The reduction in visual resolution produced by retinal blur is a universal experience. This may occur for either short (seconds or minutes) or longer periods (days or months) depending on one’s refractive state and the viewing distance of the object of regard. Although the effects of short-term blur on visual acuity (VA) have been widely investigated,1 some individuals exhibit perceptual adaptation after just 30 minutes of sustained blur, resulting in an improvement in their visual resolution.2, 3 For example, Mon-Williams et al.2 noted a change in monocular and binocular VA after a 30-minute period of induced defocus in an emmetropic population. Mean monocular VA improved by 0.12 logarithm of the minimum angle of resolution (logMAR) units after 30 minutes of viewing through a +1.00 D lens. The Snellen equivalent of the mean change was approximately 6/15 to 6/9. Further, this improvement in VA was not accompanied by any significant change in the refractive state of the eye. They concluded that this shift represented a “neural compensation” to the sustained blur to improve the perceived contrast of the blurred retinal image.
A similar adaptive effect has been demonstrated in neonatal chicks, whereby improved contrast sensitivity was observed after only 1.5 h of viewing through frosted goggles.4–6 Additionally, when the chicks were tested under cycloplegia, plus and minus lenses produced significant adaptation. After removal of the imposed blur stimulus, the time constant for recovery to baseline was slow, but resolution reached baseline levels after 1.5 days. Thus, blur adaptation may represent a general vision enhancement phenomenon present in a range of species in addition to humans.
Further, Mon-Williams et al.2 demonstrated interocular transfer of blur adaptation in humans. They reported that if a blur-inducing lens was introduced before one eye, with the fellow eye occluded during the adaptation period, then an improvement in VA was observed in both eyes. The improved resolution seen in the covered eye was 35% of that measured in the viewing eye. This interocular transfer suggests that the adaptation process occurred at central binocular sites in the visual cortex.
The improvement in visual resolution during extended periods of blur (≥30 minutes) has led some authors to claim that unnecessary refractive corrections have been prescribed.7, 8 Although these claims are difficult to substantiate and often contain serious inaccuracies and methodological flaws, they are still frequently quoted, particularly in the lay literature and media. Nevertheless, perceptual adaptation to retinal blur, resulting in improved unaided VA, may reduce the need for a full-time refractive correction, particularly if the effect is enhanced during longer periods of blur, such as for several hours or days. Alternatively, less frequent changes in the power of the corrective lenses during ongoing refractive development may be acceptable. Any reduction in the need for refractive correction would be of significant economic benefit to the patient, as well as removing a source of inconvenience.
Accordingly, the aim of the present study was to examine blur adaptation in a population of subjects with moderate myopia (between 1.00 and 3.50 D) after removal of their refractive correction. Individuals in this refractive group frequently report improvements in their visual resolution during sustained periods without their refractive correction.2, 3 High and low contrast stimuli were tested during a 3-h period of sustained blur.
The study was performed on 22 visually normal, myopic subjects, all of whom were optometry students at the SUNY State College of Optometry. Their mean age was 25.6 years (SD, ±2.63 years; range, 22.6 to 33.4 years). The mean spherical equivalent refractive correction was −1.85 D (SD, ±0.70D; range, −1.00 to −3.50 D). Seventeen of the subjects had astigmatism of <0.50 D, and the remaining five subjects had between 0.50 and 1.25 D of astigmatism (mean, 0.90 D). No subject had any manifest binocular abnormality or ocular disease.
Letter and sine wave grating VA were measured monocularly at 30-minute intervals during the course of a 3-h period, during which time the myopic subjects removed their refractive correction. Subjects were required to wear their correction for a continuous 1-h period immediately before the start of the experiment. To obtain the maximum blur effect, subjects maintained distance fixation throughout the uncorrected trial period. Binocular vision was maintained during the experimental session, apart from the brief periods when visual resolution was assessed. A television set was available, and subjects watched either broadcast television or videotapes at a viewing distance of 5 m throughout the 3-h experimental session. While providing a wide range of visual and cognitive information during the trial period, this naturalistic stimulus also helps to prevent boredom and fatigue during the adapting test period. Pesudovs and Brennan3 used a similar adapting stimulus to produce significant blur adaptation. The experimental operator periodically checked fixation, and no reading or near-work materials were available to the subjects during the trial.
Letter VA was measured using one of six Early Treatment Diabetic Retinopathy Study (ETDRS), logMAR, high contrast (∼90%) charts (Precision Vision, La Salle, IL, catalogue no. 2121, 2122, 2123, 2110, 2111, and 2122) at a viewing distance of 4 m. A different chart was used for each VA assessment to prevent subjects from memorizing the chart letters. Subjects were encouraged to read down the chart as far as possible, and credit was given for each letter identified correctly, irrespective of its position on the chart. All the VA measurements were taken from the right eye, whereas the left eye was fully occluded.
By presenting randomly orientated sine wave gratings at a viewing distance of 4 m using a Mentor B-VAT computerized visual display (model 22–4850, Mentor O & O, Norwell, MA), grating acuity was assessed. The highest spatial frequency that the subject was able to resolve for a given contrast level was determined. Subjects were required to indicate whether the grating was oriented vertically or tilted to the right or left. Contrast levels of 2.5, 4.0, 6.3, 16.0, 25.0, and 40.0% were tested. Three presentations were made for each contrast level, and subjects were considered to have resolved the grating if they were able to identify its orientation correctly on at least two of the three presentations. To determine whether the change in VA resulted from variation in either the refractive or accommodative state of the eye, noncycloplegic distance refractive error was assessed objectively from the right eye at 30-minute intervals using a Canon Autoref R-1 infrared optometer9, 10 (Canon, Tokyo, Japan) while subjects viewed the high contrast acuity chart at a distance of 4 m.
Two control conditions also were performed. First, for eight subjects, the experiment was performed as described previously, with the exception that subjects were fully corrected throughout the course of the 3-h test period, including all the measurements of VA. In a second control condition for a different group of eight subjects with a mean refractive error of −2.72 D (SD, ±1.73 D), the subjects again wore their refractive correction during the course of the 3-h test period but removed their spectacles for the assessment of letter and grating VA. These control conditions will determine whether any observed changes in visual resolution resulted from learning effects during the 3-h trial period.
Mean changes in high contrast letter VA during the 3-h trial period for the subjects with uncorrected myopia and the two control conditions are illustrated in Fig. 1. One-factor, factorial analysis of variance indicated that the change in acuity over time for the subjects with uncorrected myopia was significant [F(6,139) = 22.91; p = 0.0001]. The mean improvement in VA from the beginning to the end of the 3-h period was from 0.76 logMAR to 0.53 logMAR. The Snellen equivalent to this change is from 6/35 to 6/20. Post hoc analysis with the Fisher protected least significant difference (PLSD) test11 indicated that all the mean values of letter VA obtained after 90 minutes of sustained blur and later were significantly different from the baseline finding (p < 0.05).
No significant change in acuity was observed for either of the two control experiments. For the first control (Control 1), during which subjects wore their refractive correction throughout the test period (including all the acuity measurements), the mean shift in VA from the beginning to the end of the 3-h control period for the latter group was from −0.002 logMAR to 0.027 logMAR. The Snellen equivalent to this change is from 6/6.0 to 6/6.4. This shift was not statistically significant [F(6,55) = 0.46; p = 0.83]. For the second control condition (Control 2), during which subjects wore their refractive condition during the test period but removed it for the acuity measurements, the mean shift in VA from the beginning to the end of the 3-h control period for the latter group was from 0.92 logMAR to 0.88 logMAR. The Snellen equivalent to this change is from 6/50 to 6/46. This change was not statistically significant [F(6,55) = 1.06; p = 0.40].
Individual changes in letter VA measured at the end of the 3-h period of sustained blur, plotted as a function of the magnitude of uncorrected myopia, are shown in Fig. 2. No significant correlation was observed (r = 0.15; p = 0.51). Of the 22 subjects examined, 4 (18%) showed an improvement in VA of <0.10 logMAR, 6 (27%) changed between 0.10 and 0.20 logMAR, 8 (36%) changed between 0.20 and 0.30 logMAR, and 4 (18%) exhibited a change in VA >0.30 logMAR.
Mean values of grating VA during the 3-h trial period for the subjects with uncorrected myopia are shown in Table 1. The change in grating acuity at the beginning and end of the trial period is shown in Fig 3A. Analysis of variance indicated that the effects of time [F(6,691) = 3.42; p = 0.004] and contrast [F(5,691) = 59.14; p = 0.0001] were significant. Post hoc analysis with the Fisher PLSD test11 indicated that all the mean values obtained after ≥30 minutes of sustained blur were significantly different from the baseline values. The large error bars observed in Fig. 3A for the 2.5% contrast condition reflect the difficulty that many subjects had when attempting to resolve extremely low contrast gratings without their refractive correction.
Grating VA data for the control condition during which subjects wore their refractive correction throughout are shown in Table 2 and Fig. 3B. No significant change in contrast sensitivity was observed over time [F(6,294) = 0.34; p = 0.92]. Grating VA data for the control condition during which subjects wore their refractive correction during the trial but removed it during the VA measurements are shown in Table 3 and Fig. 3C. No significant change in grating VA was observed over time [F(6,245) = 0.31; p = 0.93]. Mean values of noncycloplegic distance refractive error, as measured by the infrared optometer, are illustrated in Fig. 4. Analysis of variance indicated no significant change over time during the 3-h session [F(6,133) = 0.04; p = 0.999].
The results of the present study demonstrate that subjects with myopia exhibit significant improvements in their unaided VA during a period of sustained blur. This improvement in visual resolution was not produced by any significant change in refractive error. Nor can it be explained by a learning effect produced during the repeated acuity measures. Further, these findings are consistent with the clinical observation that patients with uncorrected ametropia often claim better visual function than may have been predicted based on their VA alone. The results obtained in the present study are similar to those reported by Mon-Williams et al.2 Both investigations observed an improvement in letter VA of 0.12 logMAR after 30 minutes of sustained blur, although the present investigation used a greater degree of retinal defocus (mean, 1.85 D) than that adopted by Mon-Williams et al. (1.00 D).
In considering possible mechanisms that might produce this adaptation to sustained blur, Georgeson and Sullivan12 noted that channels that respond selectively to different spatial frequency bands analyze the visual stimulus. Introduction of retinal blur will attenuate the amplitude of the high spatial frequency components while producing little change in the lower spatial frequencies. Accordingly, they proposed “an active compensation process” to restore the quality of the blurred image. This conclusion was later supported by Näsänen et al.,13 who suggested that the visual system attempts to compensate for the degradation produced by early signal processing, so that perceived contrast becomes proportional to the physical contrast of the stimulus. The observation of interocular transfer2 would suggest that blur adaptation occurs at central binocular sites in the visual cortex, although Diether et al.4 suggested that some blur adaptation might also take place at the retina. This latter proposal is supported by recent investigations by Heinrich and Bach,14–16 who observed significant changes in the pattern electroretinogram (PERG) and cortical visual evoked potential (VEP) after contrast adaptation. Diether and Schaeffel6 suggested that the adaptation occurring during prolonged periods of retinal defocus might provide a retinal error signal for myopia development.
This phenomenon may have clinical implications with regard to the correction of refractive errors. For example, the improvement in visual resolution produced by this adaptive process may reduce the necessity for full-time refractive correction in some individuals. Only part-time wear or a partial correction may at times be adequate, depending on the magnitude of uncorrected refractive error, although the poor level of acuity obtained at the end of the present study (20/67) must also be noted. Furthermore, care must be taken to ensure that the level of acuity achieved while uncorrected is sufficient to protect the safety of the patient (and others) when they are engaged in any hazardous activity, such as driving or operating machinery. The study of Chung et al.17 also would suggest that caution should be exercised when promoting the undercorrection of myopia. They reported that subjects whose myopia was uncorrected by 0.75 D showed a slightly higher rate of myopia progression (0.50 D/year) compared with a group of fully corrected subjects (mean progression rate, 0.39 D/year). It is of interest to note that both the fully corrected and undercorrected groups in this latter study showed significant increases in myopia, implying that the degree of retinal defocus might play a limited role in refractive error development.
Knowledge of the magnitude of blur adaptation will provide valuable control data when evaluating methodologies designed to slow or reverse refractive error development. A wide variety of techniques have been advocated for slowing the development of myopia (see Grosvenor18 and Grosvenor and Goss19 for reviews). Many of these procedures have shown an improvement in unaided VA, without demonstrating any change in the underlying refractive error. The results of the present study indicate that blur adaptation produces significant improvements in visual resolution without the need for any therapeutic intervention. However, it should be stressed that these changes in VA do not result from an alteration of the underlying refractive state.
If blur adaptation is to be of value in the clinical setting, then it is critical that the measured improvements be transferable to the normal visual environment.20 For example, Matson et al.21 noted that subjects whose resolution improved when testing with the tumbling E chart did not show any significant improvement when assessed using a Snellen letter chart. In contrast, Leung et al.22 reported improvements in the ability to resolve Chinese and English letters after training with only Chinese characters, whereas Ricci and Collins23 found that adaptation resulted in improvements in a variety of visual tasks, including playing a video game, identifying facial expressions, and locating household items. With regard to the duration of the effect in humans, Collins et al.24 noted that improved acuity was maintained for >9 weeks after a behavioral training program. However, the investigation was conducted on only two subjects. This latter study also involved active training, whereas the protocol adopted in the present investigation was entirely passive. Data from our laboratory have indicated that the improved resolution may be maintained after passive blur adaptation for up to 10 days and is not significantly reduced by intervening periods of clear vision.25, 26
This study has observed that subjects with myopia demonstrated a significant mean improvement in uncorrected VA during a 3-h adapting period. Thus, patients with uncorrected ametropia may use blur adaptation to improve their visual resolution, thereby reducing the need for a refractive correction on a full-time basis in some instances.
Supported by NIH grant T-35 07079.
SUNY College of Optometry
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Keywords:© 2004 American Academy of Optometry
adaptation; blur; contrast sensitivity; defocus; myopia; refractive error; visual acuity