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Boris Gramatikov, Kristina Irsch, David Guyton; Pupil Size Dynamics During the First Minutes of Dark Adaptation While Fixating on a Target. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4374.
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Pupil size can directly affect the amount of light that reaches retinal structures and is returned by them in double-pass measuring technology, such as scanning laser ophthalmoscopes, retinal birefringence scanners, OCT devices etc. Numerous publications describe the acute “light-off” effect and the slower dark adaptation, but there appears to be very little information on how pupil size changes in the first several minutes after ambient lights are turned off and while the subject is fixating on a target. Fixation is known to cause pupil constriction, which counteracts pupil dilation.
Subjects were asked to fixate on a white-light accommodative target (2mm x 1mm), optically 33 cm from the eye, after which ambient illumination was turned off. Pupil diameter was measured under monocular conditions with an eye tracking apparatus using video-oculography, comprising an infrared sensitive USB video camera equipped with a 12 mm fixed-focal-length lens. Illumination was provided by a NIR LED. The camera was connected to a PC that controlled video frame capture using custom acquisition software. The recorded eye image sequences were analyzed off-line. Pupil diameters were calculated with commercial eye tracking software. Pupil area was calculated based on the diameter measured. The pupil area traces were normalized, each by its baseline value at the initial moment when the light was turned off. Five subjects, age 28-60 were studied.
Figure 1 shows the non-normalized trace from one subject as pupil area vs. time, plotted over 6 minutes after the lights were turned off. Figure 2 shows the normalized traces of all subjects. In addition to the individual traces, the average trace is also shown (thick cyan line). The dilation reaches a maximum of ca. 60% above the baseline level at a time of ca. 40-50 s. Approximately 50 s later, the average normalized curve starts descending exponentially toward the baseline.
Despite a significant variance largely due to insufficient accommodation attempt, it can be concluded that measurements between 22 s and 140 s are likely to be performed at a pupil area at least 50% larger than the baseline. This may be taken into consideration when optimizing the time window for measurements on retinal structures with double pass systems, with subjects fixating on an accommodative target.
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