The Effect of Exposure Duration on Visual Acuity for Letter Optotypes Depends On How Visual Acuity is Defined

Abstract

Purpose: Increasing exposure duration improves visual acuity (VA) for letter optotypes, as defined by the standard logarithm of the minimum angle of resolution (log MAR) scale. This study determined the extent to which VA, expressed in terms of retinal frequency (cycles per degree; cpd), improves with increasing duration after accounting for the object frequency information mediating performance (cycles per letter; cpl).

Methods: Log MAR VA of four visually-normal individuals (ages 25 to 35) was measured for a set of tumbling E optotypes presented on a CRT display for presentation durations ranging from 0.02 to 1.0 s. The Es were either unblurred or blurred through convolution with Gaussian functions of different widths (σ_stim), which permitted object frequency to be derived. Log MAR values were plotted as a function of log σ_stim and fit with the function: MAR = MAR₀[1 + (σ_stim/ σ_int)²]^0.5, where MAR₀ and σ_int set the vertical and horizontal positions of the function, respectively. The object frequencies mediating VA were derived from MAR₀ and σ_int as: cpl = 5*MAR₀*1/(2π*σ_int). The retinal frequencies mediating VA were derived from MAR₀ and cpl as: cpd = 12*cpl/ MAR₀.

Results: Log MAR for the unblurred E decreased (VA improved) significantly (p < 0.05) by a factor of 1.6 as duration was increased from 0.02 to 0.50 s, but log MAR was constant for durations longer than approximately 0.50 s. These findings are consistent with previous reports. Increasing the exposure duration had similar effects on log cpl, a relationship that has not been reported previously. That is, from 0.02 to 0.50 s, cpl decreased significantly (p < 0.05) by a factor of approximately 1.6, and was constant for longer durations. However, VA expressed in cpd was independent of exposure duration. This constancy is due to the offsetting effects of the decrease in letter size (increase in cpd) and the decrease in cpl (decrease in cpd), as indicated in the definition of retinal frequency.

Conclusions: The extent to which VA for broadband optotypes improves with increasing exposure duration depends on how VA is defined. If it is assumed that object frequency is independent of duration (i.e. the log MAR scale), then VA improves as duration is increased up to approximately 0.50 s. However, if both size and object frequency are included in the definition of VA (i.e. cpd), then VA is approximately independent of duration.

Keywords: 754 visual acuity • 730 temporal vision • 641 perception