Reading and Myopia: Contrast Polarity Matters

Abstract

In myopia the eye grows too long, generating poorly focused retinal images when people try to look at a distance. Myopia is tightly linked to the educational status and is on the rise worldwide. It is still not clear which kind of visual experience stimulates eye growth in children and students when they study. We propose a new and perhaps unexpected reason. Work in animal models has shown that selective activation of ON or OFF pathways has also selective effects on eye growth. This is likely to be true also in humans. Using custom-developed software to process video frames of the visual environment in realtime we quantified relative ON and OFF stimulus strengths. We found that ON and OFF inputs were largely balanced in natural environments. However, black text on white paper heavily overstimulated retinal OFF pathways. Conversely, white text on black paper overstimulated ON pathways. Using optical coherence tomography (OCT) in young human subjects, we found that the choroid, the heavily perfused layer behind the retina in the eye, becomes about 16 µm thinner in only one hour when subjects read black text on white background but about 10 µm thicker when they read white text from black background. Studies both in animal models and in humans have shown that thinner choroids are associated with myopia development and thicker choroids with myopia inhibition. Therefore, reading white text from a black screen or tablet may be a way to inhibit myopia, while conventional black text on white background may stimulate myopia.

Figure 1

Analysis of the relative strength of ON and OFF stimulation when looking at (A) dark text on bright background or (B) bright text on dark background. The average luminance of both pictures was matched (top). On the bottom, the output of the software is shown. Green lines indicate the relative strength of ON stimulation, red lines of OFF stimulation, plotted over a range of spatial frequencies (from 22.4 to 2.24 cyc/deg when measured with a 16 mm camera lens). Note that dark text on bright background always overstimulates OFF pathways while bright text on dark overstimulates ON pathways.

Figure 2

(A) Sample picture of the foveal area, obtained from the Spectralis OCT. The measurement procedure to determine subfoveal choroidal thickness is illustrated (yellow bar, between the two yellow lines; see also the methods section), together with the average effect size observed in the current study. (B) Absolute choroidal thickness in both eyes of the seven subjects is shown at the beginning of the reading task, and after 30 and 60 minutes. Box in yellow: average effects in all subjects. Note that reading white text on black background (ON stimulus, denoted in green) causes choroidal thickening while black text on white background caused choroidal thinning (OFF stimulus, denoted in red). Triangles denote right eyes, circles left eyes. Error bars are standard deviations. Significance levels after 60 minutes: ***p < 0.001, **p < 0.01, *p < 0.05 (two-sided t-tests, no multiple comparisons).

Figure 3

(A) Procedure to determine the relative ON and OFF input strength in a scene. Software scanned the video with 285,200 “receptive fields” spaced at regular intervals across the picture. Receptive fields had a simple structure as illustrated by the arrays of green and red dots. The average gray level of 8 peripheral pixels (red) was subtracted from the grey level of the center pixel (green). If the result was positive, the position in the image was considered ON stimulating, if negative, OFF stimulating. The analysis was done simultaneously for different receptive field sizes, and accordingly at different spatial frequencies. The highest sampled spatial frequency by receptive field “1” was about 22.4 cyc/deg with a 16 mm camera lens. Receptive field “2” sampled about 11.2 cyc/deg. The lowest sampled spatial frequency was 2.24 cyc/deg, with the peripheral pixels at a distance of 10 pixels. (B) Output (screenshot) of the software. The bottom left picture shows ON outputs in red, OFF outputs in blue. On the right, the sum of all ON and OFF outputs is shown for different spatial frequencies. For the picture above, the sum of all ON and OFF responses were very similar.