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Pupillary Light Reflex Induced by Two-Photon Vision.

Agnieszka Zielinska1, Piotr Ciacka2,3, Maciej Szkulmowski1

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Summary

Two-photon infrared light elicits a weaker human pupil reaction (pupillary light reflex) compared to visible light stimuli. This suggests reduced stimulation of photoreceptors in two-photon vision.

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Area of Science:

  • Ophthalmology
  • Vision Science
  • Photobiology

Background:

  • Two-photon vision utilizes pulsed infrared light, relying on two-photon absorption in visual pigments.
  • The pupillary light reflex (PLR) is a fundamental indicator of visual system function.

Purpose of the Study:

  • To quantify the human pupil reaction to a two-photon 1040-nm infrared (IR) stimulus.
  • To compare this response with pupil reactions elicited by 520-nm visible (VIS) stimuli of equivalent perceived brightness.

Main Methods:

  • Pupillary light reflex was measured in 14 dark-adapted subjects using fovea-centered stimuli (3.5° diameter).
  • Stimuli included spirals of 1040-nm IR laser, 520-nm VIS laser, and a uniformly filled 520-nm VIS light-emitting diode (LED) circle.
  • Visible stimulus power was adjusted to match the perceived brightness of the 800 µW IR stimulus.

Main Results:

  • The IR laser stimulus resulted in a significantly larger minimum pupil diameter (88% ± 10% of baseline) compared to VIS laser (74% ± 10%) and VIS LED (69% ± 9%).
  • Mean constriction velocity and time to maximum constriction were significantly slower for the IR stimulus.
  • Pupil response latency was similar for IR and VIS lasers, and slightly shorter for VIS LED.

Conclusions:

  • Two-photon stimulation with IR light elicits a weaker pupil response than one-photon VIS stimuli of comparable brightness and color.
  • This diminished response may stem from weaker two-photon stimulation of rods relative to cones.
  • Reduced perception of stray light in two-photon vision could further decrease photoreceptor stimulation and weaken the PLR.