Reading speed during a mesopic visual acuity task

Abstract

Corrected myopes demonstrate reduced visual acuity and less stable fixational eye movements under mesopic conditions, compared to subjects without refractive error (Coletta et al., ARVO, 2007; Coletta et al., ARVO, 2016; FVM, 2016). Reduced acuity in myopia may be related to fixation instability, perhaps analogous to amblyopia, where less stable eye movements are associated with reductions in acuity and reading speed (Kanonidou et al., IOVS, 2010). In this study, we examined whether a subject's acuity and fixation stability were related to the time they spent to read an acuity line. Measurements were made on 31 subjects, mean age 25.7 years. Spherical equivalent refractions ranged from +2.25 to -10.88 D. Mesopic (0.28 cd/m2) and photopic (325 cd/m2) acuity was measured with high contrast letters, five per line, while eye position was recorded at 500 Hz using an EyeLink 1000 video-based eye tracker. Fixations were analyzed for letter sizes corresponding to the mesopic and photopic acuity limits. Fixation stability is reported as the bivariate contour ellipse area (BCEA), taken over a 2-sec interval, representing the area of fixation for 68% of the time. At the mesopic condition, subjects who took longer to read a line at their acuity limit had higher mesopic acuity (p=0.001) and smaller loss of acuity relative to their photopic acuity (p=0.025). However, there was no correlation between reading duration at the photopic acuity limit and photopic acuity (p>0.05), nor between reading duration and refractive error (p>0.05). When the mesopic acuity letter size was read at the photopic light level, BCEA decreased with increasing reading duration (p=0.032). For an acuity task in dim light, reading speed was correlated with acuity, but in an opposite direction to that reported for amblyopia; slower reading speed was associated instead with increased visual performance. Reading speed was not associated with refractive error. Meeting abstract presented at VSS 2017