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 adaptation: clinical and refractive considerations

The human visual system is amenable to a number of adaptive processes; one such process, or collection of processes, is the adaptation to blur. Blur adaptation can be observed as an improvement in vision under degraded conditions, and these changes occur relatively rapidly following exposure to blur. The potential important future directions of this research area and the clinical implications of blur adaptation are discussed.     

Blur detection thresholds in childhood myopia: single and dual target presentation.

There is some suggestion that the ability to detect blur may be altered in adults with myopia. Here, we address the question of whether children with myopia have worse blur detection than other children, and whether blur detection in myopic children is related to the rate of myopia progression. We recruited 20 myopes and 20 non-myopes aged between 8 and 12 years. Refractive errors, visual acuity, and contrast sensitivity were measured and the change in refractive error over the past year calculated from clinic records. Blur detection thresholds for two different types of black and white targets (text and scenes), two illumination conditions and two testing protocols were determined using a computer-based forced-choice testing procedure. The two testing protocols used were: (i) dual image presentation where subjects were asked to choose the clearer of the two images, one image always having zero blur, and (ii) single image presentation in which the subject reported whether the image was clear or blurred. Blur discrimination ability under all tested conditions was similar for both refractive error groups. Blur detection thresholds were 0.27+/-0.15 D (myopes) and 0.24+/-0.07 D (non-myopes) for text images. Thresholds were similar when measured with a one log unit reduction in lighting: 0.27+/-0.31 D compared to 0.23+/-0.14 D. Blur detection thresholds were greater for photographic scenes (myopes 0.41+/-0.36 D, non-myopes 0.44+/-0.36 D) and when only a single text image (myopes 0.51+/-0.21 D, non-myopes 0.59+/-0.01 D) was presented, but this increase was measured in both refractive error groups. There was no correlation between blur thresholds and refractive error magnitude, refractive error progression over the past year, or contrast sensitivity. We found that the blur detection ability showed greater individual variation in myopic children. Further work is required to determine whether blur detection ability is of relevance to myopia development.     

Accommodative error, adaptation and myopia

Accommodative adaptation, resulting from the sustained output of slow blur-driven accommodation during the course of a sustained near-vision task, has generally been assessed under open-loop conditions. This study examined whether adaptation influences closed-loop accommodation during the course of a sustained near-vision task. Accommodative adaptation was assessed in 18 fully-corrected subjects by comparing pre- and post-task values of dark accommodation recorded objectively with an infra-red optometer. Subjects performed a continuous 10 min binocular near-vision task at a viewing distance of 33 cm, with the within-task accommodative response being assessed at 1 min intervals during this period. Subjects were categorized into adaptors (N = 11) and non-adaptors (N = 7) on the basis of whether their initial 10 sec post-task adaptation exceeded +0.30D. The adapting group exhibited a significant decline in the lag of accommodation during the first 3 min of the near-task, whereas no significant change in the within-task response over time was observed in the non-adapting group. These results indicate that accommodative adaptation increases the accuracy of the within-task, closed-loop accommodative response. Furthermore, we speculate that a deficit in accommodative adaptation, being accompanied by increased retinal defocus during near fixation, may contribute to the development of nearwork-induced myopia.     

Accommodative adaptation and age of onset of myopia

Tonic accommodation in an empty field and accommodative adaptation, defined as accommodation shift after 2 min viewing of a 4 D near task, were measured using an infrared optometer. Subjects were classified into three types: emmetropes ( n = 18). early onset myopes ( n = 18) and late onset myopes ( n = 15). The relationship between accommodative adaptation and the onset age of myopia were compared. There was no statistical difference in the pre-task ionic accommodation among the three subject groups. However, the accommodative adaptation differed in the three groups during the 5 min post-task period. The average accommodative adaptation of emmetropia. early onset myopia and late onset myopia were 0.66. 0.27 and 1.19 D. respectively. Accommodative adaptation in the late onset myopia group was significantly higher than in the emmetropia group. Furthermore, the accommodative adaptation in the early onset myopia group was significantly lower than in the emmetropia group. Two possible aetiologies of myopia are discussed: one for the group which shows low accommodative adaptation that has no relationship with the age of onset and another for the group which shows high accommodative adaptation that has a relationship to an onset age of 15 years or later. The aetiology is also considered with regard to a potentiating chemical effect on the synapses of the ciliary muscle.     

Colour vision and dark adaptation in high myopia without central retinal degeneration.

Colour vision and dark adaptation of 36 high myopes (68 eyes) without degenerative fundus changes were studied with the standard pseudoisochromatic plates part 2, Farnsworth-Munsell 100 hue (FM 100) test, Nagel anomaloscope, colour vision meter 712-anomaloscope, and Goldmann-Weekers adaptometer. The ages of the myopes ranged from 20 to 49 years, the visual acuity from 6/12 to 6/6, and the refractions from -6.0 to -20.0 D. All the colour vision tests and dark adaptation examination gave normal results. However, the error scores of the myopes in (blue) box III of the FM 100 test were significantly higher than those of the controls matched for age. In dark adaptation, the rod thresholds of the myopes were significantly higher than in the controls. The stretching of the posterior pole of the eye might explain this minimal impairment of the photoreceptor layer of the retina without any visible degenerative changes in the fundus.     

Blur sensitivity in myopes.

PURPOSE: This study compared the ability of myopes and emmetropes to detect subjectively the presence of retinal defocus. METHODS: Subjects (12 myopes, 12 emmetropes) were cyclopleged and monocularly viewed a bipartite target through an appropriate near addition lens via a 2-mm artificial pupil. One-half of the target remained fixed while the other half was alternatively moved forward or backward until subjects first reported a difference in clarity between the two halves of the target. RESULTS: The mean blur threshold for the emmetropes and myopes was +/-0.11 and +/-0.19 D, respectively (p = 0.0001). CONCLUSIONS: These results demonstrate that myopes are less sensitive to the presence of blur, and may at least partially explain why previous reports have demonstrated a larger lag of accommodation in this refractive group. Additionally, the hyperopic retinal defocus resulting from the increased accommodative error may play a significant role in myopia development and progression.     

Effect of accommodative adaptation on static and dynamic accommodation in emmetropia and late-onset myopia.

PURPOSE: Prolonged near work is considered to be an environmental factor leading to the development of late-onset myopia. Accommodation may be the specific mechanism underlying the association between near work and late-onset myopia. To determine whether late-onset myopes have abnormal accommodation, we compared accommodative static responses and dynamic facilities before and after a near task in two groups of subjects, emmetropes and late-onset myopes. METHODS: In experiment 1, the accommodative stimulus/response function with monocular viewing and the dark focus (the accommodative response in the dark) were objectively measured with a Canon R-1 infrared optometer before (preadaptation) and after (postadaptation) a 20-min near task. In experiment 2, monocular accommodative facility (AF) and dark focus were measured before and after the near task. Facility was measured as the subjective time needed to clear an accommodative target (20/40 letters) at 40 cm through +/-2.00 D lenses. The time between when the subject flipped the lenses from viewing through the +2.00 D to the -2.00 D lenses was recorded by a computer. RESULTS: In both experiments, inward shifts of the dark focus were observed after the near task. In experiment 1, after the near task, static accommodative responses also showed a small but statistically significant inward shift. Neither postadaptation effect differed between refractive groups. The only difference between groups was that late-onset myopes had a lower slope of the accommodative stimulus/response function, both pre- and postadaptation. In experiment 2, both refractive groups showed the same results. After the near task, the duration for accommodation from near to far (relaxation) increased but the duration for accommodation from far to near (stimulation) did not change. CONCLUSIONS: Late-onset myopes have shallower accommodative stimulus/response functions. As suggested in a previous study, this may be due to their reduced sensitivity to defocus. In both emmetropes and late-onset myopes, the near task causes an increase in static accommodative responses. Although our results show it to be a small increase, it is consistent with predictions of Hung and Semmlow's model of accommodation. In both emmetropes and late-onset myopes, the near task also increases the duration for relaxing accommodation, but not for stimulating accommodation. This suggests there are two subsystems which may adapt to prolonged accommodation differently.     

Dynamic accommodation and myopia.

PURPOSE: Accommodative effort during nearwork is thought to be a causative factor in the development of myopia. It has been proposed that an anomaly in autonomic control may be a precursor to the development of myopia. In the present study the closed-loop accommodation response after variations in fixation period was investigated in emmetropes, early-onset myopes and late-onset myopes to determine characteristics of reflex accommodation for each refractive group. METHODS: Closed-loop accommodation responses were measured in a group of emmetropes (n = 7), early-onset myopes (n = 7), and late-onset myopes (n = 7) by use of a dynamic tracking infrared optometer. A variation in fixation period (10 seconds, 60 seconds, and 180 seconds) before an accommodative step was used to stimulate the accommodation control mechanism differentially. RESULTS: Group results of accommodative response times showed that late-onset myopes were significantly affected by the duration of fixation before the change in stimulus vergence. Accommodative response times after 3 minutes of sustained near vision were significantly longer than those observed for other groups for the near-to-far condition. Reaction time appears to be independent of refractive grouping, prior fixation period, and direction of step change. CONCLUSIONS: Late-onset myopes showed significantly extended accommodation response times after a sustained near vision task that was demonstrable under well-controlled experimental conditions. The extended response times observed in the present study were consistent with previous reports of refractive shifts in late-onset myopes and early-onset myopes and provide a corollary between reflex and adaptive components of the accommodation response. Potential mechanisms are discussed in an attempt to explain the resultant hysteresis under closed-loop viewing conditions.     

Astigmatism and school myopia.

Changes in astigmatism were followed for a 3-year period among 238 myopic children as part of a clinical trial of myopia treatment. Children with mild myopia and no previous myopic spectacle correction and astigmatism less than or equal to 2 D were included in the study. The prevalence of astigmatism of at least 0.25 D was 55% at the beginning of the follow-up at a mean age of 10.9 years, increasing to 76% during the 3-year period. At the same time the mean astigmatism increased from -0.26 D to -0.45 D. Most of the astigmatism was against the rule; with the rule astigmatism represented 18% of the astigmatism at the beginning and 24% at the end of the study. There was a weak correlation between the spherical equivalent and astigmatism at the beginning of the follow-up (r = 0.122, n = 240, P = 0.029) but not at the end of the follow-up. Myopic progression controlled for the spherical equivalent, was not related to degree of astigmatism at the beginning of the study.     

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.     

Lower-field myopia in birds: an adaptation that keeps the ground in focus

In the lower visual field of pigeons, a myopia (near-sightedness) has been reported that progressively increases with the angle below the horizon. Previous data suggested that this lower-field myopia may be an adaptation that permits pigeons to keep the ground in focus while they forage, and simultaneously, to monitor the horizon and sky for predators. We report here a lower-field myopia in other species of birds that have a wide range of heights. A geometric model of this adaptation predicts that the amount of myopia should be systematically related to the distance from the pupil to the ground. The eyes of quail, chickens and cranes of various heights (7.0-104.1 cm) were refracted at 60 deg below the horizon. Their myopia was close to the predicted value at each height.     

A protocol for contact lenses adaptation in aphakia and myopia in children; surveillance and follow-up

The authors report their procedure of contact lens adaptation in child and particularly after traumatic cataract. Adaptation in the briefest time following cataract surgery is absolutely necessary in order to undertake jointly the orthoptic treatment of amblyopia.     

X-linked pathologic myopia.

We examined a patient with decreased visual acuity and macular changes attributable to pathologic myopia. Thirteen additional family members were examined, and historic information was obtained on their relatives. A pedigree spanning six generations was constructed that demonstrated an X-linked mode of inheritance. This finding has important genetic and therapeutic implications for similarly affected patients.