Contrast threshold recovery after bleaching during cone‐mediated mesopic adaptation: The effect of test conditions

Abstract

Aims/Purpose: To determine the optimal luminance conditions and the staircase procedures for evaluating the time course of macular contrast sensitivity recovery after photobleaching in order to decide the best protocol to assess cone‐mediated mesopic adaptation.Methods: The dynamics of contrast threshold (CT) recovery following photopigment bleach were measured psychophysically for five minutes in twenty‐four healthy adults (23.7 ± 3.7 years old). The stimuli used were sine‐wave gratings of low‐spatial frequency (1 cycle‐per‐degree). In each subject, six tests were performed, combining three luminance levels (0.1, 1.0, and 10 cd/m2) and two modified staircase procedures (3‐down/1‐up and 2‐down/1‐up) to estimate CTs. The test order was randomized, conducted in two visits, with a 15‐min washout period between tests. Outlier detection techniques were applied to verify the integrity of the sample. Non‐linear exponential‐based optimization was adopted to fit CT recovery functions to estimate the time constant (τ, seconds) of cone sensitivity recovery and the final CT (CTf, log units). Classical and heteroscedastic ANOVA analyses were performed to assess the effect of test conditions.Results: The 2‐down/1‐up staircase allowed for denser point sampling of CTs measurements than the 3‐down/1‐up. Under the lowest luminance test condition, the CT recovery was modelled by bumpier functions, for both staircases procedures, than the other luminance levels. Mean cone τ increased (21.7, 42.5, and 72.0 seconds (p < 0.001)) and CTf worsened (−1.99, −1.85, and − 1.47 log units (p < 0.001)) as luminance level decreased. The best fit (median R2 = 0.87) was achieved under 1 cd/m2 mesopic luminance conditions. There were no statistically significant differences in mean cone τ and CTf between the two staircases procedures.Conclusions: An intermediate mesopic light level and a 2‐down/1‐up staircase provided the best protocol for assessing cone‐mediated mesopic adaptation.