Characterization of two-photon vision by measurements of sensitivity threshold

Abstract

Purpose : Pulsed infrared light (IR) causes visual sensation in humans, with a color perception corresponding to light of roughly half the wavelength. The authors previously identified this phenomenon as two photon isomerization of visual pigments. In this contribution, we present an improved laboratory set-up, and data from IR psychophysical measurements in human subjects.

Methods : The improved set-up, Fig. 1, allows for the measurement of sensitivity threshold via the use of visible and IR light. Additionally, this novel set-up is equipped with the capability to measure recovery of light sensitivity after variable visible light bleaching.
Light from a pulsed laser (1045nm) is split into two beams where one is routed through the crystal for second harmonic generation (SHG) at 522.5nm. Both, IR or visible light beams are directed into the dark adapted subject’s eye pupil after reflecting off the pair of galvo-scanners. The subject can adjust the beam power by operating a gradient filter with a computer mouse. The bleaching LED diode (530nm) was providing a uniform illumination of the subject’s retina.
Retinoid content in Abca^–/^– Rdh8^–/^– mouse eyes at defined time-point after bleaching was quantified by high performance liquid chromatography (HPLC).
All tests in human subjects were conducted in compliance with ANSI Z136.1:2014 and/or EN 60825−1:2014. The study was approved by the Ethics Committee of the Collegium Medicum NCU.

Results : Mean sensitivity threshold for IR beam was 24±6 µW for 10 volunteers aged 30-42 years old – Fig.2. The threshold from the same group for visible light had large variation: 1.5 fW-25 pW, confirming that measurements of visual threshold with IR provide more consistent results. Our result indicate that compared to previous reports in BALB/c mice, recovery of retinyl esters and visual chromophore after bleaching are delayed in mouse models of Stargardt disease.

Conclusions : IR and visible light induced vision processes can now be easily compared using the same setup. Bleaching optical train allows for measuring the recovery time curves for two-photon vision. Retinoids measurement in mice supports measurements in humans by providing a way of tracking visual system kinetics trends on the molecular level.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Fig.1. Set-up for sensitivity measurements.

 

Fig.2. Sensitivity threshold results from humans.