Each stimulus was repeated six times during the test in staggered fashion. Eight stimulus intensities were presented over a 3.5 log unit range (35 dB range −44 to −9 dB of attenuation above a background of 3.1 apostilbs −44 dB attenuation = 0.13 cd/m 2, −0.9 dB = 400.7 cd/m 2). The stimulus duration was well within the latency time of the pupil light reflex so that the entrance pupil size was not affected during the time that the stimulus was on. The stimulus duration was 0.2 seconds and the time between light stimuli was 3.3 seconds. A flat black metal septum separated the right and left eye optical pathways to minimize stray light scatter. The refractive power of the instrument was adjusted by entering the refraction of the subject into the instrument's software so that the focal point was set at infinity to control accommodation. During the test, the stimulated eye was allowed to foveate on a small central cross before the ensuing light stimulus to control fixation. Each subject was adapted to a 3.1 apostilb background light for 30 seconds, then a sequence of light stimuli was presented alternately to each eye at varying intensities above the background level. Pupil responses to light stimuli were recorded using a computerized infrared pupillometer (Visual Pathways, Inc., Prescott, AZ) which presented a 30° radius light stimulus to each eye in non-Maxwellian view. This has important implications in understanding the potential influence of anisocoria on the RAPD and also greater susceptibility of lightly pigmented eyes to light toxicity. However, retinal illumination of lightly pigmented eyes is relatively independent of entrance pupil size, presumably due to extrapupillary transmission of light through the iris and sclera. In darkly pigmented eyes, entrance pupil size significantly influenced the retinal illumination. However, anisocoria correlated with RAPD only in subjects with darkly pigmented irides (Pearson correlation coefficient 0.793, P = 0.05). Induced anisocoria produced a significant change in RAPD from baseline (mean = 1.60 dB in the miotic eye, P = 0.007). The main outcome measure was the RAPD, determined by computerized pupillography, at baseline and after pilocarpine-induced anisocoria. The interocular difference in retinal illumination was assessed by computerized pupillometry from the stimulus response curve of the right and left eyes. Miosis was induced by topical 1% pilocarpine in the right eye of 14 healthy subjects with normal eyes. The influence of unilateral miosis on the magnitude of the pupil light reflex was studied to ascertain how a clinically significant anisocoria influences the relative afferent pupil defect (RAPD). We determined the effect of entrance pupil size on retinal illumination.
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