Pupillary light reflex in two-photon vision experiments

Abstract

Purpose : Two-photon vision depends on the perception of NIR pulsed laser beams as having color of half of the laser wavelength and is caused by two photon absorption in human photoreceptors. We observed the pupillary light reflex (PLR) upon two-photon stimulation and compared it with PLR for visible light. The aim of this study is to establish how a human pupil reacts to a two-photon stimulus and if this effect can be an objective measure of the perception of this kind of stimuli.

Methods : We measured PLR following 1040nm and 520nm scanning pulsed picosecond laser beams (F_rep=76MHz) for 5 healthy subjects (age 25-40, 3 female, 2 male). Power of 1040 nm beam was 0.6mW and 0.8mW (17.4 and 17.5 log phot/cm²s at pupil plane, respectively). Intensity of visible stimulus was adjusted by each subject to obtain brightness close to IR one. Spiral donut optotype of external radius 3.5deg was displayed for 10s at the center of the fovea under scotopic conditions. The eye was fixated to the red point (630nm) at the center of stimulus and illuminated with an 860nm LED. The images of pupil were registered by a CMOS camera for subsequent diameter and position determination.
The study was approved by the Ethics Committee of the Collegium Medicum, NCU.

Results : Most of the obtained time courses (Fig. 1a and b) show typical shape of PLR. In 4 of 5 subjects, mean pupil diameter decreased noticeably (by 6-11%) for both stimuli. 2 subjects exhibited more pronounced PLR for 520nm (change of 20%) than for 1040nm. One subject showed no statistically significant constriction for either stimulus. Differences in reactions to visible beam might have been caused by imperfections of subjectively adjusted intensities of visible stimuli (Fig. 1c).

Conclusions : The PLR upon two-photon stimulation was shown for the first time. Registered constrictions for IR stimuli were below 90% of initial diameter suggesting that the brightness of two-photon stimuli is relatively small: IR stimulus of 17.5 log phot/cm²s corresponds to visible stimulus of 10 log phot/cm²s. In our tests 2 subjects reacted slightly weaker to 1040nm stimulus than to the 520nm one. However, these differences might be due to inaccurate adjustment of equal brightness. Further tests with more subjects will be performed soon.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Fig. 1. a) and b) Pupil diameter vs time from stimulus for 520nm and 1040nm (mean of 3 trials for representative subject). c) Powers corresponding to subjective equal brightness of both stimuli.