The "speed" of acuity in scotopic vs. photopic vision

Abstract

Purpose: The effect of duration of optotype presentation on visual acuity measures has been extensively studied under photopic conditions. However, systematic data on duration dependence of acuity values under mesopic and scotopic conditions is scarce, despite being highly relevant for many visual tasks including night driving, and for clinical diagnostic applications. The present study aims to address this void.

Methods: We measured Landolt C acuity under photopic (90 cd/m²), mesopic (0.7 cd/m²), and scotopic (0.009 cd/m²) conditions for several optotype presentation durations ranging from 0.1 to 10 s using the Freiburg Acuity and Contrast Test. Two age groups were tested (young, 18-29 years, and older, 61-74 years).

Results: As expected, under all luminance conditions, better acuity values were found for longer presentation durations. Photopic acuity in young participants decreased by about 0.25 log units from 0.1 to 10 s; mesopic vision mimicked the photopic visual behavior. Scotopic acuities depended more strongly on presentation duration (difference > 0.78 log units) than photopic values. There was no consistent pattern of correlation between luminance conditions across participants. We found a qualitative similarity between younger and older participants, despite higher variability among the latter and differences in absolute acuity: Photopic acuity difference (0.1 vs. 10 s) for the older participants was 0.19 log units, and scotopic difference was > 0.62 log units.

Conclusion: Scotopic acuity is more susceptible to changes in stimulus duration than photopic vision, with considerable interindividual variability. The latter may reflect differences in aging and sub-clinical pathophysiological processes and might have consequences for visual performance during nocturnal activities such as driving at night. Acuity testing with briefly presented scotopic stimuli might increase the usefulness of acuity assessment for tracking of the health state of the visual system.

Keywords: Mesopic vision; Photopic vision; Scotopic vision; Temporal factors; Visual acuity.

Fig. 1

Results (logMAR) of individual young participants (thin lines) and respective median values (thick lines) for all three luminance conditions and all five presentation durations. For all luminance conditions, acuity values improved with both increasing presentation duration and increasing luminance. See text for display limitations of scotopic logMAR at 0.1 s

Fig. 2

LogMAR differences between mesopic and photopic conditions (red) and between scotopic and photopic conditions (blue) of young participants. Shaded areas indicate the 95% confidence intervals. While there is no dependence on presentation duration for the mesopic vs. photopic logMAR difference, the scotopic vs. photopic logMAR differences are larger for the short presentation durations than for the long presentation durations. This suggests scotopic processing to involve additional integration processes

Fig. 3

Scotopic (blue) and mesopic (red) logMAR values (ordinate) of young participants compared with photopic values (abscissa). The parametric covariance ellipses have been added to provide an approximate visualization of the data’s structure

Fig. 4

Results (logMAR) of individual participants above 60 years of age (thin lines) and respective median values (thick lines) for all three luminance conditions and all five presentation durations. For all luminance conditions, acuity values improved with both increasing presentation duration and increasing luminance. See text for display limitations of scotopic logMAR at 0.1 s

Fig. 5

Comparison of median logMAR values of both young (open symbols) and older (filled symbols) participants. While acuity was generally better for young participants (i.e., logMAR values were lower), the dependence on presentation duration was similar

Fig. 6

logMAR differences between mesopic and photopic conditions (red) and between scotopic and photopic conditions (blue) of participants above 60 years of age. Shaded areas indicate the 95% confidence intervals. While there is no sizable dependence on presentation duration for the mesopic vs. photopic logMAR difference, the scotopic vs. photopic logMAR differences is significantly larger for short presentation durations than for long durations