Blur Adaptation Research Articles

Improvement of Visual Acuity as An Aftereffect of Blur Adaptation through a Defocusing Lens

Our previous study has shown that blur adaptation through a foggy filter temporarily improved visual acuity (VA). Here we report the influence on VA from the adaptation to blurred images by a defocusing lens. In the experiment, a subject adapted to the blurred visual field by observing motion pictures through a trial lens either of 2.5D, 3.0D, 4.0D, or 5.0D. The visual acuity was measured in the periods of pre- and post-adaptation by a quick and simplified staircase method with Landolt C. The moment the lens was removed, subject's VA was obviously higher than the original VA and then gradually returned to the original level in a few minutes. However the degree of transient improvement in VA was not necessarily larger with longer period of adaptation. The enhancement in VA demonstrated here is presumably caused by blur adaptation and its aftereffect. The usual VA might not be the optimum performance of the hardware of an eye, or rather, be suppressed by the visual system in the level of the cortex.

Response normalization and blur adaptation: Data and multi-scale model

Adapting to blurred or sharpened images alters perceived blur of a focused image (M. A. Webster, M. A. Georgeson, & S. M. Webster, 2002). We asked whether blur adaptation results in (a) renormalization of perceived focus or (b) a repulsion aftereffect. Images were checkerboards or 2-D Gaussian noise, whose amplitude spectra had (log-log) slopes from -2 (strongly blurred) to 0 (strongly sharpened). Observers adjusted the spectral slope of a comparison image to match different test slopes after adaptation to blurred or sharpened images. Results did not show repulsion effects but were consistent with some renormalization. Test blur levels at and near a blurred or sharpened adaptation level were matched by more focused slopes (closer to 1/f) but with little or no change in appearance after adaptation to focused (1/f) images. A model of contrast adaptation and blur coding by multiple-scale spatial filters predicts these blur aftereffects and those of Webster et al. (2002). A key proposal is that observers are pre-adapted to natural spectra, and blurred or sharpened spectra induce changes in the state of adaptation. The model illustrates how norms might be encoded and recalibrated in the visual system even when they are represented only implicitly by the distribution of responses across multiple channels.

The effect of blur adaptation on accommodative response and pupil size during reading

To study the effect of blur adaptation on accommodative variability, accommodative responses and pupil diameters in myopes (n = 22) and emmetropes (n = 19) were continuously measured before, during, and after exposure to defocus blur. Accommodative and pupillary response measurements were made by an autorefractor during a monocular reading exercise. The text was presented on a computer screen at 33 cm viewing distance with a rapid serial visual presentation paradigm. After baseline testing and a 5-min rest, blur was induced by wearing either an optimally refractive lens, or a +1.0 DS or a +3.0 DS defocus lens. Responses were continuously measured during a 5-min period of adaptation. The lens was then removed, and measurements were again made during a 5-min post-adaptation period. After a second 5-min rest, a final post-adaptation period was measured. No significant change of baseline accommodative responses was found after the 5-min period of adaptation to the blurring lenses (p > 0.05). Compared to the pre-adaptation level, both refractive groups had similar and significant increases in accommodative variability right after blur adaptation to both defocus lenses. After the second rest period, the accommodative variability in both groups returned to the pre-adaptation level. The results indicate that blur adaptation has a short-term effect on the accommodative system to elevate instability of the accommodative response. Mechanisms underlying the increase in accommodative variability by blur adaptation and possible influences of the accommodation stability on myopia development were discussed.

Spatial and temporal integration in blur adaptation

Spatial and temporal integration in blur adaptation

Shape and individual variability of the blur adaptation curve

We are interested in clinical implications of adaptation to blurred and sharpened images. Therefore, we investigated repeatability, individual variability and characteristics of the adaptation curves of 39 normally-sighted individuals. The point of subjective neutrality (PSN – the slope of the spatial spectrum of the image that appears normal) following adaptation was measured for each adaptation level and was used to derive individual adaptation curves for each subject. Adaptation curves were fitted with a modified Tukey biweight function as the curves were found to be tumbled-S shaped and asymmetrical for blur and sharp in some subjects. The adaptation curve was found to be an individual characteristic as inter-subject variability exceeds test/re-test variability. The existence of individual variability may have implications for the prescription and clinical success of optical devices as well as image enhancement rehabilitation options.

Refractive error-dependent differences in accommodation after blur adaptation

Refractive error-dependent differences in accommodation after blur adaptation

Blur adaptation to spatial frequency structure

Blur adaptation to spatial frequency structure

Blur adaptation and induction in the fovea and periphery

Blur adaptation and induction in the fovea and periphery

Enhanced Contrast Sensitivity Confirms Active Compensation in Blur Adaptation

To determine the effects of defocus-induced blur adaptation on human contrast sensitivity (CS) function. Defocused (+2 D) CS was measured for spatial frequencies between 0.5 and 12 cycles per degree (cpd) before and after adaptation to +2 D blur in six subjects with normal vision. During the 30-minute adaptation period with +2 D lens, subjects were exposed to a succession of static calibrated natural images that were also used to "top-up" adaptation between postadaptation trials. After 30 minutes of blur adaptation, CS was found to be significantly reduced at 0.5 cpd (P = 0.023), though it was enhanced at 8 cpd (P = 0.007) and 12 cpd (P = 0.005). The average sensitivity reduction at 0.5 cpd was 0.20 log(10) units, whereas enhancements were 0.09 and 0.16 log(10) units at 8 and 12 cpd, respectively. The present study demonstrates a novel finding that 30 minutes of defocused viewing results in enhanced high spatial frequency CS. The concurrent observation of low spatial frequency CS reduction suggests that the changes are not caused by simple learning effects but are likely caused by neural adaptation.

Dynamic Accommodation Responses Following Adaptation to Defocus

Adaptation to defocus is known to influence the subjective sensitivity to blur in both emmetropes and myopes. Blur is a major contributing factor in the closed-loop dynamic accommodation response. Previous investigations have examined the magnitude of the accommodation response following blur adaptation. We have investigated whether a period of blur adaptation influences the dynamic accommodation response to step and sinusoidal changes in target vergence. Eighteen subjects (six emmetropes, six early onset myopes, and six late onset myopes) underwent 30 min of adaptation to 0.00 D (control), +1.00 D or +3.00 D myopic defocus. Following this adaptation period, accommodation responses to a 2.00 D step change and 2.00 D sinusoidal change (0.2 Hz) in target vergence were recorded continuously using an autorefractor. Adaptation to defocus failed to influence accommodation latency times, but did influence response times to a step change in target vergence. Adaptation to both +1.00 and +3.00 D induced significant increases in response times (p = 0.002 and p = 0.012, respectively) and adaptation to +3.00 D increased the change in accommodation response magnitude (p = 0.014) for a 2.00 D step change in demand. Blur adaptation also significantly increased the peak-to-peak phase lag for accommodation responses to a sinusoidally oscillating target, although failed to influence the accommodation gain. These changes in accommodative response were equivalent across all refractive groups. Adaptation to a degraded stimulus causes an increased level of accommodation for dynamic targets moving towards an observer and increases response times and phase lags. It is suggested that the contrast constancy theory may explain these changes in dynamic behavior.

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.

Static accommodative responses following adaptation to differential levels of blur

To investigate the effects of two levels of blur adaptation on visual resolution and steady-state accommodation responses in emmetropes and myopes. 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. 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. 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.

Effect of Blur Adaptation on Blur Sensitivity and Discrimination in Emmetropes and Myopes

To determine whether blur adaptation influences blur sensitivity and blur discrimination thresholds in young adult myopes and emmetropes. In addition, to determine whether there is a differential effect of blur adaptation on blur sensitivity and discrimination between refractive error groups. Proximal and distal blur sensitivity thresholds and blur discrimination thresholds were measured under cycloplegia with a Badal optometer in 24 young adult subjects (8 emmetropes [EMM], 8 early-onset myopes [EOM], and 8 late-onset myopes [LOM]). Adaptation to 1 D of myopic refractive blur was then undertaken for 30 minutes. Blur sensitivity and discrimination thresholds were then remeasured. After blur adaptation, blur sensitivity, and blur discrimination thresholds were found to be elevated. Blur adaptation had a significant effect on distal blur sensitivity threshold, with the largest effect being observed in the EOMs. Mean changes in distal blur sensitivity thresholds were EMMs +0.03 +/- 0.14 D, EOMs +0.30 +/- 0.21 D, and LOMs +0.08 +/- 0.13 D. Adaptation to a degraded stimulus modifies the blur detection mechanisms of the visual system in young adults. Depth of focus is expanded by prolonged exposure to defocus. EOMs are more susceptible to this phenomenon than are LOMs and EMMs.

Aging and blur adaptation

Color appearance remains remarkably stable in the aging visual system despite large changes in the spectral distribution of the retinal stimulus and losses in chromatic sensitivity (P. B. Delahunt, J. L. Hardy, K. Okajima, & J. S. Werner, 2005; J. S. Werner, 1996). This stability could reflect adaptive adjustments in peripheral or central chromatic mechanisms that compensate for sensitivity losses in senescence. We asked whether similar compensatory adjustments play a role in maintaining spatial vision--and whether the adaptation itself shows changes with aging-by examining the effects of adaptation on judgments of image focus. Perceptual aftereffects following adaptation to a uniform field and blurred or sharpened images were compared between younger adults and older observers. Subjects adapted to a sequence of blurred or sharpened images for 120 s, and a two-alternative forced-choice staircase task was used to vary the filter exponent of the test to define the subjective point of best focus. There was a small but significant difference between younger and older observers in the level perceived as best focused in all three adaptation conditions, possibly reflecting differences in the ambient blur level the groups are routinely exposed to. However, the magnitude of the blur aftereffect did not differ between the two age groups. These results suggest that although there may be small differences in the long-term adaptation to blur, younger and older observers do not differ in the strength of adaptation to transient changes in blur. The neural processes mediating adaptation to blur thus appear to remain largely intact with aging.

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 1 h 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.

Blur adaptation modulates amplitude and phase spectra of perceived image

Blur adaptation modulates amplitude and phase spectra of perceived image

Involvement of Neural Adaptation in the Recovery of Vision After Laser Refractive Surgery

To investigate whether the gradual improvement in unaided visual acuity commonly seen after laser refractive surgery is attributable, in part, to neural adaptation to blur. Unaided logMAR visual acuity was measured at presentation (blur adapted) and immediately after refraction and removal of refractive correction (correction adapted) in 26 patients with low myopic residual refractive error after laser in situ keratomileusis (LASIK). The difference in unaided visual acuity (correction adapted--blur adapted) indicates a dominance of practice effect (if negative) or blur adaptation (if positive). A combination of blur adaptation and practice effect is possible. Overall, the mean unaided visual acuity at presentation (blur adapted) was 0.16+/-0.16 (mean+/-standard deviation) logMAR, and the mean unaided visual acuity immediately after refraction and removal of refractive correction (correction adapted) was 0.14+/-0.14 logMAR, giving a difference (correction adapted--blur adapted) of -0.02+/-0.06 logMAR. This difference was not significant (analysis of variance [ANOVA] F1,25=0.204, P>.05), suggesting neither blur adaptation nor practice effect. However, during the first 10 weeks after surgery, the difference in unaided visual acuity was -0.07+/-0.05 logMAR, suggesting a practice effect. After 10 weeks, the mean difference was +0.02+/-0.05 logMAR, suggesting any practice effect is offset by blur adaptation. These values were significantly different (ANOVA F1,25= 13.53, P<.01). These data suggest that patients do not adapt to surgically induced blur, on average, until 10 weeks after LASIK. The reason for this delay is uncertain; perhaps instability of blur hinders adaptation during the early postoperative period. Part of the gradual visual improvement after LASIK appears to be due to neural adaptation to blur.

Increased accommodation following adaptation to image blur in myopes

Prolonged exposure to blurred images produces perceptual adaptation (M. A. Webster, M. A., Georgeson, & S. M. Webster, 2002). The purpose of this study is to test whether in addition to the reported change in perceived blur there is also a change in accommodation. Young adult (aged 18 to 31 years) myopic (n = 23) and emmetropic (n = 17) subjects participated in the study. Myopes were tested with contact lenses and had corrected monocular visual acuity of 20/20 or better. Accommodation was measured binocularly with a PowerRefractor, an eccentric infrared photorefractor. Accommodation for a near target (high-contrast text at 0.33 m) was measured for 2 min before and immediately after 3 min of blur exposure. Blur was induced using 0.2 Bangerter diffusing filters in front of both eyes. In addition, accommodation was measured for a far target (high-contrast letters at 4.0 m) before and after the near measurements, with each subject's initial far readings used as a baseline for calculating the accommodative responses at near. Compared to the pre-adaptation level, myopes showed a significant (p < .01) increase in the near accommodative response after 3 min of blur adaptation, while accommodation to the near target in emmetropes did not change. In a second experiment using monocular viewing, the increase of accommodation found in myopes was shown to occur during the period of blur exposure. The refractive group differences in the accommodative response may be related to differences in the habitual response to image clarity between myopes and emmetropes under normal viewing conditions.

Blur adaptation in myopes.

It has been suggested that when subjects with myopia remove their refractive correction, blur adaptation develops to produce an improvement in their visual resolution. The present study measured visual acuity (VA) using high contrast letters and gratings with contrast levels between 2.5% and 40% at 30-minute intervals over the course of a 3-h period during which the subjects remained uncorrected. Twenty-two young subjects with moderate degrees of myopia (mean refractive error, -185 D) participated in the study. Immediately after a 1-h period of full correction, subjects spent 3 h without any refractive correction, during which time they watched television and videos at a viewing distance of 5 m. A significant change in letter and grating VA was observed during the course of the 3-h period of sustained blur, with the mean uncorrected letter VA improving from 0.76 (SD, +/-0.26) to 0.53 (SD, +/-0.23) logarithm of the minimum angle of resolution (logMAR). The Snellen equivalent to this change is from 6/35 to 6/20. A significant improvement in grating acuity was also observed. However, no significant change in refractive error, measured using noncycloplegic autorefraction, was found. These results demonstrate significant blur adaptation in subjects with uncorrected myopia, which does not result from a change in refractive state. We hypothesize that the improvement in visual resolution results from perceptual adaptation to the blurred image, which may occur at central sites within the visual cortex.

Blur thresholds following blur adaptation

Blur thresholds following blur adaptation