Recent models of human myopia propose retinal defocus as a causative factor in refractive error development (Flitcroft,
1998; Jiang & Morse,
1999; Hung & Ciuffreda,
1999,
2000). In these models, the growing eye works as a feedback system designed to maintain the clarity of the retinal image by modulating eye growth according to the magnitude of retinal defocus. Retinal defocus would, therefore, serve as a stimulus regulating the rate of axial elongation in myopia.
During normal visual development the eye achieves a close match between the power of its optics and its axial length with the result that far images are focused on the retina without accommodative effort (emmetropia). This emmetropization process is partly an optical consequence of proportional eye growth, and thus passive in nature. However, experimental models of myopia also provide strong evidence for an active role of defocus in the emmetropization process (Wallman & Adams,
1987; for a review, see Wildsoet,
1997). Degrading a retinal image by frosted eye occluders produces elongated eyes and ”form-deprivation myopia” in a variety of animal species, providing evidence for a feedback system that correlates eye growth and the magnitude of retinal defocus. Partial frosting degrades the retinal image in a more subtle manner, leading to the development of lesser amounts of myopia (Bartmann & Schaefffel,
1994; Smith & Hung,
1999). In addition, a number of studies have demonstrated that experimental myopia may be induced by placing negative lenses before the eyes in various animal species (e.g., chick, guinea pig, tree shrew, and marmosets) (for reviews, see Edwards,
1996, and Norton,
1999). Although the exact mechanism controlling emmetropization remains uncertain, a number of studies (Graham & Judge,
1999; Schaeffel & Diether,
1999; Smith & Hung,
1999) highlight the fundamental role played by blur in the regulation of eye growth.
Disruption of the emmetropization process results in the development of refractive errors, of which myopia is the most common. Myopia is a highly significant problem, not only because of its increasing prevalence-more than 80% in some Asian countries-(Lin et al.,
2001), but also because it is a high risk factor for vision-threatening conditions (e.g., retinal detachment and glaucoma). These conditions are due to the stresses produced in the posterior segment of the eye as a result of the excessive increase in axial length.
According to clinical observations, the visual performance (e.g., visual acuity [VA] and contrast sensitivity function [CSF]) of corrected myopic subjects improves after a period of uncorrected vision compared to the performance when the correction is worn at all times (Pesudovs & Brennan,
1993). This phenomenon can be interpreted as an increased tolerance to blur (learning process), or an improvement in vision due to neural or optical adjustments within the visual system (Mon-Williams, Tresilian, Strang, Kochhar, & Wann,
1998).
Myopes normally have reduced sensitivity to blur in comparison to emmetropes. Rosenfield and Abraham-Cohen (
1999) showed that on average, myopes have increased blur thresholds. Various models of human myopia (Jiang,
1997; Flitcroft,
1998; Hung & Ciuffreda,
1999) suggest that higher blur thresholds may be related to increased accommodative errors and the development of myopia. The increased accommodative lag found in some myopic subjects (Gwiazda, Thorn, Bauer, & Held,
1993; Jiang,
1997; Abbott, Schmid, & Strang,
1998) would produce a hyperopic retinal defocus that may play a significant role in myopia development and/or progression.
More recently, prolonged exposure to blurred images has been shown to produce perceptual adaptation. Webster, Georgeson, and Webster (
2002) found that exposure to a blurred image caused the original image, which had previously been interpreted as clear, to appear to be too sharp. These aftereffects appeared after brief periods (a few seconds) of adaptation. Although the authors did not identify the refractive errors of the subjects, they did note that “these adaptation effects are thus important for understanding… how vision changes during development and with refractive errors.” Judgments of focus are strongly biased by adaptation to blurred or sharpened versions of an image. Adaptive tuning may be important in calibrating and maintaining the correlation between the image processing in the visual cortex and natural visual stimuli during visual development. Variations of the environment and/or the observer, such as in refractive errors, may alter this correlation. The adjustments taking place by adaptation to blur may be important in maintaining a constant perception of the world. Furthermore, these adaptation effects can potentially alter the accommodative response to the image, by altering sensitivity or responsiveness of the accommodative system to blur.
The present study was designed to test whether the reported adaptation in perceived blur produced by exposure to blurred images is accompanied by a change in accommodation, and whether that change differs between emmetropes and myopes.