Two different visual stimuli that cause axial eye shortening and choroidal thickening have no additive effect

Abstract

Aims/Purpose: Previous studies identified two visual stimuli that can make the human eye shorter and potentially inhibit myopia: [1] watching digitally filtered movies that kept the red channel in focus but were low pass filtered in the blue and [2] reading text with inverted contrast. We have tested whether the two mechanisms may be additive, perhaps causing even more inhibition of eye growth.Methods: Fifteen emmetropic (average refraction: −0.6 ± 0.7 D) and fifteen myopic (average refraction: −3.7 ± 1.8 D, range −6 to −1.5D) young subjects (average age: 26 ± 4 years) read text from a large screen (65″) in a dark room at 2 m distance. Text with inverted contrast (bright letters on dark background) was digitally filtered in real time by custom‐developed software written in Visual C++ to present the red channel of the RGB colour format unfiltered while green and blue were blurred (red in focus) accordingly to the human longitudinal chromatic aberration function. Changes in axial length before and after 30 min of reading were measured using low coherence interferometry with auto positioning system (Lenstar LS‐900).Results: Unexpectedly, in emmetropes, significant eye elongation was found after 30 min of reading text with inverted contrast, combined with the red in focus filter (+5.7 ± 15.3 μm). As previously found, unfiltered text with inverted contrast made eyes shorter (−7.9 ± 14.7 μm, p < 0.05). Strikingly, no such changes were observed in myopic eyes, neither with the red in focus filter, nor when text with inverted contrast remained unfiltered.Conclusions: In emmetropes, the two stimuli had no additive effects but rather cancelled each other out. There may be a simple explanation: the red in focus filter made the text very blurry in the blue channel. Apparently, this removed the spatial information that is needed by the retina to detect defocus in the blue. Therefore, our finding supports the view that the emmetropic retina needs some spatial information in the blue to generate eye growth inhibiting signals—and they were lost in myopes.