Blur Adaptation Research Articles

Brief Adaptation to Astigmatism Reduces Meridional Anisotropy in Contrast Sensitivity.

To investigate the effect of visual adaptation to orientation-dependent optical blur on meridional contrast sensitivity function in artificially imposed astigmatism. The study adopted a top-up adapt-test paradigm. During the blur adaptation process, the 18 non-astigmatic young adult participants were briefly presented with natural scene images (first trial, 10minutes; subsequent trials, 6 seconds). Contrast sensitivities for horizontal and vertical gratings at spatial frequencies ranging from 1 to 8 cycles per degree (cpd) were measured immediately before and after adaptation to +3.00 diopters cylinder (DC) with-the-rule or against-the-rule astigmatism. Meridional anisotropy was measured to quantify the contrast sensitivity difference between the two grating orientations. Adapting to astigmatic blur enhanced contrast sensitivity at the blurred power meridian but reduced contrast sensitivity at the least affected axis meridian. In with-the-rule conditions, contrast sensitivity for horizontal gratings was significantly increased, whereas that for vertical gratings was significantly decreased. Similarly, in against-the-rule conditions, contrast sensitivity for vertical gratings was significantly increased, whereas that for horizontal gratings was significantly decreased. These two factors together resulted in a substantial systematic reduction, averaging 34%, in meridional anisotropy of contrast sensitivity across the spatial frequency spectrum. Astigmatism adaptation occurs in natural scene viewing. Brief exposure to astigmatic blur altered contrast sensitivity in the opposite direction at the two principal meridians, indicating that the mature visual system possesses functional plasticity to recalibrate the response characteristics of orientationally tuned cortical filters and thus promote substantial reductions of meridional anisotropy in astigmatic vision, to some extent counterbalancing the elongated oval shape of astigmatic blur.

The effects of blur adaptation on visual search performance

The effects of blur adaptation on visual search performance

Frequency of adapting events affects face aftereffects but not blur aftereffects

The dynamics of visual adaptation remain poorly understood. Recent studies have found that the strength of adaptation aftereffects in the perception of numerosity depends more strongly on the number of adaptation events than on the duration of the adaptation. We investigated whether such effects can be observed for other visual attributes. We measured blur (perceived focus-sharp vs blurred adapt) and face (perceived race- Asian vs. White adapt) aftereffects by varying the number of adaptation events (4 or 16) and the duration of each adaptation event (0.25 s or 1 s). We found evidence for an effect of event number on face but not on blur adaptation, though the effect for faces was significant for only one of the two face adapt conditions (Asian). Our results suggest that different perceptual dimensions may vary in how adaptation effects accrue, potentially because of differences in factors such as the sites (early or late) of the sensitivity changes or nature of the stimulus. These differences may impact how and how rapidly the visual system can adjust to different visual properties.

Contributed Session II: Effects of event number and adaptation duration on blur and face aftereffects

Previous studies have revealed that the strength of adaptation aftereffects can depend on both the number of adaptation events (trials) and the duration of each event. However, how these effects depend on the stimulus property adapted remains unknown. In the present study, we compared the influence of adaptation event number vs event duration on the strength of adaptation for blur or face aftereffects. For blur adaptation, we filtered a natural image's amplitude spectrum over slopes between -1 (blurred) to +1 (sharpened) relative to the original image to create a series of blurred/sharpened images. For face adaptation, we morphed an Asian and Caucasian face image resulting in a finely graded series of images spanning the two ethnicities. During each top-up adapting period, we varied the number of adaptation events (4 or 16) and duration of each event (250ms or 1s), resulting in 4 event number-event duration conditions. Our results suggest that the effects of event number and event duration on the strength of aftereffects are similar for blur and face adaptation.

What are patients' beliefs about, and experiences of, adaptation to glasses and how does this affect their wearing habits?

It is well known that some patients experience difficulties adapting to new glasses. However, little is known about what patients themselves understand of the adaptation process, and how this influences their attitudes and the decisions they make when adapting to a new pair of glasses. Nor is it understood whether these factors affect their wearing habits. We conducted four focus groups. Participants were 22glasses wearers (mean±SD age 43±14years, range 21-71years) who reported they: (1) wore spectacle correction for distance vision (single vision, bifocal or progressive lenses); (2) had struggled to get used to a new pair of glasses and (3) sometimes chose not to wear their distance correction. Focus groups were audio recorded, transcribed verbatim and analysed thematically. We identified three themes. Trust is about how participants' trust in their optometrist and themselves influences the likelihood of them adapting successfully to new glasses. Conflict describes how the advice patients have received about adapting to glasses can conflict with what they have experienced and how this conflict influences their expectations. Part of Me explores how participants' experiences and feelings about their glasses are important to adaptation and this includes physical, visual, emotional and behavioural aspects. The traditional optometric perspective of adaptation to glasses is much narrower than that held by patients, and significantly underestimates the physical, behavioural and emotional adaptation that patients must go through in order to feel fully comfortable wearing their glasses. Patients should receive significantly more information about adaptation, including symptoms that may be experienced and why these happen, practical tips to aid adaptation, and when and how to raise concerns. Patients should also receive information about the day-to-day effects of blur adaptation to avoid them not wearing their glasses, including for vision-critical tasks such as driving.

Probing blur adaptation with reverse correlation

Prolonged viewing of blurred or sharpened images makes subsequently viewed images look more sharp or blurred, respectively. A possible mechanism is provided by scale-space models of edge detection, which filter the image with Gaussian derivative operators of different scale (i.e. size), and detect edges by finding responses that are peaks across both space and filter scale. The position of the peak across space indicates edge location, and the position of the peak across filter scale indicates edge blur. To create the peak across filter scale (σ), the output of each filter is multiplied by σγ. γ is conventionally chosen so that the peak occurs in a filter matched in scale to the edge blur. If γ deviates from this default value, then the peak across scale occurs in a filter whose scale is a constant multiple of the edge blur (either larger or smaller). This provides a potential mechanism for blur adaptation: prolonged viewing of blurred or sharpened images could cause the value of γ to be adjusted, causing the peak across scale to occur in a filter with scale larger or smaller than the true edge blur, thereby leading to biases in perceived blur. We estimated the scale of the filter mediating blur discrimination using a task in which participants see two noisy edges and have to say which is the most blurred. By correlating the observer’s responses with the noise samples, we obtain a classification image, a linear approximation of the filter used to do the task. Participants adapted to a blurred or sharpened square wave grating, with Fourier components of frequency f multiplied by 1/f (for blurred gratings) or f (for sharpened gratings). Surprisingly, the classification images were unaffected by blur adaptation. This suggests that the mechanism for blur adaptation is located downstream of low-level blur analysis.

Blur Adaptation to Central Retinal Disease.

PurposeThe long-term, low-resolution vision experienced by individuals affected by retinal disease that causes central vision loss (CVL) may change their perception of blur through adaptation. This study used a short-term adaptation paradigm to evaluate adaptation to blur and sharpness in patients with CVL.MethodsA variation of Webster's procedure was used to measure the point of subjective neutrality (PSN). The image that appeared normal after adaptation to each of seven blur and sharpness levels (PSN) was measured in 12 patients with CVL (20/60 to 20/320) and 5 subjects with normal sight (NS). Patients with CVL used a preferred retinal locus to view the images. Small control studies investigated the effects of long-term and medium-term (1 hour) defocus and diffusive blur.ResultsAdaptation was reliably measured in patients with CVL and in the peripheral vision of NS subjects. The shape of adaptation curves was similar in patients with CVL and both central and peripheral vision of NS subjects. No statistical correlations were found between adaptation and age, visual acuity, retinal eccentricity, or contrast sensitivity. Long-term blur experience by a non-CVL myopic participant caused a shift in the adaptation function. Conversely, medium-term adaptation did not cause a shift in the adaptation function.ConclusionsBlur and sharp short-term adaptation occurred in peripheral vision of normal and diseased retinas. In most patients with CVL, neither adaptation nor blur perception was affected by long-term attention to peripheral low-resolution vision. The impact of blur/sharp adaptation on the benefit of image enhancement techniques for patients with CVL is discussed.

Neural adaptation to peripheral blur in myopes and emmetropes

In the presence of optical blur at the fovea, blur adaptation can improve visual acuity (VA) and perceived image quality over time. However, little is known regarding blur adaptation in the peripheral retina. Here, we examined neural adaptation to myopic defocus at the fovea and parafovea (10° temporal retina) in both emmetropes and myopes. During a 60-min adaptation period, subjects (3 emmetropes and 3 myopes) watched movies with +2 diopters of defocus blur through a 6-mm artificial pupil in two separate, counter-balanced sessions for each retinal location. VA was measured at 10-min intervals under full aberration-corrected viewing using an adaptive optics (AO) vision simulator. By correcting subjects’ native optical aberrations with AO, we bypassed the influence of the individual subjects’ optical aberrations on visual performance. Overall, exhibited a small but significant improvement after the 60-min of adaptation at both the fovea (mean±SE VA improvement: −0.06±0.04 logMAR) and parafovea (mean±SE VA improvement: −0.07±0.04 logMAR). Myopic subjects exhibited significantly greater improvement in parafoveal VA (mean±SE VA improvement: 0.10±0.02 logMAR), than that of emmetropic subjects (mean±SE VA improvement: 0.04±0.03 logMAR). In contrast, there was no significant difference in foveal VA between the two refractive-error groups. In conclusion, our results reveal differences in peripheral blur adaptation between refractive-error groups, with myopes displaying a greater degree of adaptation.

The effect of interrupted defocus on blur adaptation.

Blur adaptation occurs when an observer is exposed to continuous defocus. However, it is unclear whether adaptation requires constant defocus, or whether the effect can still be achieved when the adaptation period is interrupted by short periods of clear vision. The study included 12 emmetropes and 12 myopes. All observers wore full refractive correction throughout the experiment. 1D and 3D of myopic defocus was introduced using spherical convex lenses. An automated system was used to place the blurring lens before the RE for varying periods of blurred and clear vision during adaptation. Participants watched a DVD at 3m during each 15min trial. Visual acuity was measured using Test Chart2000 before and after adaptation. Blur adaptation occurs to varying degrees depending on the periods of incremental blur exposure. Significant improvements in defocused visual acuity occur with continuous blur, equal blur and clear periods, as well as for longer blur periods. However, longer clear periods showed reduced adaptation and this trial is significantly different to the other three trials for both defocus levels (p<0.001). No refractive group differences were observed for neither 1D nor 3D defocus (p=0.58 and p=0.19 respectively). Intervening periods of clear vision cause minimal disruption to improvements in defocused visual acuity after adaptation, indicating that blur adaptation is a robust phenomenon. However, when the exposure to clear vision exceeds the defocused periods, adaptation is inhibited. This gives insight into the effects of real-world tasks on adaptation to blur.

Blur and sharpness discrimination and adaptation

Blur and sharpness discrimination and adaptation

Blur adaptation, blur sensitivity and visual load

Blur adaptation, blur sensitivity and visual load

Dynamics of blur adaptation

Dynamics of blur adaptation

Blur adaptation: Contrast sensitivity changes and stimulus extent

A prolonged exposure to foveal defocus is well known to affect the visual functions in the fovea. However, the effects of peripheral blur adaptation on foveal vision, or vice versa, are still unclear. In this study, we therefore examined the changes in contrast sensitivity function from baseline, following blur adaptation to small as well as laterally extended stimuli in four subjects. The small field stimulus (7.5° visual field) was a 30min video of forest scenery projected on a screen and the large field stimulus consisted of 7-tiles of the 7.5° stimulus stacked horizontally. Both stimuli were used for adaptation with optical blur (+2.00D trial lens) as well as for clear control conditions. After small field blur adaptation foveal contrast sensitivity improved in the mid spatial frequency region. However, these changes neither spread to the periphery nor occurred for the large field blur adaptation. To conclude, visual performance after adaptation is dependent on the lateral extent of the adaptation stimulus.

Can perceptual learning compensate for optical image blur?

Tests of target recognition under blur mostly fail to separate factors of resolution and contrast from the influences of pure blur, i.e., shallow luminance edge gradients. In experiments designed to single out blur, patterns of fixed size were convolved with a Gaussian spread function whose parameter was the variable. In addition, contrast invariance was ensured. The spread parameter was varied to measure form discrimination thresholds for simple geometrical shapes. Following determination of baseline values, observers trained for 7days, 1000 form discrimination responses with error feedback per day in a staircase procedure of the blur parameter. For four observers, thresholds improved an average of 5% (range -11% to +14%) equally for trained and untrained patterns and remained stable during subsequent training with the same targets in a related form discrimination task not involving blur. Because it transferred across target sets, the very slight improvement was indeed in the perceptual capacity to compensate for optical image degradation and not in form discrimination, but its defocus equivalent was quite minor, well less than ¼ diopter. Previous claims for blur adaptation must therefore rest on more complex factors that are not fully excluded in clinical settings.

Adaptation to blurred and sharpened video

The visual system can distinguish different levels of blur and different levels of excess sharpness. Adaptation alters this capacity so that the adapted blur (or sharp) level looks more like a normal, properly focused image. Here, we describe the more general pattern of aftereffects of blur and sharp adaptation by measuring matching functions, using video clips from a DVD movie as stimuli. Results show that blur and sharp adaptation are selective: The sharpening aftereffects of blur adaptation are strongest for blurry videos while the blurring aftereffects of sharp adaptation are strongest for sharp videos. Despite the spatiotemporal variability of our adaptor and test stimuli, we found adaptation effects similar in magnitude to previous studies using invariant static images. A recent model of blur adaptation can be simplified to explain the form of our data, leading us to conclude that what we see as blur/sharp adaptation is a consequence of narrowband contrast adaptation.

The time course of blur adaptation in emmetropes and myopes

This study examined the effect of myopic defocus on visual acuity (VA) over time, with attention being paid to the first point at which blur adaptation had a significant and measurable effect on defocused VA. Visual acuity was sampled at a higher rate than previous studies in order to assess the time course of blur adaptation processes in myopic and emmetropic observers. Participants were 24 normally-sighted observers (12 emmetropes and 12 myopes, median age: 22.5 years). All ametropic participants wore their full refractive correction throughout the experiment. 1 D and 3 D of myopic defocus were introduced in two separate, randomised sessions. Visual acuity was measured using Test Chart 2000 at 2 min intervals over a 30 min session whilst looking through defocus lenses. Recovery clear VA was also measured every 2 min for a further 20 min. Defocused VA was found to improve significantly within 4 min after the introduction of defocus for both 1 D (P < 0.0001) and 3 D conditions (P < 0.0001). The improvements reached a plateau shortly after, with no significant further improvements in defocused VA after 6 min. There were no significant differences found in the temporal blur adaptation profiles between emmetropes and myopes (P = 0.267). Data were fitted with an exponential decay function; the lowest R(2) value for this fit was 0.95. Blur adaptation has a clinically significant and measurable effect on VA within 4 min of exposure to defocus. This finding indicates that the visual system instigates the neural compensatory mechanisms shortly after the appearance of defocus. Our results relate particularly to real-life vision of uncorrected myopes or myopes who remove their correction for part of the day.

Effect of blur and subsequent adaptation on visual acuity using letter and Landolt C charts: differences between emmetropes and myopes

To compare the effects of defocus induced blur and blur adaptation on visual acuity and to evaluate any differences between emmetropes and myopes using letter and Landolt C logMAR charts. The sample consisted of 26 volunteers, with a mean age of 27 ± 3 years, comprising 13 emmetropes (spherical equivalent range: -0.63 to +0.50 D) and 13 myopes (spherical equivalent range: -0.75 to -5.00 D). Monocular visual acuity (VA) was measured in each eye using letter and Landolt C logMAR charts under the following conditions: (1) with the distance refractive correction, (2) immediately after exposure to +2.00 D defocus and (3) following 60 min of binocular adaptation to +2.00 D blur. Objective refraction at the beginning and at the end of the experimental procedure was evaluated. Averaged VA data between the two eyes were used for analysis. Deterioration in VA with +2.00 D defocus was greater in the emmetropes compared to myopes for both charts. The mean difference between the two refractive groups was more pronounced for the Landolt-C (0.17 logMAR) compared to the letter chart (0.10 logMAR). The reduction in VA with blur was related to the amount of the refractive error. Following 60 min of adaptation, a significant improvement in VA was observed in both groups that did not differ between the two charts. The improvement in VA following adaptation using the letter chart was linearly correlated with spherical equivalent refractive correction. Myopes show higher tolerance to retinal defocus compared to emmetropes, which could be attributed to previous blur experience. The effect of blur on VA is more pronounced using Landolt C optotypes than with letters. Prolonged exposure to blur results in equally improved performance for both refractive groups.

Effect of Blur Adaptation on Human Parafoveal Vision

This study was conducted to investigate whether neural compensation for induced defocus can alter visual resolution in other areas of the human retina beyond the fovea. In certain circumstances, the blur adaptation response may be influenced by refractive status. The effect of blur adaptation on the central 10° of the retina was investigated in 20 normally sighted observers (10 emmetropes and 10 myopes; median age, 21 years). Visual acuity (VA) was measured at the fovea and at five locations of the parafoveal nasal visual field (2°, 4°, 6°, 8°, and 10°) with best corrected distance vision. Myopic defocus of 1 D was introduced, and the same measurements were repeated immediately before and after a 30-minute adaptation. VA declined with increasing eccentricity in the clear, blurred, and blur-adapted viewing conditions. The rate of decline was quantified by the parameter E2, which represents the amount of eccentricity dependence of the acuity task. Foveal and parafoveal VA decreased with the introduction of optical defocus and improved significantly after a period of blur adaptation. The consistent value of E2 in each condition indicated that these changes in VA were not eccentricity dependent. Changes in VA under blurred and blur-adapted conditions were of similar magnitudes in myopic and emmetropic observers. Neural adaptation to blur improves VA under defocused conditions in the parafovea as well as the fovea, indicating that the underlying compensatory mechanism acts across a range of spatial scales and independently of retinal eccentricity. Foveal and parafoveal blur adaptation does not vary with refractive error.

Effect of blur adaptation on contrast discrimination in emmetropes and myopes

Studies have investigated visual resolution and contrast detection following a period of degradation in the retinal image (blur adaptation) and have shown it can produce increases in visual resolution and changes in contrast sensitivity. However these studies only explored a limited range of spatial frequencies of sine wave gratings or tested visual resolution alone. The current study aimed to expand on previous work by investigating contrast discrimination and contrast threshold sensitivity before and after blur adaptation for a range of different spatial frequencies. In addition both myopes and emmetropes were tested to see if they responded in different ways to the blur adaptation. Contrast discrimination thresholds were measured (monocular) for a range of spatial frequencies of sine wave gratings (0.5 to 24 cycles per degree) before and after adaptation to +2D blur (45 minutes of adaptation) in four emmetropes and four myopes (mean refraction −6DS). The contrast pedestal was set to 10% and an adaptive 2AFC procedure was employed. Contrast sensitivity and high contrast visual acuity were also measured before and after adaptation. The results showed increases in contrast sensitivity and contrast discrimination at some spatial frequencies after adaptation while others were reduced. In particular, the myopes showed the largest increases in mid to high spatial frequencies (16–20 cycles per degree) while emmetropes displayed reduced sensitivity at the higher spatial frequencies (18–24 cycles per degree). Measurements of the optics before and after adaptation ruled out any optical changes being responsible. The results therefore point to different neural adaptation strategies in the refractive groups. Possible models of how the visual system responds to periods of blur are discussed.

Effect of blur adaptation on contrast discrimination in emmetropes and myopes

Effect of blur adaptation on contrast discrimination in emmetropes and myopes