How could man-made environments influence childhood development of myopia?

Paper title: The Spatial Frequency Content of Urban and Indoor Environments as a Potential Risk Factor for Myopia Development

Authors: Daniel Ian Flitcroft (1,2), Elise N Harb (3), Christine Frances Wildsoet (3)

1. Ophthalmology, Children's University Hospital, Dublin, Ireland
2. Technological University of Dublin, Dublin, Ireland
3. School of Optometry, University of California, Berkeley, California, United States

Date: Sep 2020

Reference:   Flitcroft DI, Harb EN, Wildsoet CF. The Spatial Frequency Content of Urban and Indoor Environments as a Potential Risk Factor for Myopia Development. Invest Ophthalmol Vis Sci. 2020 Sep 1;61(11):42.

[Link to open access paper]

Animal studies^1-5 have shown that reduced retinal image quality can be a risk factor for form-deprivation myopia and can be induced by the use of diffusing filters.

The purpose of this study was to investigate how differences in spatial frequencies between natural, urban and indoor environments may feature in myopia development, where reduced spatial frequencies may act in a similar fashion to diffusing filters.

Images of differing environments taken at the campus of the University of California (UCB) were classified as being either natural, mixed urban or man-made (indoor and outdoor).  The illuminance and spatial frequency slopes of each were analysed before being compared to similar images from open access image libraries at the University of Texas (UT) and from University of Pennsylvania (UPenn).

The Berkeley and Texas images were also repeated with a 0.4 Bangerter foil filter in front of the camera.  This filter has been found to induce myopia in animal studies.^2,3 The analysis of all the images, with and without a Bangerter foil, included plotting a relationship between log amplitude and log spatial frequency in cycles per degree.

The spatial frequency analysis found that:

• Images with no man-made features had mean spatial frequency slopes of -1.08, -0.90 and -1.04 for the UCB, UT and UPenn images, respectively.
• Previously discovered slope values for natural images have recorded an average of -1.0 where there is a proportional relationship between spectral amplitude decreasing with increasing spatial frequency.
• Natural outdoor scenes were found to follow patterns seen by Field⁶ and have higher spatial frequencies and mean spatial frequency slopes from -0.90 to -1.08.
• Urban man-made outdoor settings were found to have mean slopes of -1.29 and -1.22 for UCB and UT, respectively.
• Indoor scenes gave greater slopes of -1.48 and -1.52 for UCB and UT respectively. The difference between these values and those of natural scenes were found to be consistently significant for both UCB and UT images.

The 0.4 Bangerter foil was shown to reduce the amplitude of mid-high spatial frequencies.

• A tree photographed with and without a Bangerter foil filter in front of the camera lens had a SF slope value of -0.88 without the filter, but an SF slope value of -1.47 with the filter.
• This was a similar value seen for the indoor spatial frequency slopes from both UCB and UT (-1.48 and -1.52 respectively).

Illumination was found to give the greatest difference between natural and indoor scenes.  

• At UCB, the mean values were 14,200 for outdoor compared to 420 lux for the indoor and at UT, illumination was 11,200 lux outdoors and 430 lux indoors.

In conclusion, images of natural outdoor, mixed urban and indoor settings were found to show different spatial frequency slopes, the steeper representing reduced spatial frequency for indoor environments.  The steeper slope values mirrored those found in animal studies where diffusing filters were able to induce myopia.

Reduced spatial frequency environments may therefore present a risk for myopia development.  This may help explain why time spent outdoors (higher spatial frequencies) is thought to have a protective effect and time indoors with lower spatial frequencies does not. Improving the spatial frequencies of man-made urban and indoor settings to mimic the natural outdoors may counteract a myopia stimulus.

This study showed that the more man-made an environment was, the less spatial frequency and illumination it contained and the reduction in spatial frequencies for indoor environments was comparable to that induced by 0.4 Bangerter foils shown to induce myopia in animal studies.

This indicates the following explanations of the benefits of outdoor time to parents of children at risk of myopia:

• The benefits of time spent outdoors relates to the brightness of light, but also the different type of image across the retina from a natural versus a man-made environment.
• Mixed urban areas that feature a mix of man-made and natural elements could be more beneficial to visual development (increased spatial frequency) than settings with man-made features only.

1. Could urban and indoor environments be altered to improve the spatial frequency and reduce potential myopic stimulus by incorporating more greenery and/or images of natural scenes?
2. This study only examined the spatial frequency slopes of still images. Could the motion of environments create differing signals of spatial frequency and illumination? Investigating how the eye receives information from static and dynamic retinal images could reveal more about eye growth regulation and its influence on myopia development.
3. Which is more important - higher illuminance or higher spatial frequency? Both were found outdoors. Although spatial frequency is not dependent on degree of illuminance, further research into how higher outdoor illuminance may block form-deprivation signalling could establish how these two elements could combine to reduce myopia.

Title: The Spatial Frequency Content of Urban and Indoor Environments as a Potential Risk Factor for Myopia Development

Authors: Daniel Ian Flitcroft, Elise N Harb, Christine Frances Wildsoet

Purpose: To examine the hypothesis that the spatial frequency spectra of urban and indoor environments differ from the natural environment in ways that may promote the development of myopia

Methods: A total of 814 images were analyzed from three datasets; University of California Berkeley (UCB), University of Texas (UT), and Botswana (UPenn). Images were processed in Matlab (Mathworks Inc) to map the camera color characteristics to human cone sensitivities. From the photopic luminance images generated, two-dimensional spatial frequency (SF) spectra were calculated and converted to one-dimensional spectra by rotational averaging. The spatial filtering profile of a 0.4 Bangerter foil, which has been shown to induce myopia experimentally, was also determined.

Results: The SF slope for natural scenes followed the recognized 1/fα relationship with mean slopes of -1.08, -0.90, and -1.04 for the UCB, UT and UPenn image sets, respectively. Indoor scenes had a significantly steeper slope (-1.48, UCB; -1.52, UT; P < 0.0001). Urban environments showed an intermediate slope (-1.29, UCB; -1.22, UT) that was significantly different from the slopes derived from the natural scenes (P < 0.0001). The change in SF content between natural outdoor scenes and indoors was comparable to that induced by a 0.4 Bangerter foil, which reduced the SF slope of a natural scene from -0.88 to -1.47.

Conclusions: Compared to natural outdoor images, man-made outdoor and indoor environments have spatial frequency characteristics similar to those known to induce form-deprivation myopia in animal models. The spatial properties of the man-made environment may be one of the missing drivers of the human myopia epidemic.

[Link to open access paper]