Two-photon visual sensitivity of human cones - a psychophysical study

Abstract

Purpose : Two-photon vision arises from the perception of pulsed infrared laser light as color corresponding to approximately half of the laser wavelength. The effect is generated by two-photon absorption in photoreceptors using a safe laser power. The physical process responsible for two photon vision has been identified and verified experimentally quite recently; however it is yet not clear how such phenomena could impact an ophthalmic diagnosis. Therefore, it is critical now to identify differences between two-photon and single photon vision.

Methods : A custom designed instrument for studying two-photon vision enabled to perform psychophysical tests of visual sensitivity thresholds and dark-adaptation in selected retinal locations. During the measurements visible (520 nm) and infrared (1040 nm) laser beams with pulse duration of 1-3 ps and repetition rate of 86 MHz were sequentially scanned on the subject retina. Stimuli consisted of a flickering circle of 0.7 deg diameter. The method of adjustments was used for determining visual sensitivity threshold.
All tests in human subjects complied with the Declaration of Helsinki, and ANSI Z136.1:2014. The study was approved by the Ethics Committee of the Collegium Medicum, NCU.

Results : Dark-adapted visual sensitivity thresholds were measured in 7 healthy subjects at the fovea (T_0deg) and a location 9.6 deg temporal (T_9.6deg). Both beams, i.e. 1040 nm and 520 nm were perceived as green. The ratio R of the sensitivity thresholds, R=T_0deg/T_9.6deg, was substantially smaller (3-7 times) for the 1040 nm stimulus than the 520 nm stimulus.
Dark adaptation measurements revealed the differences between 1040 nm and 520 nm in the cone-mediated portion. The increase of cone visual sensitivity threshold with respect to the rod threshold for the two-photon stimulus (1040 nm) was smaller compared to the single-photon stimulus (520 nm). The observed cone threshold increase at 1040 nm was approximately 1 order of magnitude lower than at 520 nm, although both stimuli are perceived as green.

Conclusions : Differences in visual sensitivity thresholds and in the cone-mediated portion of the dark-adaptation curves between the 520 nm and 1040 nm provide evidence that the 1040 nm stimulus, although perceived as green, is in fact processed differently by the visual system. Our data indicate that instrument based on two-photon visual sensitivity measurements provides additional information about retinal function.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.