The contrast theory in myopia: Q&A with Prof. Earl Smith, Marcella McParland and Jill Woods

In the context of vision, contrast describes the differences in color (color contrast) or brightness (luminance contrast) over time or between adjacent objects. Contrast is critical for the detection of objects on a background of a different luminance or color. The human visual system, beginning in the retina, is organized to detect contrast and is more sensitive to contrast than to absolute luminance.

It has been well established that eye growth and refractive development is a visually guided process.¹ The primary visual signals that influence eye growth are derived from defocus and contrast,^1,2 and are processed locally within the eye.³ Many of the retinal neurons are, in essence, contrast detectors. 

There are many examples of how variations in contrast can influence refractive development. In primates, imposing significant reductions in retinal image contrast under outdoor ambient lighting conditions produces hyperopia. In humans, abnormally high retinal contrast signalling evident in familial myopia promotes myopia progression. These findings suggest a potential therapeutic use of contrast management in myopia control. Recent well-structured clinical trials have demonstrated that mild reductions in retinal contrast imposed by spectacle diffusion lenses can slow the progression of myopia in children.⁴ 

While there is still much to learn about the role of vision in regulating refractive development, contrast management represents a promising new approach to myopia management.     

Our eyes are exposed to a range of contrast levels in normal everyday life. 

Artificial contrast describes higher contrast visual stimuli and environments made by humans: for example, books with dark text on a white background or digital devices and also modern urban environments. 

Natural contrast refers to lower contrast stimuli evident in the natural world: for example, the countryside or a football field and are made up of different shades of green and blue sky with clouds.  

We have observed a connection between contrast levels in different environments and myopia progression. 

Higher levels of education, greater amounts of near work and urban environments are established myopia risk factors⁵ and are all sources of higher, artificial contrast. Contrast theory states that high artificial contrast overstimulates the retina (particularly the bipolar cells which are the first retinal cells to detect contrast), leading to an overstimulation of eye growth and myopia progression.

Increased time outdoors is protective against the onset of myopia⁶ and may help control myopia progression.⁷ The natural outdoor environment provides a lower, natural contrast visual experience. This natural contrast elicits lower-level retinal activity that does not appear to cause an acceleration of eye growth. 

DOT spectacle lenses are designed to slow myopia progression by slightly lowering retinal contrast to mimic more natural contrast.

The purpose of the CYPRESS clinical trial was to evaluate the safety and efficacy of DOT spectacle lenses in reducing the progression of juvenile myopia. It was a multicentre, randomised, controlled clinical trial that enrolled 256 children aged 6-10 years across 14 North American clinical sites. The original 3-year study was extended for 1 year and the Control group was retained throughout. I led the data collection at the Centre for Research and Education (CORE) at the University of Waterloo.

The CYPRESS clinical trial provides robust evidence that DOT lenses are safe and effective in children from age 6. 

Of particular interest are the results of the children starting the study at age 6-7 years because these children are likely to progress to high levels of myopia and there is a paucity of efficacy data available for this young age group. After 3 years, myopia progression was slowed by 0.84 D in cycloplegic SER and 0.32 mm in axial length in the 23 6–7-year-old children who wore DOT lenses. This benefit continued to year 4, for the 61% who enrolled into the one-year extension study.

The CYPRESS clinical trial expands the evidence-base for myopia control spectacle lenses and has several unique aspects, as the only spectacle lens study to demonstrate myopia control efficacy in a multicentre clinical trial with North American children from age 6. 

These results support the hypothesis that myopia progression can be slowed by managing contrast.

Our research team at CORE have also investigated the choroidal response with DOT lenses, which will be presented at the 2024 Association for Research in Vision and Ophthalmology (ARVO) conference in May.