The effect of contrast polarity on visual acuity in myopes and emmetropes

Abstract

Purpose : Exposure to retinal image blur, as well as the relative stimulation of the retinal ON- and OFF-pathways are known to play a role in the regulation of eye growth and the development of myopia. This study aimed to explore the effect of relative activation of retinal ON/OFF pathways, through changing the contrast polarity of the visual acuity (VA) chart, upon measures of VA under clear and defocused conditions, in myopic and emmetropic participants.

Methods : Twenty-four healthy young adults aged 23 ± 1 years with normal corrected vision were classified as emmetropes (n = 12, mean SE = -0.07 ±0.31D) or low myopes (n = 12, mean SE = -1.59 ± 0.72D). Cycloplegic VA of the right eye was measured using a chart with positive polarity showing dark letters on a bright background to overstimulate the OFF-pathway, and with negative polarity showing light letters on a dark background to overstimulate the ON-pathway, at different levels of defocus, including +1D, 0D, and -1D. Pupil size was controlled using an artificial pupil of 3.5 mm diameter. Room lighting was maintained at 360 lux, mean chart luminance was 65 cd/m², and letter contrast was 87% across all conditions. RM-ANOVA was used to assess the effect of contrast polarity on the VA of myopes and emmetropes under different levels of defocus.

Results : VA averaged across all levels of defocus was better with negative compared to positive polarity by 0.04 ± 0.01 logMAR (p = 0.004). The main effect of defocus was significant, with both myopic (+) and hyperopic (-) defocus reducing the VA significantly from the no defocus condition by 0.28 ± 0.03 and 0.23 ± 0.02 logMAR respectively (p < 0.001), with these changes not different between contrast polarities. Mean VA was not significantly different between the refractive groups (p = 0.44). While the refractive group by contrast polarity interaction was also not significant (p = 0.27), a trend was observed indicating a relative reduction in VA under positive polarity versus negative polarity in myopes by 0.06 ± 0.02 logMAR, but not in emmetropes (difference of 0.03 ± 0.02, Fig 1).

Conclusions : These results suggest that visual acuity is improved during overstimulation of the ON-pathway, independent of low levels of retinal image defocus. Further studies with greater sample size are required to draw more definitive conclusions on the impact of myopia upon changes in VA associated with relative ON/OFF-pathway overstimulation.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.