Fig. 4: Rod-medicated dark adaptation in normal ageing and age-related macular degeneration.

With our colleague Gregory Jackson, we developed a new metric for rod-mediated dark adaptation, called the rod intercept time (RIT). Computing the slope of the second component from dark adaptation data requires model-fitting [72], but using RIT as a surrogate for the slope of the second rod component was much simpler. During dark adaptation testing, RIT is the duration required for sensitivity to recover to a criterion sensitivity value in the latter half of the second component of recovery [80]. This sensitivity criterion is illustrated by the horizontal dotted line at 3 log units of sensitivity. The arrows descending to the x-axis illustrate RIT. Higher RIT indicates slower recovery. Representative plots from two older adults in normal macular health and four with early age-related macular degeneration (AMD), depicting the time-course of recovery for a target at 5 degrees following a photo-bleach of 83%. The early component of cone-mediated recovery is followed by rod-mediated sensitivity recovery. Note the dramatically slowed rod-mediated recovery in early AMD eyes compared to those in normal macular health. In some early AMD eyes, they did not recover sensitivity during the 20-min period; some AMD eyes take much longer than 20 min to recover. Used with permission from the Association for Research in Vision and Ophthalmology through the Creative Commons.