The aetiology and possible control of myopia: the importance of whole-field visual signals. In many regions within Asia, myopia has reached epidemic proportions. The prevalence of myopia is increasing rapidly in non-Asian countries. This rapid rise in myopia suggests that changing environmental factors are influencing current patterns of refractive errors (less outdoor play, more video-game playing). Myopia is a leading cause of permanent visual impairment (myopic eyes have an increased risk of cataract, glaucoma, chorioretinal degenerations, and retinal detachments), and has become a substantial economic burden on society.
Recent data from primate shows that the quality of the peripheral retinal image can modulate axial eye growth, with the ametropic shift proportional to the sign and magnitude of peripheral image blur. In short, it is believed that hyperopic defocus in the peripheral retina may lead to axial elongation (i.e. Myopia).
There has been a long-time assumption that foveal signals dominate refractive development. This is logical given that: (i) acuity is highest at the fovea; (ii) the fovea is most sensitive to optical defocus; (iii) accommodation is largely controlled by visual signals from the fovea. However, recent studies show:
- Visual signals from the fovea are not essential for many aspects of vision-dependent growth. Indeed, foveal signals can be eliminated in young monkeys without significantly interfering with emmetropization.
- Optically imposed peripheral errors can alter the refractive state of primates. Note that pharmacological/surgical sectioning of the optic nerve doesn’t prevent form-deprivation myopia (usually achieved with lid suture or translucent goggles) or the compensating responses to optically imposed defocus.
- Changes in refractive development operate in a regionally selective manner – e.g. nasal-field form deprivation in primates results in elongation in the contralateral retina.
- When conflicting signals exist between the central and peripheral retina, peripheral visual signals can dominate refractive development.
Myopic eyes are less oblate than emmetropic eyes. In consequence, without correction subjects are left with myopic blur in the fovea and hyperopic blur in the periphery. Traditional correction of a myope with negative lenses corrects the central myopic error but typically increases the degree of peripheral hyperopia. This peripheral hyperopic defocus is thought to be a key signal in driving further elongation of the eye, increasing the severity of myopia.
- Is eye growth a visually-guided process? Only in the sense that the processes occur locally within the retina.
- Does blur detection occur in the retina? The answer appears to be YES.
- How does the retina accomplish the task of differentiating the sign of blur (i.e. hyperopic versus myopic defocus)? (a) First-order (spherical) aberrations are not sign specific, but higher-order aberrations may be. (b) There is some evidence that amacrine cells respond differentially to the sign of defocus (Fisher et al., 1999, Nature Neuroscience).
- What is the source of noise in the peripheral retina that gives rise to poor detection performance?
From the above it is clear that we need a better understanding of the role of peripheral vision and its interactions with foveal vision to improve our understanding of myopia. Experimental work using the CAVE and an eye-tracker will help us do this.