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Phillip T. Yuhas, Holly E. Moose, Andrew T. Hartwick; Flicker Sensitivity of the Human Pupillary Light Reflex. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4824.
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In addition to rods and cones, intrinsically photosensitive retinal ganglion cells (ipRGCs) contribute to the pupillary light reflex in mammals, including humans. Our aim was to characterize the flicker sensitivity of the human pupillary light reflex and isolate the ipRGC-driven component of the pupil response based on the unique temporal properties of these photoreceptors.
After 20 min dark adaptation, flickering light stimuli were presented monocularly to the dilated left eye of 10 healthy human subjects (age: 23-26). The right pupil was recorded with an infrared light-sensitive camera. In the first experiment, 6 subjects sat for two separate sessions investigating the effect of stimulus intensity (range: 1012 to 1015 phots/s/cm2) on the pupillary light response to red (625 nm) or blue (470 nm) light flickering on and off slowly (rate: 0.1 Hz). In the second experiment, 4 subjects sat for two separate sessions investigating the effect of flicker frequency (range: 0.05 to 1.0 Hz) on the pupillary light response to red or blue light (intensity: 1014 phot/s/cm2). Pupil diameters were measured offline with ImageJ (NIH) software and normalized to peak constriction.
Using Fourier analysis on the normalized data, the magnitude of the pupil flicker response decreased from 21.6 to 9.7% as the intensity of the flashing blue light intensity increased from 1x1012 to 1x1015 phot/s/cm2. The change in the magnitude of the pupil flicker response (23.1 to 15.5%) to increasing intensities of red light (7x1012 to 7x1015 phot/s/cm2) was significantly less than that observed in the blue light experiments. As the frequency of the presentation of the light flashes slowed from 1.0 Hz to 0.05 Hz, the amplitude of the pupil flicker increased from 0.7 to 7.2% with the blue light stimuli and from 0.8 to 12.3% for the red light stimuli. There was a significant difference (p < 0.05) in the amplitude of the pupil flicker response elicited by the two light stimuli (blue vs. red) when presented at frequencies slower than or equal to 0.2 Hz.
With brighter intensities of a slowly flashing light stimulus, there was a decrease in the amplitude of the flicker of the pupil response. The dampening in pupil flicker due to the bright blue light stimuli was more pronounced than that observed with bright red light stimuli. The significant difference in the pupil response to slowly flashing blue versus red light is consistent with an ipRGC critical duration longer than 2 s. We propose that the reduced pupil flicker observed with the blue light stimuli was due to an increased ipRGC contribution, which have sluggish response characteristics. A pupil flicker fusion test has potential as a clinical tool for assessing ipRGC function in patients with optic nerve disease.
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