With stimulus-locked PLR, shorter latency responses allow less time for the development of confounding influences of nonphotic processes to pupil diameter changes. This is especially true in severe LCA eyes with substantial oculomotor instability. We hypothesized that PLR latencies may be accelerated with the use of higher luminance stimuli.
Figure 4 presents a combination of results obtained with both the standard and higher luminance pupillometers in a subset of four patients (P7, P10, P11, and P14). Nondetectable PLRs were confirmed with 0.1-second-long stimuli at luminances of 1.9, 2.5, and 2.9 log scot-cd.m
−2 and with 1-second long stimuli at luminances of 0.9 and 1.9 log scot-cd.m
−2 (
Figs. 4A,
4B). On the other hand, detectable PLRs were recorded with 0.1-second-long stimuli at 3.9 log scot-cd.m
−2, and with 1-second-long stimuli at 2.9 log scot-cd.m
−2 (
Figs. 4A,
4B). These results suggested a PLR threshold occurring above 1.9 but below 2.9 log scot-cd.s.m
−2 for a range of pupillometer II white stimuli between 0.1 and 1 second in duration. Pupillometer I results demonstrating a PLR threshold above 1.5 but below 3.2 log scot-cd.s.m
−2 for white stimuli between 0.1 and 5 seconds in duration (
Figs. 4A,
4B) were consistent also. Interestingly, 0.1- and 1-second stimulus conditions appear to show near complete temporal summation and a faster response, whereas the 5-second stimulus evoked a slower (
Fig. 4C) and smaller (
Fig. 4D) response, suggesting that the reciprocity between irradiance and duration may start to break down between 1 and 5 seconds in direct PLR recordings using a natural pupil. Potential contributors to this observation may include light adaptation of ipRGCs.
45 Even though a detailed examination of the temporal tuning properties of the PLR detected in severe LCA was beyond the scope of the current work, our results provide some insight to the limits of reciprocity between stimulus irradiance and duration, and suggest an ideal stimulus duration of 1 second or shorter and a stimulus luminance of 2.9 log scot-cd.s.m
−2 or higher, which is similar to the conclusion reached by previous investigators.
8