blur method”, which measures blur discrimination thresholds while systematically increasing the external blur in the physical stimulus. Surprisingly, amblyopes do not exhibit elevated intrinsic blur when measured with an edge stimulus. Given the fundamental ways in which they differ, synthetic stimuli, such as edges, are likely to generate contrasting blur perception compared to natural stimuli, such as pictures. Because our visual system is presumably tuned to process natural stimuli, testing artificial stimuli only could result in performances that are not ecologically valid.MethodsWe tested this hypothesis by measuring, for the first time, the perception of blur added to natural images in amblyopia and compared discrimination performance for natural images and synthetic edges in healthy and amblyopic groups.ResultsOur results demonstrate that patients with amblyopia exhibit higher levels of intrinsic blur than control subjects when tested on natural images. This difference was not observed when using edges.ConclusionsOur results suggest that intrinsic blur is elevated in the visual system representing vision from the amblyopic eye and that distinct statistics of images can generate different blur perception.     

Neuronal adaptation to simulated and optically-induced astigmatic defocus

Purpose

It is well established that spatial adaptation can improve visual acuity over time in the presence of spherical defocus. It is less well known how far adaptation to astigmatic defocus can enhance visual acuity. We adapted subjects to “simulated” and optically-induced “real” astigmatic defocus, and studied how much they adapt and how selective adaptation was for the axis of astigmatism.

Methods

Ten subjects with a mean age of 26.7 ± 2.4 years (range 23-30) were enrolled in the study, three of them myopic (average spherical equivalent (SE) ± SD: −3.08 ± 1.42D) and seven emmetropic (average SE ± SD: −0.11 ± 0.18D). All had a corrected minimum visual acuity (VA) of log VA 0.0. For adaptation, subjects watched a movie at 4 m distance for 10 min that was convolved frame-by-frame with an astigmatic point spread function, equivalent to +3D defocus, or they watched an unfiltered movie but with spectacle frames with a 0/+3D astigmatic trial lenses. Subsequently, visual acuity was determined at the same distance, using high contrast letter acuity charts. Four experiments were performed. In experiment (1), simulated astigmatic defocus was presented both for adaptation and testing, in experiment (2) optically-induced astigmatic defocus was presented both for adaptation and testing of visual acuity. In all these cases, the +3D power meridian was at 0°. In experiments (3) and (4), the +3D power meridian was at 0° during adaptation but rotated to 90° during testing. Astigmatic defocus was simulated in experiment (3) but optically-induced in experiment (4).

Results

Experiments 1 and 2: adaptation to either simulated or real astigmatic defocus increased visual acuity in both test paradigms, simulated (change in VA 0.086 ± 0.069 log units; p < 0.01) and lens-induced astigmatic defocus (change in VA 0.068 ± 0.031 log units; p < 0.001). Experiments 3 and 4: when the axis was rotated, the improvement in visual acuity failed to reach significance, both for simulated (change in VA 0.042 ± 0.079 log units; p = 0.13) and lens-induced astigmatic defocus (change in VA 0.038 ± 0.086 log units; p = 0.19).

Conclusions

Adaptation to astigmatic defocus occurs for both simulated and real defocus, and the effects of adaptation seem to be selective for the axis of astigmatism. These observations suggest that adaptation involves a re-adjustment of the spatial filters selectively for astigmatic meridians, although the underlying mechanism must be more complicated than just changes in shapes of the receptive fields of retinal or cortical neurons.     

Visual regulation of refractive development: insights from animal studies

Investigations employing animal models have demonstrated that ocular growth and refractive development are regulated by visual feedback. In particular, lens compensation experiments in which treatment lenses are used to manipulate the eye''s effective refractive state have shown that emmetropization is actively regulated by signals produced by optical defocus. These observations in animals are significant because they indicate that it should be possible to use optical treatment strategies to influence refractive development in children, specifically to slow the rate of myopia progression. This review highlights some of the optical performance properties of the vision-dependent mechanisms that regulate refractive error development, especially those that are likely to influence the efficacy of optical treatment strategies for myopia. In this respect, the results from animal studies have been very consistent across species; however, to facilitate extrapolation to clinical settings, results are presented primarily for nonhuman primates. In agreement with preliminary clinical trials, the experimental data show that imposed myopic defocus can slow ocular growth and that treatment strategies that influence visual signals over a large area of the retina are likely to be most effective.     

Foveal blur discrimination of the human eye

Although the effect of retinal defocus on the foveal blur detection threshold has been well investigated, knowledge regarding the foveal blur discrimination threshold is limited. In the present study, both thresholds were assessed psychophysically using the ascending method of limits at the fovea with accommodation paralyzed. The unidirectional blur detection threshold was 0.87+/-0.18 D (+/-1 S.E.M.). The subsequent blur discrimination thresholds were relatively constant and significantly smaller than the blur detection threshold, with an average value of 0.48+/-0.006 D (+/-1 S.E.M.). We speculate that the difference in magnitude between these two thresholds may be attributed to the defocus-related change in the ocular modulation transfer function (MTF) and its interaction with contrast discrimination ability, as well as to the presence of a neuroperceptual blur buffering mechanism.     

Effect of defocusing and of distracted attention upon recordings of the visual evoked potential

Pattern reversal visual stimuli are used to evoke potentials (VEPs) for assessment of visual acuity and for localizing defects along the visual pathways. Our goal was to assess the importance of attention and defocusing to the recordings of pattern VEP. Forty-one volunteers with normal (6/6) corrected visual acuity participated in this study. Twenty-one were asked to defocus intentionally the visual stimulus (located 200cm away) by fixating at a target 25 or 50cm from the eye. Twenty other subjects performed auditory tasks to distract their attention from the visual stimulus. Pattern VEPs were elicited by different check sizes. The amplitude and time-to-peak of the P₁₀₀ wave were measured. Intentional defocusing caused amplitude reduction and prolongation of the time-to-peak in young subjects (20–34years old). With the smallest checks used (7.5) we could not record a reliable response from 43 of the young subjects (6 out of 14). In older patients (35–61years old), intentional defocusing induced negligible effects on pattern VEPs regardless of check size. There were no effects of auditory distraction upon the pattern VEPs. Our data suggest that intentional defocusing can produce false positive results (reduced VEP with prolonged time-to-peak) only when small checks are used in young subjects. Divided attention has negligible effect on the recordings of pattern VEPs. With proper controls, the pattern VEP test can be used for objective assessment of visual function.     

Emmetropic,But Not Myopic Human Eyes Distinguish Positive Defocus From Calculated Blur

PurposeDefocus blur imposed by positive lenses can induce hyperopia, whereas blur imposed by diffusers induces deprivation myopia. It is unclear whether the retina can distinguish between both conditions when the magnitude of blur is matched.MethodsTen emmetropic (average 0.0 ± 0.3 diopters [D]) and 10 subjects with myopia (−2.7 ± 0.9 D; 24 ± 4 years) watched a movie on a large screen (65 inches at 2 meters (m) distance. The movie was presented either unfiltered (“control”), with calculated low-pass filtering equivalent to a defocus of 2.5 D, or with binocular real optical defocus of +2.5 D. Spatial filtering was done in real-time by software written in Visual C++. Axial length was followed with the Lenstar LS-900 with autopositioning system.ResultsWatching unfiltered movies (“control”) caused no changes in axial length. In emmetropes, watching movies with calculated defocus caused axial eye elongation (+9.8 ± 7.6 µm) while watching movies with real positive defocus caused shorter eyes (−8.8 ± 9.2 µm; difference between both P < 0.0001). In addition, in myopes, calculated defocus caused longer eyes (+8.4 ± 9.0 µm, P = 0.001). Strikingly, myopic eyes became also longer with positive defocus (+9.1 ± 11.2 µm, P = 0.02). The difference between emmetropic and myopic eyes was highly significant (−8.8 ± 9.2 µm vs. +9.1 ± 11.2 µm, respectively, P = 0.001).Conclusions(1) In emmetropic human subjects, the retina is able to distinguish between real positive defocus and calculated defocus even when the modulation transfer function was matched, (2) in myopic eyes, the retina no longer distinguishes between both conditions because the eyes became longer in both cases. Results suggest that the retina in a myopic eye has reduced ability to detect positive defocus.     

Effects of refractive blur on the multifocal electroretinogram

A significant difference in the response density of the MF-ERG response has been suggested for every 2 diopter change of refraction. The influence of refractive blur on the MF-ERG was studied in 8 healthy volunteers using either the VERIS™ system (Group A: n=5) or Retiscan™ (Group B: n=3). For each eye recordings were obtained with a corrective lens of −3 dpt, 0 dpt, +3 dpt and +6 dpt placed in front of the contact lens electrode. The viewing distance was adjusted to compensate for the induced changes in the retinal image size. When the changes in retinal image size due to the refractive lens were compensated for, no influence due to refraction was observed in either latencies or amplitudes of (KI (P>0.05). This held true for the central response average (four degrees) as well as for the outer 6–25 degrees. In KII.1 only the peripheral amplitudes of Group B showed an influence due to refraction (P≤0.05). This may be due to adaptation as the recordings of group B were obtained in succession. As expected, significant differences were observed when the recordings obtained with the different systems were compared (P≤0.05). This revised version was published online in July 2006 with corrections to the Cover Date.